Convective heat transfer and pressure losses across novel heat sinks fabricated by Selective Laser Melting

This study presents the thermal and fluid flow characteristics of five heat sinks that have been fabricated by a rapid manufacturing technique known as Selective Laser Melting. The five heat sinks consist of two conventional designs, the cylindrical pin and rectangular fin array, for comparison purposes, and three novel heat sinks: a staggered elliptical array; a lattice; and a rectangular fin array with rounded corners. The experimental results for the rectangular fin were compared with data from the literature and were found to be consistent. The rectangular fin with rounded corners proved able to transfer the largest amount of heat whilst improving upon the pressure drop performance of the standard rectangular fin array. Although the lattice arrangement made use of the fabrication process’ ability to manufacture heat sinks with high surface area to volume ratios, its performance was limited by the lack of interaction between the cooling air and structure. In terms of both heat transfer performance and pressure drop, the staggered elliptical array, which cannot be manufactured by conventional techniques, outperformed the other heat sinks.     

Single-Phase Cryogenic Flow and Heat Transfer Through Microscale Pin Fin Heat Sinks

Single-phase heat transfer and pressure drop of liquid nitrogen in microscale heat sinks are studied experimentally in this paper. Effects of geometrical variations are characterized on the thermofluidic performance of staggered microscale pin fin heat sinks. Pitch-to-diameter ratio and aspect ratio of the micro pin fins are varied. The pin fins have square shape with 200 or 400 μm width and are oriented at 45 degrees to the flow direction. Thermal performance of the heat sinks is evaluated for Reynolds numbers (based on pin fin hydraulic diameter) from 108 to 570. Results are presented in a nondimensional form in terms of friction factor, Nusselt number, and Reynolds number and are compared with the predictions of existing correlations in the literature for micro pin fin heat sinks. Comparison of flow and heat transfer performance of the micro pin fin heat sinks reveals that at a particular critical Reynolds number of ～250, pin fin heat sinks with the same aspect ratio but larger pitch ratio show a transition in both friction factor and Nusselt number. In order to better characterize this transition, visualization experiments were performed with the Fluorinert PF5060 using an infrared camera. At the critical Reynolds number, for the larger pitch ratio pin fin heat sink, surface thermal intensity profiles suggest periodic flapping of the flow behind the pin fins at a Strouhal number of 0.227.     

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.     

Thermal Analysis for Heat Transfer Enhancement in Perforated Pin Fins of Various Shapes with Staggered Arrays

The present work investigates the enhancement of heat transfer rate through staggered pin fins of different shapes with different perforation geometries, namely circular, diamond shaped and elliptical type. Three dimensional computational fluid dynamics simulation has been carried out to analyze the effects of fin geometry and dimension of perforation as well as the shape of fin to enhance heat transfer rate against pressure loss. Results show that the heat transfer rates of perforated fins up to certain perforation number and size are always greater than the solid ones and with the change of fin shape and perforation geometry heat transfer rate also improves significantly. On the other hand pressure drop through heat sink decreases not only with increasing perforation number but also with the size of perforation. Moreover, variation of pressure drop of perforated fins is influenced with fin geometry.     

Heat transfer from multiple row arrays of low aspect ratio pin fins

Pin fin arrays are used in many applications to enhance heat transfer. In modern gas turbines, for example, airfoils are designed with sophisticated internal and external cooling techniques. One method for cooling is routing air from the compressor through intricate cooling channels embedded in turbine airfoils. Heat transfer from the blade to the coolant air can be increased by installing arrays of cylindrical pedestals often referred to as pin fins. Pin fin arrays increase heat transfer by increasing the flow turbulence and surface area of the airfoil exposed to the coolant.For the current study, experiments were conducted to determine the effects of pin spacing on heat transfer and pressure loss through pin fin arrays for a range of Reynolds numbers between 5000 and 30,000. Results showed that spanwise pin spacing had a larger effect than streamwise spacing on array pressure loss while streamwise spacing had a larger effect than spanwise spacing on array heat transfer.     

