proceedings yrc 2013

Proceedings of The 4th

Conference Young Researchers from TUCEB

ISSN 2069-1793

BUCHAREST

2013

CONTENTS CONSOLIDATION PROCEDURE FOR UNSATURATED SOILS USING A MODIFIED SETUP OF THE TRIAXIAL COMPRESSION APPARATUS........................................................................................................... 5

Adrian Liviu BUGEA

SLOPE STABILITY ANALYSIS OF UNSATURATED SOILS ......................................................................... 11

Andreea CARASTOIAN

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF INDOOR COMFORT AND ENERGY CONSUMPTION IN A TYPICAL ROMANIAN CLASSROOM FOR DIFFERENT GLAZING AREAS .... 21

Tiberiu CATALINA, Razvan POPESCU, Nicolae BAJENARU, Andrei ENE

INFLUENCE OF LONGITUDINAL VORTICES ON HEAT TRANSFER FOR AIRFLOW PASSING THROUGH AN INNOVATIVE SOLAR FACADE .............................................................................................. 27

Cristiana Verona CROITORU, Florin BODE, Ilinca NASTASE

DESIGN PROCEDURE FOR SIDE WALLS OF SOCKET FOUNDATIONS .................................................. 36

Ionu DAMIAN

CONCEPTION OF AN ADVANCED THERMAL MANIKIN FOR THERMAL COMFORT ASSESSMENT IN BUILDINGS AND VEHICLES .......................................................................................................................... 56

Angel DOGEANU, Ciprian CALIANU, George CHITARU, Matei GEORGESCU, Andrei TUDORACHE

ANALYSIS OF CREST CUTOFF WALL AT OSTROVUL MIC LEFT SIDE EMBANKMENT DAM ....... 67

Daniel Andrei GAFTOI, Ctlin POPESCU, Drago FRILESCU

ENERGY SAVING ANALYSIS INSIDE A DOUBLE SKIN FACADE ............................................................. 76

Sebastian HUDITEANU, Claudia-Florentina POENARI, Bogdan-Iulian BALINT, Monica CHERECHE,

Nelu-Cristian CHERECHE

OPEN SOURCE 3D MODELING FROM RASTER IMAGES ........................................................................... 82

Gabriel Adrian KEREKES

INFLUENCES OF OXIDATION STEP AND INITIAL METAL CONCENTRATIONS ON IRON AND MANGANESE REMOVAL EFFICIENCY .......................................................................................................... 88

Alexandru JERCAN

SEISMIC RESPONSE OF TALL BUILDINGS WITH ROCKING WALLS SYSTEM ................................... 97

Lidia MARIN, Mircea VADUVA

NUMERICAL MODELLING OF RIGID VERTICAL INCLUSIONS AS REINFORCEMENTS FOR COMPRESSIBLE SOILS..104

Iulia-Victoria TALPOS (NEAGOE)

MATHEMATICAL MODELLING OF SHOCK WAVES GENERATED BY BLAST EVENTS AND THEIR EFFECT ON CONCRETE STRUCTURES ......................................................................................................... 111

George-Bogdan NICA

MATHEMATICAL MODELING FOR THE SEDIMENTATION PROCESS IN THE RESERVOIRS ...... 120

Catalin POPESCU, Daniel Andrei GAFTOI, Drago FRILESCU

INTEGRATED SOLUTIONS FOR IMPROVING THE ENERGETIC PERFORMANCE OF BUILDINGS .................................................................................................................................................................................. 129

Ana-Maria PASRE, Paul ANGHEL, Nelu-Cristian CHERECHE, Andrei BURLACU

R2D NUMERICAL MODELING OF FLOW THROUGH GEOMEMBRANE DEFECTS IN A LANDFILL LINING SYSTEM .................................................................................................................................................. 135

Gheorghe PANTEL

NUMERICAL MODELLING OF PILED RAFT FOUNDATIONS ................................................................. 148

rpd SZERZ

INFLUENCE ASPECTS OF EXTERNAL PARAMETERS UPON EFFICIENCY OF A NATURAL SMOKE EXHAUST SYSTEM .............................................................................................................................................. 157

Andrei-Mihai STOICA

DYNAMICS OF FREE SURFACE FLOW AROUND A CYLINDER VISUALIZATIONS AND PIV MEASUREMENTS ................................................................................................................................................ 168

Nicoleta Octavia TNASE, Florin BODE

DYNAMIC BEHAVIOR OF BUCKLING RESTRAINED BRACES AND THE INFLUENCE OF COMPRESSION STRENGTH ADJUSTMENT FACTOR .............................................................................. 174

Mircea VADUVA, Lidia MARIN

FISH FRIENDLY TURBINES USED FOR VERY LOW HEAD SMALL HYDROPOWER PLANTS........181

Daniel Andrei GAFTOI, Catalin POPESCU, Andreea BELA, Dragos FRATILESCU

PROCESSING AND ANALYSES STATICAL DATA OF TROWELS CONCRETE DOUBLE...................191

Daniel Alin SERBAN

TECHNOLOGICAL PROCESS MODELING SMOOTHING OF CONCRETE FLOORS WITH THE WORKING BODY AT TROWELS.......................................................................................................................201 Daniel Alin SERBAN

CONSOLIDATION PROCEDURE FOR UNSATURATED SOILS USING A MODIFIED SETUP OF THE TRIAXIAL COMPRESSION APPARATUS

Adrian Liviu BUGEA PhD Student, Technical University of Civil Engineering, Faculty of Hydrotechnical Engineering, e-mail: [email protected]

Abstract: This paper presents the proposed diffusion procedure for obtaining controlled suction samples, in order to determine the soil water characteristic curve (SWCC). The method was mentioned by D.G. Fredlund [1] and it was adapted to the triaxial compression equipment in the geotechnical laboratory of the Technical University of Civil Engineering Bucharest. Based upon the translation axis method, initially under a theoretical form, an optimal modified setup has been designed. Moreover, a consolidation procedure permitting the determination of the volumetric deformations is being analyzed, in order to obtain a volume variation law as a function of the moisture content under an imposed value of suction.

Keywords: unsaturated soils, matric suction, volume variation law, SWCC

1. Introduction

Last decades trend in Soil Mechanics is represented by the understanding and modeling of the unsaturated soil behavior, state in which over 80% of the cases of soil samples are found. Moreover, this is of great interest as the direct foundation systems are founded on this type of soil and certain particularities of their behavior are not covered by classical saturated soils mechanics, among which we can specify collapsible or swelling-contractile properties of eolian deposits or clayey layers.

Based on the works of researchers like D.G. Fredlund, H. Rahardjo [1], A. Gens or A. Lloret [2], the knowledge accumulated in the field of agriculture has been transferred in the geotechnical engineering, while from theoretical background new methods and apparatuses have been developed ( [3], [4], [5], [6]). Initially using mercury or water burettes, in order to measure either water pressures or pore air pressures, the nowadays technology implied in these tests evolved to such a precision that almost infinitesimal fluid volumes may be measured or applied to the samples.

This paper aims at presenting an evolved methodology to determine the constitutive surface of soils, in terms of net stresses, void ratio and matric suction, using a classic triaxial compression test apparatus, modified in order to be used in unsaturated soil tests. The sensors for monitoring the sample during the test and the conditions applied to it in terms of cell pressure, back pressure and pore air pressure are not new, but the entire setup and methodology have been designed in such ways that it may offer a clear image on the behavior of the soil.

2. Variation of indices and their determination

Following the molding of the sample to a ratio between the diameter and the height of 1:2, for which it is recommended to have a 50x100mm sample due to the stiffness reason, the sample is measured and the initial mass and physical parameters are being identified. Based on the relation of the general density with respect to the solid skeleton density, porosity and moisture content, one obtains all the indices to define soils initial state.

