Pin Fin Arrays Research Articles

Heat transfer measurement in a rotating cooling channel with staggered and inline pin-fin arrays using liquid crystal and stroboscopy

Endwall heat transfer in the stationary and rotating cooling channels with pin-fin arrays (inline and staggered pattern) were experimentally measured by wide band liquid crystal. The temperature measurement was performed with the stroboscopic photography to capture the reflected color from the liquid crystal coatings on the rotating object. A detailed calibration was performed to quantify the uncertainties in the liquid crystal measurement from the stroboscope frequency, flash duration, and viewing angle. The Reynolds numbers ranged from 5000 to 20000 and the rotation numbers ranged from 0 to 0.52. This measurement technique provided detailed heat transfer variations caused by the distinct secondary flow patterns from pin-fin and rotation. Results showed that the heat transfer from the staggered array was 25–30% higher than the inline array in the stationery channel. The rotation led to unsymmetrical heat transfer distributions along the spanwise direction. The heat transfer variation caused by rotation was more noticeable for the inline array, and the heat transfer in the rotating channel was more than 50% than that in the stationary channel.

Pressure drop and heat transfer characteristics of pin fin enhanced microgaps in single phase microfluidic cooling

Microfluidic cooling with microgaps with surface area enhancements such as pin fins can potentially achieve superior thermal performance in cooling high power microsystems. In this work, the single phase thermal/fluid characteristics of microgaps of H/W around 0.02, incorporating staggered pin fin arrays are studied. The effects of the pin fin dimensions including diameter, transversal and longitudinal spacing, and height are investigated by experimentally validated model over a range of Reynolds number (Re) 22–357. Micropin fin arrays investigated have pin diameter of 100μm, pitch/diameter ratios of 1.5–2.25, and height/diameter ratios of 1.5–2.25. Correlations of friction factor (f) and Colburn j factor for these dense arrays of micro pins have been developed. The performance of circular and square micro pin fin arrays were compared, showing that the circular array has better thermal performance under the same pumping power.

Experimental Investigation on Heat Transfer Characteristics of Different Metallic Fin Arrays

The reliability of electronic equipment depends on the reliability of the system. For small applications natural convection cooling is sufficient, but for the electronic equipment having number of heat generating components, forced convection cooling is essential. In number of cases, pin fin arrangement is preferred for augmentation of heat transfer. Here, the performance of pin fin array of copper and aluminum material with in-line, as well as staggered arrangement over a flat plate is studied. Constant heat input was given to the inline, staggered arrangement of copper as well as aluminium pin fin arrays. In the present experimental study, heat input and airflow rates are the variables. It was found that the heat transfer coefficient for staggered array is 15% more than that of the in-line array, at the same time pressure drop across the staggered array is more by 10% than the in-line array. The pressure drop was observed to be increasing with increase in flow rate as expected. Endeavor of the present work is to find the optimum spacing between the fins in an array for maximum heat transfer rate, by investigating the heat transfer characteristics.

Copper vertical micro dendrite fin arrays and their superior boiling heat transfer capability

Micro pin fin arrays have been widely used in electronic cooling, micro reactors, catalyst support, and wettability modification and so on, and a facile way to produce better micro pin fin arrays is demanded. Herein, a simple electrochemical method has been developed to fabricate copper vertical micro dendrite fin arrays (Cu-VMDFA) with controllable shapes, number density and height. High copper sulphate concentration is one key point to make the dendrite stand vertically. Besides, the applied current should rise at an appropriate rate to ensure the copper dendrite can grow vertically on its own. The Cu-VMDFA can significantly enhance the heat transfer coefficient by approximately twice compared to the plain copper surface. The Cu-VMDFA may be widely used in boiling heat transfer areas such as nuclear power plants, electronic cooling, heat exchangers, and so on.