Natural convection heat transfer from a heat sink with hollow/perforated circular pin fins

Experiments were performed on natural convection heat transfer from circular pin fin heat sinks subject to the influence of its geometry, heat flux and orientation. The geometric dependence of heat dissipation from heat sinks of widely spaced solid and hollow/perforated circular pin fins with staggered combination, fitted into a heated base of fixed area is discussed. Over the tested range of Rayleigh number, 3.8 × 10⁶ ≤ Ra ≤ 1.65 × 10⁷, it was found that the solid pin fin heat sink performance for upward and sideward orientations shows a competitive nature, depending on Rayleigh number and generally shows higher heat transfer coefficients than those of the perforated/hollow pin fin ones in both arrangement. For all tested hollow/perforated pin fin heat sinks, however, the performance for sideward facing orientation was better than that for upward facing orientation. This argument is supported by observing that the augmentation factor was around 1.05–1.11, depending on the hollow pin diameter ratio, D_i/D_o. Meanwhile, the heat sink of larger hollow pin diameter ratio, D_i/D_o offered higher heat transfer coefficient than that of smaller D_i/D_o for upward orientation, and the situation was reversed for sideward orientation. The heat transfer performance for heat sinks with hollow/perforated pin fins was better than that of solid pins. The temperature difference between the base plate and surrounding air of these heat sinks was less than that of solid pin one and improved with increasing D_i/D_o.     

Numerical Study of Thermal and Hydraulic Performance of Compound Heat Sink

The present study demonstrates the numerical simulation of the compound heat sink and provides physical insight into the flow and heat transfer characteristics. The governing equations are discretized by using a control-volume-based finite-difference method with a power-law scheme on an orthogonal nonuniform staggered grid. The coupling of the velocity and the pressure terms of momentum equations are solved by the SIMPLEC algorithm. The well-known RNG k ? ε two-equations turbulence model is employed to describe the turbulent structure and behavior. The compound heat sink is composed of a plate fin heat sink and some pins between plate fins. The objective of this investigation is to examine the effects of the types and the arrangements of the pins. It is found that the compound heat sink has better synthetical performance than the plate fin heat sink. Moreover, the compound heat sink which is composed of a plate fin heat sink and circular pins performs better than the square ones.     

The hydraulic‐thermal performance of miniature compact heat sinks using SiO2‐water nanofluids

Ever since the rapid increase in both the demand for the miniature electronic devices and their applications, heat dissipation in the electronic components has been a serious issue. A miniature plate‐pin heat sink model with square, circular, and elliptic pins is considered to enhance the hydrothermal performance of this kind of compact heat sink (CHS). Water and 3% of SiO₂‐water nanofluids of volume fraction were used with different Reynolds number ranges (100‐1000). The findings show that the base temperature of heat sink reduces while the Nusselt number enhances by using nanofluids and increasing Reynolds number. The lowest value of the base temperature is nearly 25% for the square pins and circular pins CHSs compared with a plate–fin heat sink at 3% of nanofluids. Furthermore, the highest value of the Nusselt number is about 98% at 3% SiO₂ for circular pin CHSs compared with the plate–fin heat sink. However, the pressure drop of CHSs is higher than that of plate–fin heat sink. Moreover, the most significant hydrothermal performance value is about 1.44 for water and around 1.51 for SiO₂ as using the CHS with circular and elliptic pins depends on the Reynolds number. Thus, applying CHSs with nanofluids instead of the traditional heat sinks might produce a substantial enhancement in the hydrothermal performance of heat sinks.     

Exergo‐economic analysis of micro pin fin heat sinks

A parametric study of thermoeconomic performance over four micro pin fin heat sinks of different spacing and shapes was conducted. Unit cost per product exergy, relative cost difference, and exergo‐economic factor were utilized to evaluate the thermoeconomic performance. The effect of working fluid on the thermoeconomic performance was also investigated using R‐123 and water as working fluids. Unit costs per product exergy were obtained to evaluate the product costs (total exergy change between exit and inlet streams) in micro pin fin heat sinks at fixed mass flow rate and fixed pressure drop. The results of the thermoeconomic analysis were compared with the results of a past exergy performance study by the author. In the light of raw experimental data acquired from the past studies of the author, important differences between the results of exergy and exergo‐economic performances were observed. It was found that the unit cost of exergy change decreased as electrical power increased and the relative cost difference approached to unity at high electrical powers (greater than 20 W). Moreover, high exergo‐economic factor values (more than 0.5) were obtained at low electrical powers while exergo‐economic factors had a small value at high electrical powers. When looking at the effect of the working fluid, higher cost per Watts of the products (up to the double of R‐123) was obtained with water compared with R‐123 at both fixed mass flow rate and pressure drop. No significant effect of pin fin spacing on the unit cost of exergy change was observed at fixed mass flow rate, while higher unit costs (up to 102%) were recorded at fixed pressure drop for scarcely packed pin fin heat sinks. Finally, the unit cost of exergy change was found to be independent of pin fin shape at fixed mass flow rate, whereas at fixed pressure drop, the hydrofoil‐based pin fin heat sink had higher unit costs (up to 1.8 times as much) when compared with the unit costs of pin fin heat sinks having flow separation promoting pin fins. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimization of heat sink geometries for impingement air-cooling of LSI packages