During the actual consolidation test, following the setup of the sample in the modified triaxial apparatus, a cell pressure, back pressure and air pore pressure combination is applied to the sample until all the displacement and internal stresses are stabilized. At the end of the stage, the void ratio, the matric suction and the net stresses that describe the new physic and mechanical state of the sample are saved. This combination belongs to a surface that connects these three components, describing the stresses that act on an unsaturated soil sample and its deformation

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state. This surface is obtained using either odometer test monoaxial infinitely confined compression test, or triaxial compression test. The Ct compression index is related to the at or av compression index obtained using the oedometer test, determined using the saturated soil mechanics relation:

(1)

where e is the variation of the void ratio and p the change in pressure. If the unloading case is considered, the ats coefficient is obtained. The Cm

Following this stage, new stress combinations may be applied, in order to obtain more points on the

index is determined considering volumetric variation during a drying test.

net-e-(ua-uw) surface.

Fig. 1 - Constitutive surface of an unsaturated soil [1]

Although Fredlund and Rahardjo [1] implied that this surface may be determined by the aid of odometer compression test and suction test using the translation of axes technique pressure plate drying test, a new method that involves both wetting and drying paths can be developed. If the air pore pressure is lowered, the newly obtained matric suction difference between air and water pore pressures, is lowered, therefore, the soil sample will arrive to an equilibrium state by adding water to its content the wetting path. If on the contrary, the air pressure is increased, the water content in the sample will reach lower values.

3. Proposed setup of apparatus

Based on the aforementioned techniques, a modified triaxial test setup is being proposed, in order to simplify the installation methodology of the sample and increase the accuracy of the measured deformations.

Most of todays unsaturated triaxial compression apparatuses are using the double wall method, in order to reduce the error regarding the deformations of the inner cell, affecting the volume calculations of the fluid that creates the hydrostatical pressure on the sample, and, finally, the calculations of the volume variations of the sample itself. This indirect method of determining the volumetric variations of the soil is both subjected to errors and difficult to use due to its complexity degree of setting up the sample inside the two celled apparatus.

Therefore, a more simple and direct setup has been developed, involving a dynamic triaxial compression apparatus, designed initially for saturated soil mechanics. The main difference between the saturated and unsaturated apparatuses are the presence of a high air entry value ceramic stone, which can maintain a pressure difference between water and air and the possibility to control another inflow fluid the air. Therefore, using the top cap drain, as shown

in Fig. 2, an air controller is linked to the upper part of the sample and through a porous stone the air pore pressure is varied. In the pedestal, a ceramic stone is placed, in order to keep the equilibrium between the pore air pressure (denoted with ua) and water back pressure (kept constant and denoted with uw), without getting air into the water system. Moreover, the ceramic disk is always held in a saturated state, due to its high capillarity and the water pressure applied bottom-up.

Fig. 2 The proposed unsaturated single cell triaxial setup

In order to overcome the problem of varying cell volume, the direct method of using displacement transducers to measure the radial and vertical deformations of the sample is being proposed. As Fig. 2 shows, in the case of vertical displacement transducer, a LVDT (linear variable differential transformer) is attached to the sample and its vertical deformation is measured. Considering the case of isotropic consolidation, the loading ram is not touching the top cap the undocked status, the vertical deformations cannot be determined but attaching the local vertical displacement sensor. In the case of anisotropic K0 (coefficient of active earth

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pressure at rest), consolidation step, although the distance between the porous stone outer surfaces is known, as the initial value was declared to be equal to the initial height of the sample, a more accurate displacement monitoring can be done using the attached sensor.

Although it is possible to back-calculate the radial deformation of the sample, knowing the cell volume and the sample height variation, a more accurate method is to measure directly on the sample the lateral surface variation - its radial deformation. In order to measure this type of variation, another LVDT is placed on a ring attached to the sample (Fig. 2) and whose initial position is set to a null value. All the measured linear deformations are treated as the perimeter deformations of the cross section, taking into account that the expected displacement value is small.

As the tests are undergoing, diffused air may accumulate under the ceramic porous stone as Fredlund [1] presented. Although several devices have been designed in order to determine the volume of diffused air, such as the bubble pump [7], or even a simple system to flush the diffused air beneath the ceramic disks [8], a newer technique developed by Lawrence [9] is to be used. It consists of rapidly changing the pressure in the pore water line, in order to determine the air volume using the ideal gas law. It still must be taken into consideration that these tests, especially on clayey soils are running for a longer period several days to weeks, and the diffused air volume may exceed the total volume of water of the tested sample. Therefore, a flushing apparatus must be connected to the aforementioned setup, in order to relieve and keep at low values the volume of the diffused air (Fig. 3).

Fig. 3 - The flushing device according to Fredlund [1]

The flushing of the diffused air is performed by opening the ball valve and moving to the tank a sufficient amount of water which will be carrying along the air bubbles. The water level is brought back to the initial level by the pressure and volume water controller. The difference between the initial position and the new one offers the volume of diffused air in terms of pressure for the differential pressure transducer, which measures the height of the water column in the flushing device.

Moreover, several authors ( [1], [10], [11]) also emphasized the possibility of wetting by accident the sample if the system is using water as cell pressure fluid. Therefore, a double membrane system with a film of oil between and a layer of silicone grease on the top cap and the pedestal are recommended to be used, adding to the mechanical waterproofing acquired by utilizing the O-rings.

4. Methodology of the consolidation test

In order to determine the constitutive surface of an unsaturated soil, as presented in Fig. 1, a series of multiple stages are to be taken into consideration. One must bear in mind that both of the wetting and drying path may be used, so a planning of the test is needed, as the well-known hysteretic behavior of the soil with respect to wetting and drying modifies its properties (Fig. 4).

Fig. 4 - Suction curve and hysteretic behaviour of the drying and wetting paths

If applying a drying path is considered, during the consolidation stage, a higher air pore pressure is used. At the equilibrium between the matric suction and the moisture content of the sample, all the deformations will reach a constant value, obtaining one of the points of the constitutive surface. In the case of considering a wetting path, a higher pressure is applied to the pore water line, in order to impose a gradient that will reduce the suction of the sample. In both cases, it is recommended to know the soil water characteristic curve (SWCC), in order to determine the value of the moisture content at the final moment, according to the detected matric suction (ua-uwThe following steps and getting more points respectively on the surface are obtained by modifying the cell pressure, increasing its value. Each step may contain various suction values, in order to determine the form of the surface in all three separate coordinate systems: net stress (-u

).

a) matric suction (ua-uw), void ration (e) matric suction (ua-uw) and net stress (-ua

Adding to these results of the consolidation tests the radial and vertical displacements also may be plotted in different systems, with respect to the matric suction, applied stresses and moisture content or void ratio, in the end, offering a more complex view on the behavior of the

) void ratio (e). Composing these three curves into one three-dimensional surface, offers the constitutive relations used in modeling the behavior of the unsaturated soils, using mathematical models such as Barcelona Basic Model [12].

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unsaturated soils, especially used for the case of predicting the swelling and contraction properties of soils ([13], [14]).

If the case of determining the shear resistance and the dynamic parameters with respect to the soil matric suction, the considered setup may be used, but the results and the methodology is still submitted to discussions.

5. Conclusions

This paper presented a setup to modify a classic dynamic triaxial compression apparatus in order to determine unsaturated soil consolidation surface, in terms of applied net stresses, matric suction and void ratio.

The aim of this new setup is to overcome some of the difficulties presented by other authors and obtaining optimum installation, testing procedures and conditions in terms of device complexity and sample monitoring, as well as accuracy of results.

Although at first it has been designed bearing in mind only the monitoring of the swelling-contractive behavior of clayey soils under constant stress state (consolidation step), further research in terms of determining the shear resistance and even dynamic parameters of unsaturated soils is developed, aiming at offering a clearer and more precise view on the entire response of the soil material under different types of stresses that may lead to failure.

