Effects of tapered-channel design on thermal performance of microchannel heat sink

A channel with a height- or width-tapered variation is designed to improve the thermal performance of a microchannel heat sink (MCHS). To this end, a three-dimensional MCHS model is constructed to analyze the effects of the height- and width-tapered ratios on the thermal performance of the MCHS. The thermal resistance and temperature distribution are taken as the thermal performance indicators. Numerical predictions show that the relationship between the thermal resistance and the width-tapered ratio is not monotonic at the fixed pumping power. The thermal resistance first decreases and then increases. A similar behavior is also exhibited by the height-tapered ratio. However, the height-tapered ratio effects can be negligible. It is also found that the width-tapered-channel design has a lower and a relatively uniform temperature distribution compared to parallel or height-tapered channel design. Moreover, the MCHS with width-tapered channel design showed a maximum enhancement in thermal performance of around 16.7% over that of the parallel-channel design when the pumping power is larger than 0.4 W.     

Cooling performance of a microchannel heat sink with nanofluids

In this paper, the cooling performance of a microchannel heat sink with nanoparticle–fluid suspensions (“nanofluids”) is numerically investigated. By using a theoretical model of thermal conductivity of nanofluids that accounts for the fundamental role of Brownian motion, we investigate the temperature contours and thermal resistance of a microchannel heat sink with nanofluids such as 6 nm copper-in-water and 2 nm diamond-in-water. The results show that the cooling performance of a microchannel heat sink with water-based nanofluids containing diamond (1 vol.%, 2 nm) at the fixed pumping power of 2.25 W is enhanced by about 10% compared with that of a microchannel heat sink with water. Nanofluids reduce both the thermal resistance and the temperature difference between the heated microchannel wall and the coolant. Finally, the potential of deploying a combined microchannel heat sink with nanofluids as the next generation cooling devices for removing ultra-high heat flux is shown.     

Analysis of microchannel heat sink performance using nanofluids

In this study, silicon microchannel heat sink performance using nanofluids as coolants was analyzed. The nanofluid was a mixture of pure water and nanoscale Cu particles with various volume fractions. The heat transfer and friction coefficients required in the analysis were based on theoretical models and experimental correlations. In the theoretical model, nanofluid was treated as a single-phase fluid. In the experimental correlation, thermal dispersion due to particle random motion was included. The microchannel heat sink performances for two specific geometries, one with W_ch = W_fin = 100 μm and L_ch = 300 μm, the other with W_ch = W_fin = 57 μm and L_ch = 365 μm, were examined. Because of the increased thermal conductivity and thermal dispersion effects, it was found that the performances were greatly improved for these two specific geometries when nanofluids were used as the coolants. In addition to heat transfer enhancement, the existence of nanoparticles in the fluid did not produce extra pressure drop because of small particle size and low particle volume fraction.     

Experimental investigation of thermoelectric power generation versus coolant pumping power in a microchannel heat sink

The coolant heat sinks in thermoelectric generators (TEG) play an important role in order to power generation in the energy systems. This paper explores the effective pumping power required for the TEGs cooling at five temperature difference of the hot and cold sides of the TEG. In addition, the temperature distribution and the pressure drop in sample microchannels are considered at four sample coolant flow rates. The heat sink contains twenty plate-fin microchannels with hydraulic diameter equal to 0.93 mm. The experimental results show that there is a unique flow rate that gives maximum net-power in the system at the each temperature difference.     

Optimum thermal performance of microchannel heat sink by adjusting channel width and height

A three-dimensional numerical model of the microchannel heat sink is presented to study the effects of heat transfer characteristics due to various channel heights and widths. Based on the theory of a fully developed flow, the pressure drop in the microchannel is derived under the requirement of the flow power for a single channel. The effects of two design variables representing the channel width and height on the thermal resistance are investigated. In addition, the constraint of the same flow cross section is carried out to find the optimum dimension. Finally, the minimum thermal resistance and optimal channel width with various flow powers and channel heights are obtained by using the simulated annealing method.     

Analysis and optimization of electrokinetic microchannel heat sink

A mixed (electroosmotic and pressure-driven) flow microchannel heat sink has been studied and optimized with the help of three-dimensional numerical analysis, surrogate methods, and the multi-objective evolutionary algorithm. Two design variables; the ratio of the microchannel width-to-depth and the ratio of fin width-to-depth of the microchannel are selected as the design variables while design points are selected through a four-level full factorial design. The single-objective optimization is performed taking overall thermal resistance as the objective function and Radial Basis Neural Network as the surrogate model while for multi-objective optimization pumping power is considered as the objective function along with the thermal resistance. It is observed that the optimum design shifted towards the lower values of the ratio of the channel width-to-depth and the higher values of the ratio of fin width-to-depth of channel with increase of the driving source. The trade-off between the two conflicting objectives has been found and discussed in detail in light of the distribution of Pareto-optimal solutions in the design space. The ratio of channel width-to-depth is found to be higher Pareto-sensitive (sensitivity along the Pareto-optimal front) than the ratio of fin width-to-depth of the channel.     

