Three‐dimensional computational comparison of mini pinned heat sinks using different nanofluids: Part one—the hydraulic‐thermal characteristics

The hydraulic‐thermal characteristics of 3D pinned heat sink designs have been numerically compared as the first part of a three‐part investigation. Five different pin geometries (circular, square, triangular, strip, and elliptic pins) and an unpinned heat sink with three types of nanofluids (Al₂O₃–H₂O, SiO₂–H₂O, and CuO–H₂O) are considered for laminar forced convection. The range of Reynolds number is from 100 to 1000, and volume fractions vary between 0% and 5%. The finite volume method is employed to solve the Navier–Stokes and energy equations by employing a SIMPLE algorithm for a computational solution. Three parameters are presented—the Nusselt number, the bottom temperature, and the hydrothermal performance of the heat sink with pressure drop data. The findings indicated that the overall hydrothermal performance of elliptic‐pinned (EP) heat sinks produces the most substantial value of 3.10 for pure water. For different nanofluids, the SiO₂–water nanofluids with EPs have the most significant hydrothermal performance. Also, this factor is enhanced with an increase in nanofluid concentration up to nearly 3.34 for 5% of SiO₂–water. Consequently, applying the elliptic‐pinned heat sinks is recommended with pure water for considering an increase in the pressure drop, with 5% of SiO₂–water nanofluids, regardless of an enlargement of pressure drop for heat‐dissipation applications.     

Three‐dimensional computational comparison of mini‐pinned heat sinks using different nanofluids: Part two—energy and exergy characteristics

The energy and exergy characteristics of 3D‐pinned heat sink (HS) designs have been computationally compared as the second part of a three‐part investigation. Different pin profiles, such as circular, square, triangular, strip and elliptic pins, and without pin HS are conducted with three different types of nanofluids—Al₂O₃‐water, SiO₂‐water, and CuO‐water for laminar forced convection. The concentrations of nanofluids vary from 0 to 5 vol% with different Reynolds numbers ranging between 100 and 1000. The finite volume method employing the SIMPLE algorithm for a computational solution is applied to solve the Navier–Stokes and energy equations. Four criterions studies are explained—energy efficiency, exergy loss, and exergy efficiency of HSs with pressure drop. The results showed that the highest energy and exergy efficiencies are nearly 76% and 57%, respectively, for elliptic‐pinned HSs using pure water, while about 82% and 62% using 5 vol% of SiO₂‐water nanofluids. Besides, the elliptic‐pinned HSs have a favorable reduction in the exergy loss, nearly 17% using 5 vol% of SiO₂‐water nanofluids. Subsequently, the elliptic‐pinned HS is recommended to apply with pure water considering the development in pressure drop required. However, the elliptic‐pinned HSs could be employed with 5 vol% of SiO₂‐water nanofluids regardless of the development in pressure drop required for thermal energy dissipation applications with more exergy efficiency and reduction of exergy loss.     

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.     

Natural convection heat transfer from a heat sink with hollow/perforated circular pin fins

Experiments were performed on natural convection heat transfer from circular pin fin heat sinks subject to the influence of its geometry, heat flux and orientation. The geometric dependence of heat dissipation from heat sinks of widely spaced solid and hollow/perforated circular pin fins with staggered combination, fitted into a heated base of fixed area is discussed. Over the tested range of Rayleigh number, 3.8 × 10⁶ ≤ Ra ≤ 1.65 × 10⁷, it was found that the solid pin fin heat sink performance for upward and sideward orientations shows a competitive nature, depending on Rayleigh number and generally shows higher heat transfer coefficients than those of the perforated/hollow pin fin ones in both arrangement. For all tested hollow/perforated pin fin heat sinks, however, the performance for sideward facing orientation was better than that for upward facing orientation. This argument is supported by observing that the augmentation factor was around 1.05–1.11, depending on the hollow pin diameter ratio, D_i/D_o. Meanwhile, the heat sink of larger hollow pin diameter ratio, D_i/D_o offered higher heat transfer coefficient than that of smaller D_i/D_o for upward orientation, and the situation was reversed for sideward orientation. The heat transfer performance for heat sinks with hollow/perforated pin fins was better than that of solid pins. The temperature difference between the base plate and surrounding air of these heat sinks was less than that of solid pin one and improved with increasing D_i/D_o.     