References

[1] Fredlund D., Rahardjo H., 1993, Soil mechanics for unsaturated soils, New York: John Wiley & Sons Ltd.. [2] Romero E., Gens A., Lloret A., 2001, Temperature effects on the hydraulic behaviour of an unsaturated clay,

Geotechnical and Geological Engineering, nr. 19, pp. 311-322. [3] Padilla J., Houston W., Lawrence C., Fredlund D., Houston S., Perez N., 2006, An automated triaxial testing

device for unsaturated for unsaturated soils, 4th International Conference on Unsaturated Soils, Arizona,. [4] Nishimura T., Fredlund D., 2003 A new triaxial apparatus for high total suction using relative humidity, 12th

Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, Singapore. [5] Lauer C., Engel J., A triaxial device for unsaturated sand - New developments, Dresden. [6] Ho D. , Fredlund D., March-June 1982, A multistage triaxial test for unsaturated soils, Geotechnical Testing

Journal , vol. 5, pp. 18-25,. [7] Bishop A., Donald I.,1961, The experimental study of partially saturated soils in the triaxial apparatus,

Proceedings of the 5th International Conference on Soil Mechanics and Foundation Engineering, Paris,. [8] Padilla J., Perera Y., Houston W., Fredlund D., Perez P., 2006 Quantification of air diffusion through high-air

entry value ceramic disks, Proceedings of the 4th International Conference on Unsaturated Soils, Carefree,. [9] C. Lawrence, 2005, Pressure pulse technique for measuring diffused air volume, Proceedings of the

International Symposium on Advanced Experimental Unsaturated Soil Mechanics, Trento,. [10] Rahardjo, H., Fredlund D., December 1996 Consolidation apparatus for testing unsaturated soils,

Geotechnical Testing Journal, vol. 19, nr. 4, pp. 341-353,. [11] Wulfsohn, D., 1994, Triaxial testing of unsaturated agricultural soils, n International Summer Meeting,

Kansas City,. [12] Alonso E., Gens A., Josa, A., 1990 A constitutive model for partially saturated soils, Geotechnique, vol. 40,

nr. 3, pp. 405-430,. [13] Hung Q., Fredlund D., 2004, The prediction of one-, two- and three dimensional heave in expansive soils,

Canadian Geotechnical Journal, vol. 41, nr. 4, pp. 713-737,. [14] Fredlund D., 1975 Prediction of heave in unsaturated soils, n 5th Regional Conference, Banglore,.

SLOPE STABILITY ANALYSIS OF UNSATURATED SOILS

Andreea CARASTOIAN PhD student Technical University of Civil Engineering, Bucharest, e-mail: [email protected] ;

Abstract: The paper presents the slope stability analysis of unsaturated soils considering geotechnical parameters and groundwater level variations due to saturation degree. Infiltrations are one of the main factors causing slope failures. The main parameters associated with slope stability analysis are the characteristics of water flow, change of pore-water pressure and shear strength of soils. The saturation degree of soils highly influences the geotechnical parameters. The finite element method was used to evaluate the stability of the slope.

Keywords: slope analysis, unsaturated soil, groundwater, finite element method, geotechnical parameters

1. Introduction

The stability factor of a slope can be computed using the finite element method by reducing the soil strength until the slope fails. The resulting stability factor is the ratio of the actual shear strength of the to the reduced shear strength at failure. In total stress analysis of soil slopes, total stress shear strength parameters ( and ) are often used. Pore pressures are not considered. These total stress analyses are appropriate in the short term only and not in the long term where slope stability is a minimum (Simons et al., 2001). [1]

It is important to formulate a slope stability analysis method, which can track the failure process from the initial deformation to the ultimate failure

2. Slope Stability Analysis

The slope stability analysis is an analytical tool for assessing the stability of a slope by using a simple failure model in the analysis.

The required safety factor depends on the consequences of losses in terms of property, lives and cost of repair in the event of slope failure. The safety factor is also dependent on the reliability of design parameters.

A factor of safety is placed on shear strength parameters;

- The strength parameters are independent of stress-strain behavior; - Some or all of the equations of equilibrium are used to the determine the safety factor; - Forces involved in the equilibrium methods are statically indeterminate.

2.1. Stress Analysis Method (SAM)

The failure of soil slopes came in a wide variety of conceivable manners. The qualitative definition is given in a book by Terzaghi, Peck and Mesri (1996): The failure of a mass of soil located beneath a slope is called a slide. It involves a downward and outward movement of the entire mass of soil participating in the failure. [2]

Slope stability analysis is globally performed by means of two methods: the simplified method and the numerical analysis method. The limit equilibrium which belongs to the simplified methods is one way to approach slope stability problems and is most widely used in practice. However, this method cannot consider the stress hysteresis effects during the formation of slope and the variation of stress for foundations due to the groundwater. Although the finite element techniques considers the formation process of slope and the property of foundation, the method

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requires high cost and long analysis time, and the slope stability data for evaluation purposes is relatively insufficient.

In this method, stress analysis is first performed on the slope using the finite element method. Based on the stress analysis results, the factor of safety for potential sliding surface is calculated, and the critical section is determined using the minimum safety factor. [3]

The safety factor in the finite element method is generally defined as:

(1)

Where, is the shear stress, and is the shear strength according to Mohr-Coulomb failure criteria.

Fig. 1 - Stress components of sliding surface [4]

The initial failures for most of the unsaturated soil slopes have small depth-to-length ratios and form the failure planes parallel to the slope surface; hence, the use of infinite slope analysis for stability evaluation is thus justified (Collins and Znidarcic, 2004). The factor of safety of the slope is calculated by using a modified Mohr-Coulomb failure criterion (Fredlund et al., 1978; Fredlund and Rahardjo, 1993):

= c + (n ua) tan + (ua uw ) tan b (2) FOS = c + tan +(uauw ) tan bW sin cos (3) Where, c is the effective cohesion, is the effective frictional angle, is the net normal stress, is the pore-air pressure, is the pore-water pressure, is the matric suction, is the internal friction angle due to matric suction.

The unsaturated friction angle ( ) depicts the increment rate of shear strength due to an increase in suction and it can be obtained by performing a series of triaxial compression tests under various matric suction conditions. In these tests the pore air pressure ( ) control and transducer are installed to measure the matric suction . For finite slope analysis, the factor of safety (FOS) of an unsaturated slope is expressed as: where W is the weight of slice, which is the product of (the total unit weight) and (vertical depth of the assumed slip surface) and is the slope angle.

The limit equilibrium methods calculate a factor of safety which, by definition, is assumed to be the same at all points along the potential slip surface. This is reasonable only at failure, when all the slices are on verge of failure; that is, when the factor of safety equals unity for each slice. In reality, the local factors of safety will vary somewhat along the slip surface, for some slices it might be one and higher value for others. In the case of brittle materials, even small hydrostatic loads can reduce the local factors of safety to less than unity and the progressive failure mechanism may be triggered. This happens, for instance, in over-consolidated clay that can exhibit residual shear strength under drained loading and in loose, saturated sands under

undrained loading. Therefore, the methods cannot explicitly model the mechanism of progressive failure (Pyke, 1991).[4]

Modeling of slope failure by the finite element method must address the following issues:

- Occurrence of large deformations; - The effect of three dimensional conditions; - Accurate following of the equilibrium path under hardening, softening and snap-back

behavior;

- Occurrence of narrow, continuous localized zone on which the slope slides. In total stress analysis, the pore water pressure within the soils is ignored, only the free water body is considered. Free water can be modeled as a material with unit weight, but has no strength (Figure 2a) or can also be modeled as an equivalent loading on the slope (Figure 2b). Since the free water body acts as counter weight, a submerged slope is always more stable than a dry one in total stress analysis.

Fig. 2 - Total stress analysis of the slope [4]

2.2. Strength Reduction Method (SRM)

The finite element method is a precise numerical analysis method which satisfies the force equilibrium, compatibility condition, constitutive equation and boundary condition at each point of a slope. It simulates the actual slope failure mechanism and determines both the minimum factor of safety and the failure behavior. It can also reflect real in-situ conditions better than most methods. Moreover, it can determine the failure process without assuming any failure planes in advance. (Griffith 1999; Matsui, 1990).[5]

There are two types of methods used in the finite element method to analyze slope stability Strength Reduction method and Indirect method. The Strength Reduction method is a direct method, which gradually reduces the shear strength of sloped ground materials until failure takes place. Failure is assumed to occur when the analysis does not converge. Under this condition, the maximum strength reduction factor where the analysis fails to converge becomes the factor of safety. The Indirect method determines the factor of safety by applying the calculated stresses combined with the conventional Limit theory method. The Strength Reduction method uses a finite element technique first proposed by Zienkiewicz (1975). We now focus on a Gauss point, A, of an element in a sloped ground structure to calculate the factor of safety of a slope as shown in Figure 3. The stress state at this point is represented in a Mohr circle. In order to represent the sliding surface, the shear stress at the point is divided by a factor of safety, F, so that the Mohr circle for the stress state of the fictitious sliding surface becomes tangent to the failure criterion.

