空气隔热夹层围岩温度场数值计算及试验分析

随着矿产资源的开采深度日益增加,矿井高温热害问题日益突出,研究新的隔热技术、隔热工艺已经成为控制围岩传热的重要研究方向。鉴于空气夹层结构在建筑围护结构和寒区隧道中的保温隔热作用,提出应用封闭空气夹层控制巷道围岩散热量的方法。采用ANSYS Fluent软件求解了含空气夹层的围岩温度场分布。通过相似理论设计了含空气夹层的巷道围岩温度场测试试验台,并应用试验结果对数值计算结果进行了验证,两者具有很好的吻合性,证明了ANSYS Fluent模拟结果的正确性。应用计算模拟结果和试验结果分析了围岩内温度场非稳态传热过程中的变化规律,发现因空气夹层内自然对流现象的存在,顶部、中部和底部围岩温度场发展并不一致,但其调温圈无因次半径均小于无隔热巷道。并通过实例计算研究,发现空气层厚度对围岩散热量影响较小,空气夹层厚度从2 cm增加到20 cm时,围岩单位长度散热量仅下降5%;围岩导热系数对空气夹层隔热率有着显著的影响,导热系数为0.59 W/(m·K)比围岩导热系数6.18 W/(m·K)时夹层隔热率衰减速率快。封闭空气夹层隔热结构适用于新掘进巷道或高温隧道的临时性隔热或围岩导热系数高的固定或半固定作...     

非线性热环境下高温合金蜂窝板隔热性能研究

金属蜂窝板结构在高温热环境下的隔热特性是高速飞行器热防护设计的重要参数. 使用自行研制的高速飞行器瞬态气动热试验模拟系统, 对高温合金蜂窝平板结构在高达800℃的非线性热环境下的隔热性能进行实验研究, 获得了蜂窝板结构的瞬态和稳态传热特性以及在多种不同温度下金蜂窝平板结构隔热效果的实验数据. 在考虑结构内部蜂窝芯壁面间辐射、金属结构的传热以及蜂窝腔内空气传热的多重热交换条件下, 采用三维有限元计算方法对蜂窝板的隔热特性进行了数值模拟, 计算结果和试验结果的吻合性良好, 验证了数值模拟方法的可信性和有效性, 并为数值模拟方法能够在一定程度上较好地替代价格昂贵的气动热模拟试验打下了基础. 讨论了在复杂非线性高温环境下金属蜂窝板隔热效率的变化, 加热面温度的升降速度与隔热效率的关联性以及金属蜂窝板表面发射率的选取等问题, 对高速飞行器金属蜂窝结构的热防护研究具有重要的参考价值.      

光纤陀螺光纤敏感线圈三维温度瞬态响应模型

针对光纤敏感线圈的热致非互易相移问题,通过传热分析建立了光纤敏感线圈的三维温度瞬态响应模型.利用该温度瞬态响应模型全面分析了三维坐标下的温度梯度造成的热致非互易效应;结合光纤陀螺实际的工作环境,利用仿真对四极对称绕法的光纤环进行了分析,获取了温度激励造成的热致速率误差.通过温度模型计算的热致误差速率与温箱中光纤陀螺稳测数据的极差为0.48(°)/h,计算结果验证了光纤敏感线圈三维温度瞬态响应模型的有效性.利用该模型可有效指导光纤环热结构参数的设计,从而抑制光纤环的Shupe误差,提高光纤陀螺的温度性能.     

多宗量瞬态热传导反演识别

引入Bregman距离加权函数,建立了多宗量瞬态热传导反演的一种求解模式.时域上采用精细算法,分别建立了便于敏度分析的有限元正/反演模型,应用同伦算法进行反问题求解,对导热系数和边界条件等宗量进行有效的组合识别.对信息误差和计算效率作了探讨,并给出了相应的数值验证.     

激光辐照下多层圆柱体中三维瞬态温度场的解析解

考虑外表面的气流影响和层间温度与传热的协调关系,建立了激光辐照下,层合圆柱体中的三维瞬态热传导解析模型。利用特征值法和Bessel函数,导出了各层柱体中三维瞬态温度场的封闭解析解。以一维轴对称问题为例计算了柱体中的瞬态温度场,给出了柱体内部温度随时间的变化和柱体表面换热系数对温度场的影响规律。本文的理论解可进一步用于分析层合圆柱体中的三维瞬态热-力效应,并可作为相应问题的数值模拟中数值模型的修正依据。     