Heat transfer of nanofluids in the mini-rectangular fin heat sinks

In the present study, the heat transfer characteristics of nanofluids cooling in the mini-rectangular fin heat sink are studied. The heat sinks with three different channel heights are fabricated from the aluminum by the wire electrical discharge machine with the length, width and base thickness of 110, 60, and 2 mm, respectively. The nanofluids are the mixture of de-ionized water and nanoscale TiO₂ particles. The results obtained from the nanofluids cooling in mini-rectangular fin heat sink are compared with those from the de-ionized water cooling method. Effects of the inlet temperature of nanofluids, nanofluid Reynolds number, and heat flux on the heat transfer characteristics of mini-rectangular fin heat sink are considered. It is found that average heat transfer rates for nanofluids as coolant are higher than those for the de-ionized water as coolant. The results of this study are of technological importance for the efficient design of cooling systems of electronic devices to enhance cooling performance.     

Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties

Turbulent flow and heat transfer of three different nanofluids (CuO, Al₂O₃ and SiO₂) in an ethylene glycol and water mixture flowing through a circular tube under constant heat flux condition have been numerically analyzed. New correlations for viscosity up to 10% volume concentration for these nanofluids as a function of volume concentration and temperature are developed from the experiments and are summarized in the present paper. In our numerical study, all the thermophysical properties of nanofluids are temperature dependent. Computed results are validated with existing well established correlations. Nusselt number prediction for nanofluids agrees well with Gnielinski correlation. It is found that nanofluids containing smaller diameter nanoparticles have higher viscosity and Nusselt number. Comparison of convective heat transfer coefficient of CuO, Al₂O₃ and SiO₂ nanofluids have been presented. At a constant Reynolds number, Nusselt number increases by 35% for 6% CuO nanofluids over the base fluid.     

Estimation of Nusselt number and effectiveness of double‐pipe heat exchanger with Al2O3–, CuO–, TiO2–, and ZnO–water based nanofluids

This paper deals with experimental studies carried out to analyze heat transfer characteristics of Al₂O₃–, CuO–, TiO₂–, and ZnO–water based nanofluids in a double‐pipe, counter flow heat exchanger for different volume concentrations (0.025%, 0.05%, 0.075%, and 0.1%) of the nanofluids. The fabricated double‐pipe heat exchanger is made up of two different materials, viz., copper as the inner tube and unplasticized polyvinyl chloride as the outer tube. The density, viscosity, and thermal conductivity were calculated, and were used to estimate dimensionless numbers, such as Reynolds number, Prandtl number, and Nusselt number, and also to estimate heat exchanger effectiveness. High‐energy ball milling technique was used to prepare nanoparticles and were characterized using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Polyvinyl alcohol (3%) was used as a surfactant for making the nanofluids stable. It was observed from the experiment that with the increase in the volume concentration, thermal conductivity, viscosity, and friction factor increase, whereas the Reynolds number decreases. The experimentally observed data for Nusselt number were formulated into a correlation that matches the data for all these nanofluids within an error of 11.4%. It was found that the highest effectiveness was obtained while using TiO₂–water nanofluids than other nanofluids.     

Numerical Evaluation of Flow and Heat Transfer in Plate-Pin Fin Heat Sinks with Various Pin Cross-Sections

A numerical investigation of the thermal and hydraulic performance of 20 different plate-pin fin heat sinks with various shapes of pin cross-sections (square, circular, elliptic, NACA profile, and dropform) and different ratios of pin widths to plate fin spacing (0.3, 0.4, 0.5, and 0.6) was performed. Finite volume method-based CFD software, Ansys CFX, was used as the 3-D Reynolds-averaged Navier-Stokes Solver. A k-ω based shear-stress-transport model was used to predict the turbulent flow and heat transfer through the heat sink channels. The present study provides original information about the performance of this new type of compound heat sink.     

Study of heat transfer enhancement in a nanofluid-cooled miniature heat sink

This paper reports numerical solution for thermally developing temperature profile and analytical solution for fully developed velocity profile in a miniature plate fin heat sink with SiO₂–water nanofluid as coolant. The flow regime is laminar and Reynolds number varies between 0 and 800. The heat sink is modeled using porous medium approach. Modified Darcy equation for fluid flow and the two-equation model for heat transfer between the solid and fluid phases are employed to predict the local heat transfer coefficient in heat sink. Results show that the nanofluid-cooled heat sink outperforms the water-cooled one, having a considerable higher heat transfer coefficient. The effects of channel aspect ratio and porosity on heat transfer coefficient of the heat sink are studied in detail. Based on the results of our analysis, it is found that an increase in the aspect ratio or the porosity of the plate fin heat sink enhances the heat transfer coefficient.     

