Channel cross section effect on heat transfer performance of oblique finned microchannel heat sink

In the present work, the effect of channel cross section on the heat transfer performance of an oblique finned micro-channel heat sink was investigated. Water and Al₂O₃/water nanofluid of volume fraction 0.25% were used as a coolant. The oblique finned microchannels are designed with three channel cross-sections namely square, semicircle and trapezoidal. The primary work of this paper is to study the heat transfer and hydrodynamic characteristics in the oblique finned microchannel. The experimental setup and procedure are validated using water as coolant in a micro-channel heat sink. Heat transfer and flow characteristics are examined for three cross-sections of varying mass flux. The trapezoidal channel cross-section increases the considerable heat transfer rate improvement for both water and nanofluid by 3.133% and 5.878% compared to square and semicircle cross section. Also, the pressure drop is higher in the trapezoidal cross-section over the square and semicircle cross section. This is due to increase in friction loss of trapezoidal cross section. The results indicate that trapezoidal cross-section oblique finned micro-channel is more suitable for heat transfer in the electronic cooling application.     

Heatline analysis of heat recovery and thermal transport in materials confined within triangular cavities

Heat recovery from hot fluids in material processing industries is important for environmental and thermal management. Present work involves numerical visualization of heat flow in entrapped cavities filled with hot materials. The concept of heatline is used to visualize the heat energy trajectory. The system involves entrapped triangular cavities filled with hot fluid (Pr = 0.015, 0.026, 0.7 and 1000). At low Rayleigh number (Ra), it is found that the heatlines are smooth and perfectly normal to the isotherms indicating the dominance of conduction for both the triangles. As Ra increases, flow slowly becomes convection dominant. Multiple heat flow circulations with high intensity are formed within the lower triangular domain especially for low Pr numbers, whereas, less intense convective heat flow circulations are observed for the upper triangle. Multiple circulations are absent for both the triangular domains involving fluids with higher Pr numbers. It is observed that the heat transfer rates are monotonic for the upper triangle whereas a few local maxima in heat transfer rates occur for smaller Pr within lower triangular domain. Overall, fluid with any Pr may be useful for enhanced heat transfer within the upper triangle but fluid with high Pr may be preferred for the lower triangle.     

Augmentation of heat transfer through passive techniques

The thermal performance of energy preservation systems is greatly improved by increasing miniaturization and boosting. These are imaginative (or Promethean) techniques to enhance heat transfer. Enhancement methods of heat transfer draw great attention in front of the industrial sector because of their ability to provide energy savings and raise the economic efficiency of thermal systems. Three techniques these methods are categorized; those are active, passive, and compound. Different types of components are used in passive methods because of the transfer/working fluid flow path to the enhancement of the heat transfer rate. In this article, the subject of the review was the passive heat transfer enhancement methods including inserts (conical strips, winglets, twisted tapes, baffles), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), extended surfaces (fins) and nanofluids (mono and hybrid nanofluid). Recent passive heat transfer enhancement techniques are studied in this article as they are cost-effective and reliable, and also comparably passive methods do not need any extra power to promote the energy conversion systems' thermal efficiency than active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluid). From the pioneers' research work, it is clear that a lower twist ratio and lower pitch, lesser winglet angles can provide more heat transfer rate and a little bit more friction factor. In the case of nanofluids, a little bit of pumping power is enhanced. Finally, heat transfer enhancement is compared with the thermal performance factor, which is more than unity.     

The effect of geometrical parameters on heat transfer characteristics of microchannels heat sink with different shapes

The effect of geometrical parameters on water flow and heat transfer characteristics in microchannels is numerically investigated for Reynolds number range of 100–1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using finite volume method. The computational domain is taken as the entire heat sink including the inlet/outlet ports, wall plenums, and microchannels. Three different shapes of microchannel heat sinks are investigated in this study which are rectangular, trapezoidal, and triangular. The water flow field and heat transfer phenomena inside each shape of heated microchannels are examined with three different geometrical dimensions. Using the averaged fluid temperature and heat transfer coefficient in each shape of the heat sink to quantify the fluid flow and temperature distributions, it is found that better uniformities in heat transfer coefficient and temperature can be obtained in heat sinks having the smallest hydraulic diameter. It is also inferred that the heat sink having the smallest hydraulic diameter has better performance in terms of pressure drop and friction factor among other heat sinks studied.     

