Numerical investigation for improved heat transfer characteristics in micro‐fin tubes

Generally, internal micro‐fin tubes are used for increasing the life and performance of electronic devices. The micro‐fins enhance the heat transfer rate by increasing the surface area with an increase of the pressure drop. In this study, heat transfer and pressure drop are analyzed by varying Reynolds number with the increase in the number of fins in tubes. Heat transfer and pressure drop, together with turbulence kinetic energy of micro‐fin tubes (helical and straight) and a smooth tube, have been evaluated for different Reynolds numbers (60 000, 40 000, 20 000, and 2000) at a constant temperature of 350 K, which clearly establishes laminar to turbulent flow. It is observed that the helical micro‐fin tube has a better result compared with the straight micro‐fin tube and smooth tube at Reynolds numbers 60 000, 40 000, and 20 000 at velocity 2, 1, and 0.5 m/s, respectively. This study is an attempt to establish a comparison of different micro‐fin geometries with varying Reynolds numbers, concluding that a high Reynolds number is suitable for the same.     

CFD Analysis of Steady Laminar Natural Convection Heat Transfer from a Pin Finned Isothermal Vertical Plate

Heat transfer behavior with both the conductive and nonconductive fins have been analyzed by examining variations of the local and average Nusselt numbers in two‐dimensional flow. The main objective of this study is to quantify and compare the natural convection heat transfer enhancement of fin array with different fin aspect ratio and at different angles of inclination. It is found that significant heat transfer augmentation is obtained for both conductive and nonconductive fins. For conductive fins 20% higher augmentation factor is obtained when the fin aspect ratio is 6, angle of inclination is 60° and the pitch‐to‐length ratio is 0.2. For nonconductive fins, 10% higher augmentation factor is obtained when fin aspect ratio is 8, angle of inclination is 45° and pitch‐to‐length ratio at 0.5. A general correlation has been developed to predict the average Nusselt number and heat transfer augmentation factor for conductive and nonconductive fin arrays as a function of different fin configurations.     

A novel multilayer fin structure for heat transfer enhancement in hydride‐based hydrogen storage reactor

Metal hydrides are promising means for compact hydrogen storage. However, the poor heat transfer in the tank packed with metal hydride powders often hinders the system from charging or discharging hydrogen effectively. In this investigation, a tube‐fin heat exchanger is supposed to be inserted to the tank, and an optimization problem accounting for both heat transfer enhancement and cost is formulated. We solve the problem with approximate analytical methods, and the influences of fin geometry are discussed. The comparison results support using quadratic curve‐shaped fins, whose effectiveness is also proved by the numerical simulation results. Furthermore, a novel multilayer fin structure with varying width is proposed, and the key parameters of it are discussed, including the number and the arrangement of fins. This paper is expected to provide new insights for the heat transfer enhancement design of hydride‐based hydrogen storage system.     

A numerical study of flow behavior in the shell and helical finned-tube heat exchanger

Extended surfaces mostly aim to improve the heat transfer upon increasing the area of heat transfer. In this paper, the influence of using fins on flow behaviors and the heat transfer of the shell and tube heat exchanger has been investigated. In this regard, the present results are verified with available experimental data in the literature for a helical tube without fins. The effects of fin density (fin per inch), its height, and material have been studied on the heat transfer rate. In addition, the effects of radial pitch and the number of coil loops are studied. The results indicate that implementing extended surfaces significantly increases the heat transfer rate. The increase of fin density from 8 to 12 and the height from 11.5 to 13.5 mm enhances heat transfer up to 48% and 43% depending on Dean number, respectively. The rise of coil pitch augments the overall heat transfer, and it is more efficient at lower Dean numbers. The predicted results also show that the fin material does not have any significant effect on heat transfer.     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Effects of fin shapes on heat transfer in microchannel heat sinks

In the present study, compact water cooling of high‐density, high‐speed, very‐large‐scale integrated (VLSI) circuits with the help of microchannel heat exchangers were investigated analytically. This study also presents the result of mathematical analysis based on the modified Bessel function of laminar fluid flow and heat transfer through combined conduction and convection in a microchannel heat sink with triangular extensions. The main purpose of this paper is to find the dimensions of a heat sink that give the least thermal resistance between the fluid and the heat sink, and the results are compared with that of rectangular fins. It is seen that the triangular heat sink requires less substrate material as compared to rectangular fins, and the heat transfer rate per unit volume has been almost doubled by using triangular heat sinks. It is also found that the effectiveness of the triangular fin is higher than that of the rectangular fin. Therefore, the triangular heat sink has the ability to dissipate large amounts of heat with relatively less temperature rise for the same fin volume. Alternatively, triangular heat sinks may thus be more cost effective to use for cooling ultra‐high speed VLSI circuits than rectangular heat sinks.     