轻质多层热防护结构的一体化优化设计研究

大面积防热结构在航天航空领域应用广泛, 其创新结构设计是关键技术之一. 航天飞行器的工作条件要求热防护材料与结构同时具备轻质、隔热、抗冲击的特点, 因此热防护材料与结构正在朝着一体化的方向发展. 基于这种发展趋势, 提出了一种轻质多层热防护结构设计方案. 以一体化多层防热结构在航天器再入过程中的传热为研究对象, 引入大面积防热结构的一维传热假设, 依照航天器再入大气层的温度条件, 建立了防热结构一维非稳态传热的物理模型和封闭的控制方程, 使用差分方法求解方程, 进行一维非稳态的传热分析, 并采用商业有限元软件ABAQUS的传热分析进行验证. 得到了航天器再入大气过程中多层防热结构的各层温度分布, 提出了在满足一定的热约束要求的条件下, 以轻质多层热防护结构的总重量为目标函数的优化设计方法, 得到了多层结构的最优几何参数, 并将优化后的结构进行了有限元承载分析.      

金属结构航天器陨落过程三维瞬态传热有限元算法研究

构建金属桁架结构航天器陨落再入气动热环境有限元传热模型,是准确预测在轨服役期满大型航天器陨落再入解体过程温度分布的关键。本文采用四节点四面体单元对空间进行离散,依据泛函理论,将传热控制方程离散为代数方程组;利用有限单元法总体合成得到具有对称正定、高度稀疏和非0元素分布的规则性刚度矩阵,发展一维变带宽压缩存贮技术,有效解决大型稀疏矩阵的数据存贮问题;为有效抑制求解过程出现的温度在时间和空间上的振荡问题,发展集中热容矩阵系数处理方法,将热容矩阵的同行或同列元素相加代替对角线元素,使非对角线元素化为0,构造求解三维瞬态温度场的两点向后差分格式、Crank-Nicolson格式和Galerkin格式。通过对正方体瞬态传热计算验证分析,在相同条件下,采用以上三种格式均可获得一致稳定的温度解,并得到与现有ANSYS有限元软件较为吻合的计算结果,验证了所建立三维瞬态传热有限元计算模型的准确可靠性。在此基础上,对铝合金低轨航天器薄壳结构进行了传热计算,给出了类天宫飞行器两舱体陨落飞行107.5 km～90 km不同高度的瞬态温度分布,为这类寿命末期航天器陨落再入解体预报提供理论支撑与可计算模型。     

金属结构航天器陨落过程三维瞬态传热有限元算法研究

构建金属桁架结构航天器陨落再入气动热环境有限元传热模型，是准确预测在轨服役期满大型航天器陨落再入解体过程温度分布的关键。本文采用四节点四面体单元对空间进行离散，依据泛函理论，将传热控制方程离散为代数方程组；利用有限单元法总体合成得到具有对称正定、高度稀疏和非0元素分布的规则性刚度矩阵，发展一维变带宽压缩存贮技术，有效解决大型稀疏矩阵的数据存贮问题；为有效抑制求解过程出现的温度在时间和空间上的振荡问题，发展集中热容矩阵系数处理方法，将热容矩阵的同行或同列元素相加代替对角线元素，使非对角线元素化为0，构造求解三维瞬态温度场的两点向后差分格式、Crank-Nicolson格式和Galerkin格式。通过对正方体瞬态传热计算验证分析，在相同条件下，采用以上三种格式均可获得一致稳定的温度解，并得到与现有ANSYS有限元软件较为吻合的计算结果，验证了所建立三维瞬态传热有限元计算模型的准确可靠性。在此基础上，对铝合金低轨航天器薄壳结构进行了传热计算，给出了类天宫飞行器两舱体陨落飞行107.5 km~90 km不同高度的瞬态温度分布，为这类寿命末期航天器陨落再入解体预报提供理论支撑与可计算模型。     

基于特征正交分解的一类瞬态非线性热传导问题的新型快速分析方法

提出了一种基于特征正交分解(POD)和有限元法的瞬态非线性热传导问题的模型降阶快速分析方法, 建立了导热系数随温度变化的一类瞬态非线性热传导问题有限元格式的POD降阶模型. 在隐式时间推进方法的基础上有效结合单元预转换方法和多级线性化方法发展了一种加速求解瞬态非线性热传导降阶模型的新型计算方法,并通过二维和三维算例验证了该方法的准确性和高效性. 研究结果表明: (1)降阶模型解的均方根误差在经过初始时段轻微的脉动后稳定于0.01%以下, 而其计算效率比有限元全阶模型提高2$\sim $3个数量级, 并且自由度数量(DOFs)愈大提高的幅度也愈加显著; (2)新型算法解决了常规算法在计算非线性降阶模型时加速性能差的问题, 即使是在DOFs比较小的时候也能够明显提高计算效率; (3)常数边界条件下得到的POD模态可以用来建立相同求解域在各种复杂时变边界条件下的瞬态非线性热传导降阶模型, 并对其传热过程和温度场进行快速准确的分析与预测, 具有很好的工程应用价值.      

