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1 Modeling Phase Change Material in Electronics using CFD - A Case Study Paul Gauch6 and Weiran Xu Flomerics Inc. 2 Mount Royal Ave, Suite 350 Marlborough, MA Phone: (508) Fax: (508) paul@flomerics.com Abstract A computational analysis was performed for an electronic system that uses Phase Change Materials (PCMs) to provide transient thermal control. The analysis is presented as a method for modeling phase change using the conventional material properties' and a transient analysis for system level thermal models'. This' provides a reliable thermal analysis using an existing Computational Fluid Dynamics (CFD) tool. A case study is considered in which a system level CFD model of an elecovnic enclosure is' modeled without PCM and with PCM retro-fitted in three configurations. The proposed method greatly simplifies the analysis without compromising the thermal integrity of the model and solutions can be obtained quickly. Unexpected results' indicate that the system level model provides important information often lacking in an idealized (non-system) analysis. Key words: Phase Change Material, Transient, CFD, Latent Heat, and Specific Heat. Introduction The increasing power densities in electronic packages and the increased demand for reliability has prompted many new techniques for cooling both transient and steady-state thermal loads. An increasingly popular method for cooling transient thermal devices is the use of Phase Change Materials (PCMs), This is evident by the number of publications available. O'Conner and Weber [1] identified the use of PCMs as an attractive approach to the intermittent absorption of heat in electronics. The authors conducted a comprehensive study of PCMs including testing, analysis and PCM requirements for the design environment. Others such as Clarksean et. al. [2, 3] and Wirtz et. al. [4] indicate the benefit of PCMs in thermal control of electronics. PCMs are heat storage materials that undergo a phase change at a certain key temperature and are commercially available for a range of phase change temperatures. Typical phase change temperatures range from -15 C to 190 C with many PCMs in the ideal range for electronics cooling. The latent heat values for good PCMs tend to range between 150 kj/kg to 250 kj/kg. Incorporating these materials in an electronic enclosure operating under time varying thermal loads will allow a smaller, more efficient cooling system according to Wirtz et. al. [4]. PCM energy storage is based on the heat absorbed or released when a material reversibly changes phase, usually between the solid and liquid states. Otherwise referred to as latent heat storage systems, PCMs have relatively high storage capacities and require no mechanical control. An added benefit is that in either liquid or solid states, there is still a component of sensible heat storage that can act as a buffer in the thermal design. Sensible heat is the heat capacity that is modeled in a traditional Computational Fluid Dynamics (CFD) code and represented by the specific heat (co) property. In equation 1, the first term represents the latent heat, the second term, the sensible heat below the melting point and the third term, the sensible heat above the melting point [5]. where: Q= mamahm + ~"mcpdt + ~,imcpdt (1) Q = Heat absorbed [kj] m ~ Mass of PCM [kg] am = Fraction of melt Ahm = Heat of fusion (latent heat) [kj/kg] 7"1 = Initial temperature [K] 7_, -- Final temperature [K] c/, = Specific heat [kj/kgk] PCMs have improved since first being applied in thermal control environments. A list of requirements exists that make them more suitable. Factors such as cost, safety, environmental consideration, useful life (stability to segregation), temperature range, phase change temperature, high 402

2 latent heat, low thermal resistance, small volume change and availability are all considered today before a choice of PCM is made [2, 5, l ]. PCMs have been the focus of numerous studies. The use of PCM imbedded into heat spreading devices such as heat sinks has been investigated by a number of authors according to Clarksean [2]. Wirtz [4] also makes use of this combination of heat spreader with imbedded PCM and refers to the concept as a hybrid cooler. Bugby et. al. [6] describes a study involving partially expanded aluminum honeycomb in conjunction with PCM. The use of the heat-spreading medium increases the effectiveness as PCMs themselves have poor thermal resistance. The heat-spreading device is often also the sealed container for the PCM. To be practical, PCM needs to be encapsulated. This encapsulating can be done at a microscopic level [7, 4, 8] or in any practical