套管换热器换热性能仿真及实验研究

在换热器性能评估平台优化设计的研究中,为了描述套管换热器在热流体动态输入温度下的换热行为,研究其温度分布变化规律,在综合考虑了流体沿流动方向的导热扩散特性、中间墙体的金属蓄热以及流体沿径向导热对出口温度瞬态响应的影响的基础上,提出了以墙体侧蓄热量为突破点的动态相变仿真交叉迭代简化模型,得到了在热流体动态输入温度下的换热器温度响应曲线以及换热系数的分布曲线,最后设计了换热器动态仿真平台,对仿真结果进行对比分析,结果表明墙体侧温度模型预测值和实验值数据吻合较好.最大相对误差在9％范围内,其仿真模型符合换热器的换热特性,说明上述简化模型对套管换热器相变换热的研究有较好的参考作用.     

修正型平面热源法导热系数测定仪的设计

在传统的瞬态平面热源法导热系数测定仪的基础上,改进设计了新型加热与温度测量探头,研究了半无限大平面在被加热时的瞬态热传导问题,建立了平面热源法的数学修正模型,并采用数学迭代与回归分析的数据处理方法,实现了采用修正型平面热源法的导热仪对材料吸热系数和导热系数的同时测定。测试实验数据表明:仪器测量精度可达5%,测量范围为0.003～40 W/(m.K),并且可以对单试件等不具备传统瞬态平面源法测定条件的材料进行测定和试件的在线测定。     

基于飞思卡尔和TPS技术的导热仪测控系统的开发

导热系数是衡量材料热物理性质的重要参数,关系到材料在化工、能源、生物等很多领域内的应用,尤其在节能材料的研究中起着重要的作用。导热系数的常见测定方法有稳态法和瞬态法,瞬态平面热源法是一种新的瞬态测试方法,具有可测试的试样种类多,导热系数范围大,测试时间短,测试精度高且试样制备简单等优点。本文旨在设计一种测试系统,以飞思卡尔单片机为数据采集、计算和控制单元,利用瞬态平面热源法测量材料导热系数,针对现有的防护热板法、热线法和热带法等的固有缺点进行了改进。该方法中,探头既被用作加热的热源,又被用作温度传感器,通过测试探头电阻的变化,就可得到探头表面温升值,从而计算出试样导热系数。     

基于Matlab的板式换热器动态特性建模与仿真

为了正确预测板式换热器的动态特性，在合理假设的基础上，根据流道和换热平板的质量、能量守恒方程，建立了无量纲动态仿真数学模型。考虑了流体沿流动方向的导热扩散特性、换热平板的金属蓄热以及沿径向的导热对出口温度瞬态响应的影响。基于Matlab的数值计算基础，对无量纲动态仿真数学模型的空间维变量的一阶导数项采用向后差分，二阶导数项采用中心差分，然后采用Matlab的ODE求解器进行求解，得到了阶跃扰动和频率扰动下的出口温度的响应曲线。     

希望一号微小卫星的热仿真分析

微小卫星的热控制和热分析有其自己的特点.为了评估“希望一号”微小卫星的热性能,分析了其功能、构成和热控制措施.针对卫星的高温和低温极端工况,用NEVADA软件建立了卫星在轨状态的几何数学模型,获得了卫星的外热流和内部辐射换热系数;用MSC.SINDA软件建立了该卫星的热数学模型,分别进行了稳态和瞬态温度场的分析计算.计...     

连铸二冷导热反问题数学模型建立及验证

针对连铸二冷导热反问题,建立了由布置在试样内部的热电偶测得的温度值反算表面温度、表面换热系数和表面热流密度的一维非稳态导热反问题计算方法,利用热平衡法建立各节点差分方程,引入泛函和灵敏度系数,通过牛顿迭代法优化假设初始值得到边界条件,利用Matlab编程实现了反算过程,并验证了导热反问题算法的准确性,反算相对误差为9.03×10-12.     

