The role of microchannel geometry selection on heat transfer enhancement in heat sinks: A review

Extensive research has been carried out by researchers for improving the thermal efficiency of the microchannel. There are various types of methodologies that have been proposed by authors for different geometry and fluid flow. The use of microchannel in the miniature heat exchangers and microchannel heat sink (MCHS) have taken the science of heat transfer to an another level for which the field of electronic device cooling, aerospace applications, automobile sectors, biomedical engineering, and chemical engineering sectors are being keen toward further development of the technology. Since 3 decades, the microchannel has been tested numerically, experimentally, and analytically for establishing the theories of hydraulic and thermal efficiency during fluid flow. Improper geometry selection of microchannel may lead to carry various losses such as pressure drop, friction factor, wall shear stress, and temperature jump. Available investigations and results have been reviewed immensely in this paper to give a clear prospective for further research in selecting a proper channel geometry.     

Heat transfer enhancement by pin elements

Heat transfer enhancement is an active and important field of engineering research since increases in the effectiveness of heat exchangers through suitable heat transfer augmentation techniques can result in considerable technical advantages and savings of costs. Considerable enhancements were demonstrated in the present work by using small cylindrical pins on surfaces of heat exchangers. A partly quantitative theoretical treatment of the proposed method is presented. It uses simple relationships for the conductive and convective heat transfer to derive an equation that shows which parameters permit the achievement of heat transfer enhancements. Experiments are reported that demonstrate the effectiveness of the results of the proposed approach. It is shown that the suggested method of heat transfer enhancements is much more effective than existing methods, since it results in an increase in heat transfer area (like fins) and also an increase in the heat transfer coefficient.     

Mass and heat transfer enhancement of chemical heat pumps

An inert additive, expanded graphite (EG), has been prepared and used to enhance the heat and mass transfer process of chemical heat pumps. The effects of mixing ratio and mixing method on the chemical reaction time are investigated.     

Energy conservation via heat transfer enhancement

This paper describes work on a program directed toward assessing the prospects for development of more efficient heat-exchange equipment. Improved equipment leads to more efficient thermal systems and more effective waste-heat recovery. Heat-transfer enhancement is a vital part of this development effort.Sources of information on heat-transfer enhancement are briefly reviewed. Applications are discussed, with emphasis on those techniques which have made the transition from the laboratory to full-scale industrial equipment. Typical reports of the benefits of enhancement are cited. Factors affecting commercial development and use are also discussed.     

Investigation of dimpled fins for heat transfer enhancement in compact heat exchangers

Direct and Large-Eddy simulations are conducted in a fin bank with dimples and protrusions over a Reynolds number range of Re_H = 200 to 15,000, encompassing laminar, transitional and fully turbulent regimes. Two dimple-protrusion geometries are studied in which the same imprint pattern is investigated for two different channel heights or fin pitches, Case 1 with twice the fin pitch of Case 2. The smaller fin pitch configuration (Case 2) develops flow instabilities at Re_H = 450, whereas Case 1 undergoes transition at Re_H = 900. Case 2, exhibits higher Nusselt numbers and friction coefficients in the low Reynolds number regime before Case 1 transitions to turbulence, after which, the differences between the two decreases considerably in the fully turbulent regime. Vorticity generated within the dimple cavity and at the dimple rim contribute substantially to heat transfer augmentation on the dimple side, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side. While friction drag dominates losses in Case 1 at low Reynolds numbers, both form and friction drag contributed equally in Case 2. As the Reynolds number increases to fully turbulent flow, form drag dominates in both cases, contributing about 80% to the total losses. While both geometries are viable and competitive with other augmentation surfaces in the turbulent regime, Case 2 with larger feature sizes with respect to the fin pitch is more appropriate in the low Reynolds number regime Re_H < 2000, which makes up most of the operating range of typical compact heat exchangers.     

An experimental investigation of heat transfer enhancement for a shell-and-tube heat exchanger

For the purpose of heat transfer enhancement, the configuration of a shell-and-tube heat exchanger was improved through the installation of sealers in the shell-side. The gaps between the baffle plates and shell is blocked by the sealers, which effectively decreases the short-circuit flow in the shell-side. The results of heat transfer experiments show that the shell-side heat transfer coefficient of the improved heat exchanger increased by 18.2–25.5%, the overall coefficient of heat transfer increased by 15.6–19.7%, and the exergy efficiency increased by 12.9–14.1%. Pressure losses increased by 44.6–48.8% with the sealer installation, but the increment of required pump power can be neglected compared with the increment of heat flux. The heat transfer performance of the improved heat exchanger is intensified, which is an obvious benefit to the optimizing of heat exchanger design for energy conservation.     

