Thermal convection of viscoelastic fluid with Biot boundary conduction

Two-dimensional unsteady natural convection of a non-linear fluid represented by Criminale–Erickson–Filbey (CEF) fluid model in a square cavity is studied in the fluid for Rayleigh–Benard convection case. The governing vorticity and energy transport equations are solved numerically either simple explicit and ADI methods, respectively. The two-dimensional convective motion is generated by buoyancy forces on the fluid in a square cavity, when the vertical walls are either perfectly insulated or conducted with Biot boundary conduction condition. The contributions of the elastic and shear dependent characteristics of the liquid to the non-Newtonian behaviour are investigated on the temperature distribution and heat transfer. The effect of the Weissenberg (which is a measure of the elasticity of the fluid), Rayleigh and Biot numbers on the temperature and streamline profiles are delineated and this has been documented first time for the viscoelastic fluid.     

A level-set method for steady-state and transient natural convection problems

This paper introduces a topology optimization method for 2D and 3D, steady-state and transient heat transfer problems that are dominated by natural convection in the fluid phase and diffusion in the solid phase. The geometry of the fluid-solid interface is described by an explicit level set method which allows for both shape and topological changes in the optimization process. The heat transfer in the fluid is modeled by an advection-diffusion equation. The fluid velocity is described by the incompressible Navier-Stokes equations augmented by a Boussinesq approximation of the buoyancy forces. The temperature field in the solid is predicted by a linear diffusion model. The governing equations in both the fluid and solid phases are discretized in space by a generalized formulation of the extended finite element method which preserves the crisp geometry definition of the level set method. The interface conditions at the fluid-solid boundary are enforced by Nitsche’s method. The proposed method is studied for problems optimizing the geometry of cooling devices. The numerical results demonstrate the applicability of the proposed method for a wide spectrum of problems. As the flow may exhibit dynamic instabilities, transient phenomena need to be considered when optimizing the geometry. However, the computational burden increases significantly when the time evolution of the flow fields needs to be resolved.     

Topology optimization of convective heat transfer problems for non-Newtonian fluids

Structural and Multidisciplinary Optimization - We conduct topology optimization of convective heat transfer problems based on the power law type non-Newtonian fluid. A heat transfer maximization...     

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.     

Thermal optimization of a micro valve array

This paper describes the thermal optimization of an array of thermopneumatically actuated microvalves. The valve density achieved in the current prototype amounts to 100 valves per square inch. Thin valve membranes of less than 3 μm thickness allow for low temperature actuation. In order to reduce thermal cross talk between adjacent valves, cooling channels and insulating trenches have been introduced. It could be shown that these features reduce thermal cross talk and thus enhance array performance. The effectiveness of the cooling channels was characterized both experimentally and by means of finite element simulations. Since available theories for convective heat transfer showed some discrepancies for the examined device, the average heat transfer coefficient was determined experimentally. The comparison of computational simulation results and infra-red measurements showed good agreement. Finite element models allow reliable prediction and optimization of the performance of high-density valve arrays.     

Numerical instabilities in level set topology optimization with the extended finite element method

This paper studies level set topology optimization of structures predicting the structural response by the eXtended Finite Element Method (XFEM). In contrast to Ersatz material approaches, the XFEM represents the geometry in the mechanical model by crisp boundaries. The traditional XFEM approach augments the approximation of the state variable fields with a fixed set of enrichment functions. For complex material layouts with small geometric features, this strategy may result in interpolation errors and non-physical coupling between disconnected material domains. These defects can lead to numerical instabilities in the optimized material layout, similar to checker-board patterns found in density methods. In this paper, a generalized Heaviside enrichment strategy is presented that adapts the set of enrichment functions to the material layout and consistently interpolates the state variable fields, bypassing the limitations of the traditional approach. This XFEM formulation is embedded into a level set topology optimization framework and studied with “material-void” and “material-material” design problems, optimizing the compliance via a mathematical programming method. The numerical results suggest that the generalized formulation of the XFEM resolves numerical instabilities, but regularization techniques are still required to control the optimized geometry. It is observed that constraining the perimeter effectively eliminates the emergence of small geometric features. In contrast, smoothing the level set field does not provide a reliable geometry control but mainly improves the convergence rate of the optimization process.     

