表面辐射对部分填充吸湿性多孔介质的封闭腔体内热湿耦合传递的影响

根据多孔介质热质传递原理, 基于有限元的方法数值分析了具有表面热辐射的部分填充吸湿性多孔介质的封闭腔体内部自然对流流动及热湿耦合传递过程, 探讨了表面发射率、Rayleigh数和Darcy数等参数对封闭腔体内部自然对流流动及热湿耦合传递过程的影响, 研究结果表明, 壁面热辐射的作用可以提高多孔介质内部的温度, 而且随着表面发射率的增大, 多孔介质内部的水分逐步向其右上角迁移和聚集。另外, Darcy数、多孔介质与空气的热导率比对方腔内部多孔介质的热量传递和水分迁移影响较小。     

圆管束中导流器对其自然对流换热的影响

对无限大空间中5根同规格圆管组成的圆管束在管间放置导流器的自然对流换热进行了数值模拟研究。考虑了Ra在10³～10⁴范围内,导流器偏转角为0°～60°,圆管间距为2～4倍圆管直径的自然对流换热,分析了5根圆管的局部Nusselt数（Nu_loc）和平均Nusselt数（Nu_ave）。研究结果表明,导流器对管束结构的自然对流换热影响体现在两方面：一是对导流器下方圆管而言相当于障碍物削弱其换热,二是对导流器上方圆管而言隔绝了下方羽状流的影响,从而增强系统的整体换热。在圆管间距较大时,与无导流器的圆管束相比,C1～C4各圆管换热下降趋势明显放缓,并且从C4、C5圆管换热趋势上升,系统整体换热增强。圆管间距S=2D,Ra=10³,导流器主要起障碍物的作用,削弱C1～C4各圆管换热,导致系统整体换热下降。当导流器偏转角度为45°时系统换热达到最大值,较无导流器时换热最多有着21%的提升。     

Finite element analysis of natural convection in a triangular enclosure: Effects of various thermal boundary conditions

The phenomena of natural convection in a right-angled triangular enclosure is studied numerically. A penalty finite element analysis with bi-quadratic elements is used for solving the Navier-Stokes and energy balance equations. The detailed study is carried out in two cases depending on various thermal boundary conditions:

(a)

Vertical wall is uniformly or linearly heated while inclined wall is cold isothermal.

(b)

Inclined wall is uniformly or linearly heated while vertical wall is cold isothermal.

In all the cases horizontal bottom wall is adiabatic and the aspect ratio of the lengths of base and height is considered to be one. The present numerical procedure adopted in this investigation yields consistent performance over a wide range of parameters of Rayleigh number Ra(10³?Ra?10⁵) and Prandtl number Pr(0.07?Pr?1000) in all the cases mentioned above. Numerical results are presented in terms of stream functions, temperature profiles and Nusselt numbers. It has been found that at low Rayleigh numbers (Ra?10⁴), the isotherms are almost parallel near the bottom portion of the triangular enclosure while at Ra=10⁵, the isotherms are more distorted. This is because the heat transfer is primarily due to conduction for lower values of Rayleigh number. As Rayleigh number increases, there is a change from conduction dominant region to convection dominant region, and the critical Rayleigh number corresponding to on-set of convection is obtained. It has been shown that the average Nusselt number for vertical wall is times that of the inclined wall as the bottom wall is adiabatic and that verifies the thermal equilibrium of the system for all case studies.     

有内热源的多孔层中自然对流的稳定性分析

采用局部非热平衡模型, 通过数值法和Garlerkin近似法, 分析存在均匀内热源和边界浓度梯度时, 有效热导率比、流体和固相间的传热系数、浓度梯度的大小以及内热源在流体与固相内的分布情况对水平多孔层中临界内热源Rayleigh数的影响, 来研究相关参数对自然对流的稳定性的影响, 并得到临界内热源Rayleigh数的表达式。结果表明, 浓度Rayleigh数的增加可以促进自然对流的形成;内热源为正时, 自然对流的形成区域主要位于上半区域;内热源为负时, 自然对流的形成区域位于下半区域, 内热源总是促进自然对流的发生;有效热导率比、流体和固相间的内部传热系数、内热源在流体与固相内的分布情况相互耦合, 影响自然对流的稳定性, 这种影响取决于各参数的范围。     

封闭腔内水自然对流换热数值模拟

为了揭示封闭腔内非Boussinesq流体在浮力驱动下所特有的流动换热现象和形成机理,采用CFD软件Fluent对封闭腔内水的自然对流进行数值模拟,得到矩形封闭腔高宽比、Rayleigh数、倾斜角度、壁面温度差对流动和传热的影响规律。研究结果表明：由于水的密度在3.98℃达到最大,两竖壁面温度跨越这一点时会引起流动图像反转;具有流动反转的双涡结构降低了对流换热平均Nusselt数;相同Rayleigh数下,高宽比为1对应对流换热平均Nusselt数最大值;倾斜角度对平均Nusselt数影响与Rayleigh数和温度边界条件有关。     

Free convection heat transfer over a horizontal plate at low prandtl number

Boundary layer equations for free convection heat transfer along a semi-infinite horizontal plate are derived by giving more importance to the energy equation. The equations are obtained for low Prandtl number and two separate polynomials are used to approximate the temperature and velocity profiles in these regions. The rate of heat transfer is compared with the available analytical and numerical results based on conventional boundary layer equations.     

