H形对流传热脉管的熵产率最小构形优化

针对H形对流传热脉管,以脉管区域面积和通道总表面积为约束,在线热流密度恒定时,基于熵产生最小化进行了构形优化,给出了无量纲总熵产量和无量纲单位传热负荷的总熵产率与脉管组合级数、无量纲质量流量和无量纲泵功率的完整解析关系式及优化算例.结果表明:给定通道总表面积与文献中给定通道总体积约束的优化结果是不同的;无量纲单位传热负...     

最大热阻最小化的环形高导热通道三维体点导热最优构形的解析解法

基于构形理论,采用解析解法,以最大温差最小为优化目标,对基于环形高导热通道和圆柱形单元体的三维“体-点”导热模型进行构形优化,得到无量纲最大热阻最小的三维圆柱体最优构形.结果表明:增大高、低导热材料导热系数比、高导热材料占比和单元体数目均有助于提高圆柱形构造体的导热性能,但当单元体数目较大时,圆柱形构造体的导热性能改善...     

基于恒截面流道矩形单元体的积耗散率最小传质构形优化

以质量积耗散率最小为优化目标,对恒截面高渗透率通道的矩形单元体传质构形问题进行了分析和优化,得到结构体内平均传质效果最好的结构外形。结果表明:对于单元体和各级构造体,其平均传质压差均为最大传质压差的2/3。因为高渗透率材料中质流率密度符合线性分布,所以基于积耗散率最小与最大压差最小的最优构形完全一致。所得最优构形同时使得传质能力和传质安全性最好。     

Towards maximal heat transfer rate densities for small-scale high effectiveness parallel-plate heat exchangers

This paper addresses the question to what extent parallel-plate heat exchangers can be downsized without loss of thermal-hydraulic performance. It is shown that when the characteristic length scales of the channels are reduced at a constant pressure drop, the effectiveness exhibits a maximum due to axial heat conduction. The point of maximal effectiveness is found to correspond to a maximal thermal power density and thus to the minimal volume required for obtaining that effectiveness. Based on asymptotic relations for the effectiveness in the small and large channel limit, closed-form expressions are derived for the optimum geometric parameters that maximize power density in the limit of design effectiveness approaching unity. These relations are extended to a broader effectiveness range by means of dimensionless correction functions that are calculated numerically. The resulting expressions define optimal elemental units that can be used to construct parallel-plate counter-flow heat exchangers with the lowest possible core volume for effectiveness values between 0.53 and 1.     

Constructal optimisation of conjugate triangular cooling channels with internal heat generation

This paper presents the development of the three-dimensional flow architecture of conjugate cooling channels in forced convection with internal heat generation within a solid. Two types of cross-section channel geometries were used. The first involved equilateral triangles with three equal legs in length and all three internal angles of 60°. The second was isosceles right triangles with two legs of equal length and internal angles of 90°, 45° and 45°. Both the equilateral triangle and isosceles right triangle are special case of triangle that can easily and uniformly be packed and arranged to form a larger constructs. The configurations were optimised in such a way that the peak temperature of the heat generating solid was minimised subject to the constraint of a fixed global volume of the solid material. The cooling fluid was driven through the channels by the pressure difference across the channel. The degrees of freedom of the channels were aspect ratio, hydraulic diameter and channel to channel spacing ratio. The shape of the channel was allowed to morph to determine the best configuration that gives the lowest thermal resistance. A gradient-based optimisation algorithm was applied in order to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure difference. The effects of porosities, applied pressure and heat generation rate on the optimal aspect ratio and channel to channel spacing are reported. It was found that there are unique optimal design variables for a given pressure difference. The numerical results that were obtained were in agreement with the theoretical formulation using scale analysis and method of intersection of asymptotes. Results obtained show that the effects of applied dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature.     

Numerical study of heat transfer and friction factor in a tube with groove and rib on the wall

Heat transfer and friction factor in a tube with turbulator internally grooved and rib are studied using numerical simulation. Two different turbulator geometries, including only groove and both groove and rib, on the tube wall are investigated. The study is developed for different parameters of turbulator including pitch ratio (PR), groove and rib height (DR), and Reynolds number in the range of 10 000 to 38 000. The results revealed that the best Nusselt number is achieved for GR turbulator and for the tube wall with PR = 2 and DR = 1. Furthermore, for a specific turbulator parameter, Nusselt number is improved by 46% compared with the experimental result. The results show that the friction factor decreases by increase of both step and height ratios. For example, based on Reynolds number, 21% to 69% reduction in the friction factor was observed for PR = 7 and DR = 3. In addition, the correlations for Nusselt number and friction factor with acceptable deviation are presented for the studied turbulators. Finally, thermal performance enhancement factor is defined to be a good criterion for selection of the optimum turbulator and their parameters.     