基于特征正交分解的一类瞬态非线性热传导问题的新型快速分析方法

提出了一种基于特征正交分解(POD)和有限元法的瞬态非线性热传导问题的模型降阶快速分析方法,建立了导热系数随温度变化的一类瞬态非线性热传导问题有限元格式的POD降阶模型.在隐式时间推进方法的基础上有效结合单元预转换方法和多级线性化方法发展了一种加速求解瞬态非线性热传导降阶模型的新型计算方法,并通过二维和三维算例验证了该方法的准确性和高效性.研究结果表明:(1)降阶模型解的均方根误差在经过初始时段轻微的脉动后稳定于0.01%以下,而其计算效率比有限元全阶模型提高2～3个数量级,并且自由度数量(DOFs)愈大提高的幅度也愈加显著;(2)新型算法解决了常规算法在计算非线性降阶模型时加速性能差的问题,即使是在DOFs比较小的时候也能够明显提高计算效率;(3)常数边界条件下得到的POD模态可以用来建立相同求解域在各种复杂时变边界条件下的瞬态非线性热传导降阶模型,并对其传热过程和温度场进行快速准确的分析与预测,具有很好的工程应用价值.     

Heat transfer by conduction,natural convection and radiation across a rectangular cellular structure

This paper describes results on the effects of wall conduction and radiation heat exchange among surfaces on laminar natural convection heat transfer in a two-dimensional rectangular cavity modelling a cellular structure. Parametric heat transfer calculations have been performed, and numerical results are presented in graphical and tabular form. Local and average Nusselt numbers along the cavity walls are reported for a range of parameters of physical interest. The findings suggest that the local or the average Nusselt number is one of many parameters that control conjugate heat transfer problems. The results indicate that natural convection heat transfer in the cavity is reduced by heat conduction in the walls and radiation exchange among surfaces. The results obtaibed for the total heat transfer rate through the system using the two-dimensional model are compared with those based on a one-dimensional model.     

A Two-Equation Model for Heat Conduction in Porous Media (II: Application)

The formulism of a two-equation model for heat conduction in porous media, developed in a previous paper, is applied to the case of steady state one-dimensional heat transfer in a porous medium that is made up of geometrically similar units of similar size and of ordered spatial distribution. For this case study, the model-predicted locally volume-averaged temperature distributions for the solid and the fluid phase are compared to a numerical solution at a microscopic level, showing excellent agreement.     

A numerical simulation of combined radiation and natural convection heat transfer in a square enclosure heated by a centric circular cylinder

A numerical simulation of combined natural convection and radiation in a square enclosure heated by a centric circular cylinder and filled with absorbing-emitting medium is presented. The ideal gas law and the discrete ordinates method are used to model the density changes due to temperature differences and the radiation heat transfer correspondingly. The influence of Rayleigh number, optical thickness and temperature difference on flow and temperature fields along with the natural convection, radiation and total Nusselt number at the source surfaces is studied. The results reveal that the radiation heat transfer as well as the optical thickness of the fluid has a distinct effect on the fluid flow phenomena, especially at high Rayleigh number. The heat transfer and so the Nusselt number decreases with increase in optical thickness, while increases greatly with increase in temperature difference. The variation in radiation heat transfer with optical thickness and temperature difference is much more obvious as comparison with convection heat transfer.     

Numerical Solvers for Radiation and Conduction in High Temperature Gas Flows

In this paper, the authors introduce a robust numerical technique for radiation–conduction heat transfer in the high temperature fields of gas turbine combustors. The conduction and radiation effects are analyzed by a differential and an integral equation, respectively. Using discrete ordinates for the angular discretization of the integral equation for the radiation effects and a Galerkin discretization for the heat equation, the authors propose a fast multilevel algorithm to solve the fully discretized problem. The algorithm uses the same mesh hierarchy for both radiation and conduction effects, but with two different smoothing operators. Numerical results are shown for test problems in three space dimensions, and comparisons to other methods are also given.     

Interaction of surface radiation with conjugate mixed convection from a vertical channel with multiple discrete heat sources

Important results of a numerical study performed on combined conduction–mixed convection–surface radiation from a vertical channel equipped with three identical flush-mounted discrete heat sources in its left wall are provided here. The channel has walls of identical height with the spacing varied by varying its aspect ratio (AR). The cooling medium is air that is considered to be radiatively transparent. The heat generated in the channel gets conducted along its walls before getting dissipated by mixed convection and radiation. The governing equations for fluid flow and heat transfer are considered without boundary layer approximations and are transformed into vorticity–stream function form and are later normalized. The resulting equations are solved, along with relevant boundary conditions, making use of the finite volume method. The computer code written for the purpose is validated both for fluid flow and heat transfer results with those available in the literature. Detailed parametric studies have been performed and the effects of modified Richardson number, surface emissivity, thermal conductivity and AR on various pertinent results have been looked into. The significance of radiation in various regimes of mixed convection has been elucidated. The relative contributions of mixed convection and radiation in carrying the mandated cooling load have been thoroughly explored.     

