investigation of boiling heat transfer in an...

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INVESTIGATION OF BOILING HEAT TRANSFER IN AN ELECTRICALLY HEATED TUBE BUNDLE

Krzysztof Krasowski / ENERGA Kogeneracja sp. z o.o., Elbląg Janusz T. Cieśliński / Gdańsk University of Technology

1. INTRODUCTION

Boiling in tube bundles can be observed in multiple industrial devices, like vaporizers, boiling pots or wet boilers. Boiling also occurs in evaporators of absorption chillers used in polygeneration systems, which simultaneously generate heat, electricity and chilling power – by operation of a compression refrigerator or an absorption chiller.

Previous research on boiling occurring in tube bundles had indicated that thermal characteristics of indi-vidual tubes within a bundle are significantly different from values observed for a single tube due to the strong effect of steam bubbles generated at lower tube rows. In addition, a significant influence of bundle geometry was observed, including the impact of the exchanger’s jacket and hydraulic properties of the flow on heat trans-fer within the bundle. There is not much information available concerning local heat transfer coefficients for individual tubes of a bundle.

Marto and Anderson in their research [1] investigated a bundle of 15 copper pipes of which only 10 were heated. The ratio between tube spacing and tube diameter was 1.2. The investigated liquid was R113 refrige-rant. Experiments were carried out at atmospheric pressure. The researchers determined that the lowest heat exchange coefficient occurred at the bottom tube row, and that those lowest tubes intensified heat exchange at higher rows only when the heat flux values were low. The intensification coefficient for the bundle depended on the heat flux density and number of tubes within the bundle. Research also showed that the greatest impact on intensification of heat exchange of a pipe had the pipe located directly underneath. This is described by the so-called bundle effect factor [2], defined as the ratio between the heat exchange coefficient for the topmost tube of the bundle to the heat exchange coefficient of the same tube when the tubes located beneath are not heated.

Qiu and Liu [3] investigated the influence of tube spacing and pressure values on intensification of heat transfer in a bundle of 18 (3 × 6) polished copper tubes with a diameter of 18 mm each, heated with cartridge heaters. The tube layout was staggered. The researchers discovered that intensification of heat transfer in the bundle was highest for a spacing of 0.3 mm. For 0.3 mm spacing, location of the tube within the bundle did not affect the recorded heat transfer coefficients. For spacing higher than 1 mm the influence of pressure on transferred heat was negligible.

Gupta et al. [4] investigated the water boiling process at ambient pressure on smooth electrically heated tubes made of stainless steel, with a diameter of 19.05 mm. Tubes – two or three – were located one above the other, with the spacing (s) to diameter (d) ratio ranging from 1.5 to 6.0. Density of mass flow of water at satu-ration point into the tank G varied between 0 and 10 kg m-2 s-1. This allowed simulating both pool boiling (G = 0) and vaporizer (G > 0). The researchers determined that the heat transfer coefficient at the lowest tube did not depend on the presence of the tubes above. Additionally, the highest heat transfer coefficient (100% higher

Abstract

The paper presents the results of experimental research on water, methanol and R141b refrigerant bo-iling in a horizontal bundle of smooth tubes supposed to represent part of a flooded evaporator. Experiments were carried out for a bundle of 19 tubes in triangular layout for two spacing-to-diameter ratio values – 1.7 and 2.0 – in atmospheric and subatmospheric pressure conditions.

Heat transfer coefficient values – both local, tube-specific, and general, for the entire bundle – were determined. The boiling process at the bundle was visualized with a CCD camera and laser sheet technique.

A correlation equation allowing the calculation of the average heat transfer coefficient for boiling in smooth tube bundles has been proposed.

Investigation of Boiling Heat Transfer in an Electrically Heated Tube Bundle

32

than for the lowest tube) was observed at the topmost tube in the three-tube system, and – interestingly – for pool boiling (G = 0).

