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.     

A fractal model for the coupled heat and mass transfer in porous fibrous media

This paper developed a mathematical model for the coupled heat and mass transfer in porous media based on the fractal characters of the pore size distribution. According to Darcy’s law and Hagen–Poiseuille’s law for liquid flows, the diffusion coefficient of the liquid water, a function of fractal dimension, is obtained theoretically. The liquid flow affected by the surface tension and the gravity, the water vapor sorption/desorption by fibers, the diffusion of the water vapor and the phase changes are all taken into account in this model. With specification of initial and boundary conditions, distributions of water vapor concentration in void spaces, volume fraction of liquid water, distribution of water molecular content in fibers and temperature changes in porous fibrous media are obtained numerically. Effects of porosity of porous fibrous media on heat and mass transfer are analyzed. The theoretical predictions are compared with experimental data and good agreement is observed between the two, indicating that the fractal model is satisfactory.     

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.     

Analysis of heat transfer through Bi-convection fins

Heat transfer through fins subject to two different convective media is modeled and analyzed analytically in this work. The terminology “Bi-convection fin” is used to refer to this kind of fins. Five different cases are analyzed: Case A: Bi-convection thickness-wise Bi-metallic fins; Case B: Bi-convection span-wise rectangular fins; Case C: Bi-convection longitudinal-wise fins; Case D: Bi-convection perimeter-wise fins with uniform cross-section; and Case E: Bi-convection perimeter-wise permeable fins. Closed form solutions for the fin temperature and heat transfer rate are obtained. It is found that Heat transfer through Bi-convection fins may be minimized for certain designs such as those belonging to cases A, B, D and E. On the other hand, it may be maximized as for those belonging to case C at specific values of fin indices and convection heat transfer coefficients ratio. Finally, the heat transfer through Bi-convection fins is found to increase as their effective thermal conductivity, cross-sectional area, fin indices and the difference between the base and the effective free stream temperatures increase.     

Combustion and heat transfer in model two-dimensional porous burners

A two-dimensional model of two simple porous burner geometries is developed to analyze the influence of multidimensionality on flames within pore scale structures. The first geometry simulates a honeycomb burner, in which a ceramic is penetrated by many small, straight, nonconnecting passages. The second geometry consists of many small parallel plates aligned with the flow direction. The Monte Carlo method is employed to calculate the viewfactors for radiation heat exchange in the second geometry. This model compares well with experiments on burning rates, operating ranges, and radiation output. Heat losses from the burner are found to reduce the burning rate. The flame is shown to be highly two-dimensional, and limitations of one-dimensional models are discussed. The effects of the material properties on the peak burning rate in these model porous media are examined. Variations in the flame on length scales smaller than the pore size are also present and are discussed and quantified.     

Semianalytical investigation on heat transfer in porous fins with temperature-dependent thermal conductivity via the homotopy perturbation Sumudu transform approach

A unique investigation has been undertaken to analyze the heat transmission by convective and radiative mechanisms in a fully saturated penetrable fin of a longitudinal structure positioned on a leaning surface. This study introduces the fusion of the realms of Homotopy perturbation and Sumudu transform techniques to address a previously unexplored problem involving a moving fin with temperature-dependent thermal conductivity. In prior research papers, the Homotopy Perturbation Sumudu Transform Method (HPSTM) was utilized to obtain analytical solutions for fins featuring temperature-dependent thermal conductivity. However, in our current study, we employ the HPSTM to tackle a novel problem involving a moving porous fin. This fin exhibits temperature-dependent thermal conductivity and is subjected to convection and radiation effects. Through a comparison with numerical results, the present study has validated the dependability of its findings. The dimensionless temperature profile has been investigated by studying its relationship with several parameters. Here we observed that when the Peclet number $$ is augmented by 400%, there is a corresponding 1.11% increase in thermal outline at the fin's extremity. Enhancing the value of radiation parameter $$ by 400% declines the temperature of the fin tip by 14.079%. This study encourages the application of the Homotopy perturbation Sumudu transform technique in more complex fin problems.     

Analysis of flux-base fins for estimation of heat transfer coefficient

Exact solutions are given for the transient temperature in flux-base fins with the method of Green’s functions (GF) in the form of infinite series for three different tip conditions. The speed of convergence is improved by replacing the steady part by a closed-form steady solution. For the insulated-tip case, a quasi-steady solution is presented. Numerical values are presented and the conditions under which the quasi-steady solution is accurate are determined. An experimental example is given for estimation of the heat transfer coefficient (HTC) on a non-rotating roller bearing, in which the outer bearing race is treated as a transient fin.     