This paper describes the use of our previous study's prediction procedures for calculating thermal resistance and pressure drop. The procedures are used in the optimization of heat sink geometries for impingement air-cooling of LSI packages. Two types of heat sinks are considered: ones with longitudinal fins and ones with pin fins. We optimized the heat sink geometries by evaluating 16 parameters simultaneously. The parameters included fin thickness, spacing, and height. For the longitudinal fins, the optimal fin thicknesses were found to be between 0.12 and 0.15 mm, depending on which of the four types of fans were used. For pin fins, the optimal pin diameters were between 0.39 and 0.40 mm. Under constant pumping power, the optimal thermal resistance of the longitudinal fins was about 60% that of the pin fins. For both types of heat sinks, the optimal thermal resistance for four off-the-shelf fans was only slightly (maximum about 1%) higher than the theoretical optimum for the same pumping power. When manufacturing cost performance is considered, the most economical fin thickness and diameter are about 5 to 10 times higher than the optimal values calculated without respect for manufacturing costs. These values almost correspond to the actual limits of extrusion and press heat-sink manufacturing processes. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 138–151, 1999     

Cooling of Laser Slab by Forced Convection through a Heat Sink

The present study addresses a novel cooling scheme for the high-power solid-state laser slab. The scheme cools the laser slab by forced convection in a narrow channel through a heat sink. Numerical simulations were conducted to investigate the thermal effects of a Nd:YAG laser slab for heat sinks of different materials, including the undoped YAG, sapphire, and diamond. The results show that the convective heat transfer coefficient is non-uniform along the fluid flow direction due to the thermal entrance effect, causing a non-uniform temperature distribution in the slab. The heat sink lying between the coolant fluid and the pumped surface of the slab works to alleviate this non-uniformity and consequently improve the thermal stress distribution and reduce the maximum thermal stress of the slab. The diamond heat sink was found to be effective in reducing both the highest temperature and the maximum thermal stress; the sapphire heat sink was able to reduce the maximum thermal stress but not as effective in reducing the highest temperature; and the undoped YAG heat sink reduced the maximum thermal stress but tended to increase the highest temperature. Therefore, cooling with the diamond heat sink is most effective, and that with the sapphire heat sink follows; cooling with the undoped YAG heat sink may not apply if the highest temperature is a concern.     

Numerical investigation of hydrothermal performance over perforated conical pin heat sinks

Over the past few decades, researchers have shown significant interest in enhancing the thermal efficiency of heat sinks while simultaneously increasing the power generation capacity of electronic devices and reducing their size. In this study, the focus lies on the originality of employing conical perforated pin heat sinks with multiple perforations (N = 0, 1, 2, and 3) and various conical pins inclination angles (Φ = 0°, 1°, 2°, 3°, and 4°). The study aimed to numerically investigate the effects of a perforated conical pin and cone inclination angle on heat transfer, pressure drop, CPU temperature, and hydrothermal performance (HTP) across the heat sinks using a three-dimensional, turbulent flow as k–ω SST model combined with the thermal conjugate model. A validated CFD model is employed to conduct a parametric analysis of the effects of the quantity and placement of circular holes. A summary of the results reveals that Model B3 exhibited the highest HTP value, reaching approximately 1.15 at U = 10 m/s, with a commendable reduction in heat sink mass of over 18%. Ultimately, the perforated conical pin heat sink demonstrates the potential to fulfill the primary objective of this investigation, which is achieving an overall improvement in Nusselt number, CPU temperature, pressure drop, and reduced heat sink mass.     

Scroll heat sink: A novel heat sink with the moving fins inserted between the cooling fins

In this paper, a new type of a fan-integrated heat sink named a scroll heat sink is proposed and demonstrated. The most striking feature of the scroll heat sink is that heat dissipation and fluid pumping occurs simultaneously in the whole cooling space without requiring any additional space for a fan module. In the scroll heat sink, the moving fins, which rotate with two eccentric shafts, are inserted between the fixed (cooling) fins. By a relative motion between the moving fins and the cooling fins, a coolant is drawn into the space between them, takes heat away from the cooling fins, and the heated coolant is discharged out of the heat sink. In the present study, an experimental investigation is performed in order to demonstrate the concept of the scroll heat sink. Average coolant velocities and thermal resistances of the scroll heat sink are measured for various rotating speeds of the moving fins from 200 rpm to 500 rpm. Experimental results show that measured flow rates of the coolant are almost linearly proportional to the rotating speed of the moving fins. A theoretical model is also developed to estimate the required pumping power and the thermal resistance, and validated using experimental results. The theoretical model shows that optimized scroll heat sinks have lower thermal resistances than optimized plate-fin heat sinks under the fixed pumping power condition.     