Numerical investigation of hydrothermal performance over perforated conical pin heat sinks

Over the past few decades, researchers have shown significant interest in enhancing the thermal efficiency of heat sinks while simultaneously increasing the power generation capacity of electronic devices and reducing their size. In this study, the focus lies on the originality of employing conical perforated pin heat sinks with multiple perforations (N = 0, 1, 2, and 3) and various conical pins inclination angles (Φ = 0°, 1°, 2°, 3°, and 4°). The study aimed to numerically investigate the effects of a perforated conical pin and cone inclination angle on heat transfer, pressure drop, CPU temperature, and hydrothermal performance (HTP) across the heat sinks using a three-dimensional, turbulent flow as k–ω SST model combined with the thermal conjugate model. A validated CFD model is employed to conduct a parametric analysis of the effects of the quantity and placement of circular holes. A summary of the results reveals that Model B3 exhibited the highest HTP value, reaching approximately 1.15 at U = 10 m/s, with a commendable reduction in heat sink mass of over 18%. Ultimately, the perforated conical pin heat sink demonstrates the potential to fulfill the primary objective of this investigation, which is achieving an overall improvement in Nusselt number, CPU temperature, pressure drop, and reduced heat sink mass.     

Heat Transfer Performance of Heat Pipe Radiator with SiO2/Water Nanofluids

The effect of nanofluids on thermal performance of the miniature heat pipe radiator which was assembled by two heat pipes containing 0.6 vol.% SiO₂/water nanofluids and 30 pieces of rectangular aluminum fins was investigated experimentally and theoretically. The wall temperatures of the miniature heat pipe and fin surface temperatures were measured. Results showed that the utilization of SiO₂/water nanofluids as a working fluid in the heat pipe enhanced the heat performance by reducing wall temperature differences. Compared with Deionized water (DI water), the thermal resistance of the miniature heat pipe with SiO₂/water nanofluids decreased by about 23% to 40%. Furthermore, the theoretical calculation on a basis of one dimension found that the fin heat dissipation in the miniature heat pipe radiator charged SiO₂/water nanofluids was about 1.17 times of that of the DI water radiator.     

Heat transfer augmentation in concentric elliptic annular by ethylene glycol based nanofluids

In this article, laminar mixed convective heat transfer at different nanofluids flow in an elliptic annulus with constant heat flux boundary condition has been numerically investigated. The three dimensional governing equations (continuity, momentum and energy) are solved using the finite volume method (FVM). The investigation covers Reynolds number and nanoparticle volume fraction in the ranges of 200–1000 and 0–4% respectively. In the present work, four different types of nanofluids are examined in which Al₂O₃, CuO, SiO₂ and ZnO are suspended in the base fluid of ethylene glycol (EG) with different nanoparticle sizes 20, 40, 60 and 80 nm. The results show that SiO₂-EG nanofluid has the highest Nusselt number, followed by Al₂O₃-EG, ZnO-EG, CuO-EG, and lastly pure ethylene glycol. The Nusselt number increased as the nanoparticle volume fraction and Reynolds number increased; however, it decreased as the nanoparticle diameter increased. It is found that the glycerine-SiO₂ shows the best heat transfer enhancement compared with other tested base fluids. Comparisons of the present results with those available in the literature are presented and discussed.     

Thermal performance of finned aluminum heat sink filled with ERG aluminum foam: Experimental and numerical approach

The rapid improvements in electronic devices have led to a high demand for effective cooling techniques. The purpose of this study was to investigate the heat transfer characteristics and performance of different aluminum heat sinks filled with aluminum foam for an Intel core i7 processor. The aluminum foam heat sinks were subjected to water flow covering the non-Darcy flow regime (300-600 Reynolds numbers). The bottom side of the heat sinks was heated with a heat flux between 8.5 and 13.8 W/cm². Three different heat sinks were examined in this study. Models A, B, and C contained two, three and four channels, respectively. Each channel gap was filled with ERG aluminum foam. The distributions of the local surface temperature and the local Nusselt number were measured for each heat sink design. The experimental data were compared with the numerical results. The average Nusselt number was obtained for the range of Reynolds numbers, and an empirical correlation of the average Nusselt number as a function of the Reynolds number was derived for each heat sink. The pressure drop across the characteristics of each heat sink design was measured. The thermal performance of each aluminum foam heat sink was evaluated based on the average Nusselt number and the required pumping power. The experimental results revealed that model B achieved the highest average Nusselt number compared with models A and C. However, model C had the highest surface to volume ratio; the thermal boundary layers, which are formed on adjacent fin surfaces inside the aluminum foam, interface with each other causing a reduction in the overall heat transfer. The numerical results were in good agreement with experimental data of local Nusselt number and local temperature with maximum relative errors of 2% and 1%, respectively.     

