Oblique Fin Research Articles

Entropy Generation and Heat Transfer Performance in Microchannel Cooling.

Owing to its relatively high heat transfer performance and simple configurations, liquid cooling remains the preferred choice for electronic cooling and other applications. In this cooling approach, channel design plays an important role in dictating the cooling performance of the heat sink. Most cooling channel studies evaluate the performance in view of the first thermodynamics aspect. This study is conducted to investigate flow behaviour and heat transfer performance of an incompressible fluid in a cooling channel with oblique fins with regards to first law and second law of thermodynamics. The effect of oblique fin angle and inlet Reynolds number are investigated. In addition, the performance of the cooling channels for different heat fluxes is evaluated. The results indicate that the oblique fin channel with 20° angle yields the highest figure of merit, especially at higher Re (250–1000). The entropy generation is found to be lowest for an oblique fin channel with 90° angle, which is about twice than that of a conventional parallel channel. Increasing Re decreases the entropy generation, while increasing heat flux increases the entropy generation.

Influence of the oblique fin arrangement on the fluid flow and thermal performance of liquid cold plate

Electric vehicle (EV) is advancing as the transportation industry and the demand is expanding globally. The use of liquid cold plate (LCP) in EV battery is more effective in providing the desire temperature rather than using the conventional air cooling. The use of straight fins is being switched to oblique-shape fins to assist disruption of the thermal boundary layer development. The arrangement of fins in the LCP also will affect the performance of it cooling the battery. Three different arrangement of the oblique fin are developed to enhance the fluid flow and heat transfer performance. The LCP contains three arrangements of oblique fin namely as inline, incline and loureved. Experimental and numerical results reveal the good aggrement where the Nusselt number is enhanced with the louvered arrangement. Among three arrangements, loureved obtained the lowest surface temperature of the battery followed by inline and incline. It also found that Nusselt number increases as the Reynolds number increases. The LCP is able to maintain the average surface temperature of the battery below permitted working temperature of 50 °C. This shows that the present LCP with oblique fin could be a promising method for EV battery thermal management.

Numerical analysis on the thermal performances of different types of fin heat sink for high-power led lamp cooling

The efficient cooling is very important for a LED module because both the energy efficiency and lifespan decrease significantly as the junction temperature increases. Fin heat sinks are commonly used for cooling LED modules under natural convection conditions. This work proposed two novel models of fin heat sink, named the pin-plate fin (PPF) heat sink and the oblique-plate fin (OPF) heat sink, by combining plate fins with pin fins and oblique fins and their thermal performances were studied numerically. The LED module was assumed to be operated under 1 atmospheric pressure and its heat input is set to 4 W. The PPF with 8 plate fins inside (PPF-8) and the OPF with 7 plate fins inside (OPF-7) show the best thermal performances among all PPF and OPF designs. Total thermal resistances of PPF- 8 and OPF-7 decreased by 9.0% - 15.6% compared to those of three original models. Heat transfer coefficients of PPF-8 and OPF-7 were higher by 12.6% - 35.2% than those of three original heat sinks.

Coupled equivalent circuit models for fluid flow and heat transfer in large connected microchannel networks – The case of oblique fin heat exchangers

This paper presents a simplified equivalent circuit model, which exploits the electric–hydraulic analogy and electric–thermal analogy, to predict the mass flow distribution and temperature distribution in an oblique fin array used in enhanced heat transfer applications. Methods to obtain accurate correlations for calculation of flow-dependent hydraulic ‘resistances’ are outlined and developed for both primary and secondary channels in the oblique fin array. Appropriate Nusselt number correlations and thermal resistance models are also employed to predict the temperature distribution associated with the mass flow distribution. Detailed full-domain numerical (CFD) simulations are performed to obtain parameters for the hydraulic resistance correlations, and also to serve as benchmarks for the proposed equivalent circuit model. Detailed comparisons between the results of simplified model and numerical simulation showed that the simplified model can accurately predict the mass flow distribution and temperature distribution, within ±5%, for varying fin number, aspect ratio, fin pitch, fin length, oblique angle and inlet velocity. Slightly higher deviations of mass flow prediction are observed for high inlet velocities as a result of the presence of vortices close to the trailing edge of the oblique fin region.

