内置发热体的倾斜开口腔内自然对流数值模拟

本文对具有一个内置发热体的二维倾斜开口方腔自然对流问题进行了数值模拟.计算参数取值为: Prandtl数0.7,Rayleigh数范围从103～108,倾斜角度从0°～90°数值结果表明,流动形式根据开口界面底部流体的流向分为两种流动形式.平均Nusselt数为Rayleigh数的增函数,增幅亦随Rayleigh数的增加而增加.当Ra<106时,平均Nusselt数受到ψ的影响较小,当Ra≥106时,平均Nusselt数取得最大值的ψ为45°.     

倾角对方腔内热对流非线性演化与分岔的影响

倾斜封闭腔内对流换热问题是非线性非平衡系统中研究的热点问题之一.本文采用高精度数值方法对倾斜方腔内流体热对流进行了直接数值模拟,研究了腔体倾角在0°—180°之间变化时,倾角的不同变化过程对流场非线性演化、传热效率以及流动分岔的影响.所考虑的Rayleigh数范围为10~3—10~6.结果表明:表征传热效率的Nusselt数对Rayleigh数、Prandtl数及倾斜角度均具有较强依赖性,在较高Rayleigh数时, Nusselt数会在80°和100°附近产生较大幅度的变化;高Rayleigh数下流场及温度场的演变更为复杂,腔体内存在1—3个对流强度不等的涡卷;低Rayleigh数下腔体倾角接近90°时流动状态为热传导状态.当腔体倾角介于70°—110°之间时,在Rayleigh数Ra∈(4949, 314721)内存在解的两条稳定分支.     

环己烷/氧气混合物Rayleigh-Bénard对流三维数值模拟

为了了解环己烷/氧气混合物Rayleigh-Bénard(R-B)对流时的温度分布和流场结构、避免环己烷氧化过程中爆燃的潜在危险,对立方腔体内具有密度极值的环己烷/氧气混合物R-B对流进行了一系列三维数值模拟。结果表明,密度极大值现象对R-B对流具有明显的抑制作用。随着密度倒置参数的增加,对流发生的临界Rayleigh(Ra)数增加。当密度倒置参数较大时,在密度极值面以上的流体层中会出现次级流胞,流动分层现象明显。R-B对流的传热能力随Ra数的增加而增强,随密度倒置参数的增加而削弱。     

斜偏心水平圆柱环形空间自然对流传热

一、前言 对偏心水平圆柱环形空间自然对流传热的系统研究仅有十年左右的历史。本文采用数值方法对斜偏心的情况,探索瑞利数、斜偏心率、直径比、偏心斜角等因素对自然对流传热的影响,并借助Mach-Zehnder光干涉仪进行实验研究。 二、数值分析 经过双极坐标变换,控制方程变为:     

水平方管层流混合对流的三维数值模拟

本文建立了水平矩形通道三维层流混合对流的数学模型,提出了壁面导热与流动耦合的简化计算方法,采用SIMPLER算法对其流动和换热过程进行了数值模拟.计算结果表明由于浮升力的作用使水平通道内出现二次流现象,加热面的上下顶端周向导热热流密度存在极大值,下底面的局部Nusselt数最大,上底面最小.混合对流:平均Nusselt数随 Re和Gr*的增大而增大;但当Re较大时,平均Nusselt数随Gr*变化而近似不变.     

底部局部加热多孔介质自然对流传热的格子Boltzmann模拟

运用格子Boltzmann方法研究了底部局部加热多孔介质方腔的自然对流传热.方腔的上壁面为低温热源,下壁面为局部高温热源,左右壁面为绝热条件.重点分析了高温热源位置a及尺寸b对多孔介质方腔自然对流传热性能的影响,提出了平均Nusselt数Nu和位置a及尺寸b的拟合关系式.研究结果表明:高温热源位置及尺寸对多孔介质方腔内自然对流传热性质的影响很大,且存在最佳高温热源位置(a=4/16)和尺寸(b=0.75),以达到最强的对流换热强度(Nu_(max)≈10.35)和最大的对流换热量(Q_(max)≈5.69).     

