Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Numerical study of laminar heat transfer and pressure drop characteristics in a water-cooled minichannel heat sink

With the rapid development of the information technology (IT) industry, the heat flux in integrated circuit (IC) chips cooled by air has almost reached its limit about 100 W/cm². Some applications in high technologies require heat fluxes well beyond such a limitation. Therefore the search of a more efficient cooling technology becomes one of the bottleneck problems of the further development of IT industry. The microchannel flow geometry offers large surface area of heat transfer and a high convective heat transfer coefficient. However, it has been hard to implement because of its very high pressure head required to pump the coolant fluid though the channels. A normal channel could not give high heat flux although the pressure drop is very small. A minichannel can be used in heat sink with a quite high heat flux and a mild pressure loss. A minichannel heat sink with bottom size of 20 mm × 20 mm is analyzed numerically for the single-phase laminar flow of water as coolant through small hydraulic diameters and a constant heat flux boundary condition is assumed. The effects of channel dimensions, channel wall thickness, bottom thickness and inlet velocity on the pressure drop, thermal resistance and the maximum allowable heat flux are presented. The results indicate that a narrow and deep channel with thin bottom thickness and relatively thin channel wall thickness results in improved heat transfer performance with a relatively high but acceptable pressure drop. A nearly-optimized configuration of heat sink is found which can cool a chip with heat flux of 256 W/cm² at the pumping power of 0.205 W. The nearly-optimized configuration is verified by an orthogonal design. The simulated thermal resistance agrees quite well with the result of conventional correlations method with the maximum difference of 12%.     

The heat transfer characteristics of liquid cooling heat sink with micro pin fins

This paper numerically and experimentally investigated the liquid cooling efficiency of heat sinks containing micro pin fins. Aluminum prototypes of heat sink with micro pin fin were fabricated to explore the flow and thermal performance. The main geometry parameters included the diameter of micro pin fin and porosity of fin array. The effects of the geometrical parameters and pressure drop on the heat transfer performance of the heat sink were studied. In the experiments, the heat flux from base of heat sink was set as 300 kW/m². The pressure drop between the inlet and the outlet of heat sink was set < 3000 Pa. Numerical simulations with similar flow and thermal conditions were conducted to estimate the flow patterns, the effective thermal resistance. It was found that the effective thermal resistance would reach an optimum value for various pressure drops. It was also noted that the effective thermal resistance was not sensitive to porosity for sparsely packed pin fins.     

Fluid flow and heat transfer characteristics of cross-cut heat sinks

In this study, effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition are experimentally studied. To find effects of the length, position, and number of cross-cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm were fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<P_p < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. Based on experimental results, the friction factor and Nusselt number correlations for heat sinks with a cross-cut are suggested. Using the proposed correlations, thermal performances of cross-cut heat sinks are compared to those of optimized plate-fin and square pin-fin heat sinks under the constant pumping power condition. This comparison yields a contour map that suggests an optimum type of heat sink under the constraint of the fixed pumping power and fixed heat sink volume. The contour map shows that an optimized cross-cut heat sink outperforms optimized plate-fin and square pin-fin heat sinks when 0.04 < log L1 < 1.     

3-D Numerical study on the correlation between variable inclined fin angles and thermal behavior in plate fin-tube heat exchanger

In this study, the heat transfer enhancement and pressure drop values of seven different fin angles with plain fin-tube heat exchangers were investigated. The numerical simulation of the fin-tube heat exchanger was performed by using a three dimensional (3-D) numerical computation technique. Therefore, a CFD computer code, the FLUENT was used to solve the equation for the heat transfer and pressure drop analyses in the fin-tube heat exchanger. The model drawing was created and meshed by using GAMBIT software. The heat transfer and pressure drop values of the vertical fin angle (θ = 0°) were provided to compare with variable inclined fin angles (θ = 5°, 10°, 15°, 20°, 25°, 30°). The heat transfer values were normalized to compare all cases. For inclined fin angle θ = 30°, which is the optimum angle, the maximum heat transfer enhancement per segment was obtained 1.42 W (the normalized value 105.24%), the maximum loss power associated with pressure drop per segment was only 0.54 mW.     