Entropy analysis for heat transfer in a rectangular channel with suction

The purpose for this paper is to study heat transfer in a rectangular channel with suction applied at the adjacent two side walls. A two‐dimensional laminar viscous fluid flow is generated due to the application of suction/injection. The other two opposite sides are kept at a constant temperatures, and the walls with suction are maintained at a constant heat flux. The streamlines thus obtained due to the flow and isothermal lines and heat function are analyzed. The regions of high and low frictions are found by drawing contours of the entropy generation number and the Bejan number. The biharmonic equation for stream function is numerically solved by the Finite Difference Method (FDM) using a 13‐point formula, and a five‐point formula is used to solve for all other harmonic equations for temperature, heat function, and pressure. For derivative boundary conditions, the central difference formula with fictitious nodes is used. In the analysis, we note that the corner points are regions of high energy dissipation. The least dissipation of energy is near the wall, where the nondimensional temperature is 1. This paper analyzes heat transfer in the rectangular channel through the heat function and entropy generation number.     

Experimental investigation on heat transfer effect of conical strip inserts in a circular tube under laminar flow

The aim of this paper is to observe the Nusselt number and friction factor behavior of the circular tube with conical strip inserts as turbulators in a laminar flow condition, using staggered and non-staggered conical strips with three different twist ratios (Y = 2, 3 and 5). The conical strip is inserted in the forward and backward direction individually compared to the flow of water which is the working fluid. The results indicate that the conical strip inserts increases the Nusselt number when compared to the plain surface tube. It is observed that the strip geometry has a major effect on the thermal performance of the circular tube. On examination of different strips for determining the enhancement of Nusselt number, the staggered conical strip with the twist ratio of Y = 3 has given a better result compared to the other two strips. Finally, correlations have been derived using regression analysis for predicting the Nusselt number and friction factor.     

Application of heat transfer on MHD shear thickening fluid flow past a stretched surface with variable heat source/sink

The purpose of this study is to explore the viscous dissipation stimulus on the steady convective magnetohydrodynamic shear thickening liquid stream across a vertically stretched sheet. The impact of thermic heat, first-order velocity slip, and variable heat generation/absorption are considered and also ignored the effect of magnetic Reynold's number. We converted flow controlling equations into the set of dimensionless nonlinear ordinary differential equations by employing similarity variables to solve these coupled equations by R–K and shooting technique. The effect of different dimensionless variables on velocity, heat, friction factor, and local Nusselt numbers are presented through graphs and tables. Depreciation in velocity and growth in temperature distribution is detected when the Casson fluid parameter is increased. Temperature is the increasing function of the Eckert number.     

Comparative study on heat transfer enhancement of nanofluids flow in ribs tube using CFD simulation

This study presents, a numerical investigation of two‐dimensional turbulent nanofluids flow in different ribs tube configurations on heat transfer, friction, and thermal performance coefficients using ANSYS‐FLUENT software version‐16. Governing equations of mass, momentum, and energy have been solved by means of a finite volume method (FVM). Four types of nanoparticles namely; Al₂O₃, CuO, SiO₂, and ZnO with volume fraction range (1%‐4%) and different size of nanoparticles (dp = 30 nm, 40 nm, 50 nm, and 60 nm) with various Reynolds number (10 000‐30 000) in a constant heat flux tube with rectangular, triangular, and trapezoidal ribs were conducted for simulation. The results exhibit that Nusselt number for all cases enhanced with Reynolds number and nanofluid volume fraction increases. Likewise, the results also reveal that SiO₂ with volume fractions of 4% and diameters of nanoparticles of 30 nm in triangular ribs offered the highest Nusselt number at Reynolds number of Re = 30 000. In addition, the higher value of thermal performance factor was obtained at Reynolds number of Re = 10 000.     

Performance of an Atkinson cycle with heat transfer,friction and variable specific-heats of the working fluid

The performance of an air standard Atkinson cycle with heat-transfer loss, friction-like term loss and variable specific-heats of the working fluid is analyzed using finite-time thermodynamics. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of variable specific-heats of the working fluid and the friction-like term loss on the irreversible cycle performance are analyzed. The results show that the effects of variable specific-heats of working fluid and friction-like term loss on the irreversible cycle performance should be considered in cycle analysis. The results obtained in this paper provide guidance for the design of Atkinson engines.     