Investigation of fluid flow and heat transfer in wavy micro-channels with alternating secondary branches

The fluid flow and heat transfer behaviors in the novel sinusoidal micro-channels with secondary branches were examined both numerically and experimentally. Secondary branches, inspired by the oblique fin design were added in an alternating fashion (±45°) to the peaks and the troughs of the wavy micro-channel configuration to enable cross-channel mixing. Two designs with different amplitude of waviness were benchmarked against the optimal wavy channel configuration from the Reynolds number range of 50–200. An increase in heat transfer performance (when compared to the reference conventional wavy design) was observed when secondary branches were added, albeit with a pressure drop penalty. However, when another design with lower amplitude of waviness was introduced and compared to the reference geometry, superior heat transfer performance was observed without pressure drop tradeoff.The velocity and temperature profiles from the simulations were obtained to explain the hydrodynamic and heat transfer behaviors. The combination of Dean’s vortices induced by the wavy channel design and the cross-channel mixing enabled by the secondary branches showed potential to further enhance convective heat transfer (at reasonable pressure drop penalties) due to the increased intensity of span-wise fluid mixing and also boundary layer disruptions. The attractiveness of the introduction of secondary branches lies in the potential to further enhance the heat transfer performance of the already established wavy channel configuration with minimal pressure drop penalty.

Investigations on the Influence of Flow Migration on Flow and Heat Transfer in Oblique Fin Microchannel Array

In order to scrutinize the coolant mass distribution and its effect to the heat transfer in oblique fin microchannel array, extensive numerical studies are performed on planar oblique fin configuration. Full-domain simulations using common-flow down (CFD) approach are employed to provide better insights into the flow distribution, flow stability, and heat transfer performance at a global level. The flow field and temperature profile analysis shows that nonuniform coolant distribution and coolant migration occur in the oblique fin microchannel, and the heat transfer performance for both edges of the heat sink is affected due to changing secondary flow rate. However, the flow migration does not affect the local coolant velocity and temperature profiles significantly in the middle region (0.2 < Z′ < 0.8). Meanwhile, it is also found that Reynolds number affects the coolant migration, the stability of the fluid flow, and heat transfer performance significantly. Higher Reynolds number increases the percentage of secondary flow rate and, hence, enhances the heat transfer for fin surfaces in secondary channels.

Comparative Study of Temperature and Pressure Instabilities During Flow Boiling in Straight- and 10° Oblique-Finned Microchannels

Flow boiling experiments are conducted in straight- and oblique-finned microchannels with similar channel dimensions and operating conditions using the FC-72 dielectric fluid. Both the test pieces are made from a copper block with a footprint area of 25mm × 25mm and 40 parallel microchannels. The oblique cuts are created at a nominal oblique angle of 10° with a nominal width which is half of that of the parallel microchannels. Two mass fluxes at 175kg/m2 s and 370kg/m2 s, with heat fluxes from 14W/cm2 to 42W/cm2 are tested. Wall temperature and pressure fluctuations during the flow boiling process are presented in this article. It is found that the oblique-finned configuration considerably mitigates temperature and pressure instabilities at various operating conditions, apart from augmentation in heat transfer performance. The increased flow stability is attributed to the ability of the oblique fins in reducing flow reversal. These results show that the oblique-finned microchannels present an interesting thermal management technique for stabilised two-phase cooling.