紧凑式两相换热器中管式换热元件沸腾传热实验研究

以烃类物质（丙烷和正戊烷）作为工质,进行了紧凑式换热器中带有加工配置表面的管式换热元件池沸腾实验研究。其中,单管实验温度工况为２５３Ｋ￣２９３Ｋ（饱和工质）。实验中所采用的换热元件为重入式结构加工配置表面的强化传热管和光管以及低助管。针对由４５根光管或带有加工配置表面的管子所构成的叉排管束进行了实验研究,实验工质为丙烷和正戊烷,实验温度分别为两种工质在２６３Ｋ和３０８Ｋ之间的饱和和温度。并将所得实     

A review of electrohydrodynamic enhancement of heat transfer

The heat transfer duty of heat exchangers can be improved by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration etc. The passive techniques require fluid additives or special surface geometries. Electrohydrodynamic (EHD) techniques have been introduced as one of the types of active heat transfer enhancement techniques. This paper presents a review of research works on electrohydrodynamic heat transfer enhancement. This paper can be used as the first guideline for the researcher in using EHD techniques for heat transfer enhancement.     

Method to improve geometry for heat transfer enhancement in PCM composite heat sinks

Use of composite heat sinks (CHS), constructed using a vertical array of ‘fins’ (or elemental composite heat sink, ECHS), made of large latent heat capacity phase change materials (PCM) and highly conductive base material (BM) is a much sought cooling method for portable electronic devices, which are to be kept below a set point temperature (SPT). This paper presents a thermal design procedure for proper sizing of such CHS, for maximizing the energy storage and the time of operation until all of the latent heat storage is exhausted.For a given range of heat flux, q″, and height, A, of the CHS, using a scaling analysis of the governing two dimensional unsteady energy equations, a relation between the critical dimension for the ECHS and the amount of PCM used (?) is determined. For a ?, when the dimensions of the ECHS are less than this critical dimension, all of the PCM completely melts when the CHS reaches the SPT. The results are further validated using appropriate numerical method solutions. A proposed correlation for chosen material properties yields predictions of the critical dimensions within 10% average deviation. However, the thermal design procedure detailed in this paper is valid, in general, for similar finned-CHS configurations, composed of any high latent heat storage PCM and high conductive BM combination.     

Water pool boiling heat transfer enhancement for modified surface tubes

This paper deals with the method of decreasing the size of heat exchanger surfaces by increasing the heat transfer coefficients and the importance of heat transfer enhancement for vaporization. We report an experimental study on surfaces modified by passive methods applied to heat transfer surfaces mechanically processed, covered with sleeves made by metallic tissues or covered with metallic porous layers performed using welding procedures. Experiments are made to investigate the heat transfer coefficient on copper tubes with a 22 mm external diameter using heat from inner source to outer vaporizing liquid. There are developed specific heat transfer correlations for each group of enhanced surfaces. The experimental data and new proposed correlations are compared with well known correlations. The results are in best agreement with the Cornwell–Houston correlation.     

Review of convective heat transfer enhancement with nanofluids

Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this review article is to summarize the important published articles on the enhancement of the forced convection heat transfer with nanofluids.     

Electrohydrodynamic enhancement of in-tube convective condensation heat transfer

An experimental investigation of electrohydrodynamic (EHD) augmentation of heat transfer for in-tube condensation of flowing refrigerant HFC-134a has been performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. The effects of varying the mass flux (55 kg/m² s ⩽ G ⩽ 263 kg/m² s), inlet quality (0.2 ⩽ x_in ⩽ 0.83) and the level of applied voltage (0 kV ⩽ V ⩽ 8 kV) are examined. The heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor 1.5 at the same conditions of the maximum heat transfer enhancement. The improved heat transfer performance and pressure drop penalty are due to flow regime transition from stratified flow to annular flow as has been deduced from the surface temperature profiles along the top and bottom surfaces of the tube.     