A comparative study of the airside performance of heat sinks having pin fin configurations

This study performs an experimental study of pin fin heat sinks having circular, elliptic, and square cross-section. A total of twelve pin fin heat sinks with inline and staggered arrangements were made and tested. The effect of fin density on the heat transfer performance is examined. For an inline arrangement, the circular pin fin shows an appreciable influence of fin density whereas no effect of fin density is seen for square fin geometry. This is associated with the unique deflection flow pattern accompanied with the inline circular fin configuration. For the staggered arrangement, the heat transfer coefficient increases with the rise of fin density for all the three configurations. The elliptic pin fin shows the lowest pressure drops. For the same surface area at a fixed pumping power, the elliptic pin fin possesses the smallest thermal resistance for the staggered arrangement.     

Heat transfer enhancement for combined convection flow of nanofluids in a vertical rectangular duct considering radiation effects

In this paper, combined convective heat transfer and nanofluids flow characteristics in a vertical rectangular duct are numerically investigated. This investigation covers Rayleigh numbers in the range of 2 × 10⁶ ≤ Ra ≤ 2 × 10⁷ and Reynolds numbers in the range of 200 ≤ Re ≤ 1000. Pure water and five different types of nanofluids such as Ag, Au, CuO, diamond, and SiO₂ with a volume fraction range of 0.5% ≤ φ ≤ 3% are used. The three‐dimensional steady, laminar flow, and heat transfer governing equations are solved using finite volume method (FVM). The effects of Rayleigh number, Reynolds number, nanofluids type, nanoparticle volume fraction of nano‐ fluids, and effect of radiation on the thermal and flow fields are examined. It is found that the heat transfer is enhanced using nanofluids by 47% when compared with water. The Nusselt number increases as the Reynolds number and Rayleigh number increase and aspect ratio decreases. A SiO₂ nanofluid has the highest Nusselt number and highest wall shear stress while the Au nanofluid has the lowest Nusselt number and lowest wall shear stress. The results also revealed that the wall shear stress increases as Reynolds number increases, aspect ratio decreases, and nanoparticle volume fraction increases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20354     

Exergo‐economic analysis of micro pin fin heat sinks

A parametric study of thermoeconomic performance over four micro pin fin heat sinks of different spacing and shapes was conducted. Unit cost per product exergy, relative cost difference, and exergo‐economic factor were utilized to evaluate the thermoeconomic performance. The effect of working fluid on the thermoeconomic performance was also investigated using R‐123 and water as working fluids. Unit costs per product exergy were obtained to evaluate the product costs (total exergy change between exit and inlet streams) in micro pin fin heat sinks at fixed mass flow rate and fixed pressure drop. The results of the thermoeconomic analysis were compared with the results of a past exergy performance study by the author. In the light of raw experimental data acquired from the past studies of the author, important differences between the results of exergy and exergo‐economic performances were observed. It was found that the unit cost of exergy change decreased as electrical power increased and the relative cost difference approached to unity at high electrical powers (greater than 20 W). Moreover, high exergo‐economic factor values (more than 0.5) were obtained at low electrical powers while exergo‐economic factors had a small value at high electrical powers. When looking at the effect of the working fluid, higher cost per Watts of the products (up to the double of R‐123) was obtained with water compared with R‐123 at both fixed mass flow rate and pressure drop. No significant effect of pin fin spacing on the unit cost of exergy change was observed at fixed mass flow rate, while higher unit costs (up to 102%) were recorded at fixed pressure drop for scarcely packed pin fin heat sinks. Finally, the unit cost of exergy change was found to be independent of pin fin shape at fixed mass flow rate, whereas at fixed pressure drop, the hydrofoil‐based pin fin heat sink had higher unit costs (up to 1.8 times as much) when compared with the unit costs of pin fin heat sinks having flow separation promoting pin fins. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal performance of plate-fin vapor chamber heat sinks