Experimental and numerical investigation of low melting point metal based PCM heat sink with internal fins

In this paper, low melting point metal (LMPM), eutectic alloy Bi_31.6In_48.8Sn_19.6 (E-BiInSn), was adopted as phase change material for potential thermal management applications. First, E-BiInSn was prepared and its main thermophysical properties were characterized. Then, transient thermal performances of E-BiInSn based heat sinks with internal crossed fins were tested, in comparison with that of organic PCM (octadecanol) which has close melting point. Three types of heat sink structures which have different number of internal fins were studied. Three heating conditions were applied, namely 80 W (2.2 W/cm²), 200 W (5.6 W/cm²) and 320 W (8.9 W/cm²). For all of the cases, E-BiInSn exhibited much superior thermal performance than that of octadecanol. Furthermore, cyclic test of the E-BiInSn heat sink was carried out, which showed good repeatability and stability, and without supercooling. Finally, a simplified 3D conjugate numerical model was developed to simulate the melting process of LMPM heat sink, which showed good agreement with the experimental results. This simplified model would be much useful in practical thermal design and optimization of LMPM heat sink, for that it would significantly save the computational time consumption.     

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.     

Modeling and optimization of designing parameters for a parallel-plain fin heat sink with confined impinging jet using the response surface methodology

The parallel-plain fin (PPF) array structure is widely applied in convective heat sinks in order to create extended surface for the enhancement of heat transfer. In the present study, for investigating the influences of designing parameters of PPF heat sink with an axial-flow cooling fan on the thermal performance, a systematic experimental design based on the response surface methodology (RSM) is used. The thermal resistance and pressure drop are adopted as the thermal performance characteristics. Various designing parameters, such as height and thickness of fin, width of passage between fins, and distance between the cooling fan and the tip of fins, are explored by experiment. Those parameters affect the structure arrangement, geometry of fins and the status of impinging jet from an axial-flow cooling fan installed over the heat sink. A standard RSM design called a central composite design is selected as experimental plan for the four parameters mentioned above. An effective procedure of response surface methodology (RSM) has been proposed for modeling and optimizing the thermal performance characteristics of PPF heat sink with the design constrains. The most significant influential factors for minimizing thermal resistance and pressure drop have been identified from the analysis of variance. The confirmation experimental results indicate that the proposed model is reasonably accurate and can be used for describing the thermal resistance and pressure drop with the limits of the factors studied. The optimum designing parameters of PPF heat sink with an axial-flow cooling fan under constrains of mass and space limitation, which are based on the quadratic model of RSM and the sequential approximation optimization method, are found to be fin height of 60 mm, fin thickness of 1.07 mm, passage width between fins of 3.32 mm, and distance between the cooling fan and the tip of fins of 2.03 mm.     

Analysis of effectiveness and pressure drop in micro cross-flow heat exchanger

A theoretical model that predicts the thermal and fluidic characteristics of a micro cross-flow heat exchanger is developed in this study. The theoretical model is validated by comparing the theoretical solutions with the experimental data from the relative literature. This model describes the interactive effect between the effectiveness and pressure drop in the micro heat exchanger. The analytical results show that the average temperature of the hot and cold side flow significantly affects the heat transfer rate and the pressure drop at the same effectiveness. Different effectiveness has a great influence upon the heat transfer rate and pressure drop. When the micro heat exchanger material is changed from silicon to copper, the thermal conductivity changes from 148 to 400 W/m K. The heat exchanger efficiency is also similar. Therefore, the (1 1 0) orientation silicon based micro heat exchanger made using the MEMS fabrication process is feasible and efficient. Furthermore, the dimensions effect has a great influence upon the relationship between the heat transfer rate and pressure drop. Therefore, the methodology presented in this paper can be used to design a micro cross-flow heat exchanger.     

Effects of inlet geometry on heat transfer and fluid flow of tangential micro-heat sink

The paper presents the geometric optimization of the micro-heat sink with straight circular microchannels with inner diameter of D_i = 900 μm. The inlet cross-section has a rectangular shape and positioned tangentially to the tube axis with the four different geometries. The fluid flow regime is laminar and water with variable fluid properties is used as a working fluid. The heat flux spread through the bottom sink surface is q = 100 W/cm². Thermal and hydrodynamic performances of the heat sink are compared with results obtained for conventional channel configuration with lateral inlet/outlet cross-section. Besides, the results are compared with the tangential micro-heat sink with D_i = 300 μm. For all the cases, the thermal and hydrodynamic results are compared on a fixed pumping power basis.     