Numerical simulation of heat transfer enhancement in wavy microchannel heat sink

In this paper, heat transfer and water flow characteristics in wavy microchannel heat sink (WMCHS) with rectangular cross-section with various wavy amplitudes ranged from 125 to 500 μm is numerically investigated. This investigation covers Reynolds number in the range of 100 to 1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite-volume method (FVM). The water flow field and heat transfer phenomena inside the heated wavy microchannels is simulated and the results are compared with the straight microchannels. The effect of using a wavy flow channel on the MCHS thermal performance, the pressure drop, the friction factor, and wall shear stress is reported in this article. It is found that the heat transfer performance of the wavy microchannels is much better than the straight microchannels with the same cross-section. The pressure drop penalty of the wavy microchannels is much smaller than the heat transfer enhancement achievement. Both friction factor and wall shear stress are increased proportionally as the amplitude of wavy microchannels increased.     

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.     

Exploiting hydrodynamic instabilities. Resonant heat transfer enhancement

We introduce here the concept of resonant heat transfer enhancement based on excitation of shear-layer instabilities present in internal separated flows. Exploitation of natural instabilities requires: creation of a system with separated flow; determination of the system's resonant frequency; and excitation of that frequency with appropriate modulation. The resulting large scale motions lead to significant lateral mixing and correspondingly dramatic heat transfer enhancement. The method is applicable both in laminar and turbulent flows. Results, experimental and numerical, for a subcritieal grooved channel flow and a cross flow around a cylinder are presented. For the case of grooved channel, up to three-fold enhancement of heat transfer is observed when the flow is modulated at the system's natural frequency.     

强化换热的方法及新进展

介绍了强化传热技术的概念、分类方法以及发展过程,总结了近几年出现的强化换热新技术、新方法。主要包括新型强化换热材料及其工质的应用、各种异型强化换热管的应用以及内插物强化换热技术,其中以新型强化换热材料及工质的介绍为重点。这些新型材料和工质包括多孔材料、液态金属和纳米流体。最后分析了目前我国强化换热技术的实际应用情况及存在的问题,提出了推广建议,对从事强化换热技术研究的工作者提供一定的参考。     

Use of oscillating heat pipe technique as extended surface in wire-on-tube heat exchanger for heat transfer enhancement

This paper presents the performance of a wire-on-tube heat exchanger of which the wire is an oscillating heat pipe. The experiments for this heat exchanger were performed in a wind tunnel by exchanging heat between hot water flowing inside the heat exchanger tubes and air stream flowing across the external surface. R123, methanol and acetone were selected as working fluids of the oscillating heat pipe. The inlet water temperature was varied from 45 to 85 °C while the inlet air temperature was kept constant at 25 °C.     

A study on the characteristics of carbon nanofluid for heat transfer enhancement of heat pipe

For the purposes of enhancing the heat-transfer utility of the heat pipe in a solar collector, the present work attempted to improve nanoparticle dispersion stability by means of a chemical reformation process wherein nanofluid is formulated with hydroxyl radicals combined with oxidized multi-walled CNTs (MWCNTs). Experiments entailing measurements of thermal conductivity and viscosity in distilled water as functions of temperature were carried out to determine the best nanoparticle mixture ratio. The thermal conductivity increased with the increasing volumetric ratio of the oxidized MWCNTs and with increasing temperature. The viscosity also increased, slowly, until its concentration reached 0.01%, and then steeply increased, and was lower at high temperature.     

循环流态化自动清洗式换热器的传热强化

液-固流态化传热强化的机理尚未完全清楚,已有文献对流态化粒子的粒度与传热系数的影响关系有较大分歧。从传热强化角度对水-沙流态化自动清洗的运行参数进行优化试验研究,得到的结果是:沙子粒度不是愈细愈好,而是D p 2mm(8目)左右为好;粒子体积分数以2.4%比较好;流速在0.2~0.28 m/s(R e为6 000~9 000)较好。虽然粒子体积分数低,但是在优化条件下的流态化传热强化幅度几乎可以达到一倍左右,并且阻力又很低。     

Optimisation of heat transfer enhancement devices in a bayonet tube heat exchanger

This paper considers the assessment and analysis of heat transfer enhancement devices which can be considered for a bayonet tube heat exchanger. Due to restraining conditions, such as material selection, manufacturing complexity, etc., simple rib roughened surfaces in the form of rings were used on the air side flow, in the annulus. Analysis of the effect of the rings were studied, starting from cited geometries, using computational fluid dynamics. Validation was carried out using laser diagnostics. For the range of Reynolds numbers (Re_ave=160,000) considered the optimal ring configuration was a ring to annulus height ratio of 0.37 with a pitch to ring height ratio of 10. This provided the optimal heat flux to pressure drop for the given conditions.