高温吸热管内超临界CO2传热特性的数值模拟

研究超临界CO2在高温吸热管内的传热特性是将其应用于聚光太阳能热发电技术中的基础.本文对此进行了数值模拟研究,分析了流体温度、流动方向、系统压力、质量流率和热流密度对对流传热系数和Nu数的影响.结果表明:高温区(800—1050 K)的对流传热系数和Nu数受流动方向和系统压力的影响均很小,但都随着质量流率的增大以及热流密度的减小而明显增大;而随着流体温度的升高,对流传热系数近似线性增大,Nu数则近似线性减小.另外,本文研究发现在高温区可忽略浮升力对传热的影响,而由高热流密度引起的流动加速效应会明显恶化传热.最后,选取了八种管内超临界流体传热关联式与模拟结果进行对比,发现使用基于热物性修正的关联式对高温区传热数据预测的结果优于使用基于无量纲数修正的关联式得到的结果,且其中预测效果最优的关联式得到的计算结果与模拟结果之间的平均绝对相对偏差为8.1%.     

裂解吸热反应对乙烷超临界传热的影响

本文对圆管内超临界状态下乙烷的湍流传热过程进行了数值模拟,分析了乙烷高温裂解对壁面热流密度、平均温度以及对流换热Nusselt数的影响。计算结果表明:考虑乙烷高温裂解吸热反应,管内平均温度会降低,出口处温度降低可达135 K;壁面热流密度则会显著增加,热出口处可增加近2倍;热裂解反应改善了超临界对流换热效果,Nusselt数可提高约20%。在本文计算条件下,经典的对流换热关系式可准确适用于不考虑热裂解反应的情况,而对于考虑裂解吸热反应的超临界乙烷传热情况则误差较大。     

Heat exchange at laminar flow in rectangular channels

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.     

Radiative equilibrium in a rectangular enclosure bounded by gray walls

Two-dimensional temperature and heat flux distributions are calculated for an absorbing-emitting gray medium at radiative equilibrium in a rectangular enclosure. The bounding walls are gray and diffuse with arbitrary surface temperature distributions, and heat generation may take place inside the medium. As a first approximation, the problem is solved for optically thick systems (differential approximation). These results are subsequently improved by the introduction of a number of geometrical parameters to yield good accuracy for all optical thicknesses. As examples, two cases are discussed in detail: (1) uniform heat generation in a black enclosure and (2) an enclosure with one gray surface at constant temperature. Comparison with some numerical solutions generated by Hottel's zonal method shows excellent agreement.     

Wall heat transfer in static and rotating 180° turn channels by quantitative infrared thermography

The aim of the present study is to develop a new experimental methodology that allows one to perform accurate measurements of the local heat transfer distribution before, in, and after a 180° sharp turn in static and rotating channels. Preliminary measurements of convective heat transfer coefficients are performed by means of infrared thermography applied to the steady state ‘heated-thin-foil’ technique. Some preliminary results in terms of Nusselt number Nu distributions and profile, as well as averaged Nu profiles along the channel axis, are presented. Results prove that infrared thermography is capable of measuring heat flux coefficients and detecting particular phenomena linked to the fluid flow configuration such as location of separation bubbles, influence of the channel aspect ratio as well as the influence of the channel rotation.     

圆管层流脉冲流动对流换热数值分析

对等热流和等壁温边界条件下圆管内层流脉冲流动对流换热问题进行了数值模拟。在等热流边界条件下的数值计算结果与理论解吻合很好。计算结果表明:在等热流和等壁温边界下脉冲流动可引起速度、温度以及努塞尔数随时间波动,振幅越大,脉冲频率越小,波动越大。但它们的时均值均等于在相同雷诺数下稳态流动的值,脉冲流动不能强化换热。     

肋片双面带涡产生器的扁管管片式散热器数值研究

采用数值模拟的方法,研究了流道内上下两肋片均布置有涡产生器的扁管管片式散热板芯的传热与阻力特性,并与流道单面布置涡产生器的换热板芯进行了对比.结果表明,采用双面带涡产生器的肋片表面能在提高Nu的同时,降低流动阻力,换热性能得到了明显的提高,在Re=1500时,平均Nu数提高了8.6%,横向平均Nu最大提高了30%,阻力下降了6.5%.     