Orientation effects on natural convective performance of hybrid fin heat sinks

Orientation effects on thermal performances of the hybrid fin heat sinks (HFHSs) in natural convection are computationally and experimentally investigated. Hollow and solid hybrid fin heat sinks are explored as representative HFHSs and compared with a similarly-sized pin fin heat sink (PFHS). The staggered array of hollow pin fins integrated with radially-oriented plate fins and containing perforations in the vicinity of fin bases composes the hollow hybrid fin heat sink (HHFHS). The staggered array of solid pin fins with extruded radially-oriented plate fins composes the solid hybrid fin heat sink (SHFHS). CFD thermal models of the HHFHS, the SHFHS, and the PFHS are generated, experimentally-verified, and employed to investigate the orientation effects, ranging from 0° to 180°, on their thermal performances. The results show the lowest thermal resistance, Rth, values for the HHFHS, the SHFHS, and the PFHS occurred at an orientation angle of 45°, less orientation dependence of the HHFHS compared with the PFHS, and consistently smaller mass-multiplied thermal resistance, RthM, values of the HHFHS, even up to 32%, than those of the PFHS despite various orientations.

Critical Biot Numbers of Periodic Arrays of Fins

In this paper, we consider the heat transfer problems associated with a periodic array of triangular, longitudinal, axisymmetric, and pin fins. The problems are modeled as a wall where the flat side is isothermal and the other side, which has extended surfaces/fins, is subjected to convection with a uniform heat transfer coefficient. Hence, our analysis differs from the classical approach because (i) we consider multidimensional heat conduction and (ii) the wall on which the fins are attached is included in the analysis. The latter results in a nonisothermal temperature distribution along the base of the fin. The Biot number (Bi=ht/k) characterizing the heat transfer process is defined with respect to the thickness/diameter of the fins (t). Numerical results demonstrate that the fins would enhance the heat transfer rate only if the Biot number is less than a critical value, which, in general, depends on the geometrical parameters, i.e., the thickness of the wall, the length of the fins, and the period. For pin fins, similar to rectangular fins, the critical Biot number is independent of the geometry and is approximately equal to 3.1. The physical argument is that, under strong convection, a thick fin introduces an additional resistance to heat conduction.

Effect of cooling fluids on high frequency electric and magnetic fields in microelectronic systems with integrated TSVs

A fully 3D conjugate numerical analysis was performed to reveal the effects of air, R134a refrigerant and water on electromagnetic fields of electronic cooling designs made of arrays of micro pin-fins with integrated Through-Silicon-Vias (TSVs). The integrated TSV cooling configuration included 8 cylindrical TSVs with 150µm diameter each and 200µm height. The external dimensions of the silicon substrate were 900×700×280µm. Each TSV encapsulated four equally spaced copper vias each having a diameter of 40µm. The impacts of the presence of the stationary cooling fluids without heat transfer on TSVs electric and magnetic fields were examined for five different frequencies; 100MHz, 500MHz, 1GHz, 5GHz and 10GHz. Then, separately, the effects of moving cooling water with temperature-dependent physical properties were studied while exposing the cooled micro pin-fin array to a uniform heat flux of 500Wcm−2. For the case of stagnant and moving cooling fluids it was found that water influences the electric field twice as much as either R134a or air and that this influence decreases only negligibly with the increase in frequency of the electric current passing through the TSVs. The influence of the presence of the stagnant and moving cooling fluids on the magnetic field is orders of magnitude smaller and reduces rapidly with the increased frequency.

Pressure loss and heat transfer performance for additively and conventionally manufactured pin fin arrays

Pin fin heat exchangers represent a common, viable means of keeping components cool and are widely used for electronics cooling and turbine airfoil cooling. Advancements in manufacturing technology will allow these heat exchangers to be built using new methods, such as laser powder bed fusion, a form of additive manufacturing. New manufacturing approaches, however, render a direct comparison between newly and conventionally manufactured parts meaningless without a good understanding of the difference in the performance.This research study investigated microchannel pin fin arrays that were manufactured using Laser Powder Bed Fusion and compared them to studies of pin fin arrays from the literature, which are representative of traditionally-manufactured pin fin arrays, where the pin and endwall surfaces exhibited much lower surface roughness. Pin fin arrays with four different spacings were manufactured and tested over a range of Reynolds numbers; pressure loss and heat transfer measurements were taken. Additionally, the test coupons were evaluated nondestructively and the as-built geometric features were analyzed.Measured surface roughness was found to be extremely high in each one of the microchannel pin fin arrays and was found to be a function of the pin spacing in the array, as was the shape of the pin itself; with more pins in the array came higher surface roughness and more distorted pin shapes. Comparisons between the smooth pin fin arrays from literature and the rough pin fin arrays from the current study showed that the high surface roughness more strongly affected the friction factor augmentation than it did the heat transfer augmentation relative to the smooth pin fin arrays.