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Fig. 3 - Strength Reduction Method [6]

That is, the stress state of the point is corrected to the failure state. An increase in the number of points results in a global slope failure. As soon as a finite element solution diverges, the analysis stops and the limit value, F, becomes the minimum factor of safety for the slope. This method requires stability in the numerical analysis, but returns consistent results and evaluates the actual failure behavior. [6]

To determine the minimum stability factor of the slopes, the elasticity modulus and Poissons ratio are assumed to be constant. The cohesion and friction angle are simultaneously reduced, and the factor of safety, is determined at the diverging point. The factor of safety for slope failure is determined on the basis of shear failure as follows:

(4)

Where, is the shear strength of the sloped material.

This is computed according to the Mohr-Coulomb criterion.

(5) Where,

- Coefficient of shear strength;

- Coefficient of shear strength;

SRF Strength reduction failure.

In order to determine the SRF accurately, it is necessary to trace the resulting values of causing the slope to fail. The incremental parameter is increased in very small steps even though it may extend the analysis time duration. Otherwise, calculating the minimum factor of safety may face difficulties.

3. Example studies

3.1. Studied cases

The finite element stability analyses were performed on homogeneous slopes. Two sets of runs were performed on each slope. The first case was a homogeneous slope consisting of the two soil types considering the influence of a horizontal water table at various depths, such as 3m, 5m, 7m and 10m. In the second set, on the same geometry, I analyzed the influence of the variation of the shear strength parameters. For both sets I performed the slope stability analysis using stress analysis method (SAM) in 2D and shear reduction method in 3D (SRM). The analyses were performed using the finite element software Midas GTS. Table 1 indicates the parameters for the finite element slope stability analysis for the first case, while Table 2 shows the corresponding parameters for the second case.

Table 1. Table 2.

Soil parameters for case 1 Soil parameters for case 2

Soil type

Case 1 - Soil 1 30 5 20

Case 1 Soil 2 10 20 20

The other parameters such as Youngs modulus and Poissons ratio are given the nominal values of and , in both cases. The slope geometry and its finite element mesh are shown figure 4.

Fig. 4 - Geometry and finite element mesh

3.2. Numerical Results for First Case

The numerical results of the safety factor are compared to each method, SAM and SRM and are shown in Figures 5-14.

- 2D results using SAM method without water table. FOS = 1,706. -

Fig. 5 - a) Displacements contour of the slope; b) Shear strength of the slope.

- 2D results using SRM method without water table. FOS = 1,30.


30 5

25 10

20 15

15 20

16

- 2D results using SAM method with water table at 3 m depth. FOS = 1,62.


- 2D results using SRM method with water table at 3 m depth. FOS = 0,975.














We can observe the difference of the value of the safety factor using those finite element methods. Using the stress analysis method, the value of the safety factor is bigger than 1.2, which means that it is stable. In the other method, by reducing the shear strength parameters, the program finds the most unfavorable situation, reducing the value of the safety factor to slope instability.

The next figures show the comparison of FOS with the shear strength parameters, influenced by the fluctuation of water table.

Fig. 15 16 - FOS values versus shear strength parameters, influenced by fluctuation of water table.

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Numerical result for slope analysis soil 2 Following the same steps the results for soil 2 are presented in the figure 16. We observe the influence of the shear strength parameter modification. The initial value of safety factor (FOS = 1.706) is lesser than the corresponding value (FOS = 1.38) for soil 1. Applying the strength reduction method, the safety factor remains in the unstable zone.

3.3. Numerical Results for the Second Case

In the second study I present a comparison between 2D and 3D analyses. The hypothesis is to observe the evolution of the safety factor by modifying the shear strength parameters consecutively, like in table 2.

- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,706. FOS (SRM) = 1.2. and

Fig. 17 - a) 2D - Displacements contour of the slope; b) 2D - Shear strength of the slope - SAM

Fig. 18 - a) 3D - Displacements contour of the slope; b) 3D - Shear strength of the slope SRM

- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,64. FOS (SRM) = 1.2. [1]. and

Fig. 19 - a) 2D - Displacements contour of the slope; b) 2D - Shear strength of the slope SAM


- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,59. FOS (SRM) = 1.05.

and



- 2D and 3D results using SAM and SRM method. FOS (SAM) = 1,53. FOS (SRM) = 0.95.

and



The results are shown in the next figure, comparing the safety factor with the shear strength parameters.

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Fig. 25 - Comparison of FOS on SAM and SRM methods

In this case, we can observe the difference between those two types of analyses. In 2D, applying the SAM method we can observe that the value of the stability factor is in the stable zone, modifying in each steps the shear strength parameters. With the same condition, applying the strength reduction method, in 3D, the value of stability factor is calculated using the reduced strength, being in the unstable zone.

4. Conclusions

The paper presents a comparison of two methods the stress analysis method and the strength reduction method using the finite element method for analyzing the slope stability for an unsaturated case.

The slope stability analysis of unsaturated ground requires to simultaneously computing deformation and groundwater flow with time dependent boundary conditions.

According to the results we can observe the advantages of the second method. SRM slope analysis can produce insights into the failure mechanisms, and their formation, in ways that may not be as evident in limit-equilibrium analysis.

The factor of safety is calculated using reduced strength, and the critical cross section is the area where the maximum shear strain occurs.

Slope stability analysis of unsaturated ground requires to simultaneously computing deformation and groundwater flow with time dependent boundary conditions.

References

[1] Simons, N.E, Menzies, B.K. and Matthews, M.C., 2001 - A Short Course in Soil and Rock Slope Engineering, Thomas Telford, London.

[2] Terzaghi, K., Peck, R., B., and Mesri, G., 1996 - Soil mechanics in engineering practice, third ed. John Wiley & Sons, Inc.

[3] Charles, W., W., Ng., Bruce, M., 2007- Advanced Unsaturated Soil Mechanics and Engineering, Taylor & Francis Group, New York;

[4] Pyke, R., M., 1991 - TSLOPE Users Guide, TAGA Engineering Systems & Software Lafayette, California, () [5] Griffiths, D.V., and P.A. Lane, 1999- Slope Stability Analysis by Finite Elements, Geotechnique, vol. 49, no.

3, pp. 387-403. [6] Shukra, R. and Baker, R., 2003 - Mesh Geometry Effects on Slope Stability Calculation by FLAC Strength

Reduction Method Linear and Non-Linear Failure Criteria. In Proceedings of the 3rd International FLAC Symposium, Sudbury, Ontario, Canada, eds. R. Brummer et al, pp. 109-116.

[7] *** MIDAS GTS SOFTWARE, 2012, User Guide.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF INDOOR COMFORT AND ENERGY CONSUMPTION IN A TYPICAL ROMANIAN

CLASSROOM FOR DIFFERENT GLAZING AREAS

Tiberiu CATALINA Lecturer, PhD, Technical University of Civil Engineering - Romania, Faculty of Building Services, e-mail: [email protected] Razvan POPESCU - Lecturer, PhD, Technical University of Civil Engineering - Romania, Faculty of Building Services, e-mail: [email protected] Nicolae BAJENARU Eng., Technical University of Civil Engineering - Romania, Faculty of Building Services, e-mail: Andrei ENE - Student, Technical University of Civil Engineering - Romania, Faculty of Building Services, e-mail:

[email protected]

Abstract: This article is divided in two parts: in the first part the indoor conditions in a typical classroom using experimental measurements are illustrated and in the second part, using numerical simulations, the impact of glazing area on the energy consumption for heating/lighting and indoor comfort is analyzed. It was found that in this typical classroom the indoor levels of CO

[email protected]

2 do not fulfill the requirements for a good air quality. However, thermal comfort is achieved as the air temperature is around 20o

Keywords: experimental measurements, indoor environmental quality, numerical simulations, energy consumption, design optimization

C. Moreover, the illuminance level is also achieved as the window area is around 25% from the floor area. During the numerical simulations we have studied a similar classroom with the one from the experimental campaign. Four windows-to-floor-area-ratios have been studied in terms of energy consumption, thermal and visual comfort. It has been found that a larger area of window affects greatly the cooling demand, but has a slight impact on the operative temperature. Moreover, the energy needed by the artificial lighting is reduced by 51% if we compare a window-to-floor-ratio (WFR) of 15% to WFR 30%. It is recommended not to overpass a WFR of 25% as this will affect too much the energy consumption.