智能式导热系数测定仪

本文介绍依据不稳定的导热原理，应用微控制器制成的瞬态热丝法智能式导热系数测定仪，包括高精度的测试技术，数据处理方法，智能化测试与控制方法，样机测试表明，其性能揸示达到同类外国导热仪的八十年代初的水平。     

纳米薄膜面向导热系数的分子动力学模拟

基于分子动力学研究纳米薄膜的导热系数现状,建立了一种导热模型.采用非平衡态分子动力学研究了纳米薄膜长度、厚度、空穴对导热系数的影响.选取结构简单、可靠实验数据和势能函数的晶体氩为模型,计算了氩晶体薄膜面向导热系数.模拟结果表明:纳米薄膜面向导热系数随薄膜长度和厚度增加而增加,增加到一定尺寸时,不再增加,近似相等,具有显著的尺寸效应.相同条件下,存在空穴的氩晶体纳米薄膜导热系数低于理想氩晶体纳米薄膜的导热系数.     

双圆弧齿轮传动系统失油润滑瞬态热分析

双圆弧齿轮传动系统失油润滑后的生存能力依赖于瞬态温度场分布和系统摩擦副热变形量.建立了双圆弧齿轮传动系统在失油润滑条件下的热源、热阻、对流换热系数计算模型,模型考虑了辐射换热效应以及传动系统热物性参数的非线性特征.基于稳态温度场分析结果,结合传热学、摩擦学、齿轮啮合原理以及热变形理论,利用所建模型计算了在失油润滑条件下双圆弧齿轮传动系统的瞬态温度场、摩擦副的热变形.根据计算结果确定传动系统中的危险零部件,为其生存能力的预测提供依据.     

基于正则化方法的阶梯边界条件反问题研究

针对复杂的第三类周期性阶梯型边界条件,基于Tikhonov正则化方法,通过求解极坐标下的二维导热反问题,建立了计算空心圆柱体钢坯表面换热系数分布的热物理模型.考虑计算结果的精确性与稳定性,提出'斜率法'以选取正则化系数.利用数值仿真验证了斜率法的适用性,计算结果中R2值为0.998;将斜率法应用于实验数据中,计算结果与预期相符合.结果 表明,斜率法很好的平衡了解的精确性与稳定性,可以有效解决导热反问题的不适定性问题.     

Boundary layer analysis and heat transfer of a nanofluid

A theoretical model for nanofluid flow, including Brownian motion and thermophoresis, is developed and analysed. Standard boundary layer theory is used to evaluate the heat transfer coefficient near a flat surface. The model is almost identical to previous models for nanofluid flow which have predicted an increase in the heat transfer with increasing particle concentration. In contrast our work shows a marked decrease indicating that under the assumptions of the model (and similar ones) nanofluids do not enhance heat transfer. It is proposed that the discrepancy between our results and previous ones is due to a loose definition of the heat transfer coefficient and various ad hoc assumptions.     

滚珠丝杠副热变形计算分析及可视化

以传热学原理为基础,探讨温升对滚珠丝杠副的影响。首先确定热流密度系数及对流换热系数,然后借助有限元分析软件ANSYS,建立滚珠丝杠副的有限元模型,计算求解温度场及热变形。仿真云图显示：滚珠丝杠以1350r/min旋转时,其温升达到3.8℃,引起8.32m的热变形。     

升压汽水喷射器的理论模型及输出量的解耦控制

针对汽水喷射器内所产生的凝结和激波现象,使其升压过程和机理非常复杂,提出利用直接接触凝结理论建立升压汽水喷射器的理论模型,并对求解该模型的一些关键问题进行论述,如采用平均凝结换热系数计算相间质量传递及利用汽羽确定各相体积分数等.根据该理论模型给出升压式汽水喷射器出口温度和流量的调节方案,理论分析表明出口温度及流量的调节...     

Determination of a time-dependent heat transfer coefficient from non-standard boundary measurements

In this paper the determination of the time-dependent heat transfer coefficient in one-dimensional transient heat conduction from a non-standard boundary measurement is investigated. For this inverse nonlinear ill-posed problem the uniqueness of the solution holds. Numerical results obtained using the boundary element method are presented and discussed.     

