Heat transfer and pressure loss characteristics of very compact heat sinks

High-performance compact heat sinks have been developed for the effective cooling of high-density LSI packaging. Heat transfer and pressure loss characteristics of the heat sinks in both air-cross-flow and air-jet cooling have been experimentally studied. The present heat sinks were of plate-fin and pin-fin arrays with a fin pitch of 0.7 mm. The plate-fin heat sinks had higher cooling performance than the pin-fin heat sinks in the range of large airflow rates both in air-cross-flow and air-jet cooling. The thermal conductance in cross-flow cooling was 20 or 40% larger than that in jet cooling. The correlation of Colburn j-factor/Fanning friction factor versus the Reynolds number for the present heat sinks was found to be very close to that of a conventional large-size heat exchanger. © Scripta Technica, Heat Trans Asian Res, 28(8): 687-705, 1999     

Heat transfer correlations for PCM-based heat sinks with plate fins

Characterization of melting process in a Phase Change Material (PCM)-based heat sink with plate fin type thermal conductivity enhancers (TCEs) is numerically studied in this paper. Detailed parametric investigations are performed to find the effect of aspect ratio of enclosure and the applied heat flux on the thermal performance of the heat sinks. Various non-dimensional numbers, such as Nusselt number (Nu), Rayleigh number (Ra), Stefan number (Ste) and Fourier number (Fo) based on a characteristic length scale, are identified as important parameters. The half fin thickness and the fin height are varied to obtain a wide range of aspect ratios of an enclosure. It is found that a single correlation of Nu with Ra is not applicable for all aspect ratios of enclosure with melt convection taken into account. To find appropriate length scales, enclosures with different aspect ratios are divided into three categories, viz. (a) shallow enclosure, (b) rectangular enclosure and (c) tall enclosure. Accordingly, an appropriate characteristic length scale is identified for each type of enclosure and correlation of Nu with Ra based on that characteristic length scale is developed.     

Experimental study of impingement cooling by heat sinks with thin longitudinal fins for LSI packages

This paper reports on the impingement cooling characteristics of a heat sink with thin longitudinal fins of 0.2 mm thickness, which are spaced with a fin-pitch in the range 0.5 mm to 2.0 mm. The air cooling of the heat sink comes from a slot-shaped orifice positioned above the heat-sink center. The breadth of the gap between the fin tops and the inlet orifice is in the range 0 mm to 10 mm. The thermal resistance of the thin longitudinal fins used is about 50% to 57% that of the thick longitudinal fins now in commercial use. The cooling performance of the thin-plate fins is almost the same as that of optimally arranged pin-fins with the same total surface area. A maximum value of six times the heat transfer rate of a single flat plate having the same base area was observed for the thin-plate fins. A comparison of cooling performance between impingement and channel flow systems was conducted. The performance of impingement cooling systems is almost unaffected by the breadth of the gap between the fin tops and the inlet orifice (or, for channel cooling, the upper wall). On the other hand, the performance of channel-cooling systems decreases significantly as the gap widens. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(7): 449–459, 1996     

Laminar convective heat transfer in a microchannel with internal longitudinal fins

A numerical study was conducted to investigate the fluid flow and heat transfer characteristics of a square microchannel with four longitudinal internal fins. Three-dimensional numerical simulations were performed on the microchannel with variable fin height ratio in the presence of a hydrodynamically developed, thermally developing laminar flow. Constant heat flux boundary conditions were assumed on the external walls of the square microchannel. Results of the average local Nusselt number distribution along the channel length were obtained as a function of the fin height ratio. The analysis was carried out for different fin heights and flow parameters. Interesting observations that provide more physical insight on this passive enhancement technique, and the existence of an optimum fin height are brought out in the present study.     

Thermal-fluid characteristics of plate-fin heat sinks cooled by impingement jet

This work experimentally and numerically studies the thermal-fluid characteristics of plate-fin heat sinks under impingement cooling by adjusting the impinging Reynolds number, the impingement distance, and the fin dimensions. The parameters and the ranges under consideration are the impinging Reynolds number (Re = 5000–25,000), the impingement distance (Y/D = 4–28), the fin width (W/L = 0.08125–0.15625) and the fin height (H/L = 0.375–0.625). The results show that the heat transferred by the heat sink increases with the impinging Reynolds number. The thermal performance can be improved significantly even at low impinging Reynolds number. However, the improvement becomes indistinct as the impinging Reynolds number increases. The thermal resistance declines as the impingement distance increases, and is minimal at Y/D = 20 for various impinging Reynolds numbers. Additionally, the thermal resistance increases as the impingement distance increases further. Increasing the fin width can effectively reduce the thermal resistance. However, as the fin width increases beyond a particular value, the thermal resistance increases dramatically. Reducing the thermal resistance by increasing the fin height depends on a suitable impinging Reynolds number and fin width. Therefore, the effect of the fin height is weaker than that of the impinging Reynolds number or the fin width.     