多开口方腔内自然对流的流动与传热特性

以计算流体力学与传热学理论为基础,建立三开口方腔内有热源驱动自然对流的物理数学模型,利用CFD方法对多开口方腔内流体的流动特性进行数值模拟和理论分析。在4种不同的通风模式下,比较热源强度和活动开口位置对热源表面的Nusselt数和方腔的量纲1有效流通量等流体流动、传热参数的影响。结果表明,在有热源驱动的自然对流条件下,与开口通风位置相比,热源强度对方腔内热流场的均匀性有更加重要的影响,揭示了三开口方腔内流体的流动特性与换热规律。     

The effect of liquid film vaporization on natural convection heat and mass transfer in a vertical tube

A numerical analysis was carried out to investigate the effects of film vaporization on natural convection heat and mass transfer in a vertical tube. Results for interfacial Nusselt and Sherwood numbers are presented for air-ethanol and air-water systems for various conditions. Predicted results show that heat transfer along the gas-liquid interface is dominated by the transport of latent heat in association with the vaporization of the liquid film. Additionally, the predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Chang et al. (1986) and Yan and Lin (1990) is only valid for systems with small liquid mass flow rates. For systems with a high liquid film Reynolds number, Re₁₀, the assumption of an extremely thin film is seriously in error.     

永磁梯度磁场布置方式对空气自然对流换热的影响

梯度磁场可用于控制非导电弱磁性介质的自然对流换热过程。利用钕-铁-硼永磁系统的不同空间布置，构建了具有不同磁场强度分布的梯度磁场，通过数值模拟得到了不同永磁梯度磁场的磁场强度和磁加速度。对不同梯度磁场作用下的二维封闭腔内的空气自然对流换热过程进行了数值研究，获得了空气自然对流的流场和温度场，以及壁面局部Nusselt数和平均Nusselt数并进行了比较。结果表明：空气自然对流换热可以通过施加具有不同磁加速度的梯度磁场得到强化或控制。     

Numerical simulation of transient natural convection induced by electrochemical reactions confined between vertical plane Cu electrodes

A transient natural convection develops along vertical plane Cu electrodes during the galvanostatic electrolysis. The time variations of both the concentration profile of CuSO₄ and the velocity profile of a 0.6 M CuSO₄ aqueous electrolyte were measured. The mass transfer rate of Cu²⁺ ion as well as the natural convective fluid motion was analyzed. The effect of electrode spacing on the transient natural convection was focused. The overshooting phenomenon caused by the transient diffusive and convective mass transfer rate was observed. Numerical simulation may provide as better understanding of the phenomena.     

耦合表面张力的封闭腔体内管外自然对流传热特性

采用格子-玻尔兹曼方法（LBM）构建了考虑表面张力影响下封闭腔体内加热圆柱外自然对流数学模型。在分别对自然对流与表面张力模型模拟验证的基础上,探究了液固表面张力与重力场下浮升力同时作用下加热圆柱外流体自然对流的传热特性。结果表明,在给定Ra数下（10³≤Ra≤10⁶）,随着表面张力的增大（Oh数减小）,封闭腔体内管外自然对流扰动会加剧,流型会变复杂,壁面换热效率有明显提高。在Ra数为10⁵,长度比（加热圆柱半径和方腔长度之比）为1/10情况下,加入表面张力σ=0.076302N/mm（Oh=0.122）,腔体左侧壁面平均努塞尔数和加热圆柱壁面与未加表面张力相比分别提高了93.5%和60.35%。当表面张力大小和自然对流的浮升力相当时,此时的流场波动更加明显剧烈,在一定范围内增大浮升力反而减弱换热。     

Flow transition for natural convective heat transfer in a porous medium saturated with water near 4°C

In this paper, the problem of buoyancy-induced convection flow in water-saturated porous media near 4°C is examined using a numerical model. Darcy's law is used to model flow behavior and a single equation convective heat transfer model is used for the energy equation. As the Boussinesq approximation is not valid for this case, a parabolic dependence of density on the temperature is used. Natural convection is generated and sustained by a uniform heat source. Flow behavior is governed by three natural parameters appearing in the model. They are: (i) dynamical parameter, (ii) geometric parameter, γ = b/a; and (iii) wall temperature, in relation to the reference temperature at the density extremum. For certain ranges of θ_w (<0) and Gr, interesting density inversion effects are possible. Transient solutions are obtained for various aspect ratios and modified Grashof number values. For a wide range of Grashof number, steady state solutions could not be obtained. Flow mutations into periodic and chaotic solutions are investigated for a range of Grashof number (100 to 40,000) and aspect ratio values (1 to 10).     

