点加热稳态导热系数测量方法研究

针对稳态导热系数测量方法测量过程时间较长、测量装置复杂、以及样品制备和加工工艺复杂等现状,提出了一种新型的点加热稳态导热系数测量方法,构建相应的三维稳态传热物理模型,使加热面温升只与热流密度、样品导热系数和测温点位置相关。通过聚焦连续激光加热样品,缩短样品达稳态时长至分钟量级;建立对照光路消除表面发射率和激光稳定性对温度测量的影响;红外热像仪测量加热表面稳态温度分布,结合物理模型实现导热系数测量。采用多种已知导热系数的标准材料和线性法对测量方法进行验证,并应用该方法测量硅藻土导热系数为0.49～0.60 W/(m·K),误差为6.06%。该方法的测量迅速及非接触特性使其可应用于工程实地测量。     

边界点法在传热问题数值分析中的应用

将一种新的数值分析方法-边界点法应用于传热问题的研究，对无内热源稳态热传导问题，通过传统边界元法将边界积分方程离散化，发现可以不直接求解影响系数矩阵，而是通过对偶关系，由域外虚源构造方程组的特解场形成边界已知和未知温度，热流密度的系数矩阵，而且域内温度和热流密度的求解将不依赖于边界未知参数的求解，对于有内热源的问题，可以将非齐次方程的解转换为齐次方程的解和某一确定解的叠加，对于非线性问题，可以通过基尔霍夫变换，将非线性问题转化为线性问题求解，这种边界点方法不但具有边界元法降维的优势，而且不须求解奇异积分，大大节约了计算时间，计算精度极高，以有内热源非线性稳态热传导问题的实例印证了这种方法的高效性。     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

加热炉钢坯加热质量的数值模拟研究

处理不同入炉温度的钢坯需采用合理的加热时间和工艺,针对此问题,采用数值模拟的方法研究人炉温度分别为200,400,600,700℃的钢坯最佳加热时间和加热质量。通过利用Fluent流体计算软件建立了钢坯非稳态导热模型和氧化烧损模型,模拟结果表明：在相同的加热工艺下,钢坯人炉温度为27℃,炉内停留时间为180min时,位于预热段的第5块钢坯表面受到的辐射热流最大,达到92kW／m^2,出炉钢坯氧化率为1．419％;钢坯入炉温度为200℃时,合理的炉内停留时间为150min,出炉温度达到1183℃,出炉钢坯氧化率为1．307％。     

火用分析在稳态导热中的应用研究

针对平壁一维稳态导热问题开展了研究,通过时间参数τ建立了热流密度Φ与热量Q的函数关系,并对一维平壁稳态导热问题进行模型建立、量化与分析。进而得出火用函数的一般表达式与量化表达式以及以火用参数为基准的导热微分方程式。并且给出了其在实际背景下的应用分析。对导热微分方程式进行了分析,揭示了温差驱动与火用差驱动热量传递的异同,得出火用差驱动对热量传递具有力的效果,更好地体现了驱动力的概念。     

非均匀加热管内单相流动传热特性数值模拟

建立了半周加热、半周绝热的非均匀热流边界条件下圆管内热传导与热对流换热的数值模型。采用Fluent软件,通过求解三维N-S方程和能量方程,对圆管内传热过程进行模拟。对不同热流密度、管内工质流速、壁厚时壁温分布及加热侧与绝热侧的热流量比值进行了计算。分析了热负荷非均匀分布对管内换热系数及压降的影响,与均匀加热管内流动与换热特性进行了对比。结果表明热负荷非均匀分布对管内单相流动换热特性影响不大,平均换热系数及摩擦压降可采用均匀加热经验关联式计算。     

基于共轭梯度法对T型管内壁瞬态温度的识别

构建了一种基于已知T型管道外壁面瞬时温度,反演管道内壁面瞬态温度的导热反问题数学模型和求解方法.利用有限单元法对管道模型进行离散,并利用共轭梯度法求解非稳态导热反问题.利用正问题得到的外壁面瞬态温度的数值结果作为反问题的输入条件,反演得到内壁面瞬态温度,此反演结果与作为边界条件的内壁面瞬态温度值进行了对比分析,对比结果...     

热流强度测试数据采集方法的改进

本文通过分析热电偶型温度记录仪工作原理,实验得到不同温差范围内的HOBO-U12型温度记录仪的温差电动势曲线及函数关系.并用于现场实测热流数据的采集,结果与实验室确定的函数关系符合较好,完全能够满足建筑热工检测的热流数据采集要求.解决目前建筑热工测试中热流强度数据长时间无人值守采集的问题.     