垂直矩形通道内混合对流的实验研究

通过实验研究了内置均匀加热板的垂直矩形通道入口段混合对流换热的温度场和阻力特性。实验分别针对同向流动和反向流动两种情况进行了研究,其中Re=329-5565,Gr=9．7×105-7．7×106。通过实验得到了矩形通道内混和对流换热的总体平均Nusselt数和压力损失的变化规律。实验表明固体壁面的导热对总体换热具有非常重要的作用。平均Nusselt数随Re的增加而增加; Re<1000时反向流动的Nusselt数大于同向流动时Nusselt数,Re>1800时二者相差很小。压力损失与Re近似为乘幂关系,且随热流密度变化很小。     

考虑Soret和Dufour效应的方腔内双扩散自然对流格子Boltzmann模拟

采用格子Boltzmann方法,考虑Soret和Dufour效应,对内置高浓度发热圆的方腔内部双扩散自然对流现象进行数值模拟.高浓度发热圆位于方腔中心,四周壁面均为低温低浓度.在该模型中,用三个独立的LBGK方程分别模拟速度场、温度场和浓度场,并通过Boussinesq近似将它们耦合起来.分析Soret数和Dufour数对方腔内部双扩散自然对流的影响,得到流线图、等温线图、等浓度线图、发热圆表面平均Nusselt数和平均Sherwood数.结果表明：Soret和Dufour效应对方腔内双扩散自然对流影响明显,不能忽略.     

复杂多孔介质腔体内自然对流换热的数值研究

采用曲线坐标系下压力与速度耦合的SIMPLEC算法,数值研究复杂多孔介质腔体内的自然对流换热问题.腔体的曲面温度分别保持恒定,上下表面绝热.在曲线坐标系中用有限容积法离散方程,并采用Brinkman扩展达西模型及局部非热平衡模型求解,综合研究Rayleigh数,Darcy数、孔隙率等参数对腔体内自然对流换热的影响.计算结果表明:Rayleigh数和Darcy数的影响最大而孔隙率的影响很小,同时存在使得腔体内换热达到最强的最佳纵横比.     

深低温球内伪稳态自然对流换热研究

基于CFD仿真软件FLUENT,建立了封闭球形容腔内氦气关键物性参数随温度变化的深低温自然对流仿真模型。利用该模型对封闭球形容腔内20～100 K温度下不同瑞利数(10~8Ra10~(11))伪稳态自然对流换热进行了数值模拟,得到了速度场分布、温度场分布和努塞尔数(Nu).开展了液氢温区球形封闭容腔内氦气伪稳态自然对流换热试验。通过与试验数据的对比分析,验证了本文所提出的深低温自然对流模型的有效性。利用最小二乘法,获得了深低温球形容腔内氦气自然对流换热准则数方程。     

Heat Convection Between Two Confocal Elliptic Tubes Placed at Different Orientations

In this paper, transient and steady natural convection heat transfer in an elliptical annulus has been investigated. The annulus occupies the space between two horizontal concentric tubes of elliptic cross-section. The resulting velocity and thermal fields are predicted at different annulus orientations assuming isothermal surfaces. The full governing equations of mass, momentum and energy are solved numerically using the Fourier Spectral method. The heat convection process between the two tubes depends on Rayleigh number, Prandtl number, angle of inclination of tube axes and the geometry and dimensions of both tubes. The Prandtl number and inner tube axis ratio are fixed at 0.7 and 0.5, respectively. The problem is solved for the two Rayleigh numbers of $10^4$ and $10^5$ considering a ratio between the two major axes up to 3 while the angle of orientation of the minor axes varies from $0^\circ$ to $90^\circ$. The results for local and average Nusselt numbers are obtained and discussed together with the details of both flow and thermal fields. For isothermal heating conditions, the study has shown an optimum value for major axes ratio that minimizes the rate of heat transfer between the two tubes. Another important aspect of this paper is to prove the successful use of the Fourier Spectral Method in solving confined flow and heat convection problems.     