Topology optimization of a coupled thermal-fluid system under a tangential thermal gradient constraint

This paper presents a continuous adjoint approach for topology optimization of a coupled heat transfer and laminar fluid flow system under tangential thermal gradient (TTG) constraints. In this system, the thermal gradient along the boundary of multiple heat sources needs to be controlled. The design goals are to minimize the temperature of the domain, the fluid power dissipation and the TTG along the boundary of the heat sources. The first two goals are combined into a single cost function with weight variables. The TTG is constrained in one of two forms, an integral form where the integral of TTG squares along the boundaries of heat sources is constrained, or a point-wise form where TTG is constrained point-wise. A gradient-based approach is developed to obtain the optimized designs. A salient feature of our approach is the use of the continuous adjoint approach to derive gradients of both the cost function and two forms of TTG constraints. Numerical examples demonstrate that the continuous adjoint approach leads to successful topological optimization of the constrained thermal-fluid system. The use of TTG constraint is effective in lowering the TTG along the heat source boundaries. The resulting designs exhibit clear black/white contrast.     

Simulation of Stokes flow over microelectrodes with least-squares meshfree method

Microelectrode structures in alternating current (AC) electrokinetics can generate high electric field strength to manipulate, characterize and separate particles in suspending medium. It has been widely used in biological, pharmaceutical and medical fields. In this paper, a least-squares meshfree method (LSMFM) based on the first-order velocity–pressure–vorticity formulation for the Stokes flow, electric potential–electric field strength expression for electric field and temperature–heat flux equations for heat transfer problem is presented to study two-dimensional electrothermally induced fluid flow over microelectrodes. Joule heat generated from electric field acts as heat source and gives rise to electric force and buoyancy force acting on the fluid. The discretization of all system of equations is completed by the least-squares method. The equal-order moving least-squares (MLS) approximation is used with Gaussian quadrature in the background cells constructed by the quadtree algorithm. A matrix-free element-by-element Jacobi preconditioned conjugate gradient method is applied to solve the resulting systems. Finally, an example of steady heat transfer problem with analytical solution is devised to analyze the error estimates of the LSMFM, and the examples of electric field of shielded microstrip line and Stokes flow over microelectrode are also solved to investigate the features of the LSMFM.     

乙烯裂解炉燃烧传热的CFD模拟

为了掌握乙烯炉炉膛内的热场分布规律,本文采用CFD方法对SL-Ⅱ型工业乙烯裂解炉辐射段内所发生的燃烧、辐射传热、烟气流动等过程进行了耦合模拟,并且详细地分析了炉膛内流场及温度场的分布情况。其中,裂解炉的几何模型按照工程图纸1:1的比例建立;网格划分在专业网格生成软件ICEM11.0中完成;计算采用标准k-ε双方程模型模拟湍流,漩涡耗散/有限速率燃烧反应模型(EDM/FRC)模拟侧壁烧嘴的预混燃烧和底部烧嘴的非预混燃烧,离散传递模型(DTM)模拟炉膛辐射传热,计算过程在CFX5.0中完成。计算得到了裂解炉炉膛内流场、温度场分布的详细信息,模拟结果与工业实测数据吻合较好。模拟结果表明,在底部烧嘴的射流核心区周围形成了大范围的回流,过渡段拐角处有小范围的回流现象;炉膛长度、宽度、高度方向都存在明显的温度梯度,说明传统的炉膛传热计算方法假定温度分布均匀是不准确的;炉膛的高温区主要集中在炉高4.5 m处;炉膛上部的热量,由于侧壁烧嘴的加入而得到了很好地补充,说明侧壁烧嘴的安装位置合理。研究结果表明CFD模拟是目前为止研究裂解炉内热场分布规律最为有效的手段之一,对于未来乙烯裂解炉的设计与优化作用很大。     