地源热泵地下岩土热物性的测定方法

提出了可用于现场测量计算地下岩土综合热物性参数的方法。垂直埋设在地下的换热器与周围岩土换热过程可以近似地看作半无限大介质中常热流边界条件下的非稳态导热过程来处理。这种方法克服了其他常用研究模型对钻孔中埋管的具体位置、上升管及下降管之间的距离、换热器材料和回填材料的物性参数的要求,相应地消除了上述各个参数所带来的误差。通过测量地埋换热器的循环水流量、进出水口温度,加热器的加热功率等数据确定地下岩土综合的热物性参数。     

基于PID自整定的电加热炉加热温度控制方法研究

电加热炉加热温度控制过程,一直存在温度控制精度较低的问题,为解决以上问题,提出基于PID自整定的电加热炉加热温度控制方法,对电加热炉加热过程进行热力学分析,建立电加热炉的稳态热力学方程,确定电加热炉热对流牛顿冷却方程,获取热流密度、热梯度和温度场分布等物理参量;根据热力学分析结果构建电加热炉温度状态变化模型;通过遗传算法实现电加热炉加热温度控制过程参数的整定;利用PID自整定方法实现电加热炉加热温度控制。实验结果表明,与传统的电加热炉温度控制方法相比,所提方法的温度控制精度高、控制稳定性高,能够更好地适应不同的电热加热炉工况。该方法应用范围广泛,不仅适用于工业生产中的电加热炉温度控制,也可用于其他类似设备的温度控制。同时,该方法具有较强的实用性和可推广性,能够为工业生产中的温度控制提供一种新的、有效的解决方案。     

A general method for the solution of inverse heat conduction problems with partially unknown system geometries

An auxiliary problem is introduced in the solution of inverse heat conduction problems with geometries not fully specified. Resolving the position of the unknown boundary subject to a Dirichlet condition leads to the solution of a nonlinear algebraic equation. Imposing Neumann or Robin conditions at the unknown boundary requires the solution of a first-order nonlinear, ordinary differential equation. The method yields accurate results for exact data, while measurement errors render the Neumann problem insoluble. The Dirichlet and Robin problems are still solvable, and for these problems, the errors in the investigated boundaries increase with their depth, a nature of the problem being investigated.     

An inverse geometry design problem in optimizing the shape of the gas channel for a proton exchange membrane fuel cell

The inverse design problem technique presented in this paper is intended for optimizing the shape of the gas channel at the cathode side in a proton exchange membrane fuel cell (PEMFC). The technique uses the desired current densities located on a carbon plate near the outlet of the channel at the cathode side as a starting point. The desired current density distributions can be obtained by modifying the current density distributions of the existing PEMFC with rectangular gas channels. The geometry of the redesigned gas channel is generated using a B-spline curve method, which enables the shape of the fuel channel to be completely specified using only a small number of control points, thus applying the technique of parameter estimation for the inverse design problem. Results show that by utilizing the redesigned optimal gas channel, the total current of the PEMFC can be increased, and at the same time the phenomenon of saturated water accumulation in the channel can be greatly reduced.     

Application of the heat-balance integral to an inverse Stefan problem

Most phase change process controls are concerned with the inverse Stefan problem. In this paper, the heat-balance integral method is applied effectively to analyze the one-region and two-region inverse Stefan problems in Cartesian and spherical coordinates. It is shown that if the movement of the phase change boundary is specified arbitrarily the present technique to predict both the temperature and its gradient at the fixed boundary is simple and accurate. As numerical illustrations, the one-dimensional inward solidification problem in Cartesian and spherical coordinates are solved and discussed in detail when the movement of the phase change interface is specified as a power function. The accuracy of these approximate solutions, based on the heat-balance integral method, is demonstrated satisfyingly by comparison with the available exact and/or numerical solutions for the one-region and the two-region problems.     

TWO-DIMENSIONAL PROBLEM IN GENERALIZED THERMOELASTICITY WITH HEAT SOURCES

In this work, a two-dimensional problem for a half-space is solved. The problem is in the context of the theory of generalized thermoelasticity with one relaxation time. The surface of the half-space is taken to be traction free and the temperature on it is specified. Heat sources permeate the medium. Laplace and exponential Fourier transform techniques are used. The solution in the transformed domain is obtained by a direct approach. The inverse double transform is evaluated numerically. Numerical results are obtained and represented graphically.     

Constructal design of nanofluids

We develop a constructal approach that is capable of finding constructal microstructure of nanofluids for constructal system performance. The approach converts the inverse problem of optimizing the microstructure for the best system performance into a forward one by first specifying a type of microstructures and then optimizing system performance with respect to the available freedom within the specified type of microstructures. The approach is applied to constructal design of nanofluids with any branching level of tree-shaped nanostructures in a circular disc with uniform heat generation. The constructal configuration and constructal system thermal resistance have some elegant universal features for both cases of given aspect ratio of the periphery sectors and given the total number of slabs in the periphery sectors, respectively. While our focus is on the constructal design and optimization of nanofluids microstructure, the methodologies and results are equally valid for other problems such as heat conduction optimization for cooling a disc-shaped area.     

