直管束流固耦合振动的数值模拟

为了研究反应堆结构中的诸如燃料棒、蒸汽发生器和其它换热器传热管束等的流体-结构交互作用问题,利用有限体积法离散大涡模拟(large eddy simulation, LES)的流体控制方程,用有限元方法求解结构动力学方程,并结合动网格技术,建立三维流体诱发振动的数值模型,模拟直管束中流体的流动及结构振动,实现计算结构动力学(computational structure dynamics, CSD)与计算流体力学(computational fluid dynamics, CFD)之间的联合仿真．首先,基于流固耦合方法对单管的流致振动特性进行了详细分析,得到了其动力学响应与流场特性;其次基于建立的传热管束流致振动计算模型,研究了两并列管、两串列管以及3×3正方形排列管束的流致振动行为．     

Multi-objective optimization of heat exchangers using a modified teaching-learning-based optimization algorithm

Teaching-learning-based optimization (TLBO) is a recently developed heuristic algorithm based on the natural phenomenon of teaching-learning process. In the present work, a modified version of the TLBO algorithm is introduced and applied for the multi-objective optimization of heat exchangers. Plate-fin heat exchanger and shell and tube heat exchanger are considered for the optimization. Maximization of heat exchanger effectiveness and minimization of total cost of the exchanger are considered as the objective functions. Two examples are presented to demonstrate the effectiveness and accuracy of the proposed algorithm. The results of optimization using the modified TLBO are validated by comparing with those obtained by using the genetic algorithm (GA).     

Front tracking for two-phase flow in reservoir simulation by adaptive mesh

In this article, an algorithm for the numerical approximation of two-phase flow in porous media by adaptive mesh is presented. A convergent and conservative finite volume scheme for an elliptic equation is proposed, together with the finite difference schemes, upwind and MUSCL, for a hyperbolic equation on grids with local refinement. Hence, an IMPES method is applied in an adaptive composite grid to track the front of a moving solution. An object-oriented programmation technique is used. The computational results for different examples illustrate the efficiency of the proposed algorithm. © 1997 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 13: 673–697, 1997     

Numerical analysis of convection heat transfer from an array of perforated fins using the Reynolds averaged Navier–Stokes equations and large-eddy simulation method

Extended surfaces (fins) are frequently used in heat exchange devices to increase the heat transfer between a primary surface and the surrounding fluid. In the present study, we determined the thermal performance of an efficient type of perforated fin and we compared the results with those obtained for a simple solid fin and a flat surface without fins in the same working conditions. The modeled geometry comprised fins that had small channels with a circular cross section and different configurations, which were arranged stream-wise along the fin's length. The turbulent flow field around the perforated fins was modeled using the Reynolds averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) method with a suitable subgrid-scale model. The conjugate differential equations for both the solid and gas phases were solved simultaneously using the finite volume procedure with the SIMPLE algorithm. For LES, the flow and heat transfer characteristics were determined for a Reynolds number equal to 3.2×10⁴ based on the fin length and a Prandtl number of 0.71. The results indicated that among the different configurations, the fins with three openings had the best thermo-hydraulic performance. In addition, we found that although the heat transfer rates predicted by RANS and LES were in close agreement, there were noticeable differences in the important flow characteristics, such as the recirculation zone around the fins and the total drag force on them.     

Mathematical model to analyze the flow and heat transfer problem in U-shaped geothermal exchangers

In this study, we propose a mathematical model for U-shaped geothermal heat exchangers based on the unsteady Navier–Stokes problem. In the numerical solution of this problem, we divide the exchanger into two computational domains: rectilinear pipes where the temperature field is computed analytically, and a U-curved pipe where solutions for both the flow and heat exchange are calculated using a numerical procedure based on the Galerkin finite elements method. The results of some numerical simulations are provided and used to study the performance of geothermal exchangers by assessing the effective energy produced. We also present a validation analysis based on experimental measurements obtained from a real geothermal exchanger.     