汽车前舱热气流对制动盘散热性能的影响

针对制动盘对流传热计算多基于单个制动盘或基于整车外壳模型下的制动盘,未考虑汽车前舱内高温气流对制动盘散热性能影响的问题,基于含有前舱部件的某SUV整车模型,对其通风制动盘散热性能进行数值计算并分析,得到在不同前舱产热工况以及不同车速下通风制动盘的外流场、外温度场、表面对流传热系数和散热功率.在前舱热源相同时,车速变高会提高制动盘对流传热性能,而在车速固定时前舱热源越大则制动盘的散热性能越差.     

Thermophoresis and MHD mixed convection three-dimensional flow of viscoelastic fluid with Soret and Dufour effects

Heat and mass transfer effects in three-dimensional mixed convection flow of viscoelastic fluid over a stretching surface with convective boundary conditions are investigated. The fluid is electrically conducting in the presence of constant applied magnetic field. Conservation laws of energy and concentration are based upon the Soret and Dufour effects. First order chemical reaction effects are also taken into account. By using the similarity transformations, the governing boundary layer equations are reduced into the ordinary differential equations. The transformed boundary layer equations are computed for the series solutions. Dimensionless velocity, temperature, and concentration distributions are shown graphically for different values of involved parameters. Numerical values of local Nusselt and Sherwood numbers are computed and analyzed. It is found that the behaviors of viscoelastic, mixed convection, and concentration buoyancy parameters on the Nusselt and Sherwood numbers are similar. However, the Nusselt and Sherwood numbers have qualitative opposite effects for Biot number, thermophoretic parameter, and Soret-Dufour parameters.

     

Imposing maximum length scale in topology optimization

This paper presents a technique for imposing maximum length scale on features in continuum topology optimization. The design domain is searched and local constraints prevent the formation of features that are larger than the prescribed maximum length scale. The technique is demonstrated in the context of structural and fluid topology optimization. Specifically, maximum length scale criterion is applied to (a) the solid phase in minimum compliance design to restrict the size of structural (load-carrying) members, and (b) the fluid (void) phase in minimum dissipated power problems to limit the size of flow channels. Solutions are shown to be near 0/1 (void/solid) topologies that satisfy the maximum length scale criterion. When combined with an existing minimum length scale methodology, the designer gains complete control over member sizes that can influence cost and manufacturability. Further, results suggest restricting maximum length scale may provide a means for influencing performance characteristics, such as redundancy in structural design.     

The determination of the effective ambient temperature for thermal flow sensors in a non-isothermal environment

Thermal flow sensor behaviour is investigated for the condition that the direct surrounding of the sensor, such as the plate on which the sensor is mounted, is at a temperature that is different from that of the flow. In that case the convective heat transfer depends on the difference between the (average) sensor temperature and the effective ambient temperature, where the latter is a weighed average of the plate and fluid temperatures. Knowledge of which temperature level represents the true effective ambient value is vital for a correct design and operation of a thermal flow sensor. In the present theoretical study this ambient-temperature weighing effect is examined by investigating the weighing function for the laminar heat transfer from a sensor mounted on a plate where (a part of) the upstream plate length is at a constant temperature which is different from that of the flow. The analysis reveals how the weighing effect, and hence the effective ambient temperature, depends on the non-isothermal upstream length, and that this dependence is significantly different for sensing methods that rely on either total or differential convective heat transfer.     

