improvement in heat transfer inside a phase change energy … · 2018-10-22 · improvement in heat...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:18 No:05 33

180505-3737-IJMME-IJENS © October 2018 IJENS I J E N S

Improvement in Heat Transfer Inside a phase change

Energy System Maitham Jameel Zaidan and Mohammed H. Alhamdo

Al-Mustansiriya University, College of Engineering, Mechanical Engineering Department, Iraq

Abstract-- In this work, an experimental and numerical

investigation was designed for recover the waste heat from the

air conditioning system. The model includes a vertical thermal

glass capsule with diameter of 60 mm and length of 300 mm of

glass thickness 1.5 mm. The capsule containing paraffin wax as a

phase change material (PCM) with 9mm diameter copper tube

with U-shaped for passing water flow through it as a discharging

heat transfer fluid. The study focusses on the enhancement heat

transfer by adding Alumina nanoparticles (AL2O3) with volume

fractions (0.5%, 1%, 2% and 3%). The results indicate the

effects of AL2O3 decreases melting rate time a proximately

7,15,11,9% besides decreases solidification rate time

approximately 4,8,6,5% respectively as compared to pure

paraffin. Also, the effect of adding a pair of copper tubes inside

the paraffin wax was tested during melting and solidification

process.

Index Term-- Phase change material, paraffin wax, thermal

storage, thermal conductivity, nano particles

1 INTRODUCTION

The continuous growth in fuel prices, gas radiations and

the level of greenhouses are the main reasons for researchers

around the world to find alternative and renewable sources of

energy directly dependent on direct solar radiation sunlight as

well as the development of means to store this energy [1].

Flow around stationary cylinders that contains paraffin wax

draw attention many of researchers, the separation of the

boundary layer had a significant effect on the amount of the

heat transfer in additional the description of the streamline,

velocity and pressure profile was described by [2] . The heat

transfers contours from tubes and other bodies in cross flow

are determined by many parameters like stream velocity,

physical properties of fluid, thermal heat flux direction,

geometry of the bodies like smooth circular cylinders [3] [4],

corrugated cylinders.[5]

The technology of phase change materials (PCMs) are one of

these energy storage devices. This part will describe how

energy storage with phase change material technology may

play a role in tackling more of the storage problems within

energy systems [6].

Paraffin wax is a thermoplastic material that can be

reformed by heat. Paraffin is one of the components of the oil

take out through the refining process, coal and other organic

materials like bituminous shale, wood, fish tallow, lignite, etc.

The melting point of paraffin rises with increasing average

molecular weight and its range from 25°C to 68°C. However,

the applications of paraffin are limited because of it has low

thermal conductivity[7] .

According to the low thermal conductivity of the paraffin

wax, there are many passive techniques are used to enhance

thermal properties like adding the Nanoparticles to the hot

fluid [8][9], insert twisted tapes [10] [11], and continuous or

discrete ribs and other factors, [12] [13].

For many latent heat thermal energy storage systems, heat

transfer enhancement techniques are required. Several

procedures are suggested to enhance heat transfer in a latent

heat thermal energy storage system (LHTES), such as finned

tubes, fillers of metallic and matrix structures of metal, were

used to improve the thermal properties of the phase change

materials (PCMs). Enhancements followed with other

techniques are also including;

i. PCM inclosing dispersed high conductivity particles,

Lessing rings [14]

ii. Microencapsulated PCM [15].

iii. Active methods of agitators, vibrators, slurries and

scrapers.

iv. Extended surfaces such as fins and honeycombs [16].

v. PCM mixed with composite material [17] .

(Valan Arasua, et al., 2011) [18]studied the rate of heat

transfer enhancement of phase change materials of paraffin.

The results illustration that the enhancement in heat transfer

rate of paraffin wax is better for associated with the

nanomaterials.

(Kyad et al. 2016 ) [19] studied related previous methods for

enhancement the thermal conductivity of the used PCM,

mainly paraffin, to successfully charge and discharge latent

heat energy and appearances at the formulation of the phase

change problem. Also, experiences to improve the solid-liquid

phase change process were led to examine a technique of

improving the thermal conductivity of paraffin by



incorporating in it a 1% mass fraction of two additives

(Copper and Aluminum). It is found that useful heat gains

increased, and charging time was reduced by adding

aluminum Nano powder.

(Putra, Prawiro, and Amin 2016) [20] studies bees wax as a

(PCM) which has high thermal capacity, with the of studying

the thermo-physical performances and properties of bees wax

with CuO nano PCM. Adding nanoparticles of CuO improved

thermal conductivity of wax and its heat capacity will reduced.