Kumar et al. [5] investigated distilled water boiling in subatmospheric pressure conditions (35-97.5 kPa) on two smooth copper tubes with a diameter of 32 mm located one above the other, heated with cartridge he-aters. When comparing their results with those obtained by other authors no significant impact of tube materialon heat exchange coefficients can be observed.

Leong and Cornwell [6] tested an electrically heated bundle composed of 241 tubes. The test setup simu-lated a flooded evaporator. Research was carried out with R113 refrigerant at atmospheric pressure for tubeswith an external diameter of 19.05 mm and spacing (in rectangular layout) of 25.4 mm. Heat transfer coefficientsat the bundle bottom turned out to be close to those observed for a single tube, but at higher rows the values were considerably higher.

Gupta [7] carried out experiments of heat exchange for distilled water boiling at atmospheric pressure in a bundle consisting of 15 tubes (3 × 5) in linear layout, with spacing-to-diameter ratio s/d = 1.5 placed in a large volume tank. Experiments were carried out for heat flux densities between 10 and 40 kW/m² and mass flowdensities 0-10 kg/(m2s). The bundle was made of stainless steel tubes with a diameter of 19.05 mm and effective length of 190 mm each; the tubes were heated electrically.

The results obtained by Gupta show clear differences in heat exchange coefficients for individual tuberows – with the lowest value for the bottom pipe layer and the highest for the topmost layer. The maximum value of heat exchange coefficient for the topmost tube of the central column was seven times higher than the heatexchange coefficient for the lowest tube of the same column, at volume boiling and the same heat flux density of23 kW/m². Similar variations in heat transfer coefficient were obtained by Gupta for tubes in side columns, butin that case the increase of heat exchange coefficient between the bottom and top tube did not exceed 300%.

Dual-phase flows which occur during boiling in a tube bundle depend on many factors, including heat fluxdensity, liquid properties, type of tube surface and layout and spacing of tubes. Nevertheless the heat transfer coefficient for a tube bundle is usually higher than that for a single pipe in the same conditions. This pheno-menon is described by the so-called bundle factor [2] defined as the ratio between the average heat transfercoefficient for a bundle and the average heat exchange coefficient for a single tube.

Webb et al. [8] point out that the heat transport mechanism in flooded evaporators is different than invaporizers. This results from a different tube layout used in flooded evaporators, which restrict natural convec-tion, as well as from the fact that the steam aridity degree at the inlet to a flooded evaporator might even reach15%. It is also extremely difficult to apply results of experiments involving specific tube bundle, type of liquid andprocess conditions for a different case. Attempts to create a theoretical description of this process or model it encounter numerous obstacles. Therefore, calculations are usually based on various empirical or semi-empirical correlations, involving constants which need to be experimentally determined.

A key issue for calculating heat transfer coefficient is to correctly determine the outside surface tubetemperature. This measurement, extremely difficult to accomplish, is influenced by the design of the heatingsection, as well as structure of power supply and control system, and data acquisition method. Therefore, this study presents a system structure which allows simultaneously reading and recording measurement results for boiling in a horizontal tube bundle, characterized by high accuracy of both power control and measurements. Based on experimental data, some of which is presented in this study, a correlation equation has been proposed. It expresses the Nusselt number as a function of boiling number, Prandtl number and geometrical parameters of a tube bundle and allows one to calculate the average heat transfer coefficient for a boiling process in a bundleof tubes with porous coatings.

2. LABORATORY SETUP

The laboratory setup for the investigation of a boiling process in a tube bundle consists of six main sys-tems: modelled tube bundle, experimental tank, power supply and control unit, cooling system, data acquisition system, and visualization system. The cylindrical tank supposed to simulate the jacket of a real evaporator has a diameter of 0.3 m, length of 0.3 m and is made of stainless steel. It is equipped with three inspection windows

Krzysztof Krasowski / ENERGA Kogeneracja sp. z o.o., ElblągJanusz Tadeusz Cieśliński / Gdańsk University of Technology

33

– in the front wall (with a diameter equal to the jacket diameter) and at both sides (200 mm in diameter). They enable direct observation and visualization of the boiling process. The water-steam system is hermetic and al-lows carrying out the investigation in absolute pressure range 3-300 kPa.