Heat transfer and pressure drop correlations for compact heat exchangers with multi-region louver fins

An experimental study on the air-side heat transfer and pressure drop characteristics for brazed aluminum heat exchangers with multi-region louver fins and flat tubes was conducted. A series of tests were conducted for heat exchangers with different numbers of louver regions at the air-side Reynolds numbers of 400–1600 based on the louver pitch. The air-side thermal performance data were analyzed by using the effectiveness-NTU method. The characteristics of the heat transfer and pressure drop for heat exchangers with different geometry parameters were presented in terms of the Colburn j factor and Fanning friction f factor as function of the Reynolds number.     

Heat transfer analysis of lateral perforated fin heat sinks

In this article fluid flow and conjugate conduction-convective heat transfer from a three-dimensional array of rectangular perforated fins with square windows that are arranged in lateral surface of fins are studied numerically. For investigation, Navier–Stokes equations and RNG based k − ε turbulent model are used. Finite volume procedure with SIMPLE algorithm is applied to coupled differential equations for both solid and gas phases. Computations are carried out for Reynolds numbers of 2000–5000 based on the fin thickness and Pr = 0.71. Numerical model is first validated with previous experimental studies and good agreement were observed. Based on a valid numerical model, numerical solution is made to find fluid flow and temperature distribution for various arrangements. For each type, fin efficiency of perforated fins is determined and compared with the equivalent solid fin. Results show that new perforated fins have higher total heat transfer and considerable weight reduction in comparison with solid fins.     

Convective heat transfer and pressure drop of a tube with internal longitudinal fins

The fluid flows and heat transfer characteristics of a tube with internal longitudinal fins were investigated experimentally and numerically. The realizable turbulence model was adopted to simulate the problem, whose results indicate a good agreement with the measured data. Compared with the circular annulus tube, it was shown that the internal longitudinal finned tube provides excellent heat transfer performance greater than those of the circular annulus tube, with a great increase of simultaneous pressure drops. Furthermore, there exists a critical Reynolds number, about 1500, when the Reynolds number is less than the critical value, and the Nusselt number of an internal longitudinal finned tube will be smaller than that of a circular annulus tube in laminar flow. On the other hand, the transition Reynolds number for a tube with internal longitudinal fins from laminar to turbulent is greatly decreased due to the existence of internal longitudinal fins. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 57–65, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20147     

Study of impingement cooling of heat sinks for LSI packages with longitudinal fins

This paper describes an experimental and a semi-empirical study on the impingement cooling characteristics of heat sinks with longitudinal fins of a type suitable for LSI packages. The experiments were performed with a variety of different fins. To enhance impingement cooling, one long rectangular inlet orifice (slit) over the center of the heat sink was found to offer the best structure. The optimum orifice width is about 1/6 of the base width of the heat sink. The thermal resistance at a fixed volumetric flow rate and orifice width varies little with size of the gap between the fin tops and inlet orifice, but gaps near 2 mm slightly lower the resistance. Correlations are proposed between the thermal resistance of the heat sink and the geometry of the longitudinal fins. The accuracy of the predicted thermal resistances was found to be within ±25% of the experimental data. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(8): 537–553, 1996     

Studies on pumping power in terms of pressure drop and heat transfer characteristics of compact plate-fin heat exchangers—A review

Renewable energy sources like solar energy, wind energy, etc. are profusely available without any limitation. Heat exchanger is a device to transfer the energy from one fluid to other fluid for many applications in buildings, industries and automotives. The optimum design of heat exchanger for minimum pumping power (i.e., minimum pressure drop) and efficient heat transfer is a great challenge in terms of energy savings point of view. This review focuses on the research and developments of compact offset and wavy plate-fin heat exchangers. The review is summarized under three major sections. They are offset fin characteristics, wavy fin characteristics and non-uniformity of the inlet fluid flow. The various research aspects relating to internal single phase flow studied in offset and wavy fins by the researchers are compared and summarized. Further, the works done on the non-uniformity of this fluid flow at the inlet of the compact heat exchangers are addressed and the methods available to minimize these effects are compared.     