Orientation effect on heat transfer of a shrouded LED backlight panel with a plate-fin array

This study reports thermal performance of a shrouded 348 mm × 558 mm aluminum plate-fin heat sink subject to various input powers and orientations. Effects of clearance (C) and the orientation on the heat transfer of the heat sink were investigated. Results show that the clearance effect is detectable only in a “window region” between 5 mm and 10 mm where an appreciable rise of heat transfer coefficient is encountered. As the tilted angle (θ) of the LED panel is increased, the heat transfer coefficient is reduced and the clearance effect on heat transfer becomes more pronounced. The heat transfer coefficients are similar between two cases in which the tilted angles of the LED panel are supplementary irrespective of clearance and input power. Except the cases of a horizontal heat sink, heat transfer coefficient of the shrouded heat sink having a fin array facing downward is usually slightly higher than that having supplementary tilted angle.     

Investigation of a multiple piezoelectric–magnetic fan system embedded in a heat sink

Previous studies have investigated the thermal performance of embedding a single piezoelectric fan in a heat sink. Based on this work, a multiple piezoelectric–magnetic fan system (“MPMF”) has been successfully developed that exhibits lower fan power consumption, optimum fan pitch and an optimum fan gap between the fan tips and the heat sink. In this study, the cooling performance and heat convection improvement for the MPMF system embedded in a heat sink are evaluated at different fan tip locations. The results indicate that the fan tip location of the MPMF system at x/S_l = 0.5 and y/S_h = 0 is an optimum configuration, improving the thermal resistance by 53.2% over natural convection condition for the fan input power of 0.1 W. The MPMF system breaks the thermal boundary layer and causes fluctuations inside the fins of the heat sink to enhance the overall heat transfer coefficient. Moreover, the relationship between the convection improvement and the Reynolds number for the MPMF system has been investigated and transformed into a correlation line for nine different fan tip locations to provide a means of predicting the cooling performance for the MPMF system embedded in a heat sink.     

Experimental investigation and flow visualization to determine the optimum dimension range of microgap heat sinks

The rapid increase of heat flux in high performance electronic devices has necessitated the development of high capacity thermal management techniques that can support extremely high heat transfer rates. Flow boiling in microgap is very promising for this purpose due to its high heat transfer rate and ease of fabrication. However, the effects of microgap dimension on heat transfer and pressure drop characteristics along with flow visualization have not been investigated extensively. This paper focuses on flow boiling experiments of deionized water in silicon microgap heat sink for ten different microgap dimensions from a range of 80 μm–1000 μm to determine the most effective and efficient range of microgap dimensions based on heat transfer and pressure drop performance. High speed flow visualization is conducted simultaneously along with experiments to illustrate the bubble characteristics in the boiling flow in microgap. The results of this study show that confinement in flow boiling occurs for microgap sizes 500 μm and below and confined slug/annular flow is the main dominant regime whereas physical confinement does not occur for microgap sizes 700 μm and above and bubbly flow is the dominant flow regime. The microgap is ineffective below 100 μm as partial dryout strikes very early and the wall temperature is much higher for a fixed heat flux as microgap size increases above 500 μm. In addition, results show that pressure drop and pressure fluctuation decrease with the increases of gap size whereas wall temperature and wall temperature fluctuation increase with the increases of gap size. A strong dependence of heat transfer coefficient on microgap sizes is observed for microgap sizes 500 μm and below. However, the heat transfer coefficient is independent of microgap size for microgap sizes 700 μm and above.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.     

Numerical study of confined slot jet impinging on porous metallic foam heat sink

This study numerically investigates the impinging cooling of porous metallic foam heat sink. The analyzed parameters ranges comprise ε = 0.93/10 PPI Aluminum foam, L/W = 20, Pr = 0.7, H/W = 2–8, and Re = 100–40,000. The simulation results exhibit that when the Re is low (such as Re = 100), the Nu_max occurs at the stagnation point (i.e. X = 0). However, when the Reynolds number increases, the Nu_max would move downwards, i.e. the narrowest part between the recirculation zone and the heating surface. Besides, the extent to which the inlet thermal boundary condition influences the prediction accuracy of the Nusselt number increases with a decreasing H/W and forced convective effect. The application ranges of H/W and Re that the effect of the inlet thermal boundary condition can be neglected are proposed. Lastly, comparing our results with those in other studies reveals that the heat transfer performance of the Aluminum foam heat sink is 2–3 times as large as that without it. The thermal resistance is also 30% less than that of the plate fin heat sink for the same volumetric flow rate and the 5.3 mm jet nozzle width. Therefore, the porous Aluminum foam heat sink enhances the heat transfer performance of impinging cooling.     