Measurements made with this setup allow determining the average value of the heat transfer coefficientfor the entire tube bundle, heat transfer coefficient for any individual tube of the bundle, peripheral temperatu-re distribution for each tube and enable visualization of boiling structures in the tube bundle. Fig. 1 presents a schematic diagram of the setup and Fig. 2 presents its real-life view.

L1 L2 L3 0

3x230V AC

1

2

3

47

8

9

13

12

11

10

16

17

18

19

56

20

21

23

14

Cooling water inlet

15

22

Cooling water outlet

Fig 1. Diagram of the experimental setup. 1 – Tank, 2 – Tube bundle, 3 – Condenser, 4 – Pressure transducer, 5 – Vacuum gauge, 6 – Relief valve, 7 – Pressure control valve, 8 – Working fluid drain valve, 9 – Cooling water isolation valve, 10 – Flow control valve, 11 – Flowmeter, 12– Cooling water temperature indicators, 13 – Temperature indicators in the tank and tube bundle, 14 – Preheater, 15 – Preheater’s tempe-rature indicator, 16 – MPS power measurement unit, 17 – Power controllers, 18 – CCD camera, 19 – Laser sheet, 20 – Adjustable support of the laser sheet system, 21 – Adjustable support of CCD camera (3D), 22 – Auxiliary light sources, 23 – Data acquisition system.


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Fig. 2. Experimental setup. 1 – Experimental tank, 2 – Condenser, 3 – Vapour system, 4 – Condensate return system, 5 – Rotameter (cooling water flow measurement), 6 – Cooling water inlet into condenser, 7 – Cooling water outlet from condenser, 8 – Working agent drain valve, 9– Multiplexer, 10 – Reg1-Reg20 regulator panel with power supply switches, 11 – RPG1-RPG20 precise power controllers panel with voltage drop measurement switches AU, 12 – V1-V4 voltmeter panel with MPS network parameter measurement unit, 13 – Adjustable support for CCD camera, 14 – CCD camera, 15 – Laser sheet, 16 – Tubular mainframe of the setup, 17 – Tank support beams, 18 – Vacuum meter, 19 – Ice zero, 20 – Data acquisition computer.

The experimental tube bundles consisted of 19 tubes in triangular layout, with a spacing-to-diameter ratio of 1.7 and 2.0 (Fig. 3a). At one end the tubes were fixed in a punched plate made of stainless steel. The other endswere free, allowing for frontal observation of the boiling process. The active length of tubes was 150 mm.

After multiple tests carried out during construction and calibration of the heating unit, as well as litera-ture research, it was decided to realize the concept shown in Fig. 3b with two copper bushings, separated with a 4 mm wide Bakelite ring installed between a cartridge heater and inner tube surface. Thermocouples were laid in grooves (0.55 × 0.55 mm) carved (electric groove carving) in bushings, while their ends were fixed in reces-ses of Ebonite rings. The used thermocouples were of the type K, with jacket diameter of 0.5 mm. Custom-built cartridge heaters have an external diameter of 4 mm, effective heating length of 150 mm and maximum power output P = 300 W.

a) left view b) right view

17 mm(20 mm)

10 mm

180 mm

150 mm

17 mm(20 mm)

145

mm

A

A

A - A

4,0

mm

8,8m

m

10 m

m

4 mm150 mm

180 mm

21 3 4 5 6

a) Tube bundle diagram b) Individual tube diagram

Fig. 3. Diagram of the investigated tube bundle. 1 – Thermocouple, 2 – Ebonite ring, 3 – Cartridge heater, 4 – Copper bushing, 5 – Investiga-ted tube, 6 – Plug.


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Determining heat transfer coefficient for individual tubes of a bundle required individual measurements of the supplied electric power. These measurements are made with N13 – MPS unit, a digital programmable instrument designed for three-phase power grid measurements in symmetrically and asymmetrically loaded systems. It simultaneously displays measured values, digitally transmits them and converts into standardized analogue signal. Power indications take into account programmed ratio values. Each measured value is trans-mitted to a supervising system via RS-485 interface and then through a converter and RS-232 connection to a PC unit. The measurement unit can be operated from the PC screen through a series link, using dedicated software (WizPar).

