Optimization of short micro pin fins in minichannels

Finned minichannels are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Six pin fin shapes – circle, square, triangle, ellipse, diamond and hexagon – are used in a staggered array and attached to the bottom heated surface of a rectangular minichannel and analyzed. Also, using square pin fins, different channel clearance over fins are investigated to optimize the fin height of the fins with respect to that of the channel. Fin width and spacing are investigated using a ratio of fin width area to the channel width. Fin material is then varied to investigate the heat dissipation effects. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better performance. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to previous ones from recent studies. These correlations only apply to short fins in the laminar regime. Completely understanding the effects of micro pin fins in a minichannel is essential to maximizing the performance in small scale cooling apparatuses to keep up with future electronic advancements.     

Natural convection in porous triangular enclosures with a solid adiabatic fin attached to the horizontal wall

A numerical analysis has been performed to investigate the effects of fin location onto the bottom wall of a triangular enclosure filled with porous media whose height base ratio is 1. The temperature of the bottom wall is higher than that of the inclined wall while the vertical wall is insulated. Thus, the fin divides the heated bottom wall to two separate regions. Finite difference method was applied to solve governing equations which are written using Darcy method. Solutions of algebraic equations were made by Successive Under Relaxation (SUR) technique. The effective parameters on flow and temperature fields are: Rayleigh number, location center of fin, dimensionless fin height, and dimensionless fin width. The obtained results indicated that the fin can be used as a control element for heat transfer and fluid flow.     

The performance of a new gas to gas heat exchanger with strip fin

A compact gas to gas heat exchanger needs large heat transfer areas on both fluid sides. This can be realised by adding secondary surfaces. The secondary surfaces are plate fin, strip fin, and louvered fin, etc. The fins extend the heat transfer surfaces and promote turbulence.This paper presents a gas to gas heat exchanger with strip fins. The heat exchanger design and construction are based on a method to seal rectangular strip fins in slots in opposite walls of a rectangular pipe. Fins are fixed and sealed to the walls simultaneously by high temperature brazing of glass mixed with metals in a furnace. The additional advantage of glass is that it forms a coating on the heat transfer surface to protect the surface from corrosion.A number of measurements were carried out to test the performance of this heat exchanger. Not surprisingly, the measurement results indicate that heat transfer coefficient and pressure drop increase with the ratio of heat transfer area to volume (fin density). Colburn factor and friction factor versus Reynolds number are presented.     

Laminar Free Convection Around a Horizontal Cylinder with External Longitudinal Fins

The numerical solution of the laminar free convection of air around a horizontal cylinder with external longitudinal fins has been reported in this paper. The cylinder surface as well as the surfaces of each fin were assumed to be at a uniform temperature. The fluid drawn over a large angular domain moves out through a narrow, almost vertical strip known as plume, the thickness of which reduces with increasing buoyancy. The heat transfer increases with an increase in Grashof number, the number of fins, and fin length. For a constant fin surface, more fins of lower length result in a better heat transfer for Gr beyond about 10 ³ .     

Effect of geometric configuration on the laminar flow and heat transfer in microchannel heat sinks with cavities and fins

A numerical simulation is performed to investigate the characteristics of flow and heat transfer in microchannels with cavities and fins. Nine microchannels with various shaped cavities and fins are presented and compared to the smooth microchannel. The effect of cavity and fin shapes on the flow field and temperature field is analyzed. Results show that the presence of cavity and fin can increase the heat transfer area, intensify mainstream disturbance, and induce chaotic advection, which result in obvious heat transfer enhancement. The shape of cavity or fin has a great influence on the hydrodynamic and thermal performance for such micro heat sinks. Based on the performance evaluation criterion (PEC), the overall performance of the microchannel is evaluated. The combination of cavities and fins leads to lower bottom temperature, lower net temperature gradient of fluid, and better heat transfer performance, which has the potential to meet the increased heat removal requirement.     

Measurement of performance of plate-fin heat sinks with cross flow cooling

This work assesses the performance of plate-fin heat sinks in a cross flow. The effects of the Reynolds number of the cooling air, the fin height and the fin width on the thermal resistance and the pressure drop of heat sinks are considered. Experimental results indicate that increasing the Reynolds number can reduce the thermal resistance of the heat sink. However, the reduction of the thermal resistance tends to become smaller as the Reynolds number increases. Additionally, enhancement of heat transfer by the heat sink is limited when the Reynolds number reaches a particular value. Therefore, a preferred Reynolds number can be chosen to reduce the pumping power. For a given fin width, the thermal performance of the heat sink with the highest fins exceeds that of the others, because the former has the largest heat transfer area. For a given fin height, the optimal fin width in terms of thermal performance increases with Reynolds number. As the fins become wider, the flow passages in the heat sink become constricted. As the fins become narrower, the heat transfer area of the heat sink declines. Both conditions reduce the heat transfer of the heat sink. Furthermore, different fin widths are required at different Reynolds numbers to minimize the thermal resistance.     