含温度载荷的管板变形的解析解

根据换热器结构形式建立了用于计算管板变形的力学模型,参考ASMEⅧ-1计算管板的有效弹性常数,并将外筒和换热管束分别等效为会因温度和压力载荷产生轴向变形的弹簧和弹性基础.应用Ritz法建立了管板挠度的解析解,将该解析解与三个不同规模换热器的有限元分析结果进行对比.结果表明,建立的解析解与有限元分析结果吻合良好,验证了推导的管板变形解析解的正确性.研究结果对固定管板式换热器的设计有较大的指导意义.     

Heating load density optimization of an irreversible simple Brayton cycle heat pump coupled to counter-flow heat exchangers

This paper presents a theoretical investigation on the finite time thermodynamic performance for an irreversible Brayton cycle heat pump (BCHP) coupled to counter-flow heat exchangers. The heating load density, i.e. the ratio of heating load to the maximum specific volume in the cycle, is taken as the optimization objective. Relations between heating load density and pressure ratio and between COP (coefficient of performance) and pressure ratio for BCHP in which the irreversibilities of heat resistance losses in the hot and cold-side heat exchangers and non-isentropic losses in the compression and expansion processes are derived. The analytical expression obtained for the cycle performance enabled its optimization through addressing the effects of mechanical and thermal inefficiencies of all components comprising the cycle. The influences of the temperature ratio of the reservoirs, the efficiencies of the compressor and expander and the effectiveness of the heat exchangers on the heating load density are provided. The cycle performance optimizations are performed by searching the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for the fixed total heat exchanger inventory and the optimum heat capacity rate matching between the working fluid and the heat reservoirs. The BCHP design with heat loading density optimization leads to a smaller size of all equipments comprising the heat pump.     

Anisotropic Wilson element with conforming finite element approximation for a coupled continuum pipe‐flow/Darcy model in Karst aquifers

Numerical method for a coupled continuum pipe‐flow/Darcy model describing flow in porous media with an embedded conduit pipe is considered. Wilson element on anisotropic mesh is used to solve the Darcy equation on porous matrix. The existence and uniqueness of the approximation solution are obtained. Optimal error estimates in L² and H¹ norms are established independent of the regularity condition on the mesh. Numerical examples show the efficiency of the presented scheme. With the same number of nodal points, the results using Wilson element on anisotropic mesh are much better than those of the same element and Q₁ element on regular mesh. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of phase change in a spiral-fin heat exchanger

In this study the effect of a spiral fin on the melting process of a phase change material in a heat exchanger is investigated. Water as the heat transfer fluid enters the tube at 348 K and the phase change material, RT50, the melting temperature of which is in the range of 318 K to 324 K, fills the space between the tube and the shell. This study focuses on the effect of geometrical parameters of the fin including the fin thickness and fin pitch on the melting process. In addition, the effect of changing the heat exchanger angle with respect to the horizontal is examined. Results indicate that fin shape is a critical parameter in every modification. In fact, for constant fin thickness, provided the fin pitch varies from 10 mm to 20 mm, due to 58% increase in fin height, the melting time reduces about 35%. Moreover, considering constant fin pitch, when the fin thickness increases from 1.5 mm to 2.5 mm, because of the 28% reduction of fin height, the melting time increases 59%. Also, an increase in the angle of the heat exchanger from zero to 90°, affects the melting process considerably.     