Heat transfer from a vertical fin array by laminar natural convection and radiation—A quasi‐3D approach

A quasi‐3D numerical model is developed to study the problem of laminar natural convection and radiation heat transfer from a vertical fin array. An enclosure is formed by two adjacent vertical fins and vertical base in the fin array. Results obtained from this enclosure are used to predict heat transfer rate from a vertical fin array. All the governing equations related to fluid in the enclosure, together with the heat conduction equation in both fins are solved by using the Alternating Direction Implicit (ADI) method for getting the temperatures along the height of the fin and the temperature of the fluid in the enclosure. Separate analysis is carried out to calculate the heat transfer rates from the end fins in the fin array. A numerical study has been carried out for the effect of fin height, fin spacing, fin array base temperature, and fin emissivity on total heat transfer rates and effectiveness of the fin array. The numerical results obtained for an eight‐fin array show good agreement with the available experimental data. Results show that the fin spacing is the most significant parameter and there exists an optimum value for the fin spacing for which the heat transfer rate from the fin array is maximum. Correlations are presented for predicting the total heat transfer rate, average Nusselt number, and effectiveness of the fin array. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20360     

A numerical study of flow and heat transfer in internally finned rotating straight pipes and stationary curved pipes

In this article the effects of internal fins on laminar incompressible fluid flow and heat transfer inside rotating straight pipes and stationary curved pipes are numerically studied under hydrodynamically and thermally fully developed conditions. The fins are assumed to have negligible thickness with the same conditions as the pipe walls. Two cases, constant wall temperature and constant heat flux at the wall, are considered. First the accuracy of the numerical code written by a finite volume method based on SIMPLE algorithm is verified by the available data for the finless rotating straight pipes and stationary curved pipes, and then, the numerical results for those internally finned pipes are investigated in detail. The numerical results for different sizes and numbers of internal fins indicate that the flow and temperature field analogy between internally finned rotating straight pipes and stationary curved pipes still prevail. The effects of Dean number (K_L) versus friction factor, Nusselt number, and other non-dimensional parameters are studied in detail. From the numerical results obtained, an optimum fin height about 0.8 of pipe radius is determined for Dean numbers less than 100. At this optimum value, the heat transfer enhancement is maximum, and the heat transfer coefficient appears to be 6 times as that of corresponding finless pipes.     

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In this study, fully developed laminar flow and convective heat transfer in an internally finned tube heat exchanger are investigated numerically. The flow is assumed to be both hydrodynamically and thermally developed with uniform outside wall temperature. Parameters of the thickness, length, and number of fins and thermal conductivity ratio between fin and working fluid are varied to obtain the friction factor as well as Nusselt number. The results show that the heat transfer improves significantly if more fins are used; however, the pressure drop turns out to be large in this heat exchanger. In addition, it is found that the emergence of closed-loop isotherms between the areas of two neighboring fins leads to heat transfer enhancement in the internally finned tube. When the fin number is smaller than 14, there appears a maximum Nusselt number at about 0.8 of the dimensionless fin length. Finally, an experiment is conducted to verify the numerical results.     

The correlation between the number of fins and the discharge time for a finned heat pipe latent heat storage system

The concept of a solar energy heat pipe latent heat storage system is presented. In order to assure large charging and discharging rates, finned heat pipes are used to transfer heat to and from the phase-change material (paraffin in this case). The evolution of the solid - liquid interface is studied by considering the radial heat transfer (due to the heat pipe wall) and the angular one (due to the fin). Two mathematical models, corresponding to exponential, respectively polynomial functions describing the fin temperature profile are presented and the results are compared. The two models allow the evaluation of the discharge time of the storage unit for a certain number of fins for a single heat pipe. When the discharge time has a fixed value, the methods presented in the paper allow to conclude whether the number of fins is sufficiently large to assure the complete solidification of the phase-change material.     

Heat transfer enhancement in annular fins using functionally graded material

This study presents a new approach on the heat transfer enhancement of annular fins with constant thickness using functionally graded materials. The thermal conductivity of the annular fin is assumed to be graded along the fin radius as a power‐law function. The resulting fin equation is solved by an approximate analytical method using the mean value theorem. The variable coefficients of second and third terms in the second‐order differential equation of the fin are replaced with their mean values along the fin radius. Several different graphs regarding the computed temperature profile, fin tip temperature, and fin efficiency are plotted with respect to the radii ratio thermo‐geometric parameter, and inhomogeneity parameter. It is demonstrated that the inhomogeneity parameter plays an important role on the heat transfer enhancement of the annular fin. However, for large radii ratios the effect of the inhomogeneity parameter decreases. Finally, it is stated that application of the functionally graded material in the annular fins, enhances the heat transfer rate between the fin and surrounding fluid resulting from the higher fin efficiency in comparison to the homogeneous annular fin. It is hoped that the results obtained from this study arouse interest among thermal designers and heat exchanger industries. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 42(7): 603–617, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21053     

Study on forced air convection cooling for electronic assemblies

The slotted fin concept was employed to improve the air cooling performance of plate-fin in heat sinks. Numerical simulations of laminar heat transfer and flow pressure drop were conducted for the integral plate fin, discrete plate fin and discrete slotted fin heat sinks. It is found that the performance of the discrete plate fin is better than that of the integral continuum plate fin and the performance of slotted fin is better than that of the discrete plate fin at the same pumping power of the fan. A new type of heat sink characterized by discrete and slotted fin surfaces with thinner fins and smaller spaces between fins is then proposed. Preliminary computation shows that this type of heat sink may be useful for the next generation of higher thermal load CPUs. The limit of cooling capacity for air-cooling techniques was also addressed. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(11): 1241–1245 [译自: 西安交通大学学报]     