固体溶质溶解过程自然对流非稳态数值模拟

利用多物理场耦合分析软件Comsol Multiphysic 3.5及其动网格技术,结合传质学相关理论对固体溶质溶解过程进行了直接数值模拟.考虑了溶解过程中液相部分的自然对流,溶解引起的其表面形态的变化由溶解质量与相界面处的浓度梯度的关系建立的方程描述.研究结果表明:文中方法可以很好地捕捉由于溶解引起的界面运动和变形的...     

有体积热源的矩形空腔内层流自然对流的数值模拟

对具有内热源方腔的稳态层流耦合自然对流换热进行了三维的数值模拟,采用的模拟代码基于连续介质计算力学的开源库OpenFoam,解决了自然对流换热与固体传热的耦合问题.Ra数的变化从105到109.对外壁面为常温、方腔内充满含体积热源流体的自然对流计算结果表明,温度场、速度场与非耦合的工况有很大差异.     

Natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium

This article deals with natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous medium and moisture content transfer in hygroscopic porous medium were solved by the finite element method. The radiative heat transfer is calculated by making use of the radiosity of the surfaces that are assumed to be grey. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation and Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and porous medium. The mean temperature at the interface decreases, the temperature and moisture content gradients are created on the upper two corners of the porous medium region, and the moisture content in the porous medium decreases in the porous medium as Ra increases.     

Optimization of natural convection heat transfer of Newtonian nanofluids in a cylindrical enclosure

This study characterizes and optimizes natural convection heat transfer of two Newtonian Al2O3 and TiO2/water nanofluids in a cylindrical enclosure. Nusselt number (Nu) of nanofluids in relation to Rayleigh number (Ra) for different concentrations of nanofluids is investigated at different configurations and orientations of the enclosure. Results show that adding nanoparticles to water has a negligible or even adverse influence upon natural convec-tion heat transfer of water:only a slight increase in natural convection heat transfer of Al2O3/water is observed, while natural convection heat transfer for TiO2/water nanofluid is inferior to that for the base fluid. Results also reveal that at low Ra, the likelihood of enhancement in natural convection heat transfer is more than at high Ra:at low Ra, inclination angle, aspect ratio of the enclosure and nanoparticle concentration influence natural convec-tion heat transfer more pronouncedly than that in high Ra.     

双周期温度边界下多孔介质方腔内自然对流传热数值模拟

采用局部非热平衡模型及方腔左右两侧壁面温度正弦波变化边界条件,数值分析了具有内热源多孔介质方腔内的稳态非达西自然对流传热。探讨了不同温度波动参数N及波动相位差τ对方腔内自然对流传热的影响。结果表明:方腔两侧壁面出现了周期性分布的温度场,随着N值的增加,方腔内流场及温度场分布逐渐趋向于壁面均一温度边界情况。壁面局部Nu沿着高度方向呈现周期性分布。相对于均一温度的边界条件而言,正弦波温度边界条件在一定程度上强化了多孔介质方腔内的整体传热过程,随着N值的增加,方腔处于温度波动边界时的散热值Q逐渐趋向于均一温度边界时的情况。     

Transient cooling of petroleum by natural convection in cylindrical storage tanks: A simplified heat loss model

The transient cooling by natural convection of a warm crude oil contained in a large vertical cylindrical storage tank located in a cold environment is investigated. The governing mass, momentum and energy equations utilizing a temperature-dependent apparent viscosity were solved numerically, and from the results a correlation is developed between the Nusselt number at the sidewall and the instantaneous Rayleigh number driving the time-dependent flow. A simplified heat loss model is developed to predict the mean tank temperature over time, utilizing this correlation to calculate a time-dependent heat transfer coefficient for the sidewall. The results from the simplified model are compared with the mean tank temperatures results from a simulation of the full natural convection equations for the cooling of a crude oil in a 60 m diameter tank, and excellent agreement is obtained.     

吸收过程自然对流非稳态数值模拟

传质过程中液相密度变化导致的自然对流,改变了液相流动状态,对传质有着较大的影响。利用纹影仪光学方法,获得了乙醇吸收CO2过程中自然对流的纹影图像;并通过吸收传质流体CFD模型,应用有限元方法对气液吸收过程中出现的自然对流发生过程进行了数值计算,得到了液相自然对流发生时的非稳态流场分布和浓度分布信息。分析结果表明传质过程中当自然对流发生后,促进了液相表面更新,加强了传质效果,在考察相际传质过程时,应充分考虑自然对流对传质速率的影响。     

Inverse solution for a biochemical heat source in a porous medium in the presence of natural convection

The inverse problem approach by conjugate gradient with adjoint equations is adapted to the context of a non-isothermal bioprocess, controlled by internal heat generation from microbial oxidation, as could be found in a composting reactor for instance, to determine the heat source from internal temperature measurements. The volumetric heat source is assumed proportional to the rate of consumption of a substrate by a biomass, as described by a Monod model. Computations are performed for Rayleigh numbers equal to 0.25 and 25, for a representative biochemical reaction under typical boundary conditions, for constant and temperature-dependent model parameters. The influence of noisy input data is also considered. It is found that good solutions can be obtained when heat release and diffusion occur over very different time scales. The variation of the model parameters with temperature must be taken into account, but single sensor solutions are possible at relatively small Rayleigh numbers when convection is present.