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ScienceDirect

Procedia Technology 25 (2016) 1182 - 1190

Global Colloquium in Recent Advancement and Effectual Researches in Engineering, Science and

Technology (RAEREST 2016)

Thermal Performance Evaluation of a Phase Change Material Based

Heat Sink : A Numerical Study

Jesto Thomasa, P.V.S.S. Srivatsab, Ramesh Krishnan Sc, Rajesh Babyd*

aAssistant Professor, Mechanical Engineering, College of Engineering Adoor, Kerala - 691551, India bEngineer, GE Aviation, Bangalore, India aAssistant Professor, Dept. of Mechanical Engineering Rajiv Gandhi Institute of Technology (Govt. Engg. College), Kottayam, India dAssociate Professor, Dept. of Mechanical Engineering, St. Joseph 's College of Engineering and Technology, Palai -686 579, Kerala, India

Abstract

This paper numerically investigates the thermal performance of a portable electronic device using PCM based heat sink. The phase change material used in the present study is n-eicosane with a melting point of 36.50C. Considering the current trend in portable electronic devices, a group of smart phones were taken and an average dimension was selected for designing a PCM based heat sink. A constant heat flux is provided at the heat sink bottom for simulating the heat generation by the electronic circuit board. Numerical analysis were performed for a constant heat input of 4 to 6 W in steps of 0.5 W on this heat sink to study its effect in conjunction with PCM in the efficient cooling of these devices. The effect of natural convention within the melt is also discussed in this paper . All the numerical computations were performed using ANSYS FLUENT 14.0.

©2016 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of RAEREST 2016 Keywords:Portable electronic devices; phase change material; n-eicosane

Introduction

Power dissipation levels in portable electronic devices such as laptops, tablets and smart phones continues to increase due to high power applications such as media, gaming, and increased functionality associated with the

* Corresponding author. Tel.: +91-9447921482. E-mail address:rbaby55@gmail.com

2212-0173 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of RAEREST 2016 doi: 10.1016/j.protcy.2016.08.237

use of internet. The heat generated by this electronic circuitry must be dissipated in order to prevent immediate failure and improve long term reliability. Portable electronic devices at present are available with increased functionality and compactness. The compactness of these devices with more features has not only increased the power density, but also decreased the external available surface area required for dissipation of heat. In order to overcome this problem, some effective cooling strategies are needed. Application of active cooling techniques is limited considering cost, size, power consumption, reliability, weight and noise. In order to develop advanced cooling techniques that can function efficiently, phase change material (PCM) based heat sinks have been widely investigated in the recent times. Phase change materials absorb a large amount of energy during their phase transformation while maintaining isothermal conditions and this has been the major motivation for their use in many applications. PCMs have been employed for applications involving cooling of portable devices like mobile phones, reducing the junction temperatures in high power electronic packages and as panels for lining the military vehicles operating in the desert conditions.