Heat transfer enhancement analysis of tube receiver for parabolic trough solar collector with pin fin arrays inserting

Tube receiver with pin fin arrays inserting was introduced as the absorber tube of parabolic trough receiver to increase the overall heat transfer performance of tube receiver for parabolic trough solar collector system. The Monte Carlo ray tracing method (MCRT) coupled with Finite Volume Method (FVM) was adopted to investigate the heat transfer performance and flow characteristics of tube receiver for parabolic trough solar collector system. To validate the feasibility of the developed MCRT and FVM combined method, the numerical results have been compared with experimental results conducted in the DISS test facility in Spain and the max relative error is less than 5%. The numerical results indicated that the introduction of absorber tube with pin fin arrays inserting design for the absorber tube of the parabolic trough receiver can effectively enhance the heat transfer performance. The average Nusselt number can be increased up to 9.0% and the overall heat transfer performance factor can be increased up to 12.0% when the tube receiver with pin fin arrays inserting was used.

Natural Convection Heat Transfer Augmentation Factor with Square Conductive Pin Fin Arrays

Natural Convection Heat Transfer Augmentation Factor with Square Conductive Pin Fin Arrays

Detached-Eddy Simulation of Trailing-Edge (TE) Cutback Turbine Blade Cooling

This research evaluates the cooling performance of trailing-edge cutback for gas turbine blade. By using DES based on SST k-ω turbulence model, numerical investigations were performed at two steps: first, to validate simulation results from an existing TE cutback cooling with staggered pin-fin arrays inside the cooling passage against experimental measurement. Three types structured mesh from coarse (Δy + = 0.74) to fine (Δy + = 1.22) were evaluated during this step; second, to investigate the TE cutback cooling performance on various blowing ratios. Simulations were performed by keeping the same initials and boundary conditions as the experiment. The result indicates that validation can be considered acceptable by controlling grid quality resolution near wall regions. Both computational data of the adiabatic film-cooling effectiveness and the discharge coefficient are in good agreement with available experimental measurements. The averaged film-cooling effectiveness along the cutback region is highly influenced by the blowing ratios, which is to be related to the turbulent flow structures formed at the mixing region as the impact of coolant flow ejection. The increase of coolant jet velocity triggers the heat transfer process up to the downstream region of TE cutback cooling.

Investigation of coupled systems consisting of fluid movers and heat-exchange devices

ABSTRACTA heat exchanger and the fluid mover that delivers a working fluid to the exchanger inlet may experience profound interactions, which argues against treating them as separate entities. On the other hand, the design practice commonly assumes that the fluid delivered to the heat exchanger inlet is specifiable without consideration of any possible influence of the exchanger. The magnitude of the flow rate arriving at the exchanger inlet is generally based on the pressure rise—flow rate (P-Q) curve supplied by the manufacturer of the fan and coupled with the assumption that that flow is uniformly distributed across the exchanger inlet. It was found that the complexity of the fluid flow delivered by the rotating fan gives rise to a large fluid resistance within the pin-fin array, such that the delivered air flow rate was only about 37% of that for the P-Q case. On the other hand, the corresponding reduction in the rate of heat transfer was, at most, 27%.

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACTThis article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm−2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm−2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.

Inverse Design of Cooling Arrays of Micro Pin-Fins Subject to Specified Coolant Inlet Temperature and Hot Spot Temperature

ABSTRACTGiven a micro pin-fin array cooling scheme with these constraints: (a) given maximum allowable temperature of the material (the hot spot temperature), (b) given inlet cooling fluid temperature, (c) given total pressure loss (pumping power affordable), and (d) given overall thickness of the entire micro pin-fin cooling array, find the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is removed by the cooling fluid. The goal was to create an optimum performance map for a cooling micro array having specified inlet coolant temperature and maximum temperature. Fully 3D conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto set of best trade-off solutions. These Pareto optimized configurations indicate the maximum physically possible heat fluxes for specified material and constraints. Detailed off-design performance maps of such Pareto-optimized cooling arrays of micro pin-fins were calculated that demonstrate superior on-design and off-design performance of pin-fins having symmetric convex cross sections as opposed to the commonly used circular cross sections.