A comparative study of experimental flow boiling heat transfer and pressure characteristics in straight- and oblique-finned microchannels

Flow boiling experiments are conducted in straight-finned and oblique-finned microchannels with similar channel dimensions and operating conditions using the FC-72 dielectric fluid. Both heat sink geometries consist of 40 parallel microchannels, and are fabricated on a copper block with a footprint area of 25mm×25mm. The oblique fins are produced by introducing diagonal cuts with angle of 27° and width which is half of that of the parallel microchannels. The comparative study with straight fins shows significant augmentation in heat transfer and the delay in the onset of critical heat flux for the oblique-finned microchannels. This is due to enhancement in the flow boiling stability offered by the oblique fins in terms of reduced wall temperature gradients and pressure fluctuations. Flow visualisations performed on both microchannel geometries show increased bubbles generation in the nucleate boiling region and a continuously developing thin liquid-film in the convective boiling region for the oblique fins, which is believed to be the primary factor in heat transfer enhancement. However, the improved heat transfer performance incurs a higher pressure drop penalty compared to its straight-finned counterpart. This drawback could possibly be overcome by careful modifications in the oblique-finned geometry, so as to control the amount of secondary flow.

Effects of pin tip-clearance on the performance of an enhanced microchannel heat sink with oblique fins and phase change material slurry

The performance of an enhanced microchannel heat sink with sectional oblique fins and with a coolant that contains Nano-Encapsulated Phase Change Material (NEPCM) particles is investigated using a 3D conjugated heat transfer model. Three volume fractions from ξ=0 (pure water) to 0.3 are studied to investigate the effects of presence of NEPCM particles. Then the effects of introducing tip-clearance to the heat sink on thermal and hydrodynamic performance is examined at low Reynolds numbers (less than 215). Four values of tip-clearance to channel width ratio (t/Wc) are investigated ranging from a no gap (t/Wc=0) to t/Wc=0.74. On the bottom wall of the heat sink a range of uniform and fixed temperatures of 299.15K to 340.15K are applied. The cooling and hydrodynamic performance of the heat sink is studied using Nusselt and Euler numbers, respectively. It was observed that using of NEPCM slurry in contrast with pure water, enhanced the cooling performance of the heat sink but increased the Euler number. Furthermore, it was seen that the introduction of tip-clearance to the heat sink, if the clearance ratio is chosen properly, has the potential to enhance the cooling performance and reducing the Euler number simultaneously.

Fluid flow and heat transfer investigations on enhanced microchannel heat sink using oblique fins with parametric study

Abstract Enhanced microchannel heat sink with sectional oblique fin is used to modulate the flow in contrast to continuous straight fin. The re-initialization of thermal boundary layer at the leading edge of each oblique due to breakage of continuous fin into oblique sections and the secondary flow due to these oblique cuts resulted in better heat transfer and a comparable pressure drop. Extensive experimental investigations are carried out with silicon test vehicle with hydraulic diameter of 100 μm and 200 μm and de-ionized water as flowing fluid. A parametric study involving the oblique angle, fin pitch is also carried out. Appreciable heat transfer augmentation is also achieved with maximum heat transfer performance enhancement at 47% when Re = 680. Comparable pressure drop to conventional microchannel is maintained up to Re = 500. Parametric study suggests that smaller oblique angle and smaller fin pitch are beneficial for heat transfer enhancement. The performance of the microchannel with 100 μm channel width and 27° oblique angle is found to be optimum.

Experimental investigation of flow boiling heat transfer in novel oblique-finned microchannels