Convective heat transfer and pressure drop of annular tubes with three different internal longitudinal fins

Pressure drop and heat transfer characteristics of air in three annular tubes with different internal longitudinal fins were investigated experimentally at uniform wall heat flux. The tested tubes have a double‐pipe structure with the inner blocked tube as an insertion. Three different kinds of fins, plain rectangle fin, plain rectangle fin with periodical ridges and wave‐like fin, were located peripherally in the annulus. The friction factor and Nusselt number can be corrected by a power‐law correction in the Reynolds number range tested. It was found that the tube with periodical ridges on the plain fin or with wave‐like fin could augment heat transfer; however, the pressure drop was increased simultaneously. In order to evaluate the comprehensive heat transfer characteristics of the tested tubes, two criteria for evaluating the comprehensive thermal performance of tested tubes were adopted. They are: 1) evaluating the comprehensive heat transfer performance under three conditions: identical mass flow, identical pumping power, and identical pressure drop; 2) the second law of thermodynamics, i.e., the entropy generation. According to the two different evaluating methods, it was found that the tube with wave‐like fins provided the most excellent comprehensive heat transfer performance among the three tubes, especially when it was used under higher Reynolds number conditions. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 29–40, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20186     

Heat transfer and flow characteristics of offset fins in the low-Reynolds-number region

The characteristics of heat exchangers with offset-type plate fins for space stations are studied for Reynolds numbers less than 300 based on the hydraulic diameter. A three-dimensional analysis is carried out to study the effects of the following parameters on the heat transfer and the flow characteristics: (a) the thermal boundary layer developing on the bottom plate and on the fins on the plate, (b) the aspect ratio (height/pitch) of the cross section of the flow passage, the fin thickness, the fin length in the direction of the flow, the thermal conductivity of the fluid and the fins, and the Prandtl number of the fluid. The results obtained are as follows. (1) The heat-transfer coefficient on the fin surface is characterized by the thermal-conductivity ratio of fluid to fin material. When the thermal conductivity of the fin material approaches that of the fluid, the heat-transfer coefficient on the fin surface becomes low. (2) The optimum condition of the aspect ratio depends on the value of the thermal-conductivity ratio between the fluid and the fins. (3) When the aspect ratio becomes large or small, the friction factor of offset fins approaches that of fully developed duct flow with the same aspect ratio as the Reynolds number decreases. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 249–261, 1997     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

The effect of flow field and turbulence on heat transfer characteristics of confined circular and elliptic impinging jets

The flow field of confined circular and elliptic jets was studied experimentally with a Laser Doppler Anemometry (LDA) system. In addition, heat transfer characteristics were numerically investigated. Experiments were conducted with a circular jet and an elliptic jet of aspect ratio four, jet to target spacings of 2 and 6 jet diameters, and Reynolds number 10 000. The toroidal recirculation pattern was observed in the outflow region for both geometries at dimensionless jet to plate distance 2. Higher spreading rates in the minor axis direction of the elliptic jet have also been mapped. Along the target plate, different boundary layer profiles were obtained for circular and elliptic jets at H/d=2, but profiles became similar when dimensionless jet to plate distance was increased to 6. Positions of maximum radial and axial velocities and turbulence intensities have been determined for both geometries. For the confined circular and elliptic jet geometries, analysis of flow field measurements and numerical heat transfer results showed that inner peaks in local heat transfer closely relate to turbulence intensities in the jet and radial flow acceleration along the wall. Differences between the circular and elliptic jet, in terms of flow field and heat transfer characteristics, reduced with increase in the jet to plate distance.     

Thermal intensification of heat transfer characteristics on the plate‐fin heat sink with piezoelectric fan

This study applied the computational fluid dynamic (CFD) code, ANSYS Fluent for simulating the effect a piezoelectric fan installed inside the rectangular channel by numerical simulation method for transient flow field and investigating the influence of each parameter. To remove the disorganized form of energy from the electronic components, the reversible piezoelectric effect is employed to energize the piezoelectric fan. To observe the variation of fan characteristics and to predict the convective heat transfer coefficient, CFD code ANSYS Fluent 15.0 is used. The numerical simulation parameters included are Nusselt number, number of fins (n = 12 and 14), and counter‐shift (inward and outward‐phase), and distance between the upper portion of the fan tip to the front part of the low thermal reservoir. Numerical analysis was carried out to evaluate the effect of thermal flow fields on the heat sink and piezoelectric fan employed in a flow domain. the results showed that by varying the height from channel bottom to the center of piezoelectric fan improves the performance of the piezoelectric fan, piezoelectric fan swinging in a transient phenomena and also simultaneously influences fluid flow behavior on the heat source surface, the fan vibration at counter‐phase has a better rate of heat transfer than vibration in in‐phase.     

