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.     

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.     

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.     

Three-dimensional numerical simulation of heat and fluid flow in noncircular microchannel heat sinks

A three-dimensional model of heat transfer and fluid flow in noncircular microchannel heat sinks is developed and analyzed numerically. It is found that Nusselt number has a much higher value at the inlet region, but quickly approaches the constant fully developed value. The temperature in both solid and fluid increases along the flow direction. In addition, the comparison of thermal efficiencies is conducted among triangular, rectangular and trapezoidal microchannels. The result indicates that the triangular microchannel has the highest thermal efficiency.     

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.     

Influence of various base nanofluids and substrate materials on heat transfer in trapezoidal microchannel heat sinks

Numerical investigations are performed to investigate the laminar flow and heat transfer characteristics of trapezoidal MCHS using various types of base nanofluids and various MCHS substrate materials on MCHS performance. This study considered four types of base fluids including water, ethylene glycol (EG), oil, and glycerin with 2% volume fraction of diamond nanoparticle, and four types of MCHS substrate materials including copper, aluminium, steel, and titanium. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is found that the best uniformities in heat transfer coefficient and temperature among the four mixture flows can be obtained using glycerin-base nanofluid followed by oil-base nanofluid, EG-base nanofluid, and water-base nanofluid heat sinks. However, the heat transfer performance of water-base nanofluid can be greatly enhanced in steel made substrate heat sink.     

Optimal thermal design of microchannel heat sinks with different geometric configurations

In this study, three-dimensional models of microchannel heat sinks (MCHSs) with different geometric configurations (such as single-layered- (SL), double-layered- (DL) or tapered-(T)-channels) are constructed by an optimization procedure. This procedure integrates a direct problem solver with a simplified conjugate-gradient method as the optimizer. The overall thermal resistance of an MCHS is the objective function to be minimized with respect to geometric parameters, such as the number of channels, channel width ratio, channel aspect ratio and tapered ratios, as the search variables. The optimal thermal resistance is found to decrease in the following order: the initial guess parallel channel (IGP channel), SL-, DL- and T-channel designs. In addition, the T-channel design has the minimum temperature difference and the most uniform temperature distribution, followed by the DL-, SL- and IGP-channel designs. Moreover, the optimal thermal resistance reduces with the pumping power for the various channel configuration designs, and the lowest thermal resistance corresponds to the T-channel design. The larger the pumping power, the larger the decrement in thermal resistance. Therefore, the optimal T-channel is the best MCHS design when considering thermal resistance and temperature distribution uniformity.     

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.     

Effects of fin shapes on heat transfer in microchannel heat sinks

In the present study, compact water cooling of high‐density, high‐speed, very‐large‐scale integrated (VLSI) circuits with the help of microchannel heat exchangers were investigated analytically. This study also presents the result of mathematical analysis based on the modified Bessel function of laminar fluid flow and heat transfer through combined conduction and convection in a microchannel heat sink with triangular extensions. The main purpose of this paper is to find the dimensions of a heat sink that give the least thermal resistance between the fluid and the heat sink, and the results are compared with that of rectangular fins. It is seen that the triangular heat sink requires less substrate material as compared to rectangular fins, and the heat transfer rate per unit volume has been almost doubled by using triangular heat sinks. It is also found that the effectiveness of the triangular fin is higher than that of the rectangular fin. Therefore, the triangular heat sink has the ability to dissipate large amounts of heat with relatively less temperature rise for the same fin volume. Alternatively, triangular heat sinks may thus be more cost effective to use for cooling ultra‐high speed VLSI circuits than rectangular heat sinks.     

A review of heat and fluid flow characteristics in microchannel heat sinks

Heat transfer and flow characteristic in microchannel heat sinks (MCHS) are extensively studied in the literature due to high heat transfer rate capability by increased heat transfer surface area relative to the macroscale heat sinks. However, heat transfer and fluid flow characteristics in MCHS differ from conventional ones because of the scaling effects. This review summarizes the studies that are mainly based on heat transfer and fluid flow characteristic in MCHS. There is no consistency among the published results; however, everyone agrees on that there is no new physical phenomenon in microscale that does not exist at macroscale. Only difference between them is that the effect of some physical phenomena such as viscous dissipation, axial heat conduction, entrance effect, rarefaction, and so forth, is negligibly small at macroscale, whereas it is not at microscale. The effect of these physical phenomena on the heat transfer and flow characteristics becomes significant with respect to specified conditions such as Reynolds number, Peclet number, hydraulic diameter, and heat transfer boundary conditions. Here, the literature was reviewed to document when these physical phenomena become significant and insignificant.     