The selected measurement unit has a nominal input current I = 5 A, phase input voltage Un = 3 × 230 V and programmable analogue current output -20 to +20 mA and interface RS-485. It is a standard instrument with quality control attestations.

Smooth power control of each individual heater is provided by thyristor-based precision power con-trollers, RPG1-RPG20. In addition there are Reg1-Reg20 regulator units which protect cartridge heaters from excessive temperature increase and prevent exceeding a certain temperature in the working agent’s circuit. Each regulator unit is connected with one of the thermocouples installed at a heater controlled by this regulator. Connection, disconnection and alarm temperature levels were programmed into the regulator units. Each of them is equipped with two displays which show the setpoint and a real measured value, so they are also used as temperature indicators. All regulating units Reg1-Reg20, just like the MPS measurement unit, were linked to a PC unit via a converter and RS-232 standard connection. This allows one to monitor their operation, change parameters and visualize temperature variations over time for any selected heater. Connection of the MPS unit also allows generating charts showing, e.g. load on individual phases or phase voltage values.

Measured momentary values of temperatures and pressure are transmitted to a computer – at a selected sampling rate – through an AL154RX02 multiplexer. The device has been designed for measurement duties at a laboratory setup. It is equipped with 127 measurement inputs for K-type thermocouples, pressure measure-ment input (-0.1 MPa to +0.6 MPa) via a custom-made pressure sensor AR26 (maximum measurement error 0.1% of measured value). The temperature measurement error is ± 0.05 K and the sampling time is set to 0.5 s. Data is transferred to a PC unit via a USB interface, while configuration programming is made with a dedicated APEK Multiplekser software which allows one to define the number of measurement channels, program input characteristics, calibration, visualization of all channels on bar graphs, visualization of selected channels on a chart, setting sampling time/rate, limiting measurement time and transferring data directly to Microsoft Excel files.

Fig. 4 shows a block diagram of the described power supply system coupled to the temperature measure-ment system and data acquisition system.

˜3 x 230 V

RS 232 / 485

RS 232 / 485

USB

PC


Power measurement unit

Power controllers

Fig. 4. Block diagram of the power sup-ply, temperature measurement and data acquisition systems.

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The tube bundle was illuminated using a laser sheet system with a power of 1500 mW (532 nm, 10 kHz, multimode) with light sheet optics, installed perpendicularly to the investigated tubes. A CCD camera was in-stalled coaxially with tubes. It is based on the Texas TC 237 sensor and used to take photographs of the boiling process (up to 15,000 frames per second). The layout and connections of the imaging system and its real-life view are presented in Fig 5.

a) Diagram

The unit synchronizing operation of the CCD camera light sheet allows one to operate (turn on or flash) the laser when the camera is triggered. Also the flash length can be adjusted.

Supporting structures for the CCD camera and laser system are flexible so it is possible to adjust the illu-minated point and recorded spot in two planes – horizontal and vertical.

The laser lighting system is also equipped with a special optical system regulator allowing changing the laser sheet geometry (vertically-horizontally, beam width).

A detailed description of the experimental setup and measurement procedure has been published in [9].

3. INVESTIGATION RESULTS

Experiments revealed that regardless of spacing and pressure values, the highest heat transfer coefficient is observed for water – which results from its excellent thermophysical properties, above all the high heat of evaporation. For example, Fig 6 presents a boiling curve in a smooth tube bundle with s/d = 1.7, at atmospheric pressure.

b) Real-life view

Fig 5. Lighting and imaging system.


Lighting

Digital video camera

Photo camera

PC unit CCD camera Laser power supply with synchronizing

unit

Measurement tank with heating section

Laser

CCD camera

Adjustable support

Laser

Laser power supply unit

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For all investigated liquids, both in atmospheric and subatmospheric pressures, higher heat transfer coef-ficients were observed for the larger of the two investigated spacing values, i.e. for s/d = 2.0. For example, Fig 7 presents a boiling curve for R141b refrigerant at atmospheric pressure.