复合肋套管导热和对流耦合换热的数值模拟

在层流范围内,数值模拟了带均布纵肋同心套管内的三维导热和对流耦合换热。结果表明间壁温度沿管长变化 较大且分布呈非线性变化;在通道人口流体内部温度和速度分布呈现层流附面层特征,换热的入口段比例随Re增大而加 大,入口段换热的Nu也随Re增大而加大;换热的Nu随管长增加趋于不变。     

Experimental Analysis of Gas Micro-Convection Through Commercial Microtubes

In this article, an experimental campaign devoted to analyzing the forced micro-convection features of heated gas flows through commercial stainless-steel microtubes having inner diameters of 172 μm and 750 μm is described. The experimental results obtained by heating the microtubes with an imposed uniform heat flux (H-boundary condition) at the external wall, in terms of Nusselt numbers, are compared to the predictions of the classical correlations validated for conventional pipes and to the correlations proposed for gas flows through microtubes under laminar and transitional conditions (100 < Re < 4,000). The cross-sections of the tested microtubes enabled the analysis of the effects of wall axial heat conduction on the Nusselt number. It was observed that the Nusselt number is strongly dependent on the Reynolds number in the laminar regime, and this fact is explained in the article with the effects of wall axial heat conduction and the difficulties in the experimental determination of the right exit bulk temperature of the gas flow, which cannot be ignored in the thermal analysis. The agreement between the Gnielinski correlation and the experimental Nusselt number is poor, especially for low Reynolds numbers, if one uses the average gas bulk temperature, obtained as the arithmetic mean between the inlet and outlet gas bulk temperature, in the definition of the experimental Nusselt number. On the contrary, the agreement with the Gnielinski correlation improves if the local wall-gas temperature difference near the exit of the microtube is used instead. The experimental results presented in the article demonstrate that the criteria for the design of accurate micro-convection tests can be quite different from those for the analysis of forced convection through conventional pipes.     

DSMC方法下给定热流边界条件的实施

本文详细讨论了DSMC方法中流体温度、能量及边界热流的统计方法,发展了一种从边界热流求得与壁面碰撞分子反射速度的方法。该方法被称为逆温度抽样算法(ITS,Inverse Temperature Sampling)方法。在此基础上,本文发展了DSMC方法中壁面处给定热流边界条件的实施方法。计算结果表明: ITS方法能准确抽样反射分子的特征温度,进而求得分子反射速度。基于该方法的给定热流边界条件可以准确求得壁面处温度分布,以及流场内的压力、速度。     

Unsteady radiative nanofluid flow near a vertical heated wavy surface with temperature-dependent viscosity

In the present contribution, a numerical treatment is provided to describe unsteady nanofluid flow near a vertical heated wavy surface. A memorable feature of the present work is the investigation of nanofluid flow associated with thermal radiation that acts as a catalyst for heat transfer rates. Likewise, the effectiveness of variable viscosity is examined as it controls fluid flow as well as heat transfer. It is necessary to study heat and mass transfer for complex geometries because predicting heat and mass transfer for irregular surfaces is a topic of fundamental importance, and irregular surfaces frequently appear in many applications, such as flat-plate solar collectors and flat-plate condensers in refrigerators. A simple coordinate transformation from the wavy surface into a flat one is employed. The non-dimensional boundary layer equations that governing both heat transfer and nanofluid flow phenomena along the wavy surface are solved via a powerful numerical approach called the implicit Chebyshev pseudospectral (ICPS) method with Mathematica code. A comparison graph of the current numerical computation and the published data shows a perfect match. Figures depict the effect of various physical parameters on nanofluid velocities, temperature, salt concentration, nanoparticle concentration, skin friction, Sherwood, nanoparticle Sherwood, and Nusselt numbers. According to the numerical results, increasing the variable viscosity parameter value causes a drop in the local skin friction coefficient value and an increase in the steady-state axial nanofluid velocity profile near the wavy surface. Furthermore, as heat radiation is increased, the local Nusselt number decreases but the nanoparticle Sherwood number increases.     

Variable fluid properties and variable heat flux effects on the flow and heat transfer in a non-Newtonian Maxwell fluid over an unsteady stretching sheet with slip velocity

The effects of variable fluid properties and variable heat flux on the flow and heat transfer of a non-Newtonian Maxwell fluid over an unsteady stretching sheet in the presence of slip velocity have been studied. The governing differential equations are transformed into a set of coupled non-linear ordinary differential equations and then solved with a numerical technique using appropriate boundary conditions for various physical parameters. The numerical solution for the governing non-linear boundary value problem is based on applying the fourth-order Runge-Kutta method coupled with the shooting technique over the entire range of physical parameters. The effects of various parameters like the viscosity parameter, thermal conductivity parameter, unsteadiness parameter, slip velocity parameter, the Deborah number, and the Prandtl number on the flow and temperature profiles as well as on the local skin-friction coefficient and the local Nusselt number are presented and discussed. Comparison of numerical results is made with the earlier published results under limiting cases.     