环形翅片效率的简化计算方法

在保持换热效果不变的情况下,用相应的直翅片替换环形翅片,推导出了环形翅片效率的简化计算公式。该计算方法与直翅片效率的计算式相近。借助这一简化计算方法可以直接求解出环形翅片的翅片效率,而无需查取图表和曲线,为翅片管换热器的程序设计和工程计算带来很大方便。通过实例和比较计算证明了这一方法是可靠的和准确的。     

空气横掠矩形翅片椭圆管束换热规律的数值研究

采用Fluent软件对矩形翅片椭圆管束空气侧的对流换热情况进行了三维数值模拟,获得了不同流速下翅片表面温度分布,分析了迎面风速与换热系数之间的关系,随着速度的增大,空气侧的换热系数增加,并拟合了换热计算公式。同时分析了不同翅片间距对换热的影响因素,随着翅片间距的增大,空气侧换热系数增加,而且随着Rg数的增加,换热的强化更加明显。     

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

The effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20091     

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     

Improvement of heat transfer through fins: A brief review of recent developments

Fins or extended surfaces are generally used in heat exchangers to enhance heat transfer between the main surface and ambient fluid. Various types of simple‐shaped fins, namely, rectangular, square, annular, cylindrical, and tapered, have been used with different geometrical combinations. To satisfy industrial demand, different trials have also been carried out for designing optimized fins. The optimization of fins can be performed either by enhancing heat dissipation at an exact fin weight or by diminishing the weight of the fin by precise heat dissipation. Recently a notable amount of work on some typical fins, like, porous fins and perforated fins, has also been carried out. This paper presents a brief review on heat transfer enhancement using fins of different types considering variable thermophysical and geometric parameters, which will also be useful for future use of geometrical modifications of extended surfaces, based on the cost and availability of space.     

Investigating the effect of various fin geometries on the thermal performance of a heat sink under natural convection

Experimentally investigates heat dissipation by different longitudinal fins fitted to a cylindrical heat sink under natural convection conditions. Five aluminum fin configurations at base temperatures (70°C, 85°C, 100°C, and 115°C) were studied. The first fin was plain (fin1), while second fin had a triangular edge (fin2). The rest fins have the same triangular edge but with six 1cm circular perforations near the edge (fin3). While the perforations in fin4 were in the middle longitudinal fin length. The last fin (fin5) had twelve 0.5 cm circular perforations distributed into two columns. The measurements were validated with theoretical correlation with an acceptable deviation. The results showed that fin2, fin3, fin4, and fin5 dissipate more heat by 2.4%, 8.7%, 11.4%, and 5% than the flat fin with 9.8%, 11.85%, 11.85%, and 10.82% weight reduction, respectively. The heat transfer coefficient enhanced by 7.98%, 16.81%, 12.35%, and 5.44% for fin5, fin4, fin3, and fin2, respectively. Large circular perforation was more effective to dissipate heat especially when located near the heat source as in fin4 which gives the best heat dissipation with more weight reduction. The proposed fins efficiency were greater than 92%.     

异型高温热管翅的实验研究

对一结构特殊、长为6cm的异型高温热管翅在不同的工况下进行实验研究。通过改变蒸发段长度、绝热段长度、冷凝段长度、散热条件及其放置位置,观察其壁面温度分布情况,掌握其最佳工作状况。结果表明,当热管翅水平放置且在自然环境下散热时,不凝性气体的影响使热管翅冷凝段末端温度偏低;当蒸发段长度太长时,热管翅蒸发段会出现过热点,而冷凝段出现温度回升现象;若使热管翅的冷凝段在有限空间散热,则温度回升现象消失;当热管翅倾斜放置时,热管翅工作性能处于最佳状态。     

Thermal performance measurement of heat sinks with confined impinging jet by infrared thermography

In this paper, the thermal performance of heat sinks with confined impingement cooling is measured by infrared thermography. The effects of the impinging Reynolds number, the width and the height of the fins, the distance between the nozzle and the tip of the fins, and the type of the heat sinks on the thermal resistance are investigated. The results show that increasing the Reynolds number of the impinging jet reduces the thermal resistance of the heat sinks consistently. However, the reduction of the thermal resistance decreases gradually with the increase of the Reynolds number. The thermal resistance can be decreased by increasing the fin width combined with an appropriate Reynolds number. Increasing the fin height to enlarge the area of heat transfer also decreases the thermal resistance, but the effects are less conspicuous than those on altering the fin width. An appropriate impinging distance with minimum thermal resistance can be found at a specific Reynolds number, and the optimal impinging distance increases as the Reynolds number increases. Generally speaking, the thermal performance of the pin–fin heat sinks is superior to that of the plate–fin heat sinks because the pin–fin heat sinks consist of smaller volumes but greater exposure surfaces.