Regardless of the boiling liquid type and spacing, higher heat transfer coefficients were observed at at-mospheric pressure, which complies with several published experimental reports. For example, Fig 8 presents a boiling curve for methanol boiling in a bundle of tubes with porous coating for s/d = 2.0.

Fig 9 shows a relation between the average heat transfer coefficient for a specific tube row, and investi-gated heat flux values for water boiling in a smooth tube bundle, s/d = 1.7, at atmospheric pressure. The higher the tube row is, the higher the heat transfer coefficient gets. The heat transfer coefficient value also increases along with the heat flux value. Such a distribution of the heat transfer coefficient values can be explained by heat transfer intensification caused by vapour bubbles generated on lower tube rows. Visualization of this phenome-non is shown in Fig 10.

8 10 12 14 16 18 20 22 2415

20

25

30

35

40

45

50

q [k

W/m

2 ]

T [K]

��

12 14 16 18 20 22 2415

20

25

30

35

40

45

50

q [k

W/m

2 ]T [K]

Fig 6. Impact of the boiling liquid type on the boiling curve at atmospheric pressure (p = 101.2 kPa), in a tube bundle with s/d = 1.7.

Fig 7. Impact of spacing on the boiling curve for R141b refri-gerant in a smooth tube bundle, at atmospheric pressure (p = 101.2 kPa). s/d values: + - 1.7, × - 2,0

10 12 14 16 18 2015

20

25

30

35

40

45

50

q [k

W/m

2 ]

T [K]

Fig 8. Impact of pressure on the methanol boiling curve for bundle of tubes with porous coating, s/d = 2.0+ - atmospheric pressure (p = 101.1 kPa) × - subatmo-spheric pressure (p = 19.5 kPa

Fig 9. Relation between the average heat transfer coefficient for different heat flux values and the location of a tube row within a bundle – water boiling in a tube bundle, s/d = 1.7, atmospheric pressure (p = 100.5 kPa).


No. of tube row within a bundle

38

Fig 11 presents variability of the bundle factor (F) and bundle effect (WP) for methanol boiling in a tube bundle with s/d = 2.0 at atmospheric pressure. In compliance with previously published data, the bundle effect factor is somewhat higher than the bundle intensification coefficient and both values decrease as the heat fluxgrows.

Due to the fact that each tube in a bundle is equipped with its own heater, it is possible to determine the distribution of a heat exchange coefficient within the bundle. For example Fig 12 presents approximate lines ofconstant heat exchange coefficient for water boiling in a smooth tube bundle at atmospheric pressure for mini-mum and maximum investigated heat flux values. Heat transfer coefficients in the bottom part of both bundlesare almost half those observed in the topmost rows. Additionally the values for the central column are some-what higher than to the sides.

15 20 25 30 35 40 45 501,8

2,0

2,2

2,4

2,6

2,8

3,0

WP,

F [-

]q [kW/m2]

Fig 10. Visualization of intensifying steam bubble effect for water boiling process in a tube bundle s/d = 1.7, at atmospheric pressure (p = 100.5 kPa) and heat flux q = 30.25 kW/m²

Fig 11. Bundle factor (F - +) and bundle effect (WP - o) for R141b refrigerant boiling in a smooth tube bundle, s/d = 2.0 and subatmos-pheric pressure.

Application of multidimensional regression analysis for the boiling process of water, methanol and R141b refrigerant, for subatmospheric pressure and atmospheric pressure, allowed developing a correlation equation (1) which can be used to determine the average value of heat transfer coefficient for smooth tubes.