Magnetohydrodynamic flow of burgers fluid with heat source and power law heat flux

Here magnetohydrodynamic (MHD) two-dimensional (2D) flow of an incompressible Burgers material bounded by a permeable stretched surface is addressed. The boundary layer flow equations are modelled. Heat transfer is discussed for power law heat flux at the surface and heat source. Convergent series solutions are constructed. Clarification of different emerging variables is presented through graphs of velocity, temperature and local Nusselt number. The present solutions are matched with the available published work in a limiting case.     

Heat transfer for boundary layers with cross flow

An analysis is presented to study the dual nature of solutions for the forced convective boundary layer flow and heat transfer in a cross flow with viscous dissipation terms in the energy equation. The governing equations are transformed into a set of three self-similar ordinary differential equations by similarity transformations. These equations are solved numerically using the very efficient shooting method. This study reveals that the dual solutions of the transformed similarity equations for velocity and temperature distributions exist for certain values of the moving parameter, Prandtl number, and Eckert numbers. The reverse heat flux is observed for larger Eckert numbers; that is, heat absorption at the wall occurs.     

Features of heat transfer in a pre-nozzle volume of a solid-propellant rocket motor with charges of complex shapes

Local features of thermophysical processes in the channels and pre-nozzle volumes of solid-propellant rocket engines with case-bonded charges of different cross-sectional shapes are considered. The influence of the charge shape on the heat exchange in the nozzle bottom is investigated. It is shown that the value of the Nusselt number at a critical point on the multi-nozzle bottom is determined both by the charge channel form and by the geometry of the pre-nozzle volume. By processing the numerical experimental results the criterial dependences for determining the Nusselt number in the areas of local increase of heat exchange intensity are obtained. The obtained dependences are compared with the known empirical formulas [1–4]. It is found that the use of empirical relationships to estimate the Nusselt number leads to incorrect determination of the parameters of heat transfer on the armored surfaces of the charge, the nozzle covers, and the input parts of the submerged rotating nozzle.     

Analysis of radiation in a suspension of nanoparticles and gyrotactic microorganism for rotating disk of variable thickness

Here heat, concentration and motile microorganism transfer rates in radiative flow of nanofluid are investigated. Variable thicked surface of rotating disk is examined. Concept of microorganisms suspended nanoparticles is stabilized through bioconvection which has been induced by combined effects of magnetic field and buoyancy forces. For obtained nonlinear differential systems the convergent series solutions are derived. Fluid flow, temperature, concentration and motile density behaviors for different parameters are analyzed through graphs. Skin friction and Nusselt number are analyzed numerically. Clearly temperature and concentration have opposite behavior for larger Brownian motion parameter. Motile density reduces for bioconvection Peclet number and bioconvection Lewis number.     

真空冷冻干燥机冷阱中的气体换热分析

从理论上分析了螺旋盘管式冻干机冷阱中气体的换热。对真空冷阱中处于滑流区的气体换热 ,应考虑温度跳跃和速度滑移对换热的影响。基于冷阱内表面定热流和恒壁温的假设 ,建立了相应的分析模型 ,导出了冻干机冷阱内处于滑流区的气体换热 Nusselt数表达式 ,认为随着冷阱压力的降低 ,换热 Nusselt数将不断减小     

Heat transfer analysis for stretching flow over a curved surface with magnetic field

The analysis of a viscous fluid flow and heat transfer is carried out under the influence of a constant applied magnetic field over a curved stretching sheet. Heat transfer analysis is carried out for two heating processes, namely, prescribed surface temperature (PST) and prescribed heat flux (PHF). The equations governing the flow are modeled in a curvilinear coordinate system (r, s, z). The nonlinear partial differential equations are then transformed to nonlinear ordinary differential equations by using similarity transformations. The obtained system of equations is solved numerically by a shooting method using Runge-Kutta algorithm. The interest lies in determining the influence of dimensionless radius of curvature on the velocity, temperature, skin friction, and rate of heat transfer at the wall prescribed by the Nusselt number. The effects of Hartmann number are also presented for the fluid properties of interest.     

Two analytic methods applied to radiative transfer in the linear-anisotropic scattering medium with graded index and diffuse gray boundaries

The curved ray-tracing method is extended to radiative transfer in the graded index medium with diffuse gray boundary conditions instead of black boundary conditions and the pseudo-source adding method is extended to the case of the linear-anisotropic scattering medium with graded index from non-scattering medium. Furthermore, the equivalence of the two methods is verified by formulation derivation. As exact analytical solutions, both the methods have high accuracy and fast computational speed. The predicted temperature distributions and dimensionless radiative heat flux at radiative equilibrium are determined by the proposed methods, and the numerical results are compared with the data in references. The results show that the present methods have a good accuracy. Influences of various combinations of refractive index and boundary emissivities on the temperature distributions and dimensionless radiative heat flux are also investigated.