Numerical Evaluation of Flow and Heat Transfer in Plate-Pin Fin Heat Sinks with Various Pin Cross-Sections

A numerical investigation of the thermal and hydraulic performance of 20 different plate-pin fin heat sinks with various shapes of pin cross-sections (square, circular, elliptic, NACA profile, and dropform) and different ratios of pin widths to plate fin spacing (0.3, 0.4, 0.5, and 0.6) was performed. Finite volume method-based CFD software, Ansys CFX, was used as the 3-D Reynolds-averaged Navier-Stokes Solver. A k-ω based shear-stress-transport model was used to predict the turbulent flow and heat transfer through the heat sink channels. The present study provides original information about the performance of this new type of compound heat sink.     

Prediction algorithm of thermal resistance for impingement cooling of heat sinks for LSI packages with pin-fin arrays

This paper is a semi-empirical report on an algorithm for the prediction of thermal resistance for impingement cooling of pin-fin heat sinks for LSI packages when the inlet orifice is relatively large and is located over the center of the sink. We present a physical model suitable for these types of heat sinks, based on flow visualization results. The model divides the flow region into five parts: I) the top surfaces of the fins where they are directly under the inlet orifice, II) the portions of the vertical surfaces of the pin-fin cylinders, where those surfaces are directly below the inlet port, III) the surface of the base to which the fins are attached, excluding the areas occupied by the feet of the fins themselves, IV) the portions of the vertical surfaces of the fin-cylinders excluding those portions of the surfaces that are directly below the inlet port (complementary to region II), V) the portions of the top surfaces of the pins, excluding those portions directly below the inlet port (complementary to region I). We predicted thermal resistance values for heat sinks with pin-fin arrays, for a variety of orifice diameters, gaps, pin-fin diameters, and heights, and number of fins. These values agreed with experimental data within ±30%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(7): 434–448, 1996     

Computational Conjugate Heat Transfer Analysis of a Hybrid Electric Vehicle Inverter

ABSTRACT

A physics-based computational simulation of the heat transfer characteristics of an insulated gate bipolar transistor (IGBT) developmental inverter is reported. The simulation considers the fluid/thermal multiphysics interactions via a conjugate heat transfer analysis. The fluid phase includes air and liquid coolant; the solid phase, where the heat is conducted, includes various solid materials. Numerical solutions of the heat conduction and convection phenomena in and around the IGBT modules and the inverter, built as a three-dimensional computational model, are sought for by using parallel computing. Comparisons with the available experimental data show a satisfactory agreement of the inverter temperature at three power levels under two different coolant flow rates. Detailed examination of the flow field reveals that the design features of the rectangular coolant flow chamber in the heat sink and the small clearance between the tips of the pin fin and the walls lead to an evenly distributed coolant flow around most of the pin fins. The temperature distributions of the pin fins depend highly on their locations relative to the IGBT modules. The findings from the current study can be useful in future efforts to optimize the thermal performance of IGBT inverters.     

Review on Heat and Fluid Flow in Micro Pin Fin Heat Sinks under Single-phase and Two-phase Flow Conditions

This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.     

Measurement of performance of plate-fin heat sinks with cross flow cooling

This work assesses the performance of plate-fin heat sinks in a cross flow. The effects of the Reynolds number of the cooling air, the fin height and the fin width on the thermal resistance and the pressure drop of heat sinks are considered. Experimental results indicate that increasing the Reynolds number can reduce the thermal resistance of the heat sink. However, the reduction of the thermal resistance tends to become smaller as the Reynolds number increases. Additionally, enhancement of heat transfer by the heat sink is limited when the Reynolds number reaches a particular value. Therefore, a preferred Reynolds number can be chosen to reduce the pumping power. For a given fin width, the thermal performance of the heat sink with the highest fins exceeds that of the others, because the former has the largest heat transfer area. For a given fin height, the optimal fin width in terms of thermal performance increases with Reynolds number. As the fins become wider, the flow passages in the heat sink become constricted. As the fins become narrower, the heat transfer area of the heat sink declines. Both conditions reduce the heat transfer of the heat sink. Furthermore, different fin widths are required at different Reynolds numbers to minimize the thermal resistance.     

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.     

Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.