Heat transfer inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths

The heat transfer phenomena inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths was investigated numerically in the present work. The heat source is considered as a local heating element of varying length, which is embedded at the bottom wall of the enclosure and maintained at a constant temperature. The air flow enters the channel horizontally at a constant cold temperature and a fixed velocity. The other walls of the enclosure and the channel are kept thermally insulated. The flow is assumed laminar, incompressible, and two‐dimensional, whereas the fluid is considered Newtonian. The results are presented in the form of the contours of velocity, isotherms, and Nusselt numbers profiles for various values of the dimensionless heat source lengths (0.16 ≤ ε ≤ 1). while, both Prandtl and Reynolds numbers are kept constant at (Pr = 0.71) and (Re = 100), respectively. The results indicated that the distribution of the isotherms depends significantly on the length of the heat source. Also, it was noted that both the local and the average Nusselt numbers increase as the local heat source length increases. Moreover, the maximum temperature is located near the heat source location.     

Study of mixed convective heat transfer in a sintered porous channel

This investigation experimentally elucidates mixed heat convection in a rectangular channel with porous medium, and analyzes the cooling performance of electronics. The test sections are sintered copper granules with dimensions of 5 × 5 × 1 cm and average diameters of 0.704 mm and 1.163 mm. A porous channel is heated using air as the working fluid. The measured variables include the heat flux on the heat surface, the local wall temperature in the flow direction, the inlet/outlet pressure, and the flow rate. The boundary condition is that the heating surface of the lower plate of the test section is heated by an isothermal heat flux, while the other three sides are thermally insulated; the heat flux is between 0.23 and 1.86 Watt/cm². The mean air flow rate within a porous channel is between 0.3 and 4.0 m/s. The thermally developed region and the fully developed region are measured. This experiment is directed at understanding the influence on heat transfer based on the diameter of the sintered copper granules, Reynolds number, and heat flux. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 64–77, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20050     

Investigation of fluid flow and heat transfer in a vertical channel heated from one side by PV elements, part I - Numerical Study

The impetus of this paper is to analyse numerically the fluid flow and heat transfer characteristics of buoyancy-driven convection between two vertical parallel walls, heated from one side. Both convection and radiation heat exchanges are considered as the heat transfer mechanisms by which the thermal energy is transferred into the air. A steady-state two-dimensional model is used for the analysis. Numerical results are derived for a channel of 6.5 m in height and different widths of the channel. Various heat fluxes are considered in order to show the effect of the input heat on the heat transfer across the air layer. Detailed studies of the flow and thermal fields in the air are presented in order to explore the thermal behavior of air in the channel. Velocity and temperature profiles of the outlet air and the surface temperature of the heated and insulated wall are presented. In Part II of this paper the findings from an experimental study are reported.     

Conjugate heat and mass transfer in membrane-formed channels in all entry regions

Membrane-based energy recovery ventilators (or total heat exchangers) are key equipments to fresh air ventilation, which is helpful for the control of respiratory diseases like Swine flu (H1N1) and SARS. Parallel-plates narrow channels are common structure for membrane-based energy recovery ventilators. In practice, the exchanger channel lengths are limited due to the confinement in pressure drops and noises. In these channels, the hydraulically, thermally and concentrationally entry regions account for a large fraction of the total duct length. However, previous investigations neglected the entry issues for simplicity. Either hydraulically fully developed, or thermally or/and concentrationally fully developed flow were assumed, which would underestimate equipments performances seriously. This study provides a more accurate methodology: fluid flow, heat and mass transport equations were solved directly as they enter into the channel. In other words, both the fluid flow and the heat and mass transport are in simultaneously developing regions. The membrane and the two neighboring flows are considered as a conjugate problem. The conjugate heat transfer problem is solved with a commercial CFD code. Then the conjugate mass transfer problem is solved by transferring it to another conjugate heat transfer problem by heat mass analogy. The Nusselt and Sherwood numbers in the entry regions are calculated. The effects of three typical flow arrangements: cocurrent, counter and cross flow, on the boundary conditions and the consequent Nusselt and Sherwood numbers in the channels are evaluated.     