Optimum thermal design of triangular,trapezoidal and rectangular grooved microchannel heat sinks

A three-dimensional numerical simulation is conducted to investigate the effect of geometrical parameters on laminar water flow and forced convection heat transfer characteristics in grooved microchannel heat sink (GMCHS). Four geometry variables which are; the depth, tip length, pitch and orientation of the cavities are taken into account in order to optimize the aluminum heat sink design. These geometric parameters could change the cavity shape from triangular to trapezoidal and then to rectangular shape. The governing and energy equations are solved using the finite volume method (FVM). The performance of GMCHS is evaluated in terms of Nusselt number ratio, thermal/hydraulic performance (JF) and isotherm and streamlines contours. The results showed that the trapezoidal groove with groove tip length ratio of δ = 0.5, groove depth ratio β = 0.4, groove pitch ratio of ψ = 3.334, grooves orientation ratio of ζ = 0.00 and Re = 100 is the optimum thermal design for GMCHS with Nusselt number enhancement of 51.59% and friction factor improvement of 2.35%.     

3-Dimensional numerical optimization of silicon-based high performance parallel microchannel heat sink with liquid flow

A full 3-dimensional (3D) conjugate heat transfer model has been developed to simulate the heat transfer performance of silicon-based, parallel microchannel heat sinks. A semi-normalized 3-dimensional heat transfer model has been developed, validated and used to optimize the geometric structure of these types of microheat sinks. Under a constant pumping power of 0.05 W for a water-cooled microheat sink, the optimized geometric parameters of the structure as determined by the model were a pitch of 100 μm, a channel width of 60 μm and a channel depth of about 700 μm. The thermal resistance of this optimized microheat sink was calculated for different pumping powers based on the full 3D conjugate heat transfer model and compared with the initial experimental results obtained by Tuckerman and Pease in 1981. This comparison indicated that for a given pumping poser, the overall cooling capacity could be enhanced by more than 20% using the optimized spacing and channel dimensions. The overall thermal resistance was 0.068 °C/W for a pumping power of 2 W.     

Effects of different pin-fin interruptions on performance of a nanofluid-cooled zigzag miniature heat sink (MHS)

In order to optimize the thermal-hydraulic performance of a nanofluid-cooled zigzag miniature heat sink (MHS), six different pin-fin interruptions are proposed in the present experimental study. The obtained results show that interrupting the pin-fins has meaningful influences on heat transfer and pressure drop characteristics of the zigzag MHS. For instance, replacing the interrupted MHSs with the basic configuration, i.e. integral MHS, varies the overall heat transfer coefficient in the range of − 19% to + 23%. At the studied ranges, the maximum value of overall heat transfer coefficient is found as 15,255 W/m² K at the mass flow rate of 0.02 kg/s corresponding to a specific interrupted MHS. It is detected that all the interrupted MHSs have lower values of the pressure drop in comparison with the integral MHS. It is also found that the cooling performance of the nanofluid-cooled MHSs is greater than the water-cooled MHSs. Finally, the obtained results in the current work may have considerable practical significance as it relates to the design of efficient MHSs.     

Numerical study of heat-transfer enhancement by punched winglet-type vortex generator arrays in fin-and-tube heat exchangers

The potential of punched winglet type vortex generator (VG) arrays used to enhance air-side heat-transfer performance of finned tube heat exchanger is numerically investigated. The arrays are composed of two delta-winglet pairs with two layout modes of continuous and discontinuous winglets. The heat transfer performance of two array arrangements are compared to a conventional large winglet configuration for the Reynolds number ranging from 600 to 2600 based on the tube collar diameter, with the corresponding frontal air velocity ranging from 0.54 to 2.3 m/s. The effects of different geometry parameters that include attack angle of delta winglets (β = 10 deg, β = 20 deg, β = 30 deg) and the layout locations are examined. The numerical results show that for the punched VG cases, the effectiveness of the main vortex to the heat transfer enhancement is not fully dominant while the “corner vortex” also shows significant effect on the heat transfer performance. Both heat transfer coefficient and pressure drop increase with the increase of attack angle β for the side arrangements; the arrays with discontinuous winglets show the best heat transfer enhancement, and a significant augmentation of up to 33.8–70.6% in heat transfer coefficient is achieved accompanied by a pressure drop penalty of 43.4–97.2% for the 30 deg case compared to the plain fin. For the front arrangements of VGs higher heat transfer enhancement and pressure drop penalty can be obtained compared to that of the side arrangement cases; the case with front continuous winglet arrays has the maximum value of j/f, a corresponding heat transfer improvement of 36.7–81.2% and a pressure drop penalty of 60.7–135.6%.