Influence of channel shape on the thermal and hydraulic performance of microchannel heat sink

Microchannel heat sinks (MCHS) can be made with channels of various shapes. Their size and shape may have remarkable influence on the thermal and hydrodynamic performance of MCHS. In this paper, numerical simulations are carried out to solve the three-dimensional steady and conjugate heat transfer governing equations using the Finite-Volume Method (FVM) of a water flow MCHS to evaluate the effect of shape of channels on the performance of MCHS with the same cross-section. The effect of shape of the channels on MCHS performance is studied for different channel shapes such as zigzag, curvy, and step microchannels, and it is compared with straight and wavy channels. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, friction factor, and wall shear stress. Results show that for the same cross-section of a MCHS, the temperature and the heat transfer coefficient of the zigzag MCHS is the least and greatest, respectively, among various channel shapes. The pressure drop penalty for all channel shapes is higher than the conventional straight MCHS. The zigzag MCHS has the highest value of pressure drop, friction factor, and wall shear stress followed by the curvy and step MCHS, respectively.     

Numerical simulation of stacked microchannel heat sink with mixing-enhanced passive structure

The paper is focused on the investigation of numerical simulation of stacked two-layer microchannel heat sink with enhanced mixing passive microstructure. In contrast to the smooth microchannel studies in the literature, the microchannel with embedded passive microstructure is chosen. The computational fluid dynamics (CFD) will be used to simulate the flow and heat transfer in a stacked two-layer microchannels with multiple MEMS easy-processing passive microstructures. To simulate the conjugated heat transfer among the heatsink and fluid, the three-dimensional conjugated model is used to solve this problem. The important parameters (e.g. the ratio of embedded structure height to microchannel height and fluid property) are investigated. The ratio of embedded structure height to microchannel height is changed from 0.13 to 0.26. The microchannel Reynolds number is fixed at 14.8. The stacked microchannel with passive structures has better performance than the smooth microchannels.     

Numerical simulation of heat transfer enhancement in wavy microchannel heat sink

In this paper, heat transfer and water flow characteristics in wavy microchannel heat sink (WMCHS) with rectangular cross-section with various wavy amplitudes ranged from 125 to 500 μm is numerically investigated. This investigation covers Reynolds number in the range of 100 to 1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite-volume method (FVM). The water flow field and heat transfer phenomena inside the heated wavy microchannels is simulated and the results are compared with the straight microchannels. The effect of using a wavy flow channel on the MCHS thermal performance, the pressure drop, the friction factor, and wall shear stress is reported in this article. It is found that the heat transfer performance of the wavy microchannels is much better than the straight microchannels with the same cross-section. The pressure drop penalty of the wavy microchannels is much smaller than the heat transfer enhancement achievement. Both friction factor and wall shear stress are increased proportionally as the amplitude of wavy microchannels increased.     

Numerical simulation of fluid flow and heat transfer in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall

The paper is focused on the investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall. In contrast to the new microchannel heat sink, the corresponding conventional rectangular microchannel heat sink is chosen. The computational fluid dynamics is used to simulate the flow and heat transfer in the heat sinks. The steady, laminar flow and heat transfer equations are solved in a finite-volume method. The SIMPLEX method is used for the computations. The effects of flow rate and heat flux on pressure drop and heat transfer are presented. The results indicate that the microchannel heat sink with offset fan-shaped reentrant cavities in sidewall improved heat transfer performance with an acceptable pressure drop. The fluid flow and heat transfer mechanism of the new microchannel heat sink can attribute to the interaction of the increased heat transfer surface area, the redeveloping of the hydraulic and thermal boundary layers, the jet and throttling effects and the slipping over the reentrant cavities. The increased heat transfer surface area and the periodic thermal developing flow are responsible for the significant heat transfer enhancement. The jet and throttling effects enhance heat transfer, simultaneously increasing pressure drop. The slipping over the reentrant cavities reduces pressure drop, but drastically decreases heat transfer.     