The performance of PCM based approaches to thermal solutions has been extensively studied experimentally and numerically for the last few decades. Tan and Tso [1] conducted experimental studies on the cooling of mobile electronic devices, such as personal digital assistants (PDAs) and wearable computers, using a heat storage unit (HSU) filled with PCM, n-eicosane inside the device. They concluded that the use of HSU helps to stabilize the system temperature to an allowable working temperature of 500C. Kandasamy et al [2] studied the application of a PCM package for thermal management of portable electronic devices for obtaining the effects of various parameters such as input power, orientation etc. From the experimental results it was found that the PCM package can be effectively used for the transient cooling applications for electronic devices which are used intermittently. The two dimensional numerical analysis conducted for the prediction of the temperature closely agrees with the experimentally measured values. Kandasamy et al [3], experimentally and numerically, investigated the potential use of the transient thermal control of PCM based heat sink placed on a quad flat package electronic devices. They found that the thermal performance and melting rate increase at higher power levels until complete melting of the PCM (paraffin wax) takes place. Saha et al [4] studied the role of thermal conductivity enhancers on PCM based finned heat sinks and reported that the case with 8% TCE volume fraction of the PCM based plate or pin fin heat sinks found to have best thermal performance. Srivatsa et al [5] conducted a three-dimensional numerical analysis for phase change material based heat sinks equipped with thermal conductivity enhancers like aluminium metal foam and crossed plate fins. The validated computational models are then employed to investigate the feasibility of operation of the heat sink configurations under high heat fluxes. Yang and Wang [6] numerically simulated the cooling application of portable hand held electronic devices with a constant and uniform volumetric heat generation using a phase change material within a closed system. The main focus is on transient surface temperature behaviours at different power levels, various orientations at charge and discharge modes. Baby and Balaji [7] optimized the thermal performance of pin fin heat sinks using genetic algorithm coupled with artificial neural networks. Hosseinizadeh et al [8] conducted experimental and numerical investigations of PCM based heat sinks with various internal fin configurations. It is found that increasing the number of fins and height of fin resulted in a significant increase in the thermal performance. Also observed that increasing the fin thickness have only a marginal improvement in the thermal performance. Zhao et al [9] attained a reduction of more than half in the solidification time with the use of metal foams.

It is a fact that lot of numerical and experimental works are already carried out to evaluate the significance of PCM based heat sinks and thermal management of electronic equipment. But a three dimensional numerical analysis, that too taking the average dimensions of smart phones available in the market at present using PCM based heat sink were scarce in the literature. In the present study the dimension of the heat sink finalized considering a group of smart phones currently available in the market. Numerical studies were conducted for the newly designed heat sink for different input power levels. The effect of natural convection has been discussed with the help of velocity plots. Time and grid independent studies are also performed for the new model. The numerical model is validated with the experimental results [10].

2. Physical Model

The new heat sink is developed considering a group of smart phones currently available in the market. The information about the smart phones and their dimensions are given in Table 1. After considering the dimensions of

Table 1. Dimensions of the smart phones considered in the study

Brand Model Dimensions (length x breadth in cm)

SONY Xperia C 14.1,7.5

IDEA Ultra 2 15, 7.1

MOTOROLA Moto G2 13.8, 6.8

ASUS Zenfone-501 14.7, 7.1

SAMSUNG Ace-GTS5830 11 , 5.5

SAMSUNG Galaxy-young 10.4 , 5.5

the smart phones available in the market, the average of these dimensions are taken for designing the heat sink. The heat sink taken for the study has a base area of 130 x 66 mm 2 and has a vertical height of 25mm. The PCM is placed inside a cavity of area 116 x 52 mm2 and extending to a depth of 20 mm from the top surface of the heat sink. The heat sink was equipped with a slot with an area of 110 x 46 mm2 with a thickness of 2 mm to accommodate an electric heater. The physical model of the newly designed heat sink is shown in Fig. 1. The properties of aluminium and n-eicosane considered in the numerical model are available in Table 2.

Fig. 1. Physical model considered in the present study

Aluminium properties are assumed to be independent of temperature and the properties of PCM are assumed to be same for the liquid and solid phase. The specific heat of n-eicosane is not constant in the temperature range 20- 800 C. It reaches values as high as 17312 J/kg at around 38°C, and begins to fall after 39 0C to reach a minimum value of 1718J/Kg at 520 C as seen from the scanning calorimetry results[10]. Therefore, a piece wise linear function of the specific heat of the PCM with respect to temperature was chosen to represent the variation. To form a piece wise linear function, values of specific heat from 27 0C to 820C are taken at an interval of 10C.

3. Governing Equations

Melting of PCM is a moving solid liquid interface problem. The melting process of the PCM is modelled using enthalpy-porosity formulation. It is a single domain approach where a system of momentum and energy equations is solved in the entire physical domain. It includes latent heat as source term; velocity vanishes in the solid

field. This model is used when the thermo physical properties of solid and liquid are equal.