竖环形空腔内有均匀容积热源时自然对流换热的数值研究

本文对竖环形空腔内有均匀容积热源、上下底面绝热、两侧壁有温差时的自然对流换热建立了数学模型，用有限差分法进行了数值求解，对半径比接近于１时的数值结果，同前人已有结果的吻合程度是令人满意的。对固定的外、内管半径比，给出了不同的ＲａＩ／ＲａＥ值、Ａ值下几种不同的流动模式，分析了沿热壁局部热流密度的变化规律。     

圆内开缝圆自然对流的Ruelle-Takens混沌道路

使用热格子Boltzmann方法针对圆内开缝圆自然对流的流动与换热进行数值模拟，通过相空间、功率谱等进行非线性动力学特性分析，研究其流动与换热的稳定性.结果表明：随着瑞利数Ra的增加，流场的相图从开始稳定的平衡点经历Hopf分岔后转变为极限环，表明流场进入一个倍周期性振荡状态；随着瑞利数进一步增加，稳定的极限环分岔为二维环面，系统相空间结构复杂化；当瑞利数Ra大于某一临界值时，二维环面分岔突变进入混沌状态，系统在相空间中出现非常复杂的轨线结构.总体上，通过系统不同瑞利数所对应的非线性动力学特性的表现形式，表明系统经过Ruelle-Takens道路到达混沌，展现出自然对流从稳定的流动和换热发展到非线性运动特征的混沌历程.     

Étude numérique de l'influence du transfert radiatif sur la convection naturelle laminaire,bidimensionnelle, permanente,dans un espace annulaire d'axe horizontal,délimité par deux cylindres circulaires isothermes

Combined radiation and natural convection in a participating medium between concentric or vertically eccentric horizontal cylinders is investigated numerically. The annular medium is considered as a gray, emitting, absorbing, and isotropically scattering gas. The equations of steady, laminar, two-dimensional, thermal, natural convection are written by using a two-cylindrical coordinate system, the stream function, and the vorticity. The finite volumes method is used to discretize the coupled equations of momentum, energy, and radiative transfer. To solve the global nonlinear algebraic equations the successive-over-relaxation iterating scheme is applied. Numerical solutions are obtained for a Rayleigh number in the range 10³–10⁵ and radiation-conduction parameter ranging from 0 to ∞. The influences of radiation-conduction parameter, Rayleigh number and other parameters on flow and temperature distributions and heat transfer are discussed.     

Étude numérique du modèle de Boussinesq de convection naturelle laminaire axisymétrique permanente dans un espace annulaire compris entre deux sphères

Authors study numerically the axisymetric steady natural convection in the annular space between two vertically eccentric spheres applying the Boussinesq approximation and an integro-interpolation finite volumes method. In the case of two isothermal concentric spheres their results agree with those of the literature. Also, they obtain correlations between the average Nusselt number and the Rayleigh number quasi identical to those publish by Chiu and Scanlan. When the internal sphere is heated by the application of a constant heat flux and eccentric as compared to the supposed external sphere isothermal, they propose correlations between the average Nusselt number and the Rayleigh number that depend on the eccentricity and on the Prandtl number. According to initialising conditions of calculation, the external sphere being isothermal, authors show that there exists a critical Rayleigh number beyond which the flow can be found unicellular or bicellular. This critical Rayleigh number depends on others parameters of the system as the eccentricity and the aspect ratio when the internal sphere is isothermal as well as subjected to a uniform constant heat flux density.     