A review on microchannel heat exchangers and potential applications

Energy conversion and utilization are continuous but ever increasing processes for sustainability and economic development. Environmental concerns, such as thermal and air pollution, have dictated the practices of energy conservation and recovery, as well as the implementation of clean energy sources. Heat exchangers are an important component for processes where energy conservation is achieved through enhanced heat transfer. Such issues as increased energy demands, space limitations, and materials savings have highlighted the necessity for miniaturized light‐weight heat exchangers, which provide high heat transfer for a given heat duty. However, while traditional heat exchangers employ conventional tubes (?6 mm) with various cross‐sections, orientations, and even the enhanced surface textures, the technology is nearing its limits. Microchannels (broadly ?1 mm) represent the next step in heat exchanger development. They are a particular target of research due to their higher heat transfer and reduced weight as well as their space, energy, and materials savings potential over regular tube counterparts. In contrast to traditional tube heat exchangers, the heat transfer and fluid flow correlations, and the systematic design procedures are not yet well established for microchannels. It remains to be established whether the classical fluid flow and heat transfer theories and correlations are valid for microchannels. Numerous investigations are underway with researchers consolidating evidence on both sides of this question. This paper surveys the published literature on the status and potential of microchannels, and it identifies research needs, and defines the scope for long‐term research. Based on results from the review, an air‐to‐liquid crossflow experimental infrastructure has been developed and commissioned. It will be used to investigate the heat transfer and fluid flow for a variety of working fluids in different microchannel test specimens. Further information and the heat balance status of the developed test facility are also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。     

Experimental and numerical study on tube-side flow and heat transfer characteristics of superheated vapor flow in catalyst-filled LH2 spiral wound heat exchanger

High energy consumption is considered to be one of the most persistent problems in liquid hydrogen (LH₂) plants. The combination of heat exchanger and ortho-para (O–P) hydrogen conversion has attracted considerable attention as a cutting-edge technology to reduce energy consumption. The flow and heat transfer characteristics of O–P hydrogen conversion catalyst-filled spiral wound heat exchanger (SWHE) were investigated in this study in two steps. In the first step, pressure-drop experiments were performed in a tube filled with porous media. The results indicated that the pressure drop was overestimated when using Ergun's equation. Therefore, a new empirical pressure-drop correlation for a channel filled with O–P catalyst was formulated. Subsequently, a novel heat transfer model was established based on this correlation for further numerical simulations. The distributions of the temperature, pressure, and para hydrogen content in a catalyst-filled tube were determined. In addition, the influence of the flow rate on the heat exchange coefficient and outlet para hydrogen was clarified; it was found that, with an increase in the flow rate, the heat exchange coefficient increased, whereas the outlet para hydrogen content decreased. At a flow rate of 0.5 m³/h, the para hydrogen content increased by 44% after hydrogen flowed through the channel filled with the O–P catalyst. Furthermore, a prediction model for the para hydrogen content with a flow rate range of 0–1.5 m³/h was derived. This study provides promising theoretical evidence for the engineering application of SWHEs filled with O–P catalysts in large-scale hydrogen liquefaction units.     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Characteristics of combined heat transfer of superheated steam drying: Numerical study on coupled convection and gas radiation heat transfer