container. Pal and Joshi [9] provide guidelines for designing systems with PCMs. Their study indicates that a heat-spreading device (using aluminum foam in the enclosure in their case) will enhance the effect of the PCM. A number of analysis methods have been performed and evaluated for modeling PCMs. Wirtz et. al. [4] developed a semi empirical method assuming symmetry and a resistance network model. The phase change material was modeled using a lumped "slope" method used in this study and described in the next chapter. The results compared very well with measurements. Clarksean [2] on the other hand performed a numerical analysis using a general purpose CFD tool taking the melting front and convective effects of the melt into account. Clarksean [3] also performed a study comparing the lumped approach and the detailed numerical approach for the design environment. The results showed excellent correlation. Kamal [10] developed a numerical algorithm for the vertical tube type phase change system using the SIMPLE algorithm in cylindrical coordinates. This paper presents the use of CFD together with the simplicity of the lumped slope method to perform the transient analysis for an electronics system using a hybrid cooler. The method will be discussed and then illustrated using a simple system level analysis case. Numerical Model The case study presented was performed using the electronics cooling CFD package FLOTHERM by Flomerics. FLOTHERM is a computational fluid and heat transfer analysis and design package specifically for the analysis of electronic equipment. FLOTHERM makes use of the finite volume method to analyze three-dimensional geometries from chip level to system level. The conjugate heat and flow solution is performed using the Boussinesq approximation for buoyancy forces. FLOTHERM solves the steady-state as well as the transient governing equations. Turbulence is modeled with a choice of zero or two equation models. The governing equations are shown here in compact form for conservation of mass, momentum (Navier-Stokes) and energy respectively [ 11 ]: Off e 07 + e(ff" V)f = -VP +//V2f V. f = 0 (2) --[- eg~(w - Tin) (3) at Up-~-+UpK.VT=V.kVT+S (4) Phase change was modeled using solid regions to define the encapsulated PCM. The thermal properties of the phase change were converted from latent heat (or heat of fusion) to an equivalent specific heat by allowing the heat of fusion to occur over a finite temperature range. This method is also known as the "slope" method based on the gradient of the enthalpy (or internal energy) - temperature curve. h C equi v = C p ~ (5) gt This allows for an elegant and simple solution to the phase change and can be considered to be a lumped approach to modeling the phase change in an electronic enclosure. The result could also be considered to be conservative from a design perspective due to the lack of the convective component in the liquid PCM. For pure PCMs, the phase change occurs at a constant temperature. To model this would require an infinite specific heat. By allowing the latent heat absorption to artificially occur over a small finite temperature range, an artificial specific heat can be calculated and used. In this case, the average specific heat of the liquid and solid PCM can be added to the artificial specific heat to form the complete equivalent specific heat for the phase change region (equation 5). For composite PCMs, the phase change usually occurs over a finite temperature and the 403

3 thermal capacity is plotted on a specific heat - temperature graph. In this case, the phase change equivalent specific heat can still be approximated as constant over a certain temperature range around the peak change of phase to provide accurate results. The enthalpy change across the phase must be conserved. One hybrid cooling device consists of a standard extruded heat sink with sealed PCM imbedded as shown in Fig. 2. The PCM considered here is Thermasorb-175 by Frisby Technologies. The approximate properties can be found in table 1. This lumped approach therefore results in the correct heat transfer and energy absorption during the phase change. A Case Study An electronic enclosure with dimensions of 16" x 14" x 3" was considered. This compact system contains a main Printed Circuit Board (PCB) with a Central Processing Unit (CPU). This device operates under transient loading and at its peak, dissipates 50 Watts. For cooling, an extruded fin heat sink is mounted on the CPU. This mounting surface will be the focus for fitting the hybrid cooling system. PCM Fig. 2: The Extruded Heat Sink Containing PCM. Property Value Melting Temperature 79 C (r,n) Density (p) 930 kg/m ~ Latent Heat (h) 200 kj/kg Specific Heat (Cr,) 2,000 J/kgK (Assumed) Conductivity (k) 0.25 W/mK (Assumed) Table 1: Properties of Thermasorb-175. Fig. 1 : The Internal Structure of the System Considered. Flow is Indicated for Purpose of Clarity. The enclosure also contains 10 peripheral devices dissipating an additional 40 Watts on the PCB. Three Single In-line Memory Modules (SIMMs) dissipating 2 Watts each are located "downstream" of the CPU. The enclosure is cooled by forced convection through a power supply at the rear. The power supply dissipates 100 Watts and has contains the only fan in the system. The fan is rated to 10,75 CFM. In front of the power supply is a storage unit dissipating 20 Watts at peak operation. The ambient air temperature is taken to be 35 C. This system is modeled as simply as possible to allow the hybrid cooling device to be the focus of the analysis. This is done for example by considering the PCB components to be lumped objects. The equivalent specific heat can be calculated according to equation 5. The small finite temperature used for the "slope" method will be 0.5 C in this case (Fig. 3). This gives: 200, ,000 = 402,000J / kgk Cequiv g O.,o I I 1000 I Equivalent Specific Heat Temperature [ C] Fig. 3: The Equivalent Specific Heat Used in the Model

4 As can be seen, the actual specific heat component in the equivalent specific heat is almost negligible in comparison with the artificial phase change component. Three variations of thermal control will be considered for the system shown in Fig. 1 with the objective of tracking the CPU temperature in time: Base case." In this case, the system is equipped with an extruded heat sink as shown in fig. 4. The heat sink dimensions are: L = 50 ram, W = 55 ram, H = 40 ram, b = 5 ram, t = 1 rnrn, number of fins = 10. Modtfication 1: PCM is encapsulated in a hermitically sealed aluminum container between the CPU and heat sink. Fig. 5. The idea behind this design is simplicity for manufacture and easy containment of the PCM. The major drawback is the high thermal resistance between the component and the heat sink. Mod~cation 2." PCM is imbedded into the heat sink as per fig. 6. This design is somewhat more difficult to realize, it does however have the advantage of having a much lower thermal resistance than modification 1 as the thermal path from the component goes into the heat sink base and up the fins. From here, the PCM controls the temperature. H! t w Fig. 4: Extruded Heat Sink (Case 1) PCM Fig. 5: Extruded Heat Sink with PCM in Hermitically Sealed Enclosure (Case 2) E -- Fig. 6: Extruded Heat Sink with Imbedded PCM (Case 3a,b) For modification 2 (Fig. 6), the amount of PCM was increased by 50% to evaluate the possibility of further improvement in the time it takes for the CPU to reach 100 C. This stage serves to illustrate the use of CFD in performing a set of "what if?" parametric changes to the design. The user input at this point is minimal and results are achieved in little more than the time it takes to perform the solution. Discussion of Results The transient analysis performed in all cases was performed for the system shown in Fig. 1. The CFD model was constructed with the enclosure coincident with the overall solution domain. The ambient boundary condition external to the enclosure is assumed to be natural convection with a heat transfer coefficient: ham/~ = 6 W/m2K and T, mh = 35 C. The computational grid contained approximately 67,000 grid cells for the system and was computed using a laptop computer with a 366 MHz Intel Celeron processor. Fig. 7 shows the transient response of the 3 thermal designs at the component case, heat sink base and where applicable, in the PCM. Interestingly, case 2 performs quite well for the simplicity of its design, during the phase change. The drawback is the higher thermal gradient between phase change and the component, This actually caused more heat to go into the PCB and gave the PCM a longer transient phase change. Similar quantities of PCM were used in case 2 and 3. The biggest surprise in this study comes from the "what if?" study. The results for case 3 are shown in Fig. 8. The increase by 50% of the PCM did not lead to much improvement in the thermal response of the CPU. The thermal response improvement was just 6.7%. This can be attributed to a number of factors imbedded into the system level model. For example, more PCM in the heat sink implies more airflow blockage as well as a reduction 405