Table 2. Properties of materials used for the present study

Material Property

n-eicosane Thermal conductivity(W/m k) 0.1505

Density(Kg/m3) 785

Specific heat(J/ kg K) ---

Dynamic viscosity(kg/m s) 0.00355

Thermal expansion coefficient( K-1) 0.001

Latent heat of fusion( J/Kg) 237400

Melting point(0C) 36.5

Aluminium Thermal conductivity (W/m K) 202.4

Density ( Kg/m3 ) 2719

Specific Heat ( J/ Kg K ) 871

Total specific enthalpy of the PCM is equal the sum of sensible heat and the latent heat. Total specific enthalpy = sensible heat + latent heat

Average specific heat of the PCM,

hpcm — hs + hl

c = /orcp(T)dr

h, = CT =

Latent heat is defined in piece wise function form as

f С (T)dT Jn

7 Tliquidus.

Tf T > T 11 liquidus

Tf T < T < T

11 1 solidus liquidus

Tf T > Tsolidus

Similarly, the specific enthalpy of aluminium heat sink is given by

^Al = CpT

The PCM medium is considered to be a porous substance with varying degrees of porosity at different locations or computational cells. The porosity of any computational cell always acquires a value equal to its liquid fraction (f). Consequently, when the whole cell region is occupied by the liquid PCM, the porosity of the cell is 1.0. Governing equations are written in terms of superficial velocity which can be given as porosity times the liquid PCM velocity vector. Superficial velocity is an artificial velocity calculated as if, the given fluid is the only one flowing in a given cross-sectional area.

Where,

V - Л Vliquid

Continuity Equation: V.7 =0

Momentum equations:

porosity(X) = liquid fraction(f) =

If T > Tliquidus If Tsolidus < T < Tliquidus

If T > Tsolidus

T —T

1 1 liquidus

P^ = v. GuVV)- Vp+Sb+Sm Where,

V = Ui+V] + Wk

For PCM, the source term Sm is identical to Carman-Kozeny equation employed for the flow in porous media.

(1 - A)2 Sm= A ^ + c Ui + Vj + Wk

Sb = [pgP(T - Tref)]j Where Tref is the reference temperature. For aluminium heat sink: Sb=0 and Sm =0. Energy Equations: For PCM,

+ hpcm) = V.(kVT)

^ + V. (pVhs) = V. (fcVT) - ^ - V. (p? hl)

For aluminium heat sink,

M + hAl)=V.kVT The presence of the mushy zone is assumed, even for pure metals, to facilitate a smooth transition of velocity from a finite value to zero in the solid. The A in the Sm term represents the mushy zone constant and it can take any value in the range 104-107 .In the present study a value of 5 x 106 is taken for the mushy zone constant. The liquid fraction is updated after every time step. The convergence criteria employed in this study are: 10-6, 10-3 and 10-7 for the residuals on the continuity, momentum and energy equations respectively.

4. Boundary Conditions

The governing equations and the corresponding boundary conditions are formulated for the whole domain including the aluminium heat sink and the PCM inside the cavity. The boundary conditions are applied based on the experimental work [10]. Adiabatic conditions are taken for the outer and top side of the heat sink.

Initial Conditions: At t= 0

T = 300K and V = 0 Top surface:

(a) For Heat sink (Aluminium surface): = 0 at y = 0.025.

(b) For PCM surface: ^ = 0 and V = 0 at y = 0.025. Side walls of heat sink:

Where, n represents the normal direction to the heat sink outer wall surface. Bottom surface:

(a) For aluminium surface surrounding the heater slot: — = 0 at y = 0.

(b) For aluminium surface in contact with the heater: —kAl — = Q at y = 0.002.

dy ^base

where, Q is the power input from the heater, Abase is the area of the heater surface and kA/ is the thermal conductivity of aluminium.