1. Introduction

The reduction of energy consumption is an important point on the agenda of objectives to be attained by EU countries by year 2020. At the same time, we cannot neglect the true purpose of a building: to provide the occupants a comfortable and healthy indoor environment. Indoor environmental quality (IEQ) is a concept that deals not only with thermal conditions but it also goes much further, because it involves air quality, lighting and acoustics [1]. Given the many interactions between building energy performance and IEQ, these two issues must be addressed and researched in a connected manner. IEQ and energy are closely linked and only an integrated research project can ensure that improvements in energy efficiency do not reduce IEQ, and that improvements in IEQ do not decrease energy efficiency, either. This issue is clearly expressed even by the European Energy Performance of Buildings Directive 2002/92/EC which underlines that the expression of a judgment about the energy consumption of a building should be always combined with an analysis of the IEQ. The problem is even more complicated as these are conflicting criteria [2] and finding an optimal solution can be quite difficult for the current knowledge frontier. Considering this panorama, it is highly important to search for solutions of building design that provide the highest benefit for both energy saving and IEQ at the lowest cost. Educational facilities are among the most important fixtures in our community [3], where children spend around 25% of their time inside school classrooms, this area being like their second home [4]. As schools present a much higher occupancy than any other building, it is vital to have an indoor climate that will not affect the comfort, health or intellectual performance of the students [5]. In all the existing educational facilities in Romania many complaints were made about discomfort, poor air quality (health problems) and expensive bills for facility operation. This research project proposal emphasizes research on schools because these types of buildings have been underrepresented in prior research. Moreover, this category was chosen to be

22

studied because children have a greater susceptibility to some environmental parameters than adults [6] and there are limited or incomplete research studies in this area. Reported findings on the indoor air quality (IAQ) in schools [7-8], correlation between IAQ and energy [9-10] or the use of alternative ventilation systems in schools [11] have been made during the last decade. Provision of useful information on the frequency of exposure to indoor factors in the educational environment with precise survey data and experimental measurements is more than needed. In this article we tackle this important aspect and also focus on the impact of glazing area or lighting control on the energy demand and visual comfort. Finally, the study is completed with a full simulation of thermal comfort along with energy consumption for heating.

2. Experimental campaign

The experimental measurements took place in a classroom from a school situated in Rmnicu Vlcea region, more exactly in Mateeti which is a rural area. The investigation of indoor comfort was done on 1st October 2013 for the entire occupational period of that day, more specific from 08:00 am to 12:00 am. The equipment that was used is TESTO 480 that is a multi-function tool that can record the following parameters: air temperature, globe temperature, illuminance, air velocity, relative humidity and CO2 level. Moreover we have used the equipment Dylos 1100 Pro Air Quality for dust particles counting for 0.5 and 2.5 microns. For the sound measurements we used a sound meter class 2 precision. We have also recorded the outside CO2 level, the relative humidity and air temperature for the same period. The equipment used was a data logger type CO2

Meter mounted outside the school.

Fig.1 - Photos of the TESTO 480 installed in the classroom

The measurements were done with a time step of 1 min for the entire period. The room size is 8.7 m x 5.7 m x 3.48 m (L x l x h). During the measurement the room was occupied by 15 pupils and 1 professor. Three windows with a surface of 3.1 m2

each allow the daylight to enter into the occupied space. The windows are double pane glazing with a PVC frame and are considered a typical solution to replace the older windows of the school. The floor is covered with linoleum, while the rest of the walls and the ceiling are made of concrete painted in white. Three radiators with of length of 1.8 m and 0.6 m the height are installed in the room. These radiators are sized in order to ensure a good thermal comfort inside the room.

0

500

1000

1500

2000

2500

3000

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7:26 8:45 10:04 11:24 12:43Time (hh:mm:ss)

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60,0

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70,0

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Outdoorhumidity

10,0

12,0

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16,0

18,0

20,0

22,0

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Temp level [oC]

Indoorair temperature

Outdoorair temperature

Heating ON

50

150

250

350

450

550

650

750

850

950

1050

7:26 8:45 10:04 11:24 12:43Time (hh:mm:ss)

Illuminance level [lux]

Indoorilluminance

Fig.2 - Indoor comfort and air quality variation inside the classroom

From Figure 2 it can be noticed that the CO2 level before the pupils arrive to school was situated at 600 ppm and then increased up to 2800 ppm. The indoor air quality norms are not fulfilled as the maximum allowed value should be around 800 ppm, which is a translation of a poor air quality. The explanation for this high values is that the classroom does not have a ventilation system to supply the fresh air and that the windows have a good air tightness. This lack of fresh air is translated in a low energy consumption zone, but at the same time in a low air quality. Due to heavy rains the day before the measurement we have recorded a high value for the relative humidity, while the indoor levels are in the range of 55% to 60%, more than acceptable. The outside temperature during that period was low with values around 11o

Due to building thermal inertia and heat gains from the pupils the indoor air temperature was kept in a comfortable range of 20

C which triggered the heating system to start but only for 2 hours.

oC to 22o

As noise problems are concerned, it was found that the global sound pressure level was higher (45 dB(A)) than 35 dB(A) which is the limit value for a comfortable environment. This is due to the sound that is propagated from the outdoor to the indoor space mainly through the windows. The exterior environment was found to be noisy as the traffic is intense and multiple trucks are passing by the school faade.

C. The indoor illuminance was also measured and the mean value for the entire period overpasses the visual comfort standards [12]. It must be mentioned that during the measurements the sky was covered by thick clouds and the artificial lighting was turned on for the school hours. It can be concluded from this experimental investigation that the building performs well in terms of thermal and visual comfort but has a serious problem with the indoor air quality. Moreover, it was observed that the artificial lighting control is not optimized and there is a waste of electric energy.

3. Numerical simulations

After the experimental analysis we have done a theoretical study of a classroom that has the size of 8m x 6m x 3.2 m (L x l x h) using numerical simulations. We found these dimensions relevant as most of our studied classrooms had these sizes. One of the most trustworthy solutions to evaluate the indoor conditions and energy consumption of a building are the dynamic simulations. It is clear that during the design stage of a new building it is important to verify the impact of a certain parameter on the building performance. As the building faade is a key element for the indoor conditions we wanted to study the impact of the window area on the thermal comfort and energy consumption. Like for the experimental studied classroom this simulated virtual classroom has a good air tightness of 0.2 ach/h and the installed windows are double pane glazing windows. The walls are made of brick and are insulated with 7.5 cm of

24

polystyrene. The structure of the walls, the floor and the ceiling corresponds to real schools. The simulated classroom is oriented WEST and is situated at the last floor of a building in order to have the most disadvantageous case from the thermal point of view. The ceiling is adjacent with an attic insulated with 10 cm of mineral wool.

0

500

1000

1500

2000

2500

3000

3500

WFR 15% WFR 20% WFR 25% WFR 30%

Ener

gy [k

Wh/

an]

Heating demandCooling demandArtificial lighting

Fig.3 - Annual heating, cooling and electric energy consumption

The simulations were done for the entire school year (15 September to 15 June) and we have taken into account the occupation periods with the holidays/weekends/days hours, the heat gain from artificial lighting and from pupils. The time step of the simulations was set to 1 hour and they were realized using the TRNSYS 16.0 software.

The same scenarios of window-to-floor-ratios were used to investigate the impact on the heating and cooling demand during the coldest and warmest day of the school year (see Figure 4). If for the heating we do not observe large variations for the cooling demand we have a much larger difference of up to 1500 W between WFR 15% case and WFR 30% scenario.

Like it was mentioned previously the purpose of the numerical study was to check the building performance for different glazing areas. We have considered four cases of window-to-floor-ratio (WFR): 15%, 20%, 25% and 30%. From Figure 3 it can be noticed that there is an increase of 24 % for the heating demand between the WFR 15% case and WFR 30%. At the same time there is an increase of 51.4% of the cooling demand for the same comparison. On the other hand we observed a reduction by 43% of the electric energy if we increase the WFR from 15% to 30%.