The change in latent heat caused no important effects in the

performance of beeswax/CuO. Thermal conductivity

enhancement of the beeswax/CuO was reached by 0.53, 1.55,

1.91, 1.97, and 2.07 W/mK for 0.05, 0.1, 0.15, 0.2, and 0.25

wt% of nano-PCM, respectively

(Ali , et al 2013) [21] studied an experimental and numerical

different cases of enhancement thermal conductivity and the

rate of heat transfer of four pure paraffin, one of them

produced by an Iranian company (KCC Co.) ,the second

produced as a bee wax by (Haman Co., Germany) and the

others produced by (Al- Durra Refinery) of Iraqi oil company

(Grade A and Grade B). They found that the thermal

properties of the wax of Grade B are the best before and after

adding the additives. The copper network additives from 3%,

6% and 9% wt., increase the thermal conductivity of pure wax

Grade (B) by 105.14%, 257.47% and 274.76% respectively

and Grade (B) wax/copper network composite of 6% wt.,

decreased the charging and discharging times by 26.4% and

30.2% respectively.

The objective of this research is to manufacture a

Waste Heat Recovery System (WHRS) to reuse air

conditioning waste energy exiting from condenser. In

additional investigate the effect of different volume fractions

nanoparticles of AL2O3 suspended in the PCM to enhance

heat transfer performance. The experiential results will

compare with the results of ANSYS Fluent 17.2.

2 NUMERICAL SIMULATION

The physical model used in this work is a cylindrical

capsule with 0.06 m diameter, 0.3m long and thickness 1.5

mm. The capsule contains pure paraffin or paraffin spread

with 0.5%, 1% ,2% and 3% volume fraction of Al2O3

nanomaterials. The governing equations are as follows:

The continuity equation:

(1)

The equation of Momentum:

( ) ( )

(2)

The energy equation:

( ) (3)

Where: P is the static pressure, is the stress tensor, is

external body forces, ρ the gravitational body force, T is the

temperature, ρ is density, S is term of heat source, is the

velocity.

Boundary conditions 2.1

The boundary conditions are considered for the inlet air

constant temperature for the (HTF1) heat transfer fluid of 75C,

for the duct wall will set to be adiabatic because it was a well-

insulated. While for the inlet water; during discharging

process the second heat transferred fluid (HTF2) which was

water passing through the copper pipe with constant volume

flowrate and temperature as 0.01 l/s and 27 °C respectively.

Assumptions 2.2

There are some assumptions was produced in this case

like the PCM is Newtonian, incompressible and laminar, the

physical properties of paraffin wax are function of the

temperature, Transient three-dimensional models, the volume

difference resulting from the change in phase of wax is

ignored and no dissipation in power through the duct.

Generation of Mesh. 2.3

In the simulation of this work, ANSYS FLUENT 17.2

software produced to sole the Naiver- Stoke equations by

defaulting quadrilateral mesh element had been used because

of the simple geometry. To ensure high accuracy of

calculation a small grid size has been used, the model has a

mesh with range (300,000-500,000) nodes and (1,500,000-

2,000,000) elements. In this work the thermal properties of

PCMs, as viscosity and density are considered as function of

temperature variations using (Piecewise-Linear Definition)

method which determined by definite below correlations.

(

) (4)

ρ

(5)

Where A, B are constant coefficients, is the density of

PCM at the melting temperature , β is the thermal

expansion coefficient.[22]

The physical and thermal properties of the nanoPCM are

calculated from the next relations where the subscripts PCM

for paraffin wax and np for nanoparticles.

Density[23] :

Ф Ф (6)

Latent heat [24]:

Ф

(7)



Specific heat capacity [25]:

Ф Ф

(8)

Dynamic viscosity [25]:

Ф (9)

The active thermal conductivity is considered from the

correlation by Ref. [25]

( )Ф

( )Ф

Ф

√

( Ф)

Where ßk for AL2O3 = 8.4407 (100 Ф )-1.07304

ʆ

ʆ: correction factor, Ҡ

: is the Boltzmann constant = 1.381 10-

23 (J/K)

f (T.Ф) = (2.821710-2 Ф + 3.91710

-3)

+(-3.066910

-2

Ф-3.9112310-3

)

Where f (T.Ф) is obtained from the experimental data

3 EXPEREMANTAL WORK

In this work a well-insulated rectangular duct with a

single cylindrical capsule content paraffin wax as PCM inside

it. The duct having over-all length of (120cm) wit (32cm) of

square cross-sectional area. The PCM capsule locating at the

distance of (80cm) from the inlet region of the duct which

located at the end of the test system. Figure (1) shows the

physical symbol of the 3-D LHTES.