Thermal analysis of CPU with CCC and copper base plate heat sinks using CFD

This paper describes experimental and theoretical investigations of heat sinks with different base plate material mounted on CPUs. The thermal model of the computer system with heat sinks which is created using Gambit (for preprocessing) and the simulation is carried out using Fluent (for solver execution and post processing). The following parameters are considered: fin thickness, fin height, and number of fins. Primarily in this paper different base plate thickness and base plate materials are optimized for maintaining the cost and thermal performance of a heat sink. In this research work, the thermal model of the computer system with a slot parallel plate fin heat sink design has been selected, and the fluid flow and thermal flow characteristics of heat sinks are studied. The slot parallel plate fin heat sinks have been used with copper base plates and carbon carbon composite (CCC) base plates to enhance the heat dissipation. The results and conclusion obtained in this present work are found to be in good agreement with numerical results. A complete computer chassis with slot parallel plate heat sinks is investigated varying the thickness of base plate, and the performances of the heat sinks are compared. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20342     

Effect of Pressure Gradient on Heat Transfer of Impinging Submerged Circular Jets

An experimental and numerical study have been carried out to investigate the distribution of radial local heat transfer coefficients of impinging submerged circular jets. Good agreement is achieved between the experimental results and the predicted value. Results show that the outer peak usually occurs at the radial location of r/d= 1.8~2.0, in which transition from laminar to turbulence happens resulting from disappeared pressure gradient abruptly, and that the inner peak appears rigidly at r/d=0.5, where the boundary layer has a minimum thickness because of elevating pressure gradient.     

Numerical study on premixed hydrogen/air combustion characteristics and heat transfer enhancement of micro-combustor embedded with pin fins

Aimed at improving the energy output performance of the Microthermal Photovoltaic (MTPV) system, it is necessary to optimize the structure of the micro combustor. In this paper, micro combustor with in-line pin fins arrays (MCIPF) and micro combustor with both end-line pin fins arrays (MCEPF) were presented to realize the efficient combustion and heat transfer enhancement, and the influence of inlet velocity, equivalent ratio, and materials on thermal performance was investigated. The results showed that pin fins embedding is beneficial to improving combustion, and the combustion efficiency of MCIPF and MCEPF reaches 98.5% and 98.7%, which is significantly higher than that of the conventional cylindrical combustor (MCC). However, with the increase of inlet velocity from 8 m/s to 14 m/s, MCIPF exhibits the highest external wall temperature with a range of (1302–1386 K), while MCEPF maintains the best temperature uniformity. As the inlet velocity increases to 10 m/s, the external wall temperature and temperature uniformity reach the optimum. Besides, under the conditions of different equivalence ratios, both external wall temperature and heat flux increases first and then decreases, meanwhile the temperature uniformity of MCEPF is significantly improved compared with that of MCIPF, they all exhibit the highest external wall temperature with an equivalence ratio of 1.1, and the thermal performance is greatly enhanced. By comparing the heat transfer performance of combustors with different materials based on MCEPF, it is interesting to find that the application of high thermal conductivity materials can not only increase the external wall temperature, but also improve the temperature uniformity. Therefore, materials with high thermal conductivity such as Aluminum, Red Copper and Silicon Carbide should be selected for application in micro combustors and their components. The current work provides a new design method for the enhanced heat transfer of the micro combustor.     

Forced convective heat transfer by swirling impinging jets issuing from nozzles equipped with twisted tapes

Forced convective heat transfer on the impinged plate associated with swirling impinging jets (SIJ) issuing from nozzles inserted by twisted tapes has been investigated. Swirling impingement jets with several swirl rates were generated by twisted tapes at different twist ratios (y/W = 3, 4, 5, and 6). The experiments were performed by locating nozzle at 4 different jet-to-plate spacings of L/D = 2, 4, 6 and 8. A jet Reynolds number varied between 4000 and 16000. Attributing to a high momentum transfer rate, an efficient heat transfer was obtained by using the jet with a small jet-to-plate spacing and the twist tape with a large twist ratio at high Reynolds number. At small jet-to-plate spacings (L/D = 2 and 4), swirling impinging jets gave higher heat transfer rate than conventional impinging jets while at large jet-to-plate spacings (L/D = 6 and 8), the opposed result was obtained. Over the range examined, only SIJ induced by the twisted tape at a twist ratio of 6 consistently provided higher average Nusselt numbers than CIJ.