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。     

Research on the mechanism of heat transfer enhancement in microchannel heat sinks with micropin fins

In this paper, the pressure drop and heat transfer features of a microchannel applying micropin fins are investigated by numerical simulations and experiments. The microchannel, which is 20-mm long, 2.7-mm wide, and 0.3-mm deep, is fabricated with copper and consisted of staggered diamond micropin fins. The visualization experiments, by means of the advanced technology micro-particle image velocimetry (PIV), are conducted to discuss the mechanism of heat transfer by analysing the flow regimes. Meanwhile, 3D-coupled numerical simulations are applied for the combination with experiments in this research. It is found that the vortex-wake flow is stable at Reynolds number (Re) = 0 to 300, and a steady recirculating zone can be observed in the wake, where a pair of symmetrical vortices is formed. All the time, the vortex-wake flow is unstable at Re = 300 to 650. Under this situation, it is due to the decrease of vorticity that the Nusselt number (Nu) is not significantly increased as it was expected. Thus, when Nu in the pin fin microchannel is predicted, the vorticity should be considered as well as turbulent kinetic energy (TKE). Furthermore, comparative study was carried out based on the mechanism proposed in this study among three kinds of microchannel with different fins, including staggered circular pin fins (CPF), square pin fins (SPF), and diamond pin fins (DPF).     

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.     

Conjugate heat transfer in channels with heat-conducting inclined fins

Conjugate heat transfer in a finned channel with equally spaced fins placed transversely to the flow direction following in-line and staggered arrangements is evaluated. The fins and channel walls are heat-conducting and are fully coupled to a turbulent fluid flow problem. The hydrodynamic and thermal effects of the fin blockage ratio, fin angle, and flow velocity were investigated. The simulations show that the fin arrangement is of paramount importance on the performance of the heat exchanger: the staggered fin configuration provided lower pressure drop and higher heat transfer rate than the in-line fin arrangement for different flow conditions.     

Analytical model of heat transfer in porous insulation around cold pipes

A thermal insulation system is analysed that consists of a cold tube insulated with a porous material faced with a vapour retarding foil.Water vapour will diffuse through the vapour retarding foil and condense on the cold tube. To avoid build-up of water in the insulation a hydrophilic wicking cloth is wrapped around the cold tube and extended through a slit in the tubular insulation and a slot in the facing to the ambient so that condensed water can evaporate into the air. Some of the moisture in that part of the wicking cloth situated in the slit in the tubular insulation will diffuse backwards to the cold pipe and contribute to the heat uptake of the cold tube. This part is calculated for the stationary case and compared with the sensible heat transfer through the tubular shaped insulation material, using measured dry λ values and measured fictitious moist λ values.     

开缝型翅片流动与传热三维数值模拟

通过数值模拟,研究空调系统使用的开缝型翅片的传热与阻力特性。对三种型式的开缝型翅片进行模拟,得出了流场和温度场。通过对比分析发现,双边交替开缝的slit-2型翅片,换热性能最好,X型双向开缝片的性能次之,单边开缝的slit-1型翅片换热效果低于前两种。数值模拟还得出,空气流过slit-x型翅片的阻力最大,流过slit-1型翅片的阻力最小。     

Effect of fins on forced convection heat transfer around a tube

Effect of fins on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open‐type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made of paper were put on the test tube having certain fin spacing. It was clarified from the experiment that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the downstream side of the tube, and the minimum fin spacing where the effect of the fin begins to appear is the largest for the single tube and the smallest for the tube bundle. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 445–454, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10098     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

Differential transformation method to determine heat transfer in annular fins

Fins are the extended surfaces that are utilized to afford a significant increase in the surface area for heat transference between a heated source and a colder ambient liquid. To enhance the heat transference rate from the exterior surface of a circular conduit, radial, or concentric annular fins are used. Fins are utilized in heat exchanging devices like superheaters, electrical equipment, computer CPU heat sinks, car radiators, refrigeration, and heat exchangers. Motivated by these applications, the current paper explores the thermal attribute of an annular fin with variable thermal conductivity. The framed equations are articulated in terms of nonlinear ordinary differential equations. One of the most effective techniques, the differential transformation method has been implemented to find the analytical solution. The domination of nondimensional parameters on the thermal gradient of the fin has been analyzed graphically. Furthermore, the variation in radial and tangential stress in an annular fin for various dimensionless parameters has been examined with a graphical explanation. Results reveal that the thermal gradient of fin increases for improved values of variable thermal conductivity parameters. The greater values of thermogeometric parameters result in a higher heat transfer.     

A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature-time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.     

Multi-objective genetic optimization of the heat transfer from longitudinal wavy fins

In the present work, we investigate the optimization of the heat transfer from wavy fins cooled by a laminar flow under conditions of forced convection and from a multi-objective point of view. The problem is addressed by means of a finite element method which allows to compute the velocity and the temperature distributions in a finned conduit cross section under conditions of imposed heat flux. Thereafter, the fin profile is optimized by means of multi-objective genetic algorithm which aims to find geometries that maximize the heat transfer and, at the same time, minimize the hydraulic resistance. The geometry of the fins is parameterized by means of a polynomial function and several order are investigated and compared.