Heat transfer in a radiating fluid with slug flow in a parallel-plate channel

Summary As a step towards a better understanding of combined conduction, convection, and radiation, fully developed heat transfer in slug flow in a flat duct between two parallel plates is considered. The flat duct consists of two diffuse, nonblack, isothermal surfaces. The gray radiating fluid between them is capable of emitting and absorbing thermal radiation. The problem is formulated in terms of a nonlinear integrodifferential equation, and the solution is obtained by an approximate method. The differences between heating and cooling the fluid are examined. The effects of the optical thickness of the fluid, the ratio which determines the relative role of energy transport by conduction to that by radiation, and the emissivity of the duct surfaces on the temperature distribution and the heat transfer characteristics are investigated. An approximate method for calculating the radiant heat flux at the wall is presented, and the accuracy of the approximation is tested.Work done under the auspices of the U.S. Atomic Energy Commission.     

Approximate analysis for transient heat conduction with radiation in a slab

In an attempt to minimize the numerical computations associated with the solution of transient heat conduction with radiation in a slab, a perturbation type of analysis is being applied to the temperature field and radiation heat flux simultaneously. The resulting partial differential equations for the perturbation functions for the temperature are solved in explicit forms by use of the energy integral methods, while the radiation heat flux is determined by an appropriate scheme of approximating the temperature distribution in the slab. Included in the analysis are the effects of the parameters: the optical thickness, the ratio of conduction transport to radiation and the wall emissivity. It is found that, in a wide range of these governing parameters, the results compare very favorably with those obtained by the numerical solution of the formulated integro-differential equation. With the present analysis, the temperature, conduction and radiation heat fluxes can be predicted without resorting to lengthy numerical analysis.     

Coupling model for unsteady MHD flow of generalized Maxwell fluid with radiation thermal transform

This paper introduces a new model for the Fourier law of heat conduction with the time-fractional order to the generalized Maxwell fluid. The flow is influenced by magnetic field, radiation heat, and heat source. A fractional calculus approach is used to establish the constitutive relationship coupling model of a viscoelastic fluid. We use the Laplace transform and solve ordinary differential equations with a matrix form to obtain the velocity and temperature in the Laplace domain. To obtain solutions from the Laplace space back to the original space, the numerical inversion of the Laplace transform is used. According to the results and graphs, a new theory can be constructed. Comparisons of the associated parameters and the corresponding flow and heat transfer characteristics are presented and analyzed in detail.     

Flow of variable thermal conductivity fluid due to inclined stretching cylinder with viscous dissipation and thermal radiation

The aim of the present study is to investigate the flow of the Casson fluid by an inclined stretching cylinder. A heat transfer analysis is carried out in the presence of thermal radiation and viscous dissipation effects. The temperature dependent thermal conductivity of the Casson fluid is considered. The relevant equations are first simplified under usual boundary layer assumptions, and then transformed into ordinary differential equations by suitable transformations. The transformed ordinary differential equations are computed for the series solutions of velocity and temperature. A convergence analysis is shown explicitly. Velocity and temperature fields are discussed for different physical parameters by graphs and numerical values. It is found that the velocity decreases with the increase in the angle of inclination while increases with the increase in the mixed convection parameter. The enhancement in the thermal conductivity and radiation effects corresponds to a higher fluid temperature. It is also found that heat transfer is more pronounced in a cylinder when it is compared with a flat plate. The thermal boundary layer thickness increases with the increase in the Eckert number. The radiation and variable thermal conductivity decreases the heat transfer rate at the surface.     

Similarity analysis of model experiments for film cooling in gas turbines

This report investigates our present ability to predict the thermal performance of film cooling arrangements used to protect the hot components of gas turbines. The required information is usually obtained by model experiments carried out at near room temperature as opposed to the high temperature encountered in the gas turbines. Dimensional or similarity analysis is used to develope the functional relationships for film effectiveness and convective heat transfer and the use of mass transfer experiments with foreign gas injection and naphthalene sublimation based on the heat-mass transfer analogy is discussed. The law of superposition is used to describe the combined effects of film cooling, surface convection or radiation and frictional heating. An order of magnitude estimate indicates to what extent local temperature gradients are alleviated in the cooled walls by internal heat conduction.