A numerical study on a combined radiation and forced convection heat transfer of superheated steam, which is a radiation participating real gas, in thermally developing laminar flow through a parallel‐plate channel has been conducted to investigate characteristics of superheated steam drying. The integrodifferential energy equation was solved using an implicit finite‐difference technique with a marching solution procedure and an exponential wide‐band model for the treatment of the radiative transfer part. Comparison of results with and without gas radiation in various conditions shows that fluid radiation decreases the temperature of the main stream, but increases the total heat flux at a heat transfer surface. Furthermore, the results show that the fluid radiation decreases the inversion point temperature approximately to 150 to 240 °C with the increase of optical thickness. This numerical result agrees in an order of magnitude with the previous experimental studies, but is about 100 K lower than that of former theoretical predictions without considering fluid radiation. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 385–399, 2000     

Calculation of turbulent flow and heat transfer in a porous-baffled channel

This study presents the numerical predictions on the turbulent fluid flow and heat transfer characteristics for rectangular channel with porous baffles which are arranged on the bottom and top channel walls in a periodically staggered way. The turbulent governing equations are solved by a control volume-based finite difference method with power-law scheme and the k-ε turbulence model associated with wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi-implicit method for pressure-linked equation) method.The parameters studied include the entrance Reynolds number Re (1×10⁴-5×10⁴), the baffle height (h=10, 20 and 30 mm) and kind of baffles (solid and porous); whereas the baffle spacing S/H are fixed at 1.0 and the working medium is air. The numerical calculations of the flow field indicate that the flow patterns around the porous- and solid-type baffles are entirely different due to different transport phenomena and it significantly influences the local heat transfer coefficient distributions. Relative to the solid-type baffle channel, the porous-type baffle channel has a lower friction factor due to less channel blockage.Concerning the heat transfer effect, both the solid-type and porous-type baffles walls enhanced the heat transfer relative to the smooth channel. It is further found that at the higher baffle height, the level of heat transfer augmentation is nearly the same for the porous-type baffle, the only difference being the Reynolds number dependence. As expected, the centerline-averaged Nusselt number ratio increases with increasing the baffle height because of the flow acceleration.     

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.     

Theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger

This paper presented theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger, a new type of exchanger with the potential to be an alternative to a conventional exchanger. Owing to the complexity of the desiccant assisted heat and mass transfer and difficulty in determining its associated parameters, work started from the simulation of a clear fibre exchanger by developing a dedicated numerical model, and its validation by using the data from the manufacturer of the exchanger. Further to this, laboratory testing was carried out with the same exchanger, but filmed with a liquid desiccant fluid, i.e. LiCl. Comparison between the data of the clear and desiccant filmed exchangers suggested the use of correction factors for heat and mass transfer resistances with desiccant operation. A revised model for the desiccant filmed exchanger was then established taking into account the correction factors. By using the updated model, influence of geometrical sizes and operating conditions of the liquid desiccant filmed exchanger on the exchanger efficiency were studied and the optimal values of these were obtained. The results indicated that the exchanger efficiencies (heat, mass and enthalpy) are largely dependent upon the exchanger channel length, air flow rate and less related to the exchanger channel height, intake air temperature and intake‐to‐outgoing air moisture content difference. It was also suggested that the air speed across the channels should be in the range 0.5–1.5 m s^?1. The height of air channel (passage) should be set at 6.5 mm or below and its length should be 1.0 m or more. A simulation was carried out under UK typical summer operation conditions, i.e. the intake air streams at 30°C db and 70% rh and outgoing air streams at 24°C db and 50% rh, and the results indicated that the exchanger with the above recommended geometrical sizes can achieve an energy efficiency of 87%, which is 30% higher than for non‐desiccant filmed operation. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Effect of corrugation frequencies on natural convective heat transfer and flow characteristics in a square enclosure of vee-corrugated vertical walls

A numerical investigation has been carried out on natural convective heat transfer and fluid flow in a square cavity with vee-corrugated vertical surfaces. This study covers the range of corrugation frequency from 1 to 3 and Grashof number from 10³ to 10⁵. The corrugation amplitude has been fixed at 5% of the enclosure height. The vorticity stream function formulation with the control volume based finite element method has been used to analyse the effects of corrugation frequency and Grashof number. The investigation shows that the overall heat transfer through the enclosure increases with the increase of corrugation for low Grashof number; but the trend is reversed for high Grashof number.