Inverse Radiation problem in One—Dimensional Semitransparent plane—Parallel Media

The discrete ordinates method is used to develop a solution to an inverse radiation problem of source term in one-dimensional semitransparent plane-parallel media with opaque and specularly reflecting boundaries. It is assumed that, with the exception of the inhomogeneous source term, all aspects of the radiation transport problem are known. A method is developed to determine the inhomogeneous source term from specified incident radiation intensities on the boundaries. The inverse problem is solved using conjugate gradient method that minimizes the error between the incident radiation intensities calculated and the experimental data. The effects of single-scattering albedo, scattering asymmetry parameter, wall emissivity, the diffuse fraction of reflectivity, and the optical thickness on the accuracy of the inverse are investigated. The results show that the source term can be estimated accurately, even with noisy data.     

Inverse radiation problem of temperature field in three-dimensional rectangular furnaces

The discrete ordinates method is used to developed a solution to an inverse radiation problem of temperature field in rectangular furnaces. It is assumed that, with the exception of the inhomogeneous temperature field, all aspects of the radiation transport problem are known. A method is developed to determine the inhomogeneous temperature field from specified incident radiation heat fluxes at the centers of boundary walls. The inverse problem is solved using conjugate gradient method that minimize the error between the incident radiation heat fluxes calculated and the experimental data. The results of temperature estimation show that the temperature field can be estimated accurately, even with noisy data.     

Recovering both the space-dependent heat source and the initial temperature by using a fast convergent iterative method

In this paper, we solve two types of inverse heat source problems: one recovers an unknown space-dependent heat source without using initial value, and another recovers both the unknown space-dependent heat source and the initial value. Upon inserting the adjoint Trefftz test functions into Green’s second identity, we can retrieve the unknown space-dependent heat source by an expansion method whose expansion coefficients are derived in closed form. We assess the stability of the closed-form expansion coefficients method by using the condition numbers of coefficients matrices. Then, numerical examples are performed, which demonstrates that the closed-form expansion coefficient method is effective and stable even when it imposes a large noise on the final time data. Next, we develop a coupled iterative scheme to recover the unknown heat source and initial value simultaneously, under two over specified temperature data at two different times. A simple regularization technique is derived to overcome the highly ill-posed behavior of the second inverse problem, of which the convergence rate and stability are examined. This results in quite accurate numerical results against large noise.     

An effective 3-D inverse procedure to retrieve cooling conditions in an aluminium alloy continuous casting problem

The paper discusses a three-dimensional numerical solution of the inverse boundary problem for a continuous casting process of an aluminium alloy. Because verified information of heat flux distribution is crucial for a good mould design, effective cooling system and the whole caster in general, the main goal of the analysis presented within the paper is identifying of the heat fluxes along the external surface of the ingot. To model the solidification process, an enthalpy-porosity technique implemented in a commercial package was used. In this method, the phase change interface was determined based on the liquid fraction approach. In the inverse procedure, a sensitivity analysis was used to estimate the boundary condition retrieval. While the measured temperatures required to solve the problem are always burdened by measurement errors, a comparison of the measured and retrieved values showed a computational high accuracy. The average percentage error of the sensors was considerably lower than the maximum percentage error of the numerically simulated measurements. In addition, the computationally effective method was independent of the mesh size, the starting value of the assumed boundary condition and the maximum error of measurements used for calculations.     

Inverse Boundary Design Problems in Enclosures With Non-Gray Media

In this work, the inverse analysis is applied to radiative heat transfer boundary design problems with non-gray media. The objective of the inverse problem is to find the power of the heaters on the heater surface that produces the desired output, that is, temperature and heat flux distribution over the design surface. The inverse problem is formulated as an optimization problem for minimization of an objective function, which is defined by the sum of the squared difference between estimated and desired heat flux distributions over the design surface. The non-gray optimization problem is solved using the conjugate gradient method, which is a gradient-based optimization method. The spectral line weighted-sum-of-gray-gases model (SLW) is used to account for non-gray gas radiation properties. The radiative transfer equation is solved by the discrete ordinates method combined with two models for simulation of non-gray media. Enclosures with diffuse and gray walls are considered. Radiation is assumed the dominant mode of heat transfer. Example problems including homogeneous/nonhomogeneous, isothermal/nonisothermal media are considered. The results obtained using the SLW model and the gray model are compared.