Since vapor chambers exhibit excellent thermal performance, they are suited to use as bases of heat sinks. This work experimentally studies the thermal performance of plate-fin vapor chamber heat sinks using infrared thermography. The effects of the width, height and number of fins and of the Reynolds number on the thermal performance are considered. Experimental data are compared with corresponding data for conventional aluminum heat sinks. The results show that generated heat is transferred more uniformly to the base plate by a vapor chamber heat sink than by a similar aluminum heat sink. Therefore, the maximum temperature is effectively reduced. The overall thermal resistance of the vapor chamber heat sink declines as the Reynolds number increases, but the strength of the effect falls. The effect of the fin dimensions on the thermal performance is stronger at a lower Reynolds number. At a low Reynolds number, a suitable number of fins must be chosen to ensure favorable thermal performance of the vapor chamber heat sink. However, at a high Reynolds number, the thermal performance improves as the fin number increases.     

Numerical investigation of thermal and hydraulic performance in novel oblique geometry using nanofluid

To capitalize the advantage of oblique fin heat sink (OFHS) with Al₂O₃–water nanofluids of different volumetric concentration (1, 2, and 4%), a comprehensive computational analysis has been performed for OFHS with nanofluid through the single-phase modeling. The present investigation focuses on the full domain simulation because the conventional periodic computational model approach is unable to investigate the flow migration effect and predicts higher value of Nusselt number. Apart from the disruption of boundary layer, vortices are observed in the secondary oblique channel due to flow separation that promotes an additional heat transfer enhancement. Higher Severity of the flow migration and hence more non-uniformity of nanofluid flow rate through the primary and secondary channels was observed at higher Reynolds numbers. The increment observed in the average Nusselt number (Nu_avg) at Re = 750 for OFHS is about 90% and 115% for water and 4% volumetric concentration of nanofluid respectively compared to conventional SCHS. Also, Al₂O₃–water nanofluid exhibits about 30% higher enhancement at 4% volumetric concentration at Re = 750 in the OFHS with compared to water. The increase in heat transfer exceeded the pressure drop penalty at all the Reynolds numbers.     

Heat transfer and nanofluid flow characteristics through a circular tube fitted with helical tape inserts

Numerical investigations are performed using finite volume method to study laminar convective heat transfer and nanofluids flows through a circular tube fitted with helical tape insert. The wall of tube was subjected to a uniform heat flux boundary condition. The continuity, momentum and energy equations are discretized and the SIMPLE algorithm scheme is applied to link the pressure and velocity fields inside the domain for plain tube. Four different twist ratios of 1.95–4.89, two different types of nanoparticles, Al₂O₃ and SiO₂ with different nanoparticle shapes of spherical, cylindrical and platelets, and 0.5–2.0% volume fraction in base fluid (water) and nanoparticle diameter in the range of 20–50 nm were used to identify their effect on the heat transfer and fluid flow characteristics through a circular tube fitted with helical tape insert geometries. The results indicate that the four types of nanofluid achieved higher Nusselt number than pure water. Nanofluid with Al₂O₃ particle achieved the highest Nusselt number. For all the cases studied, the Nusselt number increased with the increase of Reynolds number and with the decrease of twist ratio of helical tape insert.     

Effect of CuO-, SiO2-, and Al2O3-based nanofluids on automotive cooling systems from laminar to turbulent flow regime—A CFD study

In recent studies, much attention has been given to nanofluids suggesting that adding nanoparticles in base fluids offers a higher heat transfer rate compared with conventional fluids. This study is based on the numerical investigation of different types of nanofluids, consisting of CuO (50 nm), SiO₂ (40 nm), and Al₂O₃ (15 nm) nanoparticles at different volume concentrations. Several simulations were performed from low to high Reynolds numbers, corresponding to laminar and turbulent flow regimes using ANSYS-Fluent CFD solver. Results suggest that under a laminar flow regime with the same Reynolds number of 2000, CuO-based nanofluids perform better as compared with SiO₂ and Al₂O₃-based nanofluids with Nusselt number (Nu) having percentage increase of 90% and 60% comparing with SiO₂- and Al₂O₃-based nanofluids, respectively. However, at higher Reynolds numbers when the flow is turbulent, Al₂O₃-based nanofluids demonstrate better performance having a percentage increase in Nusselt numbers equal to 40% and 23% as compared with CuO and SiO₂-based nanofluids respectively under the same Reynolds number of 15,000. This implies that turbulence has a significant effect on heat transfer rate, and is not only related to thermal conductivity. This study will help in designing more compact cooling systems for engines and the internal environment of motor vehicles.