(1)

a) b)

2,5 2,9 2,7

2,5 2,1 2,4 2,1

1,7 1,8 1,9 1,7

1,6 1,6 1,7 1,5

1,5

1,8

1,41,5

2,6

3,6 3,8 5,0

3,6 3,0 3,2 3,0

2,8 3,0 3,2 2,8

2,7 2,7 3,0

2,6

3,0

2,42,8

Fig 12. Lines of constant heat transfer coefficient for water boiling in a tubebundle s/d= 1,7a) q = 15,42 kW/m2, b) q = 49,17 kW/m2; Values given in [kW/(m2K)]

67,0

74,048,12

305,0 Prln7,521

Ds

ppBoNukr


39

where: Nusselt number

l

dNu�

� , boiling number

lv

l

rLq

Bo��

,Prandtl number

a�

Pr , charac-

teristic dimension

vlgL

��

a – thermal diffusivity, m²/sd – diameter, mg – apparent gravity, m/s2

p – pressure, N/m2

q – heat flux density, W/m2

r – heat of evaporation, J/kgs – tube spacing, mα – average heat transfer coefficient, W/m2Kλ – thermal conductivity, W/mKρ – density, kg/m³µ – dynamic viscosity, Ns/m² σ – surface tension, N/mv – kinematic viscosity, m²/skr – criticall – liquid v – vapour

Comparison of the average heat transfer coefficient observed in experiments with the values calculated with the proposed formula shows that only 4 out of 72 points do not fit the ± 20% range (Fig 13).

Fig 13.Comparison of experiment results with calculations for boiling of water, methanol and R141b refrigerant in a smooth tube bundle at atmospheric pressure and subatmospheric pressure.


Water

Methanol

R 141b

α pr

ed [

kW/m

2 K]

α exp [kW/m2K]

40

REFERENCES

4. CONCLUSION

The systematic experimental research on the boiling process of water, methanol and R141b refrigerant in a smooth tube bundle shows that:

• Regardless of tube spacing and pressure value, the highest heat transfer coefficients are observed for water

• Regardless of pressure and type of boiling liquid, higher heat transfer coefficients are observed for the larger of the two investigated spacing values, in the case of s/d = 2.0

• Regardless of the type of boiling liquid and tube spacing, higher heat transfer coefficients are observed for atmospheric pressure

• Proposed correlation for boiling of water, methanol and R141b refrigerant provides results convergent with those obtained during experiments.

1. Marto PJ., Anderson CL., Nucleate boiling characteristics of R-113 in small tube bundle. ASME J. Heat Transfer, vol. 114, p. 425-433, 1992.

2. Memory S.B., Chilman SV., Marto PJ., Nucleate pool boiling of TURBO-B bundle in R-113. ASME J. Heat Transfer, vol. 116, p. 670-678, 1994.

3. Qiu Y.H., Liu Z.H.: Boiling heat transfer of water on smooth tubes in a compact staggered tube bundle. Applied Ther-mal Engineering, vol. 24, p. 1431-1441, 2004.

4. Gupta A., Saini J.S., Varma H.K., Boiling heat transfer in small horizontal tube bundles at low cross-flow velocities. Int. Journal of Heat and Mass Transfer, vol. 38, no. 4, p. 599-605, 1995.

5. Kumar S., Mohanty B., Gupta S.C., Boiling heat transfer from vertical row of horizontal tubes. Int. Journal of Heat and Mass Transfer, vol. 45, p. 3857-3864, 2002.

6. Leong L.S., Cornwell K., Heat transfer coefficients in a reboiler tube bundle. The Chemical Engineer, 343, p. 219-221, 1979.

7. Gupta A., Enhancement of boiling heat transfer in a 3x5 tube bundle. Int. Journal of Heat and Mass Transfer, vol. 48, p. 3763-3772, 2005.

8. Webb R.L., Choi K.D., Apparao T.R., A theoretical model for prediction of the heat load in flooded refrigerant evapo-rator. ASHRAE Trans., vol. 95, Pt. 1, 326-338, 1989.

9. Krasowski K., Przejmowanie ciepła przy wrzeniu na poziomym pęku rur z powłoką porowatą. PhD thesis, Faculty of Mechanical Engineering, Gdańsk University of Technology, 2009.


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