Oblique fins are modified from its conventional straight fins counterpart via the introduction of oblique cuts at specific locations, leaving obliquely-shaped fins with secondary microchannels. The benefits of having these secondary microchannels have been conclusively demonstrated in single-phase mode, however, they have yet to be tested in two-phase mode, which forms the motivation behind this study. Flow boiling experiments are conducted in the oblique-finned microchannel heat sink, made from a copper block with a footprint area of 25×25mm2. 40 parallel microchannels, each with a nominal width of 0.3mm are machined with wire-cut electro-discharge machining process. The nominal width of the oblique cuts, on the other hand, is half of that of the parallel microchannels. Tests are performed with FC-72 dielectric fluid with an inlet temperature of 29.5°C, with a range of mass fluxes from 175kg/m2s to 350kg/m2s and heat fluxes from 6W/cm2 to 120W/cm2. Flow boiling results and discussion on its heat transfer performance and pressure drop characteristics are presented in this paper. It is found that nucleate boiling is the dominant heat transfer mechanism at low heat fluxes, with a transition to convective boiling-dominant via thin-film evaporation occurring at medium heat fluxes. A full convective boiling mechanism is observed thereafter prior to CHF. These observations agreed well with high-speed flow visualisation images. Inlet pressure fluctuations within the oblique-finned microchannels are extremely small, indicating that the oblique fins are able to considerably stabilise the flow boiling process.

Investigation on the influence of edge effect on flow and temperature uniformities in cylindrical oblique-finned minichannel array

The influences of edge effect on flow and temperature uniformities were investigated for oblique-finned structure on both planar and cylindrical heat source surface though numerical and experimental studies. The numerical simulation using CFD approach was employed to provide definitive information about the flow field distribution and flow mixing between the main and secondary channels. The flow field analysis showed that poor flow mixing exists in the draining region and filling region, while the flow regime between the middle regions is not influenced by the edge effects in the blockaded cylindrical oblique fin heat sink. For regular cylindrical oblique fin heat sink, the flow fields in both the main and secondary channels are distributed uniformly in the spanwise direction. The local temperature distribution curves for blockaded cylindrical oblique fin heat sink are shown to have a uniquely concave shaped due to edge effects. Interestingly, a uniform and lower surface temperature distribution for regular cylindrical oblique fin heat sink are observed as a result of improved flow mixing due to the absence of the edge effects. These studies prove that cylindrical oblique fin heat sink is an effective cooling solution for cylindrical heat sources, and this can provide insights for designers interested in oblique fin heat sink or other similar designs.

A parametric investigation of heat transfer and friction characteristics in cylindrical oblique fin minichannel heat sink

A novel cylindrical oblique fin minichannel heat sink was proposed and studied for cooling heat source with cylindrical surfaces. To optimize and analyze the heat transfer performance of the heat sink, a similarity analysis and parametric study on the geometric dimensions of the heat sink were performed. Three dimensional conjugated heat transfer simulation using CFD (Computational Fluid Dynamics) approach was used to analyze the laminar convective heat transfer and apparent friction characteristics for 43 different cylindrical heat sinks with varied geometric dimensions. The studies were performed by varying the oblique angle from 20° to 45°, secondary channel gap from 1mm to 5mm and Reynolds number from 200 to 900. In this paper, the flow distribution was also investigated and reported, as the secondary channel gap, oblique angle and Reynolds number were varied. Based on the 259 numerical data points, multiple correlations for the average Nusselt number and the apparent friction constant were formulated, verified and presented. The average deviation for the approximation of average Nusselt number is from −5.8% to 7.0% while it is from −5.1% to 7.1% for the apparent friction constant. These correlations can be used for further studies such as performance prediction or geometrical optimization.

Experimental investigation on heat transfer and pressure drop of a novel cylindrical oblique fin heat sink

A novel cylindrical oblique fin minichannel heat sink, in the form of an enveloping jacket, was proposed to be fitted over cylindrical heat sources. The presence of the oblique fins disrupts and reinitializes the boundary layer development at the leading edge of each fin. This results in a significant reduction of the boundary layer thickness and causes the flow to remain in the developing state. Experimental investigations were conducted to compare its heat transfer performance with conventional straight fin minichannel heat sink. The test pieces were fabricated from copper and measurements on the heat transfer characteristics were performed for Reynolds number ranging from 50 to 500. In addition, the effects of flow distribution were examined and it was found that cylindrical oblique fin structure eliminates the edge effect which is present in the planar oblique fin configuration. The uniform secondary flow generated by the cylindrical oblique fin structure improves fluid mixing and enhances heat transfer significantly. The experimental results showed that the average Nusselt number for the cylindrical oblique fin minichannel heat sink increases up to 75.6% and the total thermal resistance decreases up to 59.1% compared to the conventional straight fin heat sink. For a heat flux of 6.1 W/cm2 and Reynolds number of 300, the average surface temperature of cylindrical heat sink is reduced by 4.3 °C compared to conventional straight fin heat sink while the required pumping power remains comparable.