Thermal performance enhancement studies using graphite nanofluid for heat transfer applications

Enhanced boiling heat transfer using nanofluids is highly relevant due to its potential applications in thermal management of systems producing large heat fluxes. However, the sedimentation of nanoparticles limits their application in heat transfer systems. So, the preparation of a stable nanofluid remains a big research challenge. The stability issues arise due to the large difference in the density of nanoparticle and the base fluid. Graphite nanoparticle is selected in this study, as it has 4.5 times lower density than copper and comparable thermal conductivity. An experimental study is conducted to evaluate the suitability of graphite nanofluid in mesh wick heat pipes, which are devices that utilize boiling and condensation principles to transfer high heat fluxes. Thermal transport properties and boiling heat transfer characteristics showed enhancement and the effect of nanofluid on the device level thermal performance is thoroughly assessed. Experimental results are compared with the published literature. A reduction in thermal resistance by 32.5% and an improvement in the heat transfer coefficient by 48.02% in comparison with base fluid clearly indicate the superiority of the graphite nanofluid for heat transfer applications.     

An experimental investigation in forced convective heat transfer and friction factor of air flow over aligned round and flattened tube banks

Heat transfer coefficients and pressure drop are studied experimentally for airflow over aligned round and flattened tube configurations. The Reynolds number is based on the outer diameter of the round tube or the outside transverse diameter of the flattened tube, which is used for various flows, ranging from 133 to 800 with a constant input heat flux. In the present work, a total of 30 samples of round and flattened tubes heat exchangers with three transverse pitches, 2.0, 3.0, and 4.5, were studied to investigate their thermal performance. The results indicate that the relative gain in the overall Nusselt number is about 32.5 to 60.6% in flattened tubes, while the reduction range in the friction factor is about 11 to 30.6%. Correlations are proposed for the overall Nusselt number, friction factor, and Colburn j–factor for both round and flattened tube banks. A higher value means that a deviation error of 9.9% in the round tube banks and 10.1% in the flattened tube banks are expected. In addition, the best value for thermal performance for the flattened tube bundle was found to be coincident with a smaller Reynolds number.     

Numerical prediction of fluid flow and heat transfer in a wavy pipe

IntroductionA pipe with periodically converging-divergingcross-section is one Of the sevens devices employed forenhancing the heat and mass tusfer efficiency. Thenuid flow, to the now passages with a periodicallyvaling cross-section, attains a folly develOPed acmethat differs fundamentally from that for a convelltionalconstant-area flow channel. In the periodically vwigcross-seCtions, the ac developed VelM field repeatsitSelf at cormsponding edal locations in successivecycles. The change of…     

Analysis of turbulent heat transfer and fluid flow in channels with various ribbed internal surfaces

This paper presents results of a numerical investigation of heat transfer and flow pattern characteristics of a channel with repeated ribs on one broad wall. Numerical computations are performed for seven ribs placed on the bottom wall of a channel for Reynolds numbers ranging from 10,000 to 30,000. The newly modified ribs (the ones with convex pointing upstream/downstream rib, wedge pointing upstream/downstream rib, concave pointing upstream/downstream rib and also concave-concave rib as well as convex-concave rib), are proposed for simulation with prospect to reduce flow separation and extend reattachment area compared to the unmodified square rib. The numerical results are reported in forms of flow structure, temperature field, turbulent kinetic energy, Nusselt number, friction factor and thermal enhancement factor. The results indicate the rib with concave-concave surfaces efficiently suppresses flow separation bubble in the corner of the rib and induces large recirculation zone over those of the others, hence giving the highest Nusselt number and friction factor. On the other hand, the one with convex-concave surface provides the lowest friction factor with moderate Nusselt number. Due to the prominent effect of its low friction factor, the rib with convex-concave surface offers the highest thermal enhancement factor of 1.19.     