Fig. 3-1. Physical symbol of the numerical model.

The test rig was illustrated in Figure (2) to achieve the experimental tests.



Fig. 2. Schematic of the test rig

Fig. 3. photograph of the test rig

The test section consists of two parts. The first part is a

square channel made of iron frame as shown in figure (3).

Two sides of test section are made from transparent plastic

materials (Perspex), with dimensions of 120 cm* 30 cm and 6

mm thickness. The second part is the PCM capsule under

investigation which is made from 6 cm diameter of Pyrex

glass with a length of 30 cm and a thickness of 1.5 mm. (U

shape) are inserted through wax inside capsules. The tubes

dimensions are 9 mm diameter and 35 cm length, as shown in

figure (4)



Various types of PCM capsules have been investigated, this

include;

1- PCM capsule with one copper U tube and pure

paraffin wax, and then mixed with four different

volume ratios of nano AL2O3 (nPCM) as shown in

figure (5).

2- PCM capsule with two copper tube (U shape) and

optimum volume ratio of (nPCM) as shown in figure

(6) (7).

Fig. 5. Capsule with one copper U tube

Fig. 6. Capsule with two copper U tube

Fig. 7. Capsule with two copper U tube (solid work software

program 2017)

A 24,000 BTU / h air conditioner was used at three fan

speeds of the condenser to supply the test section with waste

hot air. The main purpose of water supply system is to

discharge the heat from paraffin wax in the test section by

passing water through the copper tube. To measure PCM, air

and water temperatures, 16 thermocouples with range of (-100

to 800) ℃ of k-type were fixed as shown below: (Fig. 8)

Fig. 3.Test section



Fig. 8. Thermocouples location inside PCM capsule

Thermal conductivity apparatus. 3.1

Thermal conductivity can be measured for the pure wax based

on the ASTM standard (C518-04) titled "[Standard Test

Method for Steady State Thermal Transmission Properties by

Means of Heat Flow Apparatus]"[26] (Fig. 9), and according

to the method which described by [27]. Dynamic viscosity

where measured for different volume fractions of the

Nanoparticles.

Fig. 9. Thermal conductivity apparatus

The Calibration 3.2

Calibration was formed to set the values of the thermocouples

and the curve of the thermocouples reading calibration shown

in (Figure 10) and the error was 2.97%.

Fig. 10. Thermocouple Calibration Curve

Mixing of nanoPCM. 3.3

NanoPCM, with volume fraction 0.5%,1%,2% and 3% was

produced by using the ultrasonic device. The slow mixing of

AL2O3 nanoparticles inside paraffin wax is necessary for good

thermal properties of nanoPCM[20] .

4 RESULTS AND DISCUSSIONS

The thermal conductivity results with respect to the volume

fraction of the nanoparticles are shown in figure (11). It is

clear that pure PCM has lower heat transfer rate related to the

same amount of nanoPCM because the thermal conductivity

of pure PCM is lower than the nanoPCM. The results show the

thermal conductivity of the nanoPCM increased when the

volume fraction increased because of high thermal

conductivity of AL2O3 nano particles, that means the thermal

conductivity enhancement was directly proportional to the

volume fraction of the nanoPCM. Thermal conductivity

enhancement of the paraffin wax/AL2O3 was reached by 0.22,

0.23, 0.235 and 0.237 W/mK for 0.05, 0.1, 0.2, and 0.3 V% of

nano-PCM, respectively.

y = 0.9997x R² = 0.9977

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Ther

mo

met

er t

emp

erat

ure

°C

Thermocouple temperature °C



Fig. 11. Effect of nanoparticles on thermal conductivity of nanoPCM

The viscosity of paraffin is found to increase when the

volumetric ratio of AL2O3 nanoparticles increasing, as shown

in Figure (12) likewise, with increasing temperature, the

dynamic viscosity will decrease for the nanoPCM.

Fig. 12. Transient dynamic viscosity of paraffin wax mixed with various

concentration of nano Al2 O3

Figure (13) show the time of melting the wax with and

without nanoparticles. The enhancing of the thermal

conductivity decreases the melting time of nanoPCM.