Discussion on the Effect of a Fluid Domain around Fins and Grid Discretization in Buoyancy-Driven Convection

In the numerical study of heat sinks, it is known that a sufficient amount of fluid domain should be added at each side of the heat sink. However, the question in this context is: what can be defined as sufficiently far away from the heat sink? Different authors use different sizes of the computational domain around the heat sink. In this work the impact of the size and location of the fluid domain on the calculated heat transfer coefficient is investigated. Three fin row types are studied: a rectangular, an interrupted rectangular, and an inverted triangular fin row. First, the influence of adding fluid domain to the sides of the heat sink is studied. A large decrease of the heat transfer coefficient on both sides and bottom is observed. Next, the influence of adding fluid domain on both the top and the sides is studied. For the rectangular fins, the impact on the lumped heat transfer coefficient is +12% compared to the case without any fluid domain added. For the inverted triangular fin shape, no net effect is observed on the lumped heat transfer coefficient. So the impact of adding fluid domain depends on fin shape that is investigated. For the sides only, a small amount of fluid needs to be added, while for the fluid domain on top of the heat sink, 130% of the equivalent fin height is found as a good option to simulate the fin in computational fluid dynamics.     

Thermal Analysis for Heat Transfer Enhancement in Perforated Pin Fins of Various Shapes with Staggered Arrays

The present work investigates the enhancement of heat transfer rate through staggered pin fins of different shapes with different perforation geometries, namely circular, diamond shaped and elliptical type. Three dimensional computational fluid dynamics simulation has been carried out to analyze the effects of fin geometry and dimension of perforation as well as the shape of fin to enhance heat transfer rate against pressure loss. Results show that the heat transfer rates of perforated fins up to certain perforation number and size are always greater than the solid ones and with the change of fin shape and perforation geometry heat transfer rate also improves significantly. On the other hand pressure drop through heat sink decreases not only with increasing perforation number but also with the size of perforation. Moreover, variation of pressure drop of perforated fins is influenced with fin geometry.     

Development of Colburn j Factor and Fanning Friction Factor Correlations for Compact Surfaces of the Triangular Perforated Fins Using CFD

The necessity of increased heat transfer surface area has resulted in the development of compact heat exchangers, which are widely used in the aerospace and automobile industries. Hence perforations are made on triangular plain fins to study the effects on the heat transfer coefficient. A numerical model has been developed for the perforated fin of a triangular plate fin heat exchanger. Perforated fin performance has been analyzed with the help of computational fluid dynamics (CFD) by changing the various parameters of the fin. The Colburn j factor and the Fanning friction factor are calculated for different Reynolds numbers. The values of the Colburn j factor and the Fanning friction factor are validated for known geometric fins with available data in the literature and extended to triangular perforated fins. The correlations have been developed between Reynolds number, Colburn j factor, and Fanning friction factor by taking into account fin height, fin thickness, and fin spacing. The present numerical analysis is carried out for air media.     

An analysis of entropy generation through a circular duct with different shaped longitudinal fins for laminar flow

The present study focuses on the entropy generation analysis in a circular duct with internal longitudinal fins of different shape for laminar flow. Three different fin shapes are chosen for the analysis: Thin, triangular and V-shaped fins. Calculations are performed for various dimensionless lengths and number of fins, dimensionless temperature difference and fin angle for triangular and V-shaped fins. It is found that the number of fins and dimensionless length of the fins for both thin fins and triangular fins, and the fin angle for triangular and V-shaped fins have significant effect on both entropy generation and pumping power. Further, both entropy generation and pumping power also are influenced by dimensionless temperature difference.     