Bubble simulations with an interface tracking technique based on a partitioned fluid-structure interaction algorithm

Numerical techniques frequently used for the simulation of one bubble can be classified as interface tracking techniques and interface capturing techniques. Most of these techniques calculate both the flow around the bubble and the shape of the interface between the gas and the liquid with one code. In this paper, a rising axisymmetric bubble is simulated with an interface tracking technique that uses separate codes to determine the position of the gas-liquid interface and to calculate the flow around the bubble. The grid converged results correspond well with the experimental data.The gas-liquid interface is conceived as a zero-mass, zero-thickness structure whose position is determined by the liquid forces, a uniform gas pressure and surface tension. Iterations between the two codes are necessary to obtain the coupled solution of both problems and these iterations are stabilized with a fluid-structure interaction (FSI) algorithm. The flow around the bubble is calculated on a moving mesh in a reference frame that rises at the same speed as the bubble. The flow solver first updates the mesh throughout the liquid domain given a position of the gas-liquid interface and then calculates the flow around the bubble. It is considered as a black box with the position of the gas-liquid interface as input and the liquid forces on the interface as output. During the iterations, a reduced-order model of the flow solver is generated from the inputs and outputs of the solver. The solver that calculates the interface position uses this model to adapt the liquid forces on the gas-liquid interface during the calculation of the interface position.     

Effects of heat transfer and space porosity on peristaltic flow in a vertical asymmetric channel

This article discusses the effect of heat transfer on the peristaltic flow of a Newtonian fluid through a porous space in a vertical asymmetric channel. Long wavelength approximation is used to linearize the governing equations. The system of the governing nonlinear PDE is solved by using the perturbation method. The solutions are obtained for the velocity and the temperature fields. The flow is investigated in a wave frame of reference moving with velocity of the wave. Numerical calculations are carried out for the pressure rise, frictional forces, and the features of the flow and temperature characteristics are analyzed by plotting graphs and discussed in detail. © 2009 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2010     

Object-oriented modelling and simulation of heat exchangers with finite element methods

The paper describes the derivation of finite-element models of one-dimensional fluid flows with heat transfer in pipes, using the Galerkin/least-squares approach. The models are first derived for one-phase flows, and then extended to homogeneous two-phase flows. The resulting equations have then been embedded in the context of object-oriented system modelling; this allows one to combine the fluid flow model with a model for other phenomena such as heat transfer, as well as with models of other discrete components such as pumps or valves, to obtain complex models of heat exchangers. The models are then validated by simulating a typical heat exchanger plant.     

Topology Optimization of Fins for Rapid Heat Storage and Release in Triangular-Inside Tube Units北大核心CSCD

针对传统相变蓄热器传热速率低的问题,提出了一种内三角管式的蓄热器,并基于拓扑优化原理,以强化换热为目的,对其进行肋片设计,重构了拓扑结果,进而提取其拓扑特征重新设计肋片,分析了不同肋片设计对传热能力的影响。结果表明:内三角管式蓄热器相比传统圆管式蓄热器,蓄放热性能大大提高;安装拓扑重构肋片的蓄热器可以使蓄、放热时间缩短,传热效率提高;在蓄热过程中,分叉的拓扑特征可以提高自然对流作用;在放热过程中,安装拓扑重构肋片的蓄热器(火 积)耗散更小,可逆性更好,换热效率更高。     

Modeling of thermal phenomena in single laser beam and laser-arc hybrid welding processes using projection method

This paper concerns mathematical and numerical modeling of thermal phenomena accompanying single laser and laser-arc hybrid butt welding of steel sheets. Coupled heat transfer and fluid flow in the fusion zone were described respectively by transient heat transfer equation and Navier–Stokes equation. Laser beam and electric arc heat sources were modeled using different heat source power distributions. Latent heat associated with the material’s state changes, buoyancy forces and liquid material flow through a porous medium were taken into account in considerations. Differential governing equations were numerically solved using projection method combined with finite volume method. Elaborated solution algorithm was implemented into computer solver used for simulation of heat transfer and fluid flow during welding. The geometry of the weld and heat affected zone as well as cooling rates were estimated on the basis of numerically obtained temperature field.     

Multigrid fictitious boundary and grid deformation methods for flow-induced rotation of wing

Numerical simulations of flow-induced rotation of wing by multigrid fictitious boundary and grid deformation methods are presented. The flow is computed by a special ALE formulation with a multigrid finite element solver. The solid wing is allowed to move freely through the computational mesