Thermal analysis of a phase change material based heat sink for cooling protruding electronic chips

This work aims to numerically study the melting natural convection in a rectangular enclosure heated by three discreet protruding electronic chips. The heat sources generate heat at a constant and uniform volumetric rate. A part of the power generated in the heat sources is dissipated to a phase change material (PCM, n-eicosane with melting temperature, Tm = 36°C). Numerical investigations were carried out in order to examine the effects of the plate thickness on the maximum temperature of electronic components, the percentage contribution of plate heat conduction on the total removed heat and temperature profiles in the plate. Correlations for the dimensionless secured working time (time to reach the threshold temperature, Tcr = 75°C) and the corresponding liquid fraction were derived.     

基于压电驱动的电子芯片散热器试验研究

电子芯片的散热一直是影响计算机技术发展的关键问题,液冷方式中采用微喷射结构散热器是满足电子芯片高效散热的有效方法。设计了利用叠堆式压电驱动器散热系统,利用压电陶瓷产生的中低频振动,实现对电子芯片冷却器散热板的脉动喷射,进行了同参数下微喷射散热器与微槽道散热器散热对比试验研究。试验表明,利用叠堆式压电陶瓷可以更有效的满足散热器槽道及微喷微小尺度的流体驱动问题。微喷射散热器具有优良的稳态下的散热性能和瞬态下的储热性能。     

A comparative analysis of innovative microchannel heat sinks for electronic cooling

In this work, a comparative analysis of innovative microchannel heat sinks such as two-layered and multi-layered microchannel heat sinks (MCHS), or thin films within flexible complex seals and cooling augmentation using microchannels with rotatable separating plates, is presented. A compilation of the numbers of layers, main characteristics, setups, advantages and disadvantages, thermal resistance, pumping power in double-layer (DL-MCHS) and multi-layer MCHS (ML-MCHS) is presented. In addition, the thermal resistance is analyzed in order to present a comparison between the single-layer MCHS (SL-MCHS) and multi-layer microchannels. The results of comparison indicates that double-layer and multi-layer MCHS have lower thermal resistance and require smaller pumping power and they resolve the high streamwise temperature rise problem of SL-MCHS.     

Novel heat transfer fluids for direct immersion phase change cooling of electronic systems

We describe the use of computer-aided molecular design (CAMD) and figure of merit (FOM) analysis to identify new heat transfer fluids for direct immersion cooling of electronic systems. Thirty-five new fluids, with thermophysical properties in the range 320 K < T_b < 370 K, k > 0.09 W m^?1 K^?1 and H_vap > 35 kJ mol^?1, were identified via a CAMD approach. Further analysis of these 35 fluids led to the selection of 1,1,1-trifluoro-3-methylpentane (C₆H₁₁F₃) for experimental evaluation. C₆H₁₁F₃ was synthesized from commercially available precursors, and its thermophysical properties were measured to verify its FOM. Next, the pool boiling performance of a mixture of 7 wt.% C₆H₁₁F₃ + 93 wt.% HFE 7200 was determined using a 10 mm × 10 mm grooved Si thermal test chip coated with copper. An improvement of 7% in the critical heat flux (CHF) was obtained, suggesting that C₆H₁₁F₃ is worth further examination as a candidate for direct immersion phase change cooling applications.     

Experimental studies on the use of a phase change material for cooling mobile phones

This paper is to examine the cooling of mobile phones using a phase change material (PCM). Experimental prototypes of mobile phones were fabricated using aluminum heat sinks with different numbers of fins filled with n-eicosane. The devices were subjected to steady-state and transient charging and discharging at different power settings. It was found that increasing the power increased the PCM melting rate. The internal fins also helped to lower the maximum device temperature. These findings indicated that the use of PCM-based heat sinks were effective for the cooling of mobile phones under intermittent moderate usage conditions.     

Natural convection in an internally finned phase change material heat sink for the thermal management of photovoltaics

Elevated operating temperatures reduce the solar to electrical conversion efficiency of building integrated photovoltaic devices (BIPV). Phase change materials (PCM) can be used to passively limit this temperature rise although their effectiveness is limited by their low thermal conductivities and crystallisation segregation during solidification. This paper presents an experimental evaluation of the effects of convection and crystalline segregation in a PCM as a function of efficiency of heat transfer within the finned PV/PCM system. The thermal performances of bulk PCM with crystallisation segregation for different internal fin arrangements are presented. It is noted that the addition of internal fins improves the temperature control of the PV in a PV/PCM system.     

Constructal cooling channels for micro-channel heat sinks

This paper documents the geometric optimisation of a three-dimensional micro-channel heat sink. The objective is to minimise the peak temperature from the walls to the coolant fluid. The optimisation is performed numerically by using the finite volume method. The numerical simulation was carried out on a unit cell with volume ranging from 0.1 mm³ to 0.9 mm³ and pressure drop between 10 kPa and 75 kPa. The axial length of the micro-channel heat sink was fixed at 10 mm. The cross-sectional area of the micro-channel heat sink is free to morph with respect to the degree of freedoms provided by the aspect ratio and the solid volume fraction. The effect of the total solid volume fraction and the pressure drop on the aspect ratio, channel hydraulic diameter and peak temperature is investigated. The numerical results show that the degrees of freedom have a strong effect on the peak temperature and the maximum thermal conductance. The optimal geometric characteristics obtained numerically (the aspect ratio and the optimal channel shape (hydraulic diameter)) are reported and compared with those obtained from approximate relationships using scale analysis. The predicted trends are found to be in good agreement with the numerical results.     

相变微胶囊悬浮液传热特性实验研究

相变微胶囊(microencapsulated phase change material,MPCM)在建筑节能领域应用广泛,为研究其传热特性,搭建了以水为换热流体(heat transfer fluid,HTF),微胶囊悬浮液为储能介质的潜热储能(latent thermal energy storage,LTES)系统。在实验过程中,通过改变换热流体的进口初始温度以及搅拌器的搅拌速率,获得了MPCM悬浮液的温度变化规律并计算了MPCM悬浮液的平均充放冷速率。实验结果表明:在充冷过程中,MPCM相变时温度变化速率减缓,相变温度区间较大,而在放冷过程中,MPCM相变时温度保持恒定,相变温度区间较小;未搅拌时,MPCM悬浮液中温度梯度较大,传热能力较差;搅拌时,MPCM悬浮液混合均匀,其温度梯度很小,传热能力较强;增加搅拌器的搅拌速率及水与相变微胶囊悬浮液的温差均可以提高MPCM的充放冷速率。     

Analysis of microchannel heat sinks for electronics cooling

This paper presents an analytical and numerical study on the heat transfer characteristics of forced convection across a microchannel heat sink. Two analytical approaches are used: the porous medium model and the fin approach. In the porous medium approach, the modified Darcy equation for the fluid and the two-equation model for heat transfer between the solid and fluid phases are employed. Firstly, the effects of channel aspect ratio (α_s) and effective thermal conductivity ratio (k?) on the overall Nusselt number of the heat sink are studied in detail. The predictions from the two approaches both show that the overall Nusselt number (Nu) increases as α_s is increased and decreases with increasing k?. However, the results also reveal that there exists significant difference between the two approaches for both the temperature distributions and overall Nusselt numbers, and the discrepancy becomes larger as either α_s or k? is increased. It is suggested that this discrepancy can be attributed to the indispensable assumption of uniform fluid temperature in the direction normal to the coolant flow invoked in the fin approach. The effect of porosity (ε) on the thermal performance of the microchannel is subsequently examined. It is found that whereas the porous medium model predicts the existence of an optimal porosity for the microchannel heat sink, the fin approach predicts that the heat transfer capability of the heat sink increases monotonically with the porosity. The effect of turbulent heat transfer within the microchannel is next studied, and it is found that turbulent heat transfer results in a decreased optimal porosity in comparison with that for the laminar flow. A new concept of microchannel cooling in combination with microheat pipes is proposed, and the enhancement in heat transfer due to the heat pipes is estimated. Finally, two-dimensional numerical calculations are conducted for both constant heat flux and constant wall temperature conditions to check the accuracy of analytical solutions and to examine the effect of different boundary conditions on the overall heat transfer.     

Effect of orientation for phase change material (PCM)-based heat sinks for transient thermal management of electric components

Phase change material (PCM)-based heat sink, consisting of a conventional, extruded aluminum sink embedded with appropriate PCMs, can potentially be used for cooling of mobile electronic devices such as personal digital assistants (PDAs) and notebooks which are operated intermittently. During the use of such mobile devices, the orientation changes from time to time. A numerical investigation was carried out to study the effect of orientation of heat sink on the thermal performance of the combined cooling system to determine if it affects the thermal performance of a PCM-based cooling system significantly.     

Experimental analysis and FEM simulation of finned U-shape multi heat pipe for desktop PC cooling

This paper presents the performance analysis of a finned U-shape heat pipe used for desktop PC-CPU cooling. The experiments are conducted by mounting the system vertically over a heat source situated inside a rectangular tunnel, and force convection is facilitated by means of a blower. The total thermal resistance (R_t) and heat transfer coefficient are estimated for both natural and forced convection modes under steady state condition, by varying the heat input from 4 W to 24 W, and the air velocity from 1 m/s to 4 m/s. The coolant velocity and heat input to achieve minimum R_t are found out and the corresponding effective thermal conductivity is calculated. The transient temperature distribution in the finned heat pipe is also observed. The experimental observations are verified by simulation using ANSYS 10. The results show that the air velocity, power input and heat pipe orientation have significant effects on the performance of finned heat pipes. As the heat input and air velocity increase, total thermal resistance decreases. The lowest value of the total thermal resistance obtained is 0.181 °C/W when heat input is 24 W and air velocity 3 m/s. The experimental and simulation results are found in good agreement.     

A numerical model for heat sinks with phase change materials and thermal conductivity enhancers

The effectiveness of thermal conductivity enhancers (TCEs) in improving the overall thermal conductance of phase change materials (PCMs) used in cooling of electronics is investigated numerically. With respect to the distribution of TCE and PCM materials, the heat sink designs are classified into two types. The first type of heat sink has the PCM distributed uniformly in a porous TCE matrix, and the second kind has PCM with fins made of TCE material. A transient finite volume method is used to model the heat transfer; phase change and fluid flow in both cases. A generalized enthalpy based formulation and numerical model are used for simulating phase change processes in the two cases. The performance of heat sinks with various volume fractions of TCE for different configurations is studied with respect to the variation of heat source (or chip) temperature with time; melt fraction and dimensionless temperature difference within the PCM. Results illustrate significant effect of the thermal conductivity enhancer on the performance of heat sinks.     

Experimental study of impingement cooling by heat sinks with thin longitudinal fins for LSI packages

This paper reports on the impingement cooling characteristics of a heat sink with thin longitudinal fins of 0.2 mm thickness, which are spaced with a fin-pitch in the range 0.5 mm to 2.0 mm. The air cooling of the heat sink comes from a slot-shaped orifice positioned above the heat-sink center. The breadth of the gap between the fin tops and the inlet orifice is in the range 0 mm to 10 mm. The thermal resistance of the thin longitudinal fins used is about 50% to 57% that of the thick longitudinal fins now in commercial use. The cooling performance of the thin-plate fins is almost the same as that of optimally arranged pin-fins with the same total surface area. A maximum value of six times the heat transfer rate of a single flat plate having the same base area was observed for the thin-plate fins. A comparison of cooling performance between impingement and channel flow systems was conducted. The performance of impingement cooling systems is almost unaffected by the breadth of the gap between the fin tops and the inlet orifice (or, for channel cooling, the upper wall). On the other hand, the performance of channel-cooling systems decreases significantly as the gap widens. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(7): 449–459, 1996     

Experimental study of convective heat transfer coefficient for pulsed fluidized bed using microcapsule phase change material

The convective heat transfer coefficient in a pulsating cylindrical fluidized bed using microcapsule particle-phase change material (MPCM) was examined experimentally. A solenoid on and off valve was used to provide the pulsation and was varied from 1 to 10 Hz. The test section in the bed was loaded with granular microparticle-phase change material (MPCM) with an average diameter of 200 µm. The pulsation effect on the thermal field, thermal storage, and heat transfer coefficient was investigated for 1.5, 2, and 2.5 of minimum fluidization velocity. Results indicated that the amplitude of the oscillation decreases with increasing in pulsation frequency. An increase in heat transfer rate was shown to be related to the superficial velocity. The maximum performance was obtained for the frequency of 7 Hz at the velocity ratio of 2.5. The convective heat transfer increased by 17% for frequency pulsation of 7 Hz. Furthermore, the Duty cycle (η) is defined as the ratio of turn-on duration to the total turn-on and off duration for entering airflow to the solenoid valve. It was shown that the η > 0.4 led to better mixing and higher heat transfer. However, the pulsation effect was shown to disappear for the duty cycle of higher than 0.8. Comparison with the available experimental data of others for continuous flow was in good agreement.     

Experimental study on the phase change behavior of phase change material confined in pores

An experimental study on the phase change behavior of organic phase change materials (PCMs) in porous building materials is reported. Three kinds of porous materials and two kinds of PCMs were used. The phase change behavior of organic PCMs and phase change composites was measured by means of differential scanning calorimetry (DSC). The pore structure of the porous materials was characterized by means of mercury intrusion porosimetry (MIP). X-ray fluorescence spectrometry (XRF) and Fourier transformation infrared spectroscopy (FTIR) were used to characterize the chemical properties of porous materials and phase change materials. Quite different phase change behaviors were found for these two kinds of PCMs in porous materials. For capric acid with a functional group of –COOH, a remarkable elevation of melting temperature was found when confined in porous materials. But for paraffin with only inactive functional groups of –CH₂ and –CH₃, no elevation or depression of the melting temperature was found when confined in the porous materials. The interaction between functional groups of PCM molecules and alkaline spots on the inner pore surface of the porous materials and the Clapeyron equation were used to explain the different shift of the phase change temperature of capric acid and paraffin in porous materials.     

Experimental study of natural convection heat transfer in a microencapsulated phase change material slurry

A new microencapsulated PCM (Phase Change Material) slurry (MEPCS) at high concentration (45% w/w) was developed based on microencapsulated Rubitherm RT6. Its heat storage and heat transfer characteristics have been experimentally investigated in order to assess its suitability for integration into a low temperature heat storage system for solar air conditioning applications. Differential scanning calorimetry tests have been conducted to evaluate the cold storage capacity and phase change temperature range. An experimental setup was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the MEPCS. First, tests were carried out using water in order to obtain natural convection heat transfer correlations and then a comparison was made with the results obtained for the MEPCS. It was found that inside the phase change interval the values of the heat transfer coefficient for the MEPCS were significantly higher than for water, under identical temperature conditions.     

Review on phase change material emulsions and microencapsulated phase change material slurries: Materials, heat transfer studies and applications

Phase change materials (PCM) in the form of slurries have had an increasingly important role as heat transfer fluids and as thermal energy storage media. Although it is a recent technology in the field of thermal energy storage with phase change materials, the volume of literature begins to be significant. This investigation carries out a compilation of information on two latent thermal fluids: PCM emulsions and microencapsulated PCM slurries (mPCM slurries). This review presents tables containing information on the different PCM emulsions and mPCM slurries studied by different researchers, as well as commercially available products. Thermophysical and rheological properties are analyzed, making a special effort to analyze heat transfer phenomena, concluding with the enumeration of application examples available in literature.     

Experimental assessment of heat storage properties and heat transfer characteristics of a phase change material slurry for air conditioning applications

A new microencapsulated phase change material slurry based on microencapsulated Rubitherm RT6 at high concentration (45% w/w) was tested. Some heat storage properties and heat transfer characteristics have been experimentally investigated in order to assess its suitability for the integration into a low temperature heat storage system for solar air conditioning applications. DSC tests were conducted to evaluate the cold storage capacity and phase change temperature range. A phase change interval of approximately 3 °C and a hysteresis behaviour of the enthalpy were identified. An experimental set-up was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the phase change material slurry. First, tests were carried out using water in order to obtain natural convection heat transfer correlations. Then a comparison was conducted with the results obtained for the phase change material slurry. It was found that the values of the heat transfer coefficient for the phase change material slurry were higher than for water, under identical temperature conditions inside the phase change interval.     

Performance enhancement of finned heat pipe assisted latent heat thermal energy storage system in the presence of nano-enhanced H2O as phase change material

In this research, Latent Heat Thermal Energy Storage Systems (LHTESS) containing Nano-Enhanced Phase Change Material (NEPCM) in the presence of novel shape finned heat pipe is numerically investigated from the viewpoint of discharging process. In recent years, LHTESS have been used to establish a balance between energy supply and demand. Since conventional PCMs are characterized with high latent heat and low thermal conductivity, these systems are capable of storing large amount of energy, but storage and retrieval processes cannot be achieved in the desired time duration. In this paper, CFD simulation and multi-objective Response Surface Method (RSM) optimization is used simultaneously to find the optimum configuration of novel shaped fin, which is then attached to a heat pipe and immersed into the LHTESS. The performance of finned heat pipe assisted LHTESS is compared to the LHTESS containing NEPCM, and LHTESS with other common fin structures. Since the immersion of finned heat pipe into the system decreases the amount of employed PCM, the maximum energy storage capacity of the LHTESS drops subsequently. Thus, energy storage capacity, as one of the objectives of optimization procedure of this research is studied quantitatively, which is proposed as the novelty here. Results indicate that employing maximum energy storage capacity as an evaluation parameter, leads to efficient design of LHTESS. Also it is inferred that immersing finned heat pipe into LHTESS as a heat transfer enhancement technique is superior to nanoparticles dispersion.