0100020003000400050006000700080009000

0 2 4 6 8 10 12 14 16 18 20 22

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ing

dem

and[

W]

Heating demand in the coldest day of the school yearWFR15%WFR20%WFR25%WFR30%

School hours

0

1000

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3000

4000

5000

0 2 4 6 8 10 12 14 16 18 20 22

Cool

ing

dem

and[

W]

Hour[h]

Cooling demand for the hottest day of the school year

Occupation period

Fig.4 - Heating and cooling demand for the coldest/warmest day of the school year

Thermal comfort is defined by ASHRAE [13] as that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation. In our case we express the thermal comfort using the operative temperature which translates the influence of both air temperature and radiant surrounding temperature. The set point temperature for the air was considered of 20oC and it can be observed that during the coldest day the operative temperature is lower than 20oC, but also equal for the WFR scenarios. For the summer period the differences between the four scenarios of WFR are higher but are still low with values around 0.5oC.

15

16

17

18

19

20

21

0 2 4 6 8 10 12 14 16 18 20 22

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pera

ture

[C]

Hour[h]

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WFR15%WFR20%WFR25%WFR30%

Occupationperiod

25,0

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Tem

pera

ture

[C]

Hour[h]

Operative temperature [oC] -Warmest day

WFR15%WFR20%WFR25%WFR30%

Occupationperiod

Fig. 4 - Heating and cooling demand for the coldest/warmest day of the school year

The illuminance level was obtained by using DIALUX software and several simulations were necessary to have the data for different outdoor illuminance. Once the correlation between indoor levels and outdoor illuminance was obtained, then by using a regression equation we were able to calculate for each hour of the school year what the indoor illuminance level was. If the 300 lux level was not attained then the artificial lighting was turned on and the energy consumption would be of 872 W. The simulations were conducted for an overcast sky, as mentioned by Comission Internationale de lEclairage [14]. The parameters of the room were introduced in the software: the reflection coefficients (walls and ceiling of 80% and 52% for the floor), the indoor objects and the window type (double pane glazing with a visible transmittance of 90%). In Figure 6 is illustrated only a single simulation that was realized for the date of 21 September at 12:00. Between the four scenarios of WFR the indoor illuminance varies greatly from a mean value of 400 lux for WFR 15% to 797 lux for WFR 30%. We observe a clear increase of the natural lighting if increasing the window area. This increase of visual comfort has also a repercussion on the electric energy consumption which will decrease drastically if the WFR is higher.

Fig. 6 - Illuminance level for the WFR 15%, 20%, 25% and 30% - Simulation time 21 September at 12:00

4. Conclusions

15% 20%

25% 30%

26

The data obtained in this article were found from experimental measurements and from the numerical analysis of the window to floor area ratio (WFR). In this research paper the indoor comfort is studied experimentally during one day using professional equipment that allowed the measurement of air quality, air temperature, humidity and illuminance level. The measurements showed that the air temperature is comfortable for the intellectual activities even if the outdoor air was around 11oC. The CO2

Acknowledgements

level was found to be much higher that the value proposed by the standards with values of 2800 ppm. Despite the cloudy day, the indoor visual comfort is achieved by both natural and artificial lighting. The glazing area is a key element in the design of new classrooms, and that is the reason for studying different cases of WFR. If for the operative temperature there is a slight difference between the WFR 15% and WFR 30% for the heating/cooling consumption we have larger differences. A larger area of window signifies lower energy consumption for the artificial lighting. The energy reduction can go up to 51% if using a WFR of 30% compared to 15%. It can be concluded that a low WFR is not sufficient for a good daylight while a higher WFR will increase the energy consumption. It is recommended that the WFR should not overpass 25% or to be lower than 20%.

This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS UEFISCDI, project number PN-II-RU-TE-2012-3-0108.

References

[1] A.C.K. Lai, K.W. Mui, L.T. Wong, L.Y. Law, An evaluation model for indoor environmental quality (IEQ) acceptance in residential buildings, Energy and Buildings, Year 2009, Volume 41, Pages 930936.

[2] Tiberiu Catalina, Vlad Iordache, IEQ assessment on schools in the design stage, Building and Environment, Volume 49, Pages 129-140, Year 2012

[3] Adelman H.S., Taylor L., Classroom climate, Encyclopedia of school psychology, Thousand Oaks, CA: Sage, Year 2005.

[4] Grimsrud D., Bridges B., Schulte R., Continuous measurements of air quality parameters in schools, Building Research and Information, Year 2006, Volume 34, Issue 5, Pages 447458.

[5] Bartlett K.H., Martinez M. and Bert j., Modeling of occupant-generated CO2 dynamics in naturally ventilated classrooms, Journal of Occupational and Environmental Hygiene, Volume 1, Issue 3, Year 2004, Pages 139148.

[6] Faustman EM, Silbernagel SM, Fenske RA, Burbacher TM, Ponce RA., Mechanisms underlying children's susceptibility to environmental toxicants, Environmental Health Perspectives, Year 2000, Volume 108, Issue 1, Pages 13-21.

[7] World Health Organization Methods for monitoring indoor air quality in schools, Report of a meeting, Bonn, Germany, 4-5 April 2011, Regional office for Europe.

[8] Daisey JM, Angell WJ, Apte MG., Indoor Air Quality, ventilation and health symptoms in schools: an analysis of existing information, Indoor Air, Year 2003, Volume 13, Pages 53-64.

[9] J.Jalas, K.Karjalainen, P.Kimari., Indoor air and energy economy in school buildings, Proc. Of Healthy Buildings,Year 2000, Volume 4, Pages 273-278.

[10] Becker R., Goldberger I., Paciuk M., Improving energy performance of school buildings while ensuring indoor air quality ventilation, Building and Environment Volume 42, Issue 9, September 2007, Pages 32613276

[11] Mikola A., Voll. H, Koiv T., Rebane M., Indoor climate of classrooms with alternative ventilation systems, GEMESED Proceedings of the 4th WSEAS international conference on Energy and development - environment biomedicine, Year 2011.

[12] European Standard EN 15251, Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics, August 2007.

[13] ASHRAE. ANSI/ASHRAE Standard 55-2004 Thermal Environmental Conditions for Human Occupancy. Atlanta : American Society of Heating, Refrigirating and Air-Conditioning Engineers, 2004. ISSN 1041-2336.

[14] CIE Technical Comittee 4.2, The Availability of Daylight, Technical Report No. NR, Comission Internationale de lEclairage, Paris, 1975

INFLUENCE OF LONGITUDINAL VORTICES ON HEAT TRANSFER FOR AIRFLOW PASSING THROUGH AN INNOVATIVE SOLAR

FACADE

Cristiana Verona CROITORU-Lecturer, PhD, Technical University of Civil Engineering Bucharest, Faculty of Building Services, Politehnica University of Bucharest; Florin BODE-Lecturer, PhD, Technical University of ClujNapoca; Ilinca NASTASE-Associate Professor, PhD, Technical University of Civil Engineering Bucharest, Faculty of Building Services, e-mail: [email protected]

Abstract: Renewable energy represents an attractive solution to fulfil two requirements: indoor environmental quality and energy efficiency. Passive solar systems are easy to implement and effective in areas with high solar potential. The Unglazed Transpired Solar Wall (UTSW) is made of metal cladding with perforations, installed at several centimetres from a building wall, creating a cavity. The air is forced to pass through this heated perforation, and thus a heat transfer between the fluid and the metal takes place. Several measurements and CFD simulations were performed on an innovative perforated solar wall. This study is a preliminary analysis approach on the importance of the orifice shape of the perforated panel as a heat transfer parameter. The results found in literature were compared with experimental and CFD results. A good agreement was found. Changing the geometry of the perforations will increase on one hand the perforation perimeter and on the other hand will generate complex fluid dynamics, resulting in a higher efficiency of heat recovery of these devices.