Optimum thermal design of microchannel heat sinks by the simulated annealing method

By adopting the simulated annealing method, a three-dimensional numerical simulation is executed to minimize the thermal resistance of the microchannel heat sink corresponding to the optimum specification under the fixed flow power. The depths of the microchannel heat sink in this study are fixed at either 1 cm or 2 cm. Based on the theory of the fully developed flow, the pressure drop between the inlet and exit in each single channel can be analytically derived if the flow power and the associated specification of the microchannel heat sink are fixed in advance. Then, this pressure drop will be used as the input condition to calculate the temperature distribution of the microchannel heat sink. For the first part of the optimum analysis, the fin width, and channel width are chosen as the design variables to find their optimum sizes. As to the second part of the present analysis, three design variables including channel height, fin width and channel width are individually prescribed as a suitable range to search for their optimum geometric configuration when the other specifications of the microchannel heat sink are fixed as 24 different cases.     

Analysis of flexible microchannel heat sink systems

In this work, single layered (SL) and double layered (DL) flexible microchannel heat sinks are analyzed. The deformation of the supporting seals is related to the average internal pressure by theory of elasticity. It is found that sufficient cooling can be achieved using SL flexible microchannel heat sinks at lower pressure drop values for softer seals. Double layered flexible microchannel heat sinks provide higher rate of cooling over SL flexible microchannel heat sinks at the lower range of pressure drops. Single layered flexible microchannel heat sinks are preferred for large pressure drop applications while DL flexible microchannel heat sinks are preferred for applications involving low pressure drops.     

复合热沉环路热管的热点散热性能模拟研究

针对芯片功耗与集成度提高而导致的局部热点问题,设计了一种用于芯片散热的复合热沉环路热管系统。建立了环路热管蒸发段模型,通过数值模拟的方法,证明了复合热沉环路热管系统能够降低热点温度,提高散热表面的温度均匀程度,且散热效果与热点的分布位置有关。当热点的热流密度为160W/cm2,热沉横向、纵向导热率分别为1500W/(m?K)、24W/(m?K)时,热点温度为88.88°C,相比于无热沉时降低了5.96°C。研究了不同热沉导热率下的热沉厚度对热点温度的影响,结果表明：若导热率各项同性,热点温度随热沉厚度的增加而降低,之后趋向不变;若为各项异性,存在最优的热沉厚度,使热点温度最低。     

Thermal performance of optimized interrupted microchannel heat sink (IMCHS) using nanofluids

An interrupted microchannel heat sink (IMCHS) using nanofluids as working fluids is analyzed numerically to increase the heat transfer rate. The rectangular IMCHS is designed with length and width of 10 mm and 0.057 mm respectively while optimum cut section number, n_c = 3. The three dimensional governing equations (continuity, momentum and energy) were solved using finite volume method (FVM). Parametric study of thermal performance between pure water-cooled and nanofluid-cooled IMCHS are evaluated for particle diameter in the range of, 30 nm to 60 nm, volume fraction in the range of, 1% to 4%,nanofluid type of Al₂O₃, CuO, and SiO₂ at Reynolds number range of 140 to 1034 are examined. The effects of the transport properties, nanofluid type, nanoparticle volume fraction and particle diameter are investigated on the IMCHS performance. It is inferred that the Nu number for IMCHS is higher than the conventional MCHS with a slight increase of the pressure drop. It is found that highest thermal augmentation is predicted for Al₂O₃, followed by CuO, and finally for SiO₂ in terms of Nu_nf/Nu_pw in the IMCHS. The Nu number increased with the increase of nanoparticle volume fraction and with the decrease of nanoparticle diameter.     

Modulating fluid rheology and confinement toward augmenting the performance of a double layered microchannel heat sink

The improvement of the cooling performance of liquid-cooled microchannel heat sinks used for densely packed electronic circuits is sorted via passive techniques like tuning substrate or coolant properties. We propose a design for enhancing heat sink performance by simulataneously modifying the channel geometry and tuning the fluid rheology. By modeling the coolant as a power law fluid, its rheological behavior is varied ranging from shear-thinning to shear-thickening, alongside Newtonian fluid. We introduced tapering to the middle wall that separates the bottom and top channels of a double layered microchannel heat sink (DL-MCHS), causing both channels to converge. This convergence not only increases the flow velocity within the downstream microchannel but also reduces the apparent viscosity of the shear-thinning fluid being subjected to shear, resulting in enhanced thermal and hydraulic performance. We analyze the results from both the first and the second law of thermodynamics context, demonstrating that a tapered DL-MCHS with shear-thinning fluid outperforms a straight partition wall DL-MCHS with Newtonian coolant. However, we also discovered that extreme tapering compromises thermodynamic viability, but by fine-tuning the extent of tapering, we inferred that a DL-MCHS with shear-thinning fluid can become viable with little compromise in the thermal performance.     

