The Effect of Micro Pin-Fin Shape on Thermal and Hydraulic Performance of Micro Pin-Fin Heat Sinks

Single micro pin-fin configurations having the same chord thickness/diameter but different shapes are numerically modeled to assess their heat transfer and hydraulic performances for Reynolds number values changing between 20 and 120. The configurations are three-dimensionally modeled based, and their heat transfer performances are evaluated using commercially available software COMSOL Multiphysics 3.5a. Navier–Stokes equations and continuity and energy equations are solved under steady-state conditions for single-phase water flows. To increase the computational efficiency, half of the domain consisting of a micro pin-fin located inside a microchannel is modeled using a symmetry plane. To validate the model, experimental data available in the literature are compared to simulation results obtained from the model of the same geometrical configuration as the experimental one. Accordingly, the numerical and experimental results show good agreement. Furthermore, performance evaluation study is performed using three-dimensional (3D) numerical models in the light of flow morphologies around micro pin-fins of various shapes. According to the results obtained from this study, the rectangular-shaped micro pin-fin configuration has the highest Nusselt number and friction factor over the whole Reynolds number range. However, the cone-shaped micro pin-fin configuration has the best thermal performance index, indicating that it could be more preferable to use micro pin-fins of unconventional shapes in micro pin-fin heat sinks.     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

Experimental Investigation on Fluid Flow Maldistribution in Plate-Fin Heat Exchangers

The plate-fin heat exchanger is normally designed with the assumption that the fluid is uniformly divided among all the parallel passages. In practice, however, the design of the exchanger, the heat transfer process, the operation of the external system, etc., may create high flow maldistribution. The performance deterioration of plate-fin heat exchangers due to flow maldistribution may be serious. In this review, the flow distribution performance in a plate-fin heat exchanger has been experimentally studied and the distribution performance of different distributors' inlet angles has been measured. The combined effects of the inlet angle and mass flow rate on flow maldistribution have been studied. The study is useful in the optimum design of plate-fin heat exchangers.     

Heat Transfer and Fluid Flow Analysis for Plate-Fin and Oval Tube Heat Exchangers With Vortex Generators

This article investigates the effectiveness of embedded vortex generators in enhancing the heat transfer performance of a plate-fin heat exchanger with a four-row staggered oval tube bundle. Two different types of vortex generator are considered, namely annular and inclined block. Numerical simulations are performed to analyze the effects of the three-dimensional turbulence induced by the vortex generators on the heat transfer and fluid flow characteristics of the heat exchanger. The results indicate that compared to a plate-fin heat exchanger with circular tubes, the use of oval tube fins and vortex generators increases the heat transfer rate by 3 to 16% and reduces the pressure drop by 17 to 35% for inlet velocities in the range of 1 to 8 m/s. Furthermore, the vortex generators make possible an average area reduction ratio of 14 to 18%. Overall, the results show that the inclined block shape vortex generators yield the greatest improvement in the heat transfer performance at medium to high inlet velocities.     

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.     

The Use of Effectiveness Concepts to Calculate the Thermal Resistance of Parallel Plate Heat Sinks

With this study, a new and more adaptable approach to the thermal design of the large heat sinks used in power electronics is proposed. This method, supported by the results from an extensive experimental program, recognizes that (1) the heat sink fins and the airflow adjacent to them form a simple cross-flow heat exchanger, and (2) conventional NTU-effectiveness methods can be adapted for use in the thermal analysis of the heat sink. This adaptation requires the development and evaluation of an equivalent heat capacity to describe the energy conducted along the fin.

This method was initially used to evaluate the convective heat transfer coefficients between the fin and the cooling air. In this geometry, the developing airflow conditions make the prediction of representative values difficult. The correlation found to describe the test results was then used in an inverted analysis to predict and compare the experimental values for the heat sinks thermal resistance. The method is finally used in a design example where the fin spacing is optimized for a particular test duty. It is concluded that this new approach will make the design of large heat sinks more robust and reliable.     

Heat Transfer and Flow Pattern Characteristics for HFE-7100 Within Microchannel Heat Sinks

This study investigates the heat transfer characteristics and flow pattern for the dielectric fluid HFE-7100 within multiport microchannel heat sinks with hydraulic diameters of 480 μm and 790 μm. The test results indicate that the heat transfer coefficient for the smaller channel is generally higher than that of the larger channel. It is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality for a modest mass flux ranging from 200 to 400 kg m^?2 s^?1 at a channel size of 480 μm and there is a noticeable increase of heat transfer coefficient with heat flux for hydraulic diameters of 790 μm. The difference arises from flow pattern. However, for a smaller mass flux of 100 kg m^?2 s^?1, the presence of flow reversal at an elevated heat flux for hydraulic diameters of 480 μm led to an appreciable drop of heat transfer coefficient. For a larger channel size of 790 μm, though the flow reversal is not observed at a larger heat flux, some local early partial dryout still occurs to offset the heat flux contribution and results in an unconceivable influence of heat flux. The measured heat transfer coefficients for hydraulic diameters of 790 μm are well predicted by the Cooper correlation. However, the Cooper correlation considerably underpredicts the test data by 35–85% for hydraulic diameters of 480 μm. The influence of mass flux on the heat transfer coefficient is quite small for both channels.     