Experimental studies on natural convection boiling of water in a vertical annulus of a closed-loop thermo-siphon

Experimental studies on natural convection boiling of water in an internally heated narrow vertical annulus, with the liquid circulating through a cold leg forming a closed-loop thermo-siphon, have been carried out. The radius and aspect ratios of the annulus are 1.184 and 352, respectively. The experimental data, which consist of wall and liquid temperatures, liquid and vapor flow rates, and differential pressure across the test section, are recorded on a data logging system. The experiments have been performed for a constant heat flux of 15–35 kW/m² from the startup period until the steady state to study the transient behavior of the system. The boiling and non-boiling zones in the annulus have been identified and presented graphically through the liquid and wall temperatures for the steady state. They have been also verified through the visual photographs of the flow patterns in the annulus. The flow is found to be oscillatory in nature with no particular trend. Although the experimental data seems to be scattered, but when analyzed for a short duration, they are found well within the ±3σ (three sigma). This confirms the quasi-steady-state condition of the system. The steady-state values of Reynolds number and liquid circulation rate come out to be 133.1–453.5 and 7.0–23.87 g/s, respectively, while the Nusselt number and heat transfer coefficient are 7.98–13.57 and 1433.57–2435.35 W/m²K, respectively. Mathematical correlations for liquid mass flow rates, heat transfer coefficient, Reynolds number, and Nusselt number have been developed and compared with the existing correlations, which are in good agreement.     

黑腔冷冻靶传热与自然对流的数值模拟研究

惯性约束聚变的设计要求在靶丸内形成均匀光滑的氘氚冰层, 靶丸周围的热环境对冰层的质量特别是低阶粗糙度有很大的影响. 本文对自主研发的黑腔冷冻靶实验装置中的热物理问题展开了数值模拟, 重点考察了黑腔冷冻靶的传热和流体力学特性. 通过参数分析得到了自然对流对靶丸温度均匀性产生影响的临界条件. 比较了黑腔不同布置朝向时的流场和温度分布, 结果显示黑腔水平布置时自然对流更加强烈, 造成的靶丸温度不均匀性也更大. 在此基础上, 讨论了消除自然对流影响的可能性, 结果发现仅当黑腔垂直布置时利用黑腔分区方法能够消除对流效应对靶丸温度不均匀性的影响而黑腔水平布置时不能消除. 研究结论对于实验中冷冻靶结构的设计、改进和实验的开展等具有指导意义.     

基于原油最高温度点变化规律分析停输相变传热

采用分区法数学模型,利用等价比热容法处理析蜡潜热问题,追踪停输相变过程中管道内原油最高温度点位置的变化轨迹,确定管道内原油最终凝固位置,提出管道内原油全凝的判断依据。基于最高温度点位置的变化规律将传热过程分为四个阶段,重点讨论了各阶段中自然对流、潜热释放、传热方式转化对最高温度位置点变化的影响,结合Ra数变化分析了自然对流换热影响从强到弱的变化过程。深入探讨了停输相变过程中管道内固相率的变化及不同位置处原油温降曲线的特点。     

自然对流换热的高精度非结构网格有限体积法

用非结构网格有限体积法求解自然对流换热时,传统的对流项离散格式难以兼顾数值精度与计算效率,我们发展了一种耦合高精度格式的延迟修正方法,用于对流项的离散.高Re数下方腔驱动流数值计算验证了该方法具有较高的计算精度和较好的稳定性.Boussinesq流体的自然对流换热数值模拟,表明该方法能有效克服高Ra数时数值计算发散,可准确捕捉自然对流换热问题中不同偏心率下的等温线和流线分布特征.     

Convection driven zonal flows and vortices in the major planets

The dynamical properties of convection in rotating cylindrical annuli and spherical shells are reviewed. Simple theoretical models and experimental simulations of planetary convection through the use of the centrifugal force in the laboratory are emphasized. The model of columnar convection in a cylindrical annulus not only serves as a guide to the dynamical properties of convection in rotating sphere; it also is of interest as a basic physical system that exhibits several dynamical properties in their most simple form. The generation of zonal mean flows is discussed in some detail and examples of recent numerical computations are presented. The exploration of the parameter space for the annulus model is not yet complete and the theoretical exploration of convection in rotating spheres is still in the beginning phase. Quantitative comparisons with the observations of the dynamics of planetary atmospheres will have to await the consideration in the models of the effects of magnetic fields and the deviations from the Boussinesq approximation.