5. Result and Discussions

In order to understand the thermal performance of the heat sink, numerical analysis is performed for a constant heat input of 4 to 6 W in steps of 0.5 W. In order to have a better understanding of the melting of PCM inside the heat sink cavity, the contours of liquid fraction at a sectional plane at different instants of time at a constant heat input of 6 W is shown in Fig. 2. 5.1 Time and grid independent study

For a heat input of 6 W and a mesh with a grid size of 21457 elements, the variation of liquid fraction with time is compared for three different time steps of 1, 0.75 and 0.5 s. From Fig. 3(a), it is seen that, for time steps of 0.5, 0.75 and 1s, the liquid fraction is almost the same at any instant. Similarly, for the same heat input and for a time step of 0.75 s, simulations were done with three grid sizes with 11920, 21457, and 32385 elements, the results of which are shown in Fig. 3(b). It can be seen that a finer grid of 32385 elements yields liquid fraction profiles similar to that obtained with 21457 cells. Hence a grid with 21457 elements and a time step 0.75 s are adequate. All further numerical simulations involving this geometry have been performed on this grid for a time step of 0.75 s.

Fig. 2. Contours of liquid fraction at different time instants

4000 Time(s)

Fig. 3(a). Time independent study

5.2 Effect of various heat inputs

The effect of different heat inputs on the thermal performance of the heat sink filled with PCM is next examined. The effect of heat load is examined using three temperatures; heat sink base temperature (TB), PCM temperature inside the heat sink cavity (Ti) and the heat sink wall temperature (TW). Fig. 4(a) shows the variation of heat sink base temperature at different heat inputs from 4 to 6 W in steps of 0.5 W. Fig. 4(b) shows the variation of heat sink interior temperatures. The interior of the heat sink is filled with the PCM, n-eicosane.

Fig. 3(b). Grid independent study

1000 2000 3000 4000 5000 6000 7000 8000 Time(s)

Fig. 4(a). Variation of base temperature at different heat inputs

Time(s)

Fig. 4(b). Variation of interior temperature at different heat inputs

It is seen that the PCM temperature is increasing from ambient to the melting temperature (sensible heating phase), then the PCM continues to be at the melting temperature of 36.50C (latent heating phase). Once the latent heating phase is over the temperature keeps increasing due to sensible heating. Stretching the duration of operation of the latent heating phase is the key to prolong the operation duration of the portable electronic devices. A decrease in the latent heating phase is observed with the increase in the input power levels.

5.3 Effect of natural convection

In this study, the maximum velocity of PCM inside the heat sink cavity was studied in order to quantify the effect of natural convection currents on the melting process. The evolution of the maximum velocity of the PCM medium in a heat sink with at different heat inputs are shown in Fig. 5. All the maximum velocity curves exhibit similar features such as progressive increase in the magnitude of maximum velocity once the melting starts. The peak maximum velocity in all heat inputs occur when the liquid fraction reaches a value close to its maximum value (i.e. liquid fraction=1). A peak maximum velocity of 5.24 e-03 m/s is observed in the heat sink cavity when the heat input is 6 W. In order to understand the variation of temperature for the entire heat sink in conjunction with the PCM, contours of static temperature is taken. The contours of static temperature at 7011 s is shown in the Fig. 6.

Time(s)

Fig. 5. Velocity variation inside heat sink cavity at different heat inputs

6. Conclusions

Fig. 6. Contour of static temperature

A three dimensional numerical model for the PCM based heat sink subjected to a constant heat flux has been developed. The temperature variation for different heat inputs, contours of static temperature and liquid fraction contours are recorded. It is found that the PCM based heat sinks can stretch the operation duration of the electronic equipment considerably in comparison with heat sinks without phase change materials. The maximum velocity of PCM inside the heat sink cavity was studied in order to quantify the effect of natural convection currents on the melting process. The peak maximum velocity for the various heat loads considered occur when the liquid fraction reaches a value close to its maximum value (i.e. liquid fraction=1). A peak maximum velocity of 5.24 e-03 m/s is observed when the heat input is 6 W.

References

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