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.     

High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 μm-square fins separated by 50 μm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel micro-channel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.     

Thermal Compensation Effect of Passage Arrangement Design for Inlet Flow Maldistribution in Multiple-Stream Plate-Fin Heat Exchanger

ABSTRACT

Passage arrangement design in fin channels is an efficient methodology for the reduction of the thermal deterioration influence of inlet flow maldistribution in multiple-stream plate-fin heat exchangers. In this work, the thermal compensation effects of passage arrangement design under different statistical parameters of inlet flow maldistribution are investigated. The inlet flow maldistribution in inlet header is analyzed and represented with distribution types, mean, and standard deviation of inlet mass flow rate entering the fin channels. A thermal calculation model based on integer–mean temperature difference method is established, and then, the passage arrangement under inlet flow maldistribution is optimized using a hybrid particle swarm algorithm. The thermal compensation effects for different inlet flow maldistributions and passage arrangements are calculated and compared. The results indicate that the compensation effect of optimization design of passage arrangement increases from 1.1% to 3.9% as the standard deviation of inlet mass flow rate increases from 0.06 kg/s to 0.12 kg/s. The results presented in this study can be used by other researchers to guide the passage arrangement design of actual heat exchanger with inlet flow maldistribution.     

Investigation of Flow Distribution in Microchannels Heat Sinks

The thermal efficiency of microchannel-based heat sinks relies on uniform fluid flow distribution between channels. Maldistribution, whether caused by poor manifold design or blockage of individual microchannels, can lead to hotspots and consequent thermal damage. This work considers design of manifolds for even flow distribution and the effect of channel blockage on the flow. An approximate model was used to evaluate the effect of manifold geometry on the flow distribution, and the results were compared with computational fluid dynamics (CFD) simulation. Various parameters, which influence the flow distribution such as the shape of distributing and collecting manifolds and position of inlet and outlet holes, have been studied for different inlet flow rates. The effect of channel blockage on flow distribution and pressure drop has been investigated. It was found that good agreement between results of the approximate model and results of CFD simulations are shown only for low Reynolds numbers. Results obtained by approximate model and CFD simulations were used to assist design of manifolds for uniform flow distribution between microchannels.     

Methods for Thermal Optimization of Microchannel Heat Sinks

This article presents a quantitative analysis of the effects of sand–bentonite backfill materials on the thermal performance of borehole heat exchangers (BHEs). Laboratory thermal probe tests were conducted to measure the thermal conductivity of sand–bentonite mixtures under different mixed ratios. Based on microscopic observations, the mechanism of bentonite affecting heat conduction between the sand grains was analyzed. Then field tests were carried out to compare the thermal performance of two double U-shaped BHEs with different backfill materials. Test results showed that the thermal conductivity of sand–bentonite mixtures first increased with increasing percentage of bentonite by dry mass, then reached a peak at the range from 10% to 12%, beyond which the thermal conductivity decreased quickly. For the BHE with an optimal sand–bentonite backfill material, the heat injection and heat extraction rate were enhanced on average by 31.1% and 22.2%, respectively, compared with the case with a common sand–clay material. These results can provide helpful guide for the design of ground source heat pump systems.     

Prediction of Heat Transfer Coefficient in Saturated Flow Boiling Heat Transfer in Parallel Rectangular Microchannel Heat Sinks: An Experimental Study

This study focuses mainly on the prediction of saturated flow boiling heat transfer in microchannels. A wide range of experiments has been carried out with de-ionized water to obtain a comprehensive data set. Experiments of mass fluxes of 51–728.7 kg/m²s, wall heat fluxes of 36–221.7 kW/m², vapor qualities of 0.01–0.69, liquid Reynolds number of 7.72–190, aspect ratios of 0.37–5.00 (with a constant hydraulic diameter of 100 µm) and hydraulic diameters of 100–250 µm (for constant aspect ratio = 1). A new correlation including the aspect ratio effect is proposed to predict the heat transfer coefficient for saturated flow boiling in microchannels. The proposed correlation shows very good predictions with an overall mean absolute error of 16.9% and 86.4%, 96.2% and 99.5% of the predicted data falling within ±30, ±40 and ±50% error bands, respectively.     