Iteration method for analysis of write-current-induced thermal protrusion

This article presents a detailed iteration method for coupled air bearing and thermal-structure analysis, and introduces a node-to-node schematic for mapping the heat transfer coefficient into the thermal-structure analysis as boundary conditions. A three-dimensional finite element model is developed. The heat transfer coefficient on the air bearing surface of slider is calculated by a quasi-steady heat transfer model and applied to the thermal-structure analysis through the proposed node-to-node mapping schematic. The results show that the iteration process is indispensable for the air-bearing-thermal-structure analysis under high write power, but changeable under low write power. Among iterations, the values and distribution of the heat transfer coefficient vary significantly, especially around the protruding regions. The corresponding thermal protrusion induced by write current and its effects on flying attitude are different and the difference is increased with the heating power. For the application of high heating power and large thermal protrusion, at least a three-time iteration of the heat transfer coefficient is needed.     

机车车轮对流传热系数计算

对流传热系数的准确计算对研究机车车轮温度场和应力场及其疲劳寿命预测有重要意义.针对HXD2机车车轮踏面制动过程,建立车轮及其绕流流场计算模型,应用CFD方法通过仿真得到机车车轮在不同运行速度下的对流传热系数.结果表明:由于车轮自身旋转,车轮表面不同位置处的对流传热系数不同;车轮上半迎风面的对流传热系数较大,下半迎风面较小,且都大于背风面数值.计算结果为研究机车车轮对流传热、蠕滑和制动等传热过程提供参考.     

A method for the identification of nonlinear components of the thermal conductivity tensor for anisotropic materials

A method is proposed for the numerical solution of coefficient inverse problems on nonlinear heat transfer in anisotropic materials used for heat shielding in the aerodynamic heating of hypersonic craft (HC). The main characteristics of anisotropic heat-shielding materials at high temperatures are the components of a nonlinear thermal conductivity tensor that are to be restored based on the results of the experimental measurements of the temperature in space-time nodes. The proposed method is based on the alternating direction method with extrapolation of the numerical solution of heat transfer problems, the method of parametric identification, and the gradient descent method. The results have been obtained for the reconstruction of components of a thermal conductivity tensor for carbon-carbon composite material using the experimental values of the nonlinear heat conduction of these materials. The method can be employed for restoring other numerous thermophysical characteristics of composite materials. The obtained results of the numerical experiments on the identification of components of a thermal conductivity tensor of composite materials in a two-dimensional space are presented and discussed.     

Nonlinear convective flow with variable thermal conductivity and Cattaneo-Christov heat flux

An analysis is introduced to investigate the salient features of nonlinear convective flow of thixotropic fluid in the version of Cattaneo-Christov heat flux theory. The stagnation point flow is present. The flow phenomenon is by an impermeable stretching sheet. The energy expression is modeled through the theory of Cattaneo-Christov heat flux. Characteristics of heat transfer phenomenon are described within the frame of variable thermal conductivity. Suitable variables reduced to the nonlinear partial differential expressions to the ordinary differential expressions. Series solutions of resulting systems are acquired within the frame of homotopy theory. Convergence analysis is achieved and suitable values are determined by capturing the so-called ℏ−curves. Graphical results for velocity and temperature are displayed and argued for sundry physical variables. Expression of skin friction coefficient is calculated through numerical values. Higher values of mixed convection parameter, Prandtl number, and thermal relaxation time lead to decay the temperature and layer thickness.

     

The contrived transient-explicit method for solving steady-state flows: application to a rotating, recirculating flow

A method is described for solving steady-state fluid flow and heat transfer problems which are governed by elliptic-type differential equations. A contrived transient version of the steady-state problem is constructed by appending time derivatives to all the participating equations, regardless of whether or not such terms have physical reality. Each time derivative is multiplied by a fictive diffusivity coefficient which is varied during the course of an explicit marching procedure in order to achieve rapid, stable convergence to the steady state. The solution method is applied to a three-component laminar flow in a cylindrical enclosure having one rotating wall and coolant throughflow. Recirculation patterns are set up in the enclosure due to the shearing action of the throughflow and to the rotation of the disk. The surface heat transfer is found to decrease as the Reynolds number of the throughflow increases.