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.     

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.     

3D numerical optimization of a heat sink base for electronics cooling

In this paper, the possible optimal thickness of a heat sink base has been explored numerically with different convective heat transfer boundary conditions in a dimensionless three dimensional heat transfer model. From the numerical results, relations among different heat transfer mechanisms (natural or forced, air or liquid), different area ratios of a heat sink to a heating source, and the lowest thermal resistance have been obtained and discussed. Also a simple correlation for these three parameters from data fitting is given for guiding a heat sink design.     

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.     

Partially heated heat sinks in a zero-bypass

The need to improve the thermal performance of heat sinks remains a design priority for thermal engineers. Most of the considerations so far have involved various shapes and sizes of the fin designs. This numerical study evaluates the influence of the heating position on the thermal performance of a plate-fin heat sink in a zero-bypass characterized by the thermal resistance data obtained from experiments. For the heating positions evaluated, the central heating position offers the least thermal resistance. In practice, many cooling systems, like in electronic components, involve heat sinks with partially heated sections; therefore, evaluating the best position to place the heat sinks could be a fairly cheap way of enhancing the thermal performance.     

Effect of tip clearance on the cooling performance of a microchannel heat sink

In this paper, the effect of tip clearance on the cooling performance of the microchannel heat sink is presented under the fixed pumping power condition. The thermal resistance of a microchannel heat sink is defined for evaluating its cooling performance. The effect of tip clearance is numerically investigated by increasing tip clearance from zero under the fixed pumping power condition. From the numerical results, the optimized tip clearance is determined, for which the thermal resistance has a minimum value. Finally, we show that the presence of tip clearance can improve the cooling performance of a microchannel heat sink when tip clearance is smaller than a channel width.     

Investigation of heat transfer in rectangular microchannels

An experimental investigation was conducted to explore the validity of classical correlations based on conventional-sized channels for predicting the thermal behavior in single-phase flow through rectangular microchannels. The microchannels considered ranged in width from 194 μm to 534 μm, with the channel depth being nominally five times the width in each case. Each test piece was made of copper and contained ten microchannels in parallel. The experiments were conducted with deionized water, with the Reynolds number ranging from approximately 300 to 3500. Numerical predictions obtained based on a classical, continuum approach were found to be in good agreement with the experimental data (showing an average deviation of 5%), suggesting that a conventional analysis approach can be employed in predicting heat transfer behavior in microchannels of the dimensions considered in this study. However, the entrance and boundary conditions imposed in the experiment need to be carefully matched in the predictive approaches.     

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.     

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.     

Influence of nanofluids on parallel flow square microchannel heat exchanger performance

The effects of using various types of nanofluids and Reynolds numbers on heat transfer and fluid flow characteristics in a square shaped microchannel heat exchanger (MCHE) is numerically investigated in this study. The performance of an aluminum MCHE with four different types of nanofluids (aluminum oxide (Al₂O₃), silicon dioxide (SiO₂), silver (Ag), and titanium dioxide (TiO₂)), with three different nanoparticle volume fractions of 2%, 5% and 10% using water as base fluid is comprehensively analyzed. The three-dimensional steady, laminar developing flow and conjugate heat transfer governing equations of a balanced MCHE are solved using the finite volume method. The MCHE performance is evaluated in terms of temperature profile, heat transfer rate, heat transfer coefficient, pressure drop, wall shear stress pumping power, effectiveness, and overall performance index. The results reveal that nanofluids can enhance the thermal properties and performance of the heat exchanger while having a slight increase in pressure drop. It was also found that increasing the Reynolds number causes the pumping power to increase and the effectiveness to decrease.