Keywords: unglazed transpired solar wall; solar energy; energy efficiency; CFD modelling

1. Introduction

The new European Directives concerning energy performance of buildings imposes significant reduction of the energy consumption. For this reason, the EU Members have adopted drastic regulation in order to achieve high building performance. On the other hand, the indoor quality has become an important parameter when conceiving residential or office buildings. The requests of the occupants are more exigent and achieving the indoor comfort is one of the most important challenges for civil engineers. Generally, the buildings sector consumes 35.3% from the total energy demand. This energy demand is caused mainly by the HVAC (Heating Ventilating and Air Conditioning) Systems. During the winter season in cold countries, the heat demand of the building represents the highest percentage from the total amount of energy demand, while during the summer, air treatment or ventilation is a major consumer of electrical energy. In this context, the use of renewable energies is an attractive solution for fulfilling the two requirements: indoor environmental quality and energy efficiency. Among renewable energies, the use of solar passive systems are easy to implement and efficient from the accessibility point of view in the zones with solar potential.

The Unglazed Transpired Solar Wall (UTSW) is made of metal cladding with perforations, installed at several tens of centimetres from a building wall, thus creating a cavity through which air circulates.

The schematic drawing of this type of solar collector is as illustrated in Fig. 1. The metal cladding is heated by the solar radiation from the Sun and ventilation fans create negative pressure in the air cavity, drawing in the solar heated air through the perforated panel. The air is generally taken off the top of the wall (due to air temperature gradients in the cavity) ensuring that all of the produced solar heat is collected, and then distributed in the building via the ventilation system. In the summer conditions, the system can work only during the night for free cooling ventilation, while during the day the air layer has an insulation role.

28

A literature survey led us to some interesting conclusions: (i) a consequent part of the heat transfer between the air and the solar collector is occurring during its passage through the perforation orifices; (ii) it is preferable to have a non-uniform flow on the back of the plate.

a) b)

Fig.5 - a) Schematic drawing of an unglazed transpired wall, b) Innovative perforated panel developed at ULR [1]

On the other hand, passive mixing techniques applied to HVAC air diffusion terminal units have been developed greatly during the past decade, since a collaborative research team from the University of La Rochelle and UTCB dedicated numerous studies to these devices [2-11]. A new research direction has been started at La Rochelle University (ULR) regarding the possibility of using passive control for enhancing heat transfer in impinging jet flows [12]. All these studies use a special geometry of nozzles, ailerons or orifices, which is called lobed geometry. An example of such geometry is the lobed orifice. In Fig.1 b a perforated panel with lobed orifices (cross shaped or 4-lobed orifices) is presented. For the same effective area (same equivalent diameter) the perimeter of the lobed jet is much larger than the one of the circular orifice, increasing the contact boundary between the air flow passing through the orifice and the orifice thickness. Under low or moderate Reynolds numbers, such as the one characterizing the flows in the UTSW, the analysis of the elementary lobed nozzle and orifice jets shows that the lobed shape introduces a transverse shear in the lobe troughs [7, 13-15].

2. Methods

2.1. Experimental approach

The perforated panels were placed on a rectangular box with thermally insulated walls. The box is connected through a circular pipe to an exhausting fan, forcing the ambient air to pass through the perforated panel. After positioning each perforated cladding, the box is sealed with sanitary silicone, in order not to have leaks which might perturb the tests.

At a distance of 30 cm of the cladding, four Metal Halide Flood Lights were placed, each corresponding to a lightning level of 400 W, which are simulating the Sun radiation. The aspiration fan creates the negative pressure necessary to force the air to pass through the perforations, so the convective heat transfer takes place. Velocity, pressure and temperature probes were placed in strategic points, either for controlling the conditions, or obtaining the results of the measurements, as can be seen in the next figure. An acquisition data box was employed to record the measurement values for a certain stabilisation period. The acquisition time step is 60 seconds.

a)

b) Fig.6 - a) Schematic drawing of the experimental facility b) Experimental set-up photograph: Radiation lamps,

exhaust pipe and the perforated panel

The cladding has a perforated surface of 0.49m2 (0.7m x 0.7m) and receives only a certain percentage of the total radiative intensity emitted by the lamps. Four temperature probes were placed in strategic points: t1- exit temperature; t2- ambiental temperature; t3- cladding surface temperature; t4- black globe temperature. The velocity and pressure probes were used to evaluate the airflow and pressure loss of the perforated sheet. The flow rate was evaluated using the omnidirectional velocity probe from TSI, placed inside the exhaust pipe. For all the measurements, we waited for the stabilisation time, each time three different readings of the values being done.

The indoor temperature and relative humidity were permanently monitored. Several types of perforated panels were tested: the baseline panel with round shape perforations and the innovative panel with lobed cross-shaped perforations. The equivalent diameter De of both geometries of orifices was 5mm. The porosity for each type of tested perforated panel is given by the distance between two adjacent orifices, from centre to centre, of 13.5 mm (3C and 3R) and 19 mm (4C and 4R) for each type of perforation.

The perforated panels are positioned on a rectangular box with thermally insulated walls. The box is attached through a circular pipe to an exhausting fan, forcing the air to pass through the perforated panel, fact that conducted to the heating of the air.

The plate collects only a certain percentage of the total radiative intensity emitted by the lamps. The radiation transmitted effectively between the source of light and the plate was considered to be around 800 W/m2 (value in agreement with the experimental conditions from [16]).

2.1. Numerical approach

The numerical simulations by the CFD approach using a RANS (Reynolds Averaged Navier Stokes) model were performed to study the airflow and heat transfer through the two types of perforations for different values of the airflow.

t1

t2

t3

t4

Fan Insulation

Perforated panel

30

Given some considerations of symmetry and in order to save numerical resources for a finer mesh, the numerical study was performed for a smaller perforated cladding corresponding to a part of the experimental panel. The model comprises 25 perforations (Fig. 3), with the same space between two adjacent centers of orifices, as in the experimental case. The metal cladding has a size of 10 cm by 10 cm positioned at 15 cm from the exhaust surface. This approach will test the capabilities of the CFD models to reproduce from the global point of view our previous experimental results [17] in order to investigate in a next step which the influence of the flow dynamics is on the heat transfer enhancement.

s Fig. 7 - Section through the used grid and detail of the cross perforated cladding

The boundary conditions on the perforated cladding considered an imposed thermal flux such as in the experimental case. The rest of the walls are considered with 0 W heat-fluxes. The air is aspired through the perforations as in the experimental case. A value of turbulence intensity of 9.8%, calculated with empirical relation proposed by [18], was imposed for the inlet boundary condition.

The accuracy of a CFD simulation depends, in a high percentage, on the way of replicating the geometry that defines the calculation domain and the heat sources, with specific boundary conditions. The final computational domain comprises 4.5 million hybrid cells: both tetrahedral and hexahedral cells for a better characterization of the flow. Inside the orifices a first layer of 0.2 mm was applied with a growth factor of 1.15. Outside the orifices, the mesh on the plate has a first layer of 5 mm, with a growth factor of 1.15. The viscous model was chosen to be k-omega SST in agreement to previous studies performed on the lobed perforations[19].

3. Results

The studies performed for solar walls systems showed good results in terms of energy efficiency. In this context the use of solar passive systems is encouraged by national regulations as they can have a significant contribution to achieve high performances and to save energy for winter heating and for summer cooling. Table 1 summarizes some of the case studies available in the literature. A quick survey allows us to be aware of the huge possibilities of such devices in energy recovery. For instance, the CFD study of Arulanadam et al. [20] concludes that not only metal cladding could be used for the perforated absorber but even low conductivity materials can lead to acceptable thermal efficiency of the system, for low porosity of the transpired plate absorbers and for low velocity flow situations. But studies such as the ones of Van Decker et al. [21], Gunnewieck et al. [22, 23] are very interesting from our point of view, given the information related on the direct possibilities of improvement of these devices. The early numerical study of Gunnewieck et al. [22] highlights the importance of a non-uniform flow and of a low velocity on the efficiency of unglazed transpired solar air heaters of large area. Van Decker et al. [21] show that in no-wind conditions, about 62% of the ultimate temperature rise of the air is predicted to occur on front-of plate, 28% in the hole and 10% on the back of the plate.

Cordeau and Barrington [24] in their study of an UTSW, used for bringing fresh air in a broiler barn, reveal that the efficiency of the solar air pre-heaters reached 65% for wind velocities under 2 m/s, but dropped below 25% for wind velocities exceeding 7 m/s, with an annual return on investment of 4.7%. Different other case studies of UTSW [16, 22-32] pointed out an energy efficiency of the system used from 52% to 68%, being an important benefit in terms of fossil energy consumptions savings.