5 in the finned region to the airflow. The flow direction and the flow rate can only be modeled accurately at a system level due to the non-linear nature of the fundamental flow phenomena. The system impedance is also needed so as to produce the correct operating point on the fan curve == 90 = 80 '- 70 E Temperature Response for System Time [s] = CPU: Case 1 = Heat Sink: Case 1 0 CPU: Case 3 PCM: Case 3 # Heat Sink: Case 3 CPU: Case 2 A Heat Sink: Case 2 PCM: Case 2 Fig. 7: Transient Thermal Response for 3 Cooling Designs. The CPU Case Temperature, PCM Temperature and Heat Sink Base Temperature are given where applicable. ; O 100 b,ml 90 = Q- 60 E 50 I Temperature Response vs. Mass of PCM O0 Time [s] CPU: Case 3b,.O,. - - PCM: Case 3b * Heat Sink: Case 3b [] CPU: Case 3a D - - PCM: Case 3a -,I---. Heat Sink: Case 3a Fig. 8: Transient Thermal Response for the Original Case 3(a) and Case 3(b) with the Increased Mass of PCM (50% Increase). Conclusion As phase change materials become more commonly used to perform the thermal management of electronic packages, the need increases for reliable system level thermal analysis in the design process. Systems that use PCM thermal control are typically transient in operation. Either the system can only remain fully powered for a short time or the power in specific components undergoes periodic fluctuations where the peak flux needs to be absorbed. Examples of systems include portable equipment such as laptop computers and portable phones, power electronic 406

6 equipment, aviation data recorders and space-based electronics. The size of an electronics enclosure can be reduced significantly when using PCMs and the passive nature of the PCM can prevent reliability issues that can be a burden in mechanical/active cooling. The case study analysis performed in this work provides evidence of these benefits. The purpose of this study was to illustrate that the analysis of hybrid cooling devices incorporating heat spreaders and PCMs is possible at a system level design stage using existing commercial CFD packages. In fact, the system level modeling is encouraged due to findings in this work. The assumptions made for the system level analysis simplify the computational and pre-processing effort but do not compromise in the validity of the results. References [1] O'Conner, J.P., and Weber, R.M., "Thermal Management of Electronic Packages Using Solid-to- Liquid Phase Change Techniques", Proceedings of the 1997 International Systems Packaging Symposium (IMAPS), San Diego, California, December 2-5, [2] Clarksean, R., Chen, Y., and Marongiu, M., "An Analysis of Heat Flux Limits for Electronic Components on a Finned Substrate Containing a PCM", Proceedings of the Pacific Rim/ASME International Intersociety Electronic & Photonic Packaging Conference (InterPACK '99), Maui, Hawaii, June 13-19, Vol. 2, pp , [3] Clarksean, R., and Chen, Y., "The Use of Phase Change Material for Electronic Cooling Application: Thermal Design Issues and Example", Proceedings of the Pacific Rim/ASME International Intersociety Electronic & Photonic Packaging Conference (InterPACK '99), Maui, Hawaii, June 13-19, Vol. 2, pp , [6] Bugby, D.C., Krein, S.J., and Kim, J.H., " Development and Testing of an Ambient Thermal Storage Unit for Pulsed Load Applications", Proceedings of the Pacific Rim/ASME International Intersociety Electronic & Photonic Packaging Conference (InterPACK '99), Maui, Hawaii, June 13-19, Vol. 2, pp , [7] Fosset, A.J., et. al., "Avionics Passive Cooling With Microencapsulated Phase Change Materials", Journal of Electronic Packaging, Vol. 120, pp , September, [8] Mulligan, J.C., Colvin, D.P., and Bryant, Y.G., " Use of Two-Component Fluids of Microencapsulated Phase-Change Materials for Heat Transfer in Spacecraft Thermal Systems", Proceedings of the 6 th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, Colorado Springs, Colorado, June 20-23, [9] Pal, D., and Yoshi, Y.K., " Thermal Control of Horizontally Mounted Heat Sources using Phase Change Material", Proceedings of the Pacific Rim/ASME International Intersociety Electronic & Photonic Packaging Conference (InterPACK '99), Maui, Hawaii, June 13-19, Vol. 2, pp , [10] Ismail, K.A.R., and Abugderah, M.M., "Transient Numerical Modeling for Phase Change Thermal Storage System Analysis and Design", Proceedings of the Pacific Rim/ASME International Intersociety Electronic & Photonic Packaging Conference (InterPACK '99), Maui, Hawaii, June 13-19, Vol. 2, pp , [11] Mills, A.F., "Heat and Mass Transfer", Irwin, First Edition, Chicago, pp , [4] Wirtz, R.A., Zheng, N., and Chandra, D., "Thermal Management Using "Dry" Phase Change Materials", Proceedings of the 1999 Semiconductor Thermal Measurement and Management Symposium (SEMI-THERM), San Diego, California, March 9-11, pp , [5] Guyer, E.C., "Handbook of Applied Thermal Design", McGraw-Hill Publishers, First Edition, New York, Part 6, Chapter 1, pp I 1,