Numerical study of unsteady hydromagnetic Generalized Couette flow of a reactive third-grade fluid with asymmetric convective cooling

Unsteady hydromagnetic Generalized Couette flow and heat transfer characteristics of a reactive variable viscosity incompressible electrically conducting third grade fluid in a channel with asymmetric convective cooling at the walls in the presence of uniform transverse magnetic field is studied. It is assumed that the chemical kinetics in the flow system is exothermic and the convective heat transfer at the channel surface with the surrounding environment follow the Newton’s law of cooling. The coupled nonlinear partial differential equations governing the problem are derived and solved numerically using an unconditionally stable and convergent semi-implicit finite difference scheme. Both numerical and graphical results are presented and physical aspects of the problem are discussed with respect to various parameters embedded in the system.     

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。     

基于空气流体动力学的高速列车制动盘散热性能模拟

基于CFD法分析高速列车制动盘的散热情况,得到制动盘各个区域的平均对流换热系数.在此散热条件下,用顺序耦合法模拟制动盘的温度场,实现复杂六边形热载荷的加载,并将二次制动温度场的模拟结果与试验数据进行对比分析.结果表明：模拟结果与试验数据在冷却阶段吻合度较高,且在制动阶段中最高温度值误差不超过5%.     

内置转子换热管强化传热数值模拟

为分析内置转子换热管的传热效果,建立光管和内置转子换热管的三维模型,对换热管内流场、温度场、压力场以及传热过程进行模拟,得到管内流体的阻力特性和传热特性.模拟结果表明:内置转子换热管内的三维流动比较复杂,转子与管壁之间缝隙内的流体有明显的环绕流动,切向速度和径向速度也增大到一定范围;相同雷诺数条件下,内置转子换热管压降...     

GPU-based model predictive control for continuous casting spray cooling control system using particle swarm optimization

Model predictive control (MPC) for spray cooling control system requires a repeated online solution of an optimization problem that includes partial differential equations (PDEs). To simulate the future temperature behavior of steel billets, 3D dynamic heat transfer model is used. The special solution domain of PDEs has led to large computation cost, which is the main challenge in the real-time practical application of spray cooling control system. Meanwhile, the heat transfer coefficients need to be identified using the measured surface temperature. This work presents a two-level parallel solution method implemented on a Graphics processing unit (GPU) for MPC of spray cooling control systems and a weighted least squares modified conjugate gradient method (WLS–MCG) for identification of heat transfer coefficients. Two-level parallel solution method consists of parallel-based heat transfer model and stream parallel particle swarm optimization (PSO). PSO is used to solve the optimization problem. WLS–MCG consists of the weighted least squares (WLS) and modified conjugate gradient method (MCG). The experimental results show that the two-level parallel solution method has good computational performance and achieves satisfactory control performance.     

Magnetohydrodynamic three-dimensional nonlinear convective flow of viscoelastic nanofluid with heat and mass flux conditions

The present research focuses on three-dimensional nonlinear convective flow of viscoelastic nanofluid. Here, the flow is generated due to stretching of a impermeable surface. The phenomenon of heat transport is analyzed by considering thermal radiation and prescribed heat flux condition. Nanofluid model comprises of Brownian motion and thermophoresis. An electrically conducting fluid is accounted due to consideration of an applied magnetic field. The dimensionless variables are introduced for the conversion of partial differential equations into sets of ordinary differential systems. The transformed expressions are explored through homotopic algorithm. Behavior of different dimensionless parameters on the non-dimensional velocities, temperature and concentration are scrutinized graphically. The values of skin friction coefficients, Nusselt and Sherwood numbers are also calculated and elaborated. It is visualized that the heat transfer rate increases with Prandtl number and radiation parameter is higher.

     

基于接触测温的太阳能重力热管温差测量方法与装置研究

热管温差是太阳能重力热管传热性能中的重要标志.通过正交实验研究温差随热管倾斜角度、充液比、水浴温度和热管蒸发段加热长度变化对太阳能重力热管温差的影响,得到了影响温差显著性因素.为解决太阳能重力热管温差尚无自动化在线监测的现状,提出了太阳能重力热管温差测量方法,该方法一次能够检测多支太阳能重力热管.结合生产厂家和国标要求,设计了一套太阳能重力热管温差批量化测量方案,并对该方案进行了不确定度评估.