Flow Passage Arrangement and Surface Selection in Multistream Plate-Fin Heat Exchangers

This paper presents a general design methodology for multistream plate-fin heat exchangers that incorporates the consideration of operability aspects through the manipulation of stream flow passage arrangement. The main features of the design approach are uniform heat load per passage and secondary surface or fin selection. Surface selection is implemented as a means to achieve uniformity in the heat transfer rate of the various streams that take part in the heat exchange process. Uniform heat load content per passage is a design consideration through which an equal number of hot and cold passages is achieved. Under these conditions, the number of passages allocated to a given stream is directly proportional to its heat capacity mass flow rate. A simple model for the steady-state simulation of multistream exchangers is also presented. This model can be used to determine the exchanger response to changes in temperature and flow rate that may take place during operation. Results indicate that flow passage arrangement is a design consideration that can be manipulated to reduce the effect of these types of disturbances upon the target temperatures of specific streams.     

Recent Studies on Fluid Flow and Heat Transfer in Thermal Microdevices

Control and measurement of fluid flow and heat transfer in microdevices is of great importance to the development and application of MEMS and Bio-MEMS such as thermal inkjet printer heads, microchemical reactors, and PCR. Thus, the detailed flow behavior, in particular the two-phase flow in microdevices, has attracted much attention in recent years. Several types of thermal micropumps have been developed, although there is still room for further development. Various techniques for measuring the temperature, which are applicable to microscale devices, have also been proposed. As the cooling problem in microdevices becomes increasingly significant, a prospective view on integrated heat and mass transfer is quite necessary. Thus, in this work, some issues and future prospects for fluid dynamics and heat transfer of thermal microdevices are presented and discussed, in terms of thermocapillary pump, temperature measurement in microdevices, and flow near an evaporating meniscus.     

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.     

Periodically Fully Developed Heat and Fluid Flow Characteristics in a Furrowed Wavy Channel

Periodically fully developed two-dimensional (2D) flow in a furrowed wavy channel is investigated numerically at various Reynolds numbers (100–2123). For the laminar and transitional flow regime, the study is done for six geometrically different channels; corresponding to various nondimensional amplitude (0.05, 0.075, and 0.1) and wavelength (0.5 and 1). Critical Reynolds number—for the onset of periodic flow—decreases with increasing amplitude and wavelength. A flow regime map—demarcating steady and unsteady flow regime—is proposed. It is shown that the size of the vortex in streamlines and waviness in isotherms increase with increasing Reynolds number, amplitude and wavelength. The performance of wavy as compared to straight channel is studied with the help of ratio of Nusselt number, friction factor and area-goodness factor, and thermal-performance factor. With increasing Reynolds number, all these parameters remain almost constant in the steady regime and increase almost linearly in the unsteady regime. For the largest Reynolds number (close to 2000) studied here, the increase in the Nusselt number ratio—within the periodic flow regime—is 11.21% and 133% for the amplitude equal to 0.075 and 0.1, respectively, at a wavelength of 0.5; at a wavelength of 1.0, the increase is 101%, 134%, and 181% for the amplitude of 0.05, 0.075, and 0.1, respectively.     

Comparative Study of the Flow and Thermal Performance of Liquid-Cooling Parallel-Flow and Counter-Flow Double-Layer Wavy Microchannel Heat Sinks

Applications of microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be an alternative cooling technology to enhance the heat transfer, and are successfully applied in heat exchangers. In this article, three kinds of liquid-cooling double-layer microchannel heat sinks, such as a rectangular straight microchannel heat sink, a parallel-flow wavy microchannel heat sink, and a counter-flow double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude and volumetric flow ratio on heat transfer, pressure drop, and thermal resistance are also observed. Results show that the counter-flow double-layer wavy microchannel heat sink is superior at a larger flow rate, and a more uniform temperature rise is achieved. For a slightly larger flow rate, the parallel flow layout shows better performance. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu₀)/(f/f₀) and (Nu/Nu₀)/(f/f₀)^1/3, are applied and similar results are obtained.     

Effects of Distributor Configuration on Flow Maldistribution in Plate-Fin Heat Exchangers

In this paper, an original concept of a design that adds a complementary fluid cavity in the distributor is presented. The experimental investigation of the effects of distributor configuration parameter on the fluid flow maldistribution in the plate-fin heat exchanger is completed. The correlation of the dimensionless flow maldistribution parameter and the Reynolds number is obtained under different distributor configuration parameters. The experimental studies prove that the performance of flow distribution in heat exchangers can be effectively improved by the optimum design of the distributor's configuration parameter. The ratio of the maximum velocity and the minimum velocity in the channels of the plate-fin heat exchanger can drop from 2.57–3.66 to 2.08–2.81 for various Reynolds numbers. The conclusions are of great significance on the optimum structure design of the plate-fin heat exchangers and can effectively improve the performance of the heat exchangers.