The Influence of Strip Location on the Pressure Drop and Heat Transfer Performance of a Slotted Fin

In this article, a numerical study is conducted to predict the air-side heat transfer and pressure drop characteristics of slit fin-and-tube heat transfer surfaces. A three-dimensional steady laminar model is applied, and the heat conduction in the fins is also considered. Five types of slit fins, named slit 1, slit 2, slit 3, slit 4, and slit 5, are investigated, which have the same global geometry dimensions and the same numbers of strips on the fin surfaces. The only difference among the five slit fins lies in the strip arrangement. Slit 1 has all the strips located in the front part of the fin surface, then, following the order from slit 1 to slit 5, the strip number in the front part decreases and, correspondingly, the strip number in the rear part increases, so that all the strips of slit 5 are located in the rear part. Furthermore, slit 1 and slit 5, slit 2 and slit 4, have a symmetrical strip arrangement along the flow direction. The numerical results show that, following the order from slit 1 and slit 5, the heat transfer rate increases at first, reaching a maximum value at slit 3, which has the strip arrangement of “front coarse and rear dense”; after that, it begins to decrease, as does the fin efficiency. Although they have the symmetrical strip arrangement along the flow direction, slit 5 has 7% more Nusselt number than slit 1, and slit 4 also has 7% more Nusselt number than slit 2, which shows that strip arrangement in the rear part is more effective than that in the front part. Then the difference of heat transfer performance among five slit fins is analyzed from the viewpoint of thermal resistance, which shows that when the thermal resistances in the front and rear parts are nearly identical, the optimum enhanced heat transfer fin can be obtained. This quantitative rule, in conjunction with the previously published qualitative principle of “front sparse and rear dense,” can give both quantitative and qualitative guides to the design of efficient slotted fin surfaces. Finally, the influence of fin material on the performance of enhanced-heat-transfer fins is discussed.     

Flow and heat transfer in compact offset strip fin surfaces

Experimental studies of air-side heat transfer and pressure drop characteristics of offset strip fins and flat tube heat exchangers were performed. A series of tests were conducted for 9 heat exchangers with different fin space, fin height, fin strip length and flow length, at a constant tube-side water flow rate of 2.5 m³/h. The characteristics of the heat transfer and pressure drop of different fin space, fin height and fin length were analyzed and compared. The curves of the heat transfer coefficients vs. the pumping power per unit frontal area were then plotted. Moreover, the enhanced heat transfer mechanism of offset strip fins was analyzed using field synergy theory. The results showed that fin length and flow length have more obviously effect on the thermal hydraulic characteristics of offset strip fins. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 366–369, 375 [译自: 上海交通大学学报]     

基于遗传算法的燃气轮机叶片内部柱肋蒸汽冷却通道的优化分析

针对叶片尾缘内部柱肋冷却方式进行数值仿真和优化分析。采用CFX软件进行数值仿真计算,建立圆形柱肋、水滴形柱肋和正方形柱肋3种柱肋形状下,不同柱肋间距的矩形通道模型,验证数值模型的正确性以及网格无关性。分析了顺排和叉排的排列方式下,柱肋形状和柱肋间距对下底面努塞尔数以及整个通道内压力损失的影响,最后通过MATLAB的遗传算法对仿真结果进行优化。研究表明：柱肋模型中,横向和纵向柱肋间距最小时,换热效果最佳,压力损失最大;在顺排和叉排中,正方形柱肋对通道的换热强度的提升效果最明显,圆形柱肋提升效果最小。     

Heat transfer augmentation in a plate‐fin heat exchanger using a rectangular winglet

This study presents numerical computation results on laminar convection heat transfer in a plate‐fin heat exchanger, with triangular fins between the plates of a plate‐fin heat exchanger. The rectangular winglet type vortex generator is mounted on these triangular fins. The performance of the vortex generator is evaluated for varying angles of attack of the winglet i.e., 20, 26, and 37° and Reynolds number 100, 150, and 200. The computations are also performed by varying the geometrical size and location of the winglet. The complete Navier–Stokes equation and the energy equation are solved by the (Marker and Cell) MAC algorithm using the staggered grid arrangement. The constant wall temperature thermal boundary conditions are considered. Air is taken as the working fluid. The heat transfer enhancement is seen by introducing the vortex generator. Numerical results show that the average Nusselt number increases with an increase in the angle of attack and Reynolds number. For the same area of the LVG, the increase in length of the LVG brings more heat transfer enhancement than increasing the height. The increase in heat transfer comes with a moderate pressure drop penalty. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20318     

Optimization of a Thermally Asymmetric Convective and Radiating Annular Fin

In this study, convective and radiating annular fins of rectangular profile under thermally asymmetric conditions are examined using an analytical method. For the fin base condition, it is assumed that heat transfer from the fluid to the inside surface of the pipe is equal to the heat transfer through the fin base. The temperature distribution along the fin height at the fin tip is presented to demonstrate the effects of the thermally asymmetric condition. The heat loss and fin tip radius for fixed fin height are optimized as a function of the fin top convection characteristic number. Also, for fixed fin volume, the heat loss and fin dimensions are optimized based on the top, bottom, and tip convection characteristic numbers, radiation characteristic numbers, fin base radius, and fin volume. The fin effectiveness as a function of the top convection characteristic number and annular fin length are also presented.     