However, it is clear that 1% nano particles is the best

concentration for enhancing the melting time of the PCM (i.e

1% the best concentration for increasing thermal conductivity

and decreasing melting time because when the volumetric

concentration increased larger than 1% lead to rise viscosity

and decrease the specific heat of nanoPCM.

The results show that addition of AL2O3 nanoparticles to a

paraffin wax as shown in (table1), decrease meting point and

enhance the other thermo-physical properties as shown in

table (2)

Table I The weight of AL2O3 nanoparticles on each volume fraction

Volume

fraction%

AL2O3

(g)

0

0

0.5

11.5

1

23

2

46

3

69

There are not important changes on rate of heat transfer in the

beginning of the melting in the four concentrations of

nanoparticles because the heat transfer by conduction only not

enough to make marked change in heat transfer, then when

paraffin begins to melt, the heat transfer turn to the convection

and the effect of viscosity is greatest because of the viscosity

of wax in liquid phase larger than in solid phase. Then for

higher AL2O3 nanoparticle concentration, the rate of heat

transfer will decrease because of increasing the viscosity

. Fig. 13. Melting processes of pure paraffin wax and wax/ Al2O3

nanoparticles

0.21

0.215

0.22

0.225

0.23

0.235

0.24

0.0% 1.0% 2.0% 3.0%

Ther

mal

co

nd

uct

ivit

y (w

/mk)

Volume fraction%

Maxwell eq.3.15[32]

Expermantal

0.002

0.007

0.012

25 40 55 70Dyn

amic

vis

cosi

ty n

s/m

2

Temperature ºC

Ф=0%AL2O3 nano particles

Ф=0.5%AL2O3 nano particles

Ф=1%AL2O3 nano particles

2025303540455055606570

0 30 60 90 120

Tem

per

atu

re (

°C)

Time (minute)



Table II Properties of pure and nano paraffin under investigation

Ф= 3% Ф= 2% Ф= 1% Ф=

0.5%

Pure

paraffin Property

2808 2829 2850 2860 2871 Cp

(J/kg.K)

0.233 0.227 0.220 0.217 0.214 K

(W/m.K)

0.0136 0.0133 0.0130 0.0128 0.0127 µ

(kg/m.k)

62 62.4 62.5 63 63.4

Melting

point °C

Melting process of PCM 4.1

During melting process, for all the cases under investigation

the temperature of the upper plane (T7, T8 andT9) were

record higher temperature because of the different in densities

between hot and cold paraffin since the buoyancy force lift the

hot wax as shown in figure (14) (15). The thermocouples in

middle plane (T4, T5 and T6) were record temperatures less

than upper plane and higher than lower plane (T1, T2 and T3).

On the other hand, each thermocouple between the glass wall

and U tube (T1, T4 and T7) was record temperatures higher

than the others in the same plane because of its relative

neighboring to the wall which is exposed to the flow of hot air

during melting process of PCM. All thermocouples in the

center of capsule (T3, T5and T8) record temperature less than

others in the same plane because of relative remoteness from

the flow of hot air during melting process. Due to low thermal

conductivity of paraffin wax that make the upper plan store

more heating during charging process, so more time is needed

to make heat travel downwards. This difference in

temperatures of points in PCM was minimized by

enhancement the thermal conductivity of paraffin wax by

adding nano particles of (AL2O3 to enhance the rate of heat

transfer.

Fig. 14. Time of melting during charging process

Fig. 15. melting process in the PCM capsule

solidification process of PCM 4.2

The discharging process is achieved by passing a cold

water as a cold heat transfer fluid in the copper pipes and

stated directly after the melting process is completed. At initial

time, (approximately up to 10 minutes) the temperature of

PCM decreases sharply, because of large temperature

difference between paraffin wax and HTF2. After that the

temperature of paraffin wax will reduces slowly until reach to

constant value. It is observed that solidification starts around

60◦C. It can be seen, by adding AL2O3 nano particles of

(0.5,1,2 and 3%) to the pure paraffin wax, the solidification

time decrease by (4,8,6,5%) respectively, and by adding pair

of U-tube the solidification time decrease by 60% less than

when adding single U-tube because of doubled quantity of

high thermal conductivity copper U tube inside PCM. The

solidification occurs from the top of the capsule to the bottom

because the entry of cold water is from the top which absorbs

a high amount of heat from liquid PCM. this lead to drop in

temperature relatively fast in upper part of capsule. The other

common phenomenon in all cases is the solidification the of

paraffin wax that surrounding the copper pipes is faster than

the other paraffin in capsule due to the rapid loss of heat to the

water a cross copper pipe as shown in figure (16). This is one

of the main problems that prevent the utilization of all the heat

in the melting paraffin wax. Then, the temperature reaches

steady state very quickly, after complete solidification. It

necessity be noted that the whole required melting time is

leaser than the solidification time.