The impact of various nanofluid types on triangular microchannels heat sink cooling performance

This paper discusses the impact of using various types of nanofluids on heat transfer and fluid flow characteristics in triangular shaped microchannel heat sink (MCHS). In this study, an aluminum MCHS performance is examined using water as a base fluid with different types of nanofluids such as Al₂O₃, Ag, CuO, diamond, SiO₂, and TiO₂ as the coolants with nanoparticle volume fraction of 2%. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is inferred that diamond-H₂O nanofluid has the lowest temperature and the highest heat transfer coefficient, while Al₂O₃-H₂O nanofluid has the highest temperature and the lowest heat transfer coefficient. SiO₂-H₂O nanofluid has the highest pressure drop and wall shear stress while Ag-H₂O nanofluid has the lowest pressure drop and wall shear stress among other nanofluid types. Based on the presented results, diamond-H₂O and Ag-H₂O nanofluids are recommended to achieve overall heat transfer enhancement and low pressure drop, respectively, compared with pure water.     

Constructal design and thermal analysis of microchannel heat sinks with multistage bifurcations in single-phase liquid flow

Based on constructal theory, five different cases with multistage bifurcations are designed as well as one case without bifurcations, and the corresponding laminar fluid flow and thermal performance have been investigated numerically. All laminar fluid flow and heat transfer results are obtained using computation fluid dynamics, and a uniform wall heat flux thermal boundary condition is applied all heated surfaces. The inlet velocity ranges from 0.66 m/s to 1.6 m/s with the corresponding Reynolds number ranging from 230 to 560. The pressure, velocity, temperature distributions and averaged Nusselt number are presented. The overall thermal resistances versus inlet Reynolds number or pumping power are evaluated and compared for the six microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with multistage bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sink with a long bifurcation length in the first stage (Case 1A) is superior. The usage of multistage bifurcated plates in microchannel heat sink can reduce the overall thermal resistance and make the temperature of the heated surface more uniform (Case 3). It is suggested that proper design of the multistage bifurcations could be employed to improve the overall thermal performance of microchannel heat sinks and the maximum number of stages of bifurcations is recommended to be two. The study complements and extends previous works.     

The tangential micro-heat sink with multiple fluid inlets

The paper presents the thermal and hydrodynamic analysis of the micro-heat sink with straight microtubes and multiple inlet jets. The inlet cross-sections have the rectangular shape and positioned tangentially to the tube axis. Four different configurations are considered: three of them with three inlet jets and one with five inlet jets. 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 surface of the heat sink is q = 100 W/cm². Thermal and hydrodynamic results of the micro-heat sink with multiple inlets are compared with results obtained for single tangential inlet configuration. For all cases, the analysis is made on a fixed pumping power basis. The strong dependence of the position and number of the inlet jets on thermal and hydrodynamic characteristics of the micro-heat sink is observed.     

The effect of geometrical parameters on heat transfer characteristics of microchannels heat sink with different shapes

The effect of geometrical parameters on water flow and heat transfer characteristics in microchannels is numerically investigated for Reynolds number range of 100–1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using finite volume method. The computational domain is taken as the entire heat sink including the inlet/outlet ports, wall plenums, and microchannels. Three different shapes of microchannel heat sinks are investigated in this study which are rectangular, trapezoidal, and triangular. The water flow field and heat transfer phenomena inside each shape of heated microchannels are examined with three different geometrical dimensions. Using the averaged fluid temperature and heat transfer coefficient in each shape of the heat sink to quantify the fluid flow and temperature distributions, it is found that better uniformities in heat transfer coefficient and temperature can be obtained in heat sinks having the smallest hydraulic diameter. It is also inferred that the heat sink having the smallest hydraulic diameter has better performance in terms of pressure drop and friction factor among other heat sinks studied.     

Superposition method to investigate the thermal performance of heat sink with embedded heat pipes

This article utilizes the thermal performance experiment with superposition method to investigate the thermal performance of heat sinks with one and two pairs of embedded heat pipes. A heat sink with embedded heat pipes transfers the total heat capacity from the heat source to both the base plate and heat pipes, and then disperses heat into the surrounding air via the forced convection. The heat capacity carried by embedded heat pipes can be found using the thermal resistance analytical approach stated in this article. The results show that two and four heat pipes embedded in the base plate carry 36% and 48% of the total dissipated heat respectively; in addition, when the total heating power of the heat sink with two embedded heat pipes is 140 W, the total thermal resistance reaches its minimum value of 0.27 °C/W, while for the heat sink with four embedded heat pipes, when the total heating power is between 40 W and 240 W, the total thermal resistance is 0.24 °C/W, meaning that the thermal performance is better than that of heat sink with two embedded heat pipes.