Constructal design of vertical multiscale triangular fins in natural convection

Constructal design of vertical multiscale triangular fins in natural convection is investigated in this paper. The design consists of two parts. The first part is for single-scale triangular fins. The objective in the first design is to reach to the highest heat transfer density from the fins for three fin angles (15°, 30°, and 45°). The single-scale fins are placed in a horizontal array and considered as isothermal fins. The degrees of freedom are the fin angle, and the fin-to-fin spacing. The constraint is the fin height. The second part is for multiscale fins where small fins are placed between the large fins which are optimized in the first part. In the second part, the angles of the large and small scales fins are kept constant at (15°). The optimal fin-to-fin spacing which is obtained in the first part is considered a constraint in the second part. The Rayleigh numbers in this design are (Ra = 10³, 10⁴, and 10⁵). The two-dimensional mass, momentum, and energy equations for natural convection are solved with the finite volume method. The results show that there is a benefit of placing the small-scale fins where the percentage increase in the heat transfer density is (10.22%) at (Ra = 10³), and (50.6%) at (Ra = 10⁵) due to existence of the small fins between the large fins.     

A simple design of fins for boiling heat transfer

This study presents the thermal characteristics of a fin with excavation at base when various types of boiling occur simultaneously at adjacent locations on its surface experimentally and analytically. The heat transfer coefficient of each boiling mode is taken as a power function of wall superheat. Continuity of temperature and the heat transfer rate at the intersection of the two different modes on fin surface are employed to obtain the one-dimensional temperature distribution and total heat transfer of the excavated fin. Both heating and cooling cases are investigated in the analysis. Compared with solid pin fins, the proposed fins can extend the operating condition to a higher temperature of the heat transfer surface. In addition, the experimental data compare favorably with the analytical results.     

Effects of thin fin on natural convection in porous triangular enclosures

A two-dimensional solution of natural convection in solid adiabatic thin fin attached to porous right triangular enclosures has been analyzed numerically. The vertical wall of the enclosure is insulated while the bottom and the inclined walls are isothermal. The temperature of the bottom wall is higher than the temperature of the inclined wall. Governing equations, which are written using Darcy model, are solved via the finite difference technique. The Successive Under Relaxation (SUR) method was used to solve linear algebraic equations. Dimensionless location of the thin fin from 0.2 to 0.6, the aspect ratio of triangular enclosure from 0.25 to 1, Rayleigh number from 100 to 1000 and the dimensionless height of the fin from 0.1 to 0.4 are used as governing parameters that are effective on heat transfer and fluid flow. Results for the mean Nusselt number, velocity profiles, the contour maps of the streamlines and isotherms are presented. It is observed that the thin fin can use as a passive control element for flow field, temperature distribution and heat transfer.     

Optimum design of a longitudinal fin array with convection and radiation heat transfer using a genetic algorithm

In the present work, the optimization of a longitudinal fin array is investigated. Heat is transferred by conduction along the fins and dissipated from the fin surface via natural convection to the ambient and radiation to other fin surfaces and surrounding. The aim of the optimization is to find the optimum geometry and the number of fins in such a way that the rate of heat transfer from the array is maximized. A modified genetic algorithm is used to maximize the objective function which is defined as the net heat rate from the fin surface for a given length. The fin profile is represented by B-spline curves, where the shape of fin is determined by the positions of a set of control points. The effects of the base temperature, the fin length and the height of array on the optimum geometry and on the number of fins are investigated by comparing the results obtained for several test cases. In addition, the contributions of convective heat transfer and radiative heat transfer in net heat transfer are studied for these cases. The enhancement of heat transfer due to the optimum fin geometry is examined by comparing the results obtained for the optimum fin profile with those with conventional profiles.     

基于砖窑余热回收的斯特林发动机及其翅片性能分析

针对小型砖窑的烟气余热利用问题,基于斯特林循环回收烟气余热,建立翅片传热模型。针对矩形、三角形和圆形3种典型翅片,研究相同工况下翅片间距、高度和厚度对换热器传热及阻力性能的影响规律,并利用综合性能指标(PEC)来进行评价。结果表明:在翅片间距与翅片高度恒定的情况下,PEC由高到低依次为圆型、三角形和矩形翅片;当翅片厚度大于1.5 mm时,PEC由高到低依次为三角形、圆形和矩形翅片;翅片厚度小于1.5 mm时,PEC由高到低依次为圆型、三角形和矩形翅片。