5

minutes

10

minutes

15

minutes

20

minutes

25

minutes

Fig. 16. The process of solidification in PCM

5 Comparison between the numerical and experimental

results .

Practical experiments and theoretical simulations have

been compared as shown in figure (17). The results show

that there was an acceptable approximation between

experimental and theoretical results. The percent of

difference between the experimental and numerical data for

melting processes were about 10% increase in time period

of melting for numerical results. The difference between

both results (numerical and experimental) might be because

of some assumptions have been assumed in the ANSYS

FLUENT program software. This include thermal and

physical properties of the paraffin wax to simplify the

solution also the location of the thermocouples inside the

PCM may be changed because of the solidification or

melting of process.

Fig. 17. Comparison between the numerical and experimental results

6 NUMERICAL RESULTS

The results of the simulation in this work for the

melting and solidification of the PCM were performed using

ANSYS FLUENT 17.2 software program. Figures (18) to

(21) show the result of the temperature distribution inside

paraffin wax during charging and discharging process for

various water tube arrangements. At the beginning of the

melting process, the paraffin wax inside the capsule in the

solid state at a temperature of 300 k. The paraffin wax

begins to gain heat from hot heat transfer fluid (air) with

temperature of 342k. It is clearer from results that at the

beginning of this process the wax melts near the wall and

gradually continues to melt towards the center of the

capsule until the melting process is fully completed.

After melting process, the process of the discharging

(solidification) begins. This is done by using the second heat

transfer fluid (water) that enters to the copper pipes at a

temperature of 300 k. The results show that wax is gradually

starting to solidify, starting from the copper pipe wall

towards the wall of the capsule until the solidification

process is fully completed.

It is clear that the PCM temperature contours are non-

uniform during charging process due to the difference in

density and wax low thermal conductivity. In the beginning,

the conduction heat transfer is dominated in the system

(since the PCM in solid state) till the phase change of the

PCM occurred after melting; then both heat conduction and

convection are affected inside the system.

The conduction heat transfer occurs between the solid

surface of the PCM and the capsule wall. The melting

process early occurred near capsule wall and shaped a thin

layer of liquid PCM in a narrow melting area as a result of

heat transfer after about 30 mints. Due to the effect of free

convection, the liquid part of PCM is pushed to the top of

the capsule because of the buoyancy. The solid part of the

PCM is pressed down due to the different densities between

the liquid and solid of PCM. The melting time depends

largely on the number of copper pipes inside the PCM, since

the melting time decrease when the number of pipes

increase and vice versa, that is occur because the conduction

heat transfer depends on the contact area of the suffuses.

30

40

50

60

70

0 20 40 60 80

Tem

per

atu

re °

C

Time (minute)

EXP.

Num.



30 min 60 min 90 min.

Plane A

Plane B

Plane C

Fig. 18. Temperature contours inside PCM during charging process through 1 U tube

B

C

A




Plane A

Plane B

Plane C

Fig. 19. Temperature contours inside PCM during charging process through 2 U tube

B

A

C




Plane A

Plane B

Plane C

Fig. 20. Temperature contours inside PCM during dischcharging process through 1 U tube

B

C

A



A

B

C


Plane A

Plane B

Plane C

Fig. 21. Temperature contours inside PCM during dischcharging process through 2 U tube



7 CONCLUSIONS

1- In present work, the effect of volume fraction of

nanoparticles, of high thermal conductivity metal

were studied and discussed. The results in general

show a reduction in the melting and solidification

time of paraffin wax.

2- Diffusion of AL2O3 nanoparticles of AL2O3 in the

wax of paraffin enhances its low thermal

conductivity and therefore when the nanoparticles

had been added, the rate of thermal energy in

charging and discharging significantly enhancing.

3- Adding nanoparticles of AL2O3 to paraffin wax by

volume fraction 0.5%,1%,2% and 3% decreases

melting rate time approximately 7,15,11,9%

respectively compared with pure wax of paraffin.

4- Adding nanoparticles of AL2O3 to paraffin wax by

volume fraction 0.5%,1%,2% and 3% decreases

solidification rate time approximately,4,8,6,5%

respectively compared with pure wax of paraffin.

5- The period decreases for whole melting of the

paraffin wax as the thermal conductivity of

container material increases.

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