Numerical simulations and optimization of porous pin fins in a rectangular channel

ABSTRACTIn this study, the multiparameter constrained optimization procedure integrating the design of experiments (DOE), genetic algorithm (GA), and computational fluid dynamics (CFD) is proposed to design three-dimensional porous pin fins in a rectangular channel. The Forchheimer–Brinkman extended Darcy model and the two-equation energy model are adopted to describe the fluid flow and heat transfer characteristics in the porous media. The elliptical, coupled, steady-state, and three-dimensional governing partial differential equations for laminar forced convection with porous pin fins in a rectangular channel are solved numerically using the finite volume approach. The numerical optimization provides a reliable and economic mean of designing a heat transfer channel with porous pin fin arrays.

Embedded single phase microfluidic thermal management for non-uniform heating and hotspots using microgaps with variable pin fin clustering

The presence of variable heat fluxes, such as localized hotspots in integrated circuit (IC) architectures poses a key challenge for thermal management of existing (2D) and emerging three-dimensionally (3D) stacked chips. The use of conventional microchannel or uniform pin fin arrays for microfluidic cooling do not provide adequate surface area for heat transfer in the vicinity of regions of concentrated high power (hotspots), and therefore significant temperature gradients might arise in such zones. In the present investigation, the concept of using a single microfluidic loop for the combined and efficient cooling of hotspot and moderate power (background) areas is proposed, experimentally demonstrated, and supported by a comprehensive numerical model. Two different thermal device vehicles (TDVs) are considered for a range of operating conditions, in which the surface area is locally increased by clustering a dense array of pin fins in the hotspot region for one configuration, while for the other the clustering is uniform in the spanwise direction. De-ionized (DI) water is used as the coolant through a silicon (Si) microgap with 200μm spacing; the hotspot heat flux is varied from 250 to 750W/cm2, while the background heat flux is fixed at 250W/cm2. Results indicate the capability of the proposed designs to keep the maximum temperature of the combined device below 65°C for an inlet water temperature of 21.3°C, with moderate temperature gradients and pressure drop. In addition, a robust computational fluid dynamics/heat transfer (CFD–HT) model capable of predicting spatially resolved temperature fields arising from heterogeneous heating is validated with relevant experimental data. Detailed benchmark simulations are provided, so they can be reproduced and used for reference in numerical studies with variable pin fin densities. The described methodology represents a cost-effective thermal modeling technique that may be applicable to virtually any type of heat flux distribution or power map, and IC architecture.

Experimental study on thermal performance of micro pin fin heat sinks with various shapes

This paper presents a visualization experimental study on the heat transfer characteristics of various shapes of micro pin fins, including the circular, ellipse, diamond, square and triangle shape micro pin fin arrays with various equivalent diameters and pin fin density. The influences study of different sizes and shapes of pin fin on Nusselt number and heat transfer coefficient have been conducted. The results show that with the increase of the flow rate, the temperature of the bottom of the experimental section decreases. And the Nusselt number of different shapes of micro pin fins increases with the increase of Re. In which, the heat transfer performance of the ellipse shape pin fin appears better among the other shapes of pin fins. However, the higher pin fin of the ellipse shape density leads to a weaker flow performance. Besides, the micro-scale heat transfer correlation between the Nusselt number and the Reynolds number is fitted based on the experimental data.

Effect of compressible volume on flow boiling instability of water in the pin-fin microchannel

Flow boiling instability induced by compressible volume is significant for the premature occurrence of the limiting critical heat flux (CHF) condition in microchannles. To suppress the flow boiling instability, some methods have been studied, including increasing inlet restrictors on the microchannel inlet, forming reentrant cavities on the microchannel inner surface, and et al. These methods focused on microchannels themselves, while little attention was given to compressible volume in upper stream of the microchannels. In this paper, flow boiling instability of water in the silicon-based pin-fin microchannel was studied when there existed compressible volume in the upper stream of the microchannel. This pin-fin microchannel had circular/staggered pin-fin arrays arranged on the undersurface of the microchannel with a hydraulic diameter of 210.4 μ m. During the experiments, heating power was unchanged, and working conditions were adjusted by decreasing mass flux. The experimental results showed that the mass fluxes of onset of flow instability (OFI) for the compressible volumes of V =0, 0.5, 1.0 and 2.0 cm3 were always the same. This indicated that the compressible volume has no effect on OFI. After OFI, flow boiling instability occurred in the microchannel with the periodic oscillations of temperature and pressure when there existed compressible volume in the upper stream of the microchannel. The amplitudes and periods of these periodic oscillations increased with the increase of the compressible volume. In this unstable boiling region, slight decrease of mass flux could result in significant increase in pressure drop through the microchannel, and the amplification increased with the increase of compressible volume. However, when there was no compressible volume in the upper stream of the microchannel, the periodic oscillations of temperature and pressure did not occur. The compressible volume has several forms, such as driving the water flow in the experimental system by a pressure tank and connecting the pipes in the upper stream of the microchannel with PVC soft-pipes, and they all introduced the compressible volume and induced the flow boiling instability.

Porous-wall microchannels generate high frequency “eye-blinking” interface oscillation, yielding ultra-stable wall temperatures

The proposed gradient–porous-wall microchannels consist of bare channels and pin–fin array regions, fabricated by MEMS (microelectricalmechanicalsystem) technique. Boiling experiments were performed with acetone as the working fluid. Ultra-stable wall temperatures are achieved with oscillation amplitudes in the range of 0.02–0.18°C. Bubble nucleation is found to happen in the porous wall. The generated vapor flows towards bare channels due to surface tension driving flow. The vapor ejection direction is periodically switched between neighboring channels, called the “bubble emission switch”. The bubble confinement ratio is newly defined. Bubbles become fat and slim in bare channels to generate high frequency “eye-blinking oscillation”. Bubble confinement ratios display sine function, and out-of-phase characteristic between neighboring channels. We confirm the “eye-blinking” oscillation as a density wave oscillation, propagating in the channel width direction. Because the porous-wall width is much smaller than the channel length, the “eye-blinking” frequencies are 10–100 times higher than that of the axially propagated density wave oscillation. The “integration parameter model” establishes the connection between “eye-blinking” oscillation and wall temperatures. The convective heat transfer intensity in bare channels is assumed to follow the bubble confinement ratio variation. The wall temperature oscillation amplitude is inversely proportional to the “eye-blinking” frequency. The phase angle between bubble confinement ratios and wall temperatures are 3π/2, being the negative feedback mechanism to inhibit wall temperature oscillations. The porous-wall microchannels open a new way to eliminate flow instabilities for heat exchangers and thermal energy systems.

Novel Heat Exchangers with Cross-Runners for Air and Water Cooling

This work experimentally investigated the pressure drops and heat transfer characteristics of cross-runner heat exchangers. Three kinds of configurations employed were, (1) the aluminum alloy heat exchanger with a staggered rectangular-fin array (Model A); (2) the aluminum alloy heat exchanger with a staggered round pin fin array (Model B); and (3) the copper heat exchanger sintered by multiple copper sheets with rectangular punched holes forming cross-runners (Model C). The results indicate that increasing heat-transfer area (AHT) and decreasing porosity (ε) of the heat exchanger significantly enhanced the heat transfer capacity of the cross-runner heat exchanger, but also increased flow resistance. At the same pumping power for the air-cooled heat transfer experiment, the Nu of Model C was 2.27 and 1.67 times that of Models A and B respectively. Additionally, this study proposed the semi-empirical correlations of dimensionless pressure drop and Nusselt number in terms of Reynolds number for air-cooling measurements. Finally, the feasibility of using the present cross-runner heat exchangers for an instantaneous water heater was also investigated according to the water-cooled heat transfer experiment and the results showed great commercial potential of Model C.