Characteristics of heat transfer and fluid flow in a channel with single-row plates array oblique to flow direction for photovoltaic/thermal system

This study has been carried out to investigate the characteristics of convective heat transfer and fluid flow for a single row of oblique plates array to the flow direction inside a channel. The flow inside the channel is laminar and the plates array have spanwise distance between the plates and heated by radiation. This configuration has been designed to be used for Photovoltaic/Thermal system (PV/T) applications. The theoretical results are validated with measured values, and a good agreement prevailed. The results show that an increase in the plate oblique angle (γ) in the range from 0 to 15 degrees, leads to an increase in the Nusselt number (Nu) up to a maximum value and then decreases. The oblique angle at the maximum value of Nu depends on the flow Reynolds Number (Re), and (?_w/?_pl), where (?_w/?_pl) is defined as the ratio of the plates’ spacing at zero oblique angle to the plate length. Furthermore, increasing (?_w/?_pl) results in a significant increase in the heat transfer coefficient depending on the values of Re, and plate oblique angle (γ). In addition, increasing (γ) from 0 to 15 degrees results in a decrease in the friction factor up to a certain value, after which the friction value approaches a constant value depending on Re value and (?_w/?_pl). It was found that for any value of the plate oblique angle (γ), the friction factor decreases with the increase of the values of (?_w/?_pl) and Re, respectively.     

矩形微通道中流体流动阻力和换热特性实验研究

以去离子水为流体工质,对其在矩形微尺度通道中的流动阻力和传热特性进行了实验研究。通过测量流量、进出口压力和温度等参数,获得了流体流过微通道时的摩擦阻力系数、对流换热过程中的热流通量和N u等。微尺度通道中流体流动的摩擦阻力系数较常规尺度通道中的摩擦阻力系数小,仅是常规尺度通道中摩擦阻力系数的20%~30%;且流动状态由层流向湍流转捩的临界R e也远小于常规尺度通道的。微尺度通道中对流换热的N u与常规尺度通道的显著不同。流量较小时,N u较常规尺度通道中充分发展段的小;随着水流量的增加,微通道的N u迅速增加,并很快超过常规尺度通道的N u,表现出微尺度效应。热流通量对微尺度通道中对流换热N u存在影响,其影响规律在不同流速条件下呈不同趋势,流速较小时,N u基本保持不变;而在流速较大时,N u随热流通量增加而呈增加趋势。     

Numerical investigation of turbulent heat transfer in channels with detached rib‐arrays

A numerical investigation is conducted to analyze the flow‐field and heat transfer characteristics in a rectangular passage of width‐to‐height ratio of 6:1 with detached ribs on one wall, where constant wall temperature condition is applied. The effect of detached‐rib geometry on heat transfer coefficient, friction factor, and thermal enhancement factor is investigated covering the range of the detached‐clearance ratios (c/a) of 0.1, 0.2, 0.3, and 0.4, the Reynolds number based on the channel hydraulic diameter ranges from 8000 to 24,000. The numerical results show that the flow‐field, temperature pattern, local Nusselt number distribution, average Nusselt number, and friction factor are strongly dependent on the detached‐clearance ratios. The thermal enhancement factor (TEF) under the same pumping power constraint is calculated in order to examine the overall effect of the detached‐clearance ratio. For the present range investigated, the maximum TEF of 1.22 is achieved by the use of the ribs with c/a of 0.1 at Reynolds number of 8000. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20357     

Convective heat transfer and second law analysis of non-Newtonian fluid flows with variable thermophysical properties in circular channels

This study presents an analytical solution, for fully developed non-Newtonian fluid flows in circular channels under isoflux thermal boundary conditions based on perturbation techniques. Since the physical properties are generally a function of temperature and may not be assumed constant under certain circumstances, the change in viscosity and thermal conductivity with temperature was taken into account. Viscous dissipation term was also included in the performed analysis. In this study, first closed form expressions for velocity, temperature distributions, and Nusselt numbers corresponding to constant thermophysical properties were given in terms of governing parameters. Then, numerical calculation was performed to obtain the values of Nusselt number and global entropy generation for variable thermophysical properties. The results revealed that neglecting the property variation significantly affects heat transfer characteristics and entropy generation, in which the deviation from the constant physical property assumption may reach up to about 32.6%.     

Analytical modeling of electrokinetic effects on flow and heat transfer in microchannels

A fundamental understanding of electrolytic flow in microchannels is essential for the design of microfluidic devices. Hence, an analytic investigation is presented on the effects of electrostatic potential in microchannels. Solving the Navier–Stokes equations, an expression for the C_fRe product is presented. Solving the energy equation the Nusselt number for constant wall heat flux and constant wall temperature boundary conditions are presented with analytic expressions over a wide range of operating conditions.