Table 1

Current studies on UTSW

Reference Collector area and type Airflow rate

[m3/h/m2

Temperature rise

]

Efficiency Energy saving

[33] 1877 m2 125 ; vertical wall; 2% porosity; 1% canopy;

12.5 C 57% 917 kWh/m2/year

[33], [34] 420 m2 72 gross; 2%porosity; 1% canopy

13 C 52% 754 kWh/m2/year

[33] 27.9 m2 N/A ; 2% porosity N/A 63-68% N/A [35] 335 m2 N/A ; corrugated dark brown

aluminum N/A N/A 195 700 kWh/m2

[35] Solar wall panel area=1.1664 m2 100 ; PV cells covered 24% of entire surface

N/A Thermal efficiency 48% Combined efficiency 51%

500-1000 kWh/m2From which electricity

/year

50-100 kWh/m2/year [36] 2 m; Transpired solar collector 117 13.2C Present study

1 m2 10-150 [m

; Transpired solar collector; 0.6 % - 10% porosity; black aluminum sheet

3/h/m29C-30C

] 60-70% for airflows larger than 50 [m3/h/m2

N/A

]

In the present study, the heat transferred from the plate to the air (P) was quantified by the air temperature rise, using:

P=mair* cp *(Tpipe-Tambwhere m

) (1)

air

Four cases were studied in comparison with a standard configuration of a commercial perforated panel for UTSW systems. The studied cases are: 3R - Round orifices with S= 13.5 mm, 4R - Round orifices with S= 19 mm, 3C Cross shaped orifices with S=13.5 mm and 4C Cross shaped orifices with S=19 mm.

is the mass flow rate.

The efficiency of the panel was defined as:

plT AIP

= (2)

where P is the heat transferred from the plate to the air, IT is the irradiation provided by the lamps to the plate level and A is the surface of the plate of 1 m2

In Fig. 4 we represented the evolution of the thermal efficiency for the four cases investigated, in comparison with the data obtained by using a commercial UTSW and the two models proposed by Belusko et al. et Shukla et al. for UTSW without wind and with circular perforations with the same equivalent diameter as in our case [

.

30, 37]. For both perforation rates studied in the present experimental campaign we can see an advantage of the innovative perforated plate with lobed orifices compared to the baseline round orifices panels. They present also a clear advantage for high flow rates when compared to the analytical models of Belusko et al. et Shukla et al. for panels with circular perforations.

32

a) b) Fig. 8 - UTSW efficiency for different volumetric flow rates: a) S=13.5, b) S=19 mm

If we compare the heat transfer that occurs, we can observe that the cross shape performs better, for both cases: 4C and 3C.

Fig. 9 - UTSW efficiency for different volumetric flow rates: a) S=13.5, b) S=19 mm

We wanted further to test a larger domain of the airflow employed in the experimental setup for the metal cladding with perforation of 13.5 mm (3C and 3R). We evaluated the heat transfer for airflows ranging from 30m3/h up to 270 m3/h. We can observe that after a stagnant zone of heat transferred, for values of airflow of 100 m3

/h we obtain a significant increase in thermal power. These findings will be treated in a further study.

Fig. 10 - Heat transfer for a larger airflow domain: 3C and 4R comparison

Let us take a look to the CFD results compared to the experimental data. In Fig. 6 and 7 we present typical velocity and temperature fields in a plane passing through one of the perforation rows adjacent to the median plane, for both geometries: cross and round shape. We can observe an increase in temperature between inlet and outlet, intensified in the case of the cross perforation.

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70

0.00 50.00 100.00 150.00 200.00

Effi

cien

cy (%

)

Q (m3/h/m)

3R

3C

Commercial UTSW

3R Dymond et al. 1999

3R Belusko et al 2009

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20

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60

70

0.00 50.00 100.00 150.00 200.00

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Commercial UTSW

4R Dymond et al. 1998

4R Belusko et al 2008

50

100

150

200

250

300

0 20 40 60 80 100 120 140

P (W

atts

)

Q (m3/h)

Round Orifices 3R

Cross Orifices 3C

Round orifices 4R

Cross orifices 4C

Reference perforated pannel from commercial system

50

100

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350

400

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500

550

-20 30 80 130 180 230 280

P (W

atts

)

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3R

a) b)

Fig. 11 - CFD fields for the round perforations at a flow rate of 96 m3

a) b)

/h: a) velocity, b) temperature

Fig. 12 - CFD fields for the cross-shaped perforations at a flow rate of 101 m3

In Fig. 8 we superposed experimental and numerical data for the temperature differences obtained for the two types of perforations. While in the case of the round shape perforation the temperature difference between the ambient temperature and the temperature of the airflow aspirated through the perforated plate are little underestimated, in the case of the lobed perforation they are rather overestimated. We notice however the similarity between the experimental data of Leon et al. [

/h: a) velocity, b) temperature

16] and our numerical data.

a b

Fig. 13 - Temperature difference as a function of the airflow: a) circular perforations b) cross shaped perforations

In Fig. 9 thermal efficiencies for the two UTSW are given from experimental and numerical cases, in comparison with the data obtained by using a commercial UTSW and the two models proposed by Belusko et al. et Shukla et al. for UTSW without wind and with circular perforations with the same equivalent diameter as in our case [30, 37]. For both perforation rates studied in the present experimental campaign we can see an advantage of the innovative perforated plate with lobed orifices compared to the baseline round orifice panels.

02468

101214161820

0 50 100 150 200 250

Delta

T(K)

Q(m3/h)

Experimental

CFD

Leon et al. (2007)

02468

101214161820

0 50 100 150 200 250

Delta

T(K)

Q(m3/h)

Experimental

CFD

34

Fig. 14 - UTSW efficiency for different volumetric flow rates; Comparison with literature

They also present a clear advantage for high flow rates when compared to the analytical models of Belusko et al. and Shukla et al. for panels with circular perforations.

4. Conclusions

The study evaluated the energy efficiency of several types of unglazed transpired solar collector (UTSW) by experimental and numerical means. The physical model used shows good results in agreement with literature. In addition, the comparison of a conventional UTSW with a new geometry with innovative perforation leads to interesting results, with over 15% increase in thermal efficiency since the literature shows a lack of the geometry study for the perforations. These effects still need to continue the investigation. The CFD study on the unglazed transpired solar collector (UTSW) which is equipped with cross shape perforations shows that the experimental conclusions can be also found by numerical means. Because such geometries require very fine meshes, a scaled model of the experimental would be the answer to numerical modelling for such case. The results showed very good agreement with the experimental study, fact that validated our model. The efficiencies calculated proved the advantage of cross-shaped models in comparison to classical ones. More of that, the comparison of a classical UTSW with a new one with innovative perforation geometries leads to interesting results, with more than 15% increase in thermal efficiency for volumetric flow rates higher than 100 m3

Acknowledgements: This work was supported by the grants of the Romanian National Authority for Scientific Research, CNCS UEFISCDI, project number PN-II-RU-PD-2012-3-0144, PN-II-ID-PCE-2011-3-0835.

/h. Further studies regarding the best configuration still have to be conducted, including complementary measurement techniques.

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36

DESIGN PROCEDURE FOR SIDE WALLS OF SOCKET FOUNDATIONS

Ionu DAMIAN - Assistant, Technical University of Civil Engineering, Faculty of Civil, Industrial and Agricultural Structures, Department of Reinforced Concrete, e-mail: [email protected]

Abstract: Single storey structures having simple structural systems, jointed roof on cantilever columns are widely used nowadays for commercial buildings. To minimize the execution time and construction effort, a popular solution is to install precast columns on the so called socket foundations. The column is a linear element characterized by flexural behavior, the design of which does not produce difficulties. However, the foundation is made of several short elements, the behavior of which may be difficult to assess. One web element is the side wall of the socket. This element behaves like a short cantilever with the load suspended at the bottom. The new version of the Romanian code for shallow foundations, NP 112/2012 proposes a strut and tie model for the design of the side wall. Anyway, the model is not fully described and the strut c

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