Hotspot Mitigating With Obliquely Finned Microchannel Heat Sink—An Experimental Study

Sectional oblique fins are employed, in contrast to continuous fins, in order to modulate the flow in a microchannel heat sink. The breakage of continuous fin into oblique sections leads to reinitialization of boundary layers and generation of secondary flows that significantly enhance the cooling performance of the heat sink. In addition, an oblique finned microchannel heat sink has the flexibility to tailor local heat transfer performance by varying its oblique fin pitch. Clusters of oblique fins at higher density can be created in order to promote a greater degree of boundary layer redevelopment and secondary flow generation to provide more effective cooling at the high heat-flux region. Thus, the variation of oblique fin pitch can be exploited for hotspot mitigation. Experimental studies of a silicon chip with two hotspot scenarios show that the temperature hike and the temperature difference for the enhanced microchannel heat sink with variable pitch are reduced by as much as 17.1 °C and 15.4 °C, respectively. As a result, temperature distribution across the silicon chip is more uniform. In addition, the associated pressure drop penalty is much smaller than the achieved heat transfer enhancement, rendering it as an effective hotspot mitigating strategy for the single-phase microchannel heat sink.

Ultra-thin minichannel LCP for EV battery thermal management

The development of electric vehicles (EVs) demands for complementary technologies in battery thermal management. To achieve fast charging/discharging capacity, liquid cooling is an effective means of maintaining temperature of a battery in operation within a narrow optimal range. In conventional straight channels, convective heat transfer deteriorates along the axial direction with the development of the hydrodynamic boundary layer, resulting in elevated maximum temperature and significant temperature gradient in the fully developed region. This is a serious problem as temperature uniformity is of utmost importance to the performance and lifespan of a Li–ion battery. In this research, a simple configuration of oblique cuts across the straight fins of a conventional straight channel design was developed, to enhance the performance of the conventional channel with minimal pressure penalty. These oblique cuts across the straight fins form an oblique fin array. The designed liquid cold plate (LCP) contains these simple oblique fins with optimized angle and width. This segmentation of the continuous fin into oblique sections leads to the re-initialization of boundary layers, providing a solution to the elevated temperatures caused by a thick boundary layer in the fully developed region. Experimental results show that heat transfer coefficients of oblique minichannel are higher than those of conventional straight minichannel. The oblique LCP is able to maintain the battery surface average temperature below 50°C for a heat load of 1240W at a flow rate lower than 0.9l/min. This implies that a proper designed minichannel cold plate could be a promising solution for EV battery thermal management.

Numerical Study of Fluid Flow and Heat Transfer in the Enhanced Microchannel With Oblique Fins

Sectional oblique fins are employed in contrast to continuous fins in order to modulate flow in microchannel heat sink. The breakage of continuous fin into oblique sections leads to the reinitialization of both hydrodynamic and thermal boundary layers at the leading edge of each oblique fin, effectively reducing the thickness of boundary layer. This regeneration of entrance effect causes the flow to be always in a developing state thus resulting in better heat transfer. In addition, the presence of smaller oblique channels diverts a small fraction of flow into the adjacent main channels. The secondary flows thus created improve fluid mixing which serves to further enhance the heat transfer. Detailed numerical study on the fluid flow and heat transfer of this passive heat transfer enhancement technique provides insight to the local hydrodynamics and thermal development along the oblique fin. The uniquely skewed hydrodynamic and thermal profiles are identified as the key to the highly augmented and uniform heat transfer performance across the heat sink. The associated pressure drop penalty is much smaller than the achieved heat transfer enhancement, rendering it as an effective heat transfer enhancement scheme for single phase microchannel heat sink.

A simulation and experimental study of fluid flow and heat transfer on cylindrical oblique-finned heat sink

A novel cylindrical oblique fin minichannel heat sink was proposed to fit over cylindrical heat sources in the form of an enveloping jacket. The periodic cylindrical oblique fin causes the hydrodynamic boundary layer development to be reinitialized at the leading edge of the next downstream fin. This decreases the average thermal boundary layer thickness, enhances the heat transfer performance and yields negligible pressure drop penalty due to combined effect of thermal boundary layer re-development and flow mixing. Its cooling effectiveness is compared with conventional straight fin minichannel heat sinks through experimental and numerical approach for the Reynolds number ranged from 50 to 500. The results showed that the averaged Nusselt number, Nuave for the cylindrical oblique-cut fin minichannel heat sink increases up to 75.6% and the total thermal resistance decreases up to 59.1% when compared with the conventional straight fin minichannel heat sink. It is also found firstly that a flow recirculation zone will form at larger Reynolds number in the secondary channel however this recirculation is insignificant in the present low Reynolds number study. Heat transfer enhancement (ENu) and pressure drop penalty (Ef) show that a significant improvement of the cylindrical oblique fin minichannel over conventional straight fin minichannel overall.

Enhanced Thermal Transport in Microchannel Using Oblique Fins

Sectional oblique fins are employed, in contrast to continuous fins in order to modulate the flow in microchannel heat sinks. The breakage of a continuous fin into oblique sections leads to the reinitialization of the thermal boundary layer at the leading edge of each oblique fin, effectively reducing the boundary layer thickness. This regeneration of entrance effects causes the flow to always be in a developing state, thus resulting in better heat transfer. In addition, the presence of smaller oblique channels diverts a small fraction of the flow into adjacent main channels. The secondary flows created improve fluid mixing, which serves to further enhance heat transfer. Both numerical simulations and experimental investigations of copper-based oblique finned microchannel heat sinks demonstrated that a highly augmented and uniform heat transfer performance, relative to the conventional microchannel, is achievable with such a passive technique. The average Nusselt number, Nuave, for the copper microchannel heat sink which uses water as the working fluid can increase as much as 103%, from 11.3 to 22.9. Besides, the augmented convective heat transfer leads to a reduction in maximum temperature rise by 12.6 °C. The associated pressure drop penalty is much smaller than the achieved heat transfer enhancement, rendering it as an effective heat transfer enhancement scheme for a single-phase microchannel heat sink.

Computational fluid dynamics (CFD) analysis of micro-reactor performance: Effect of various configurations

The objective of this study is to examine via mathematical modeling several microreactor configurations which may yield enhanced overall performance. A computational fluid dynamic analysis was carried out for a microreactor in the form of a square cross-section channel of eight different configurations. These include parallel, pin-hole, wavy, oblique fin, serpentine, coiled, coiled with serpentine and coiled with double serpentine channel geometries. The performance of these micro-channels in terms of the flow field and reaction rates was investigated numerically using a commercial package Fluent, and compared with that of a conventional straight microchannel of the same cross-section as well as other reactor geometries reported in the literature. This study was carried out for a single phase catalytic reaction between methane over a range of Reynolds numbers and selected geometric parameters. The reactor performance was compared in terms of the figure of merit (FoM) based on reaction throughput per unit pumping power and catalyst active area. The pinhole channel design was found to give the best overall performance over the range of Reynolds numbers studied. The effect was more pronounced at higher Reynolds numbers. It was observed that for each geometry a higher Reynolds number results in lower FoM. The relative advantages and limitations of the various micro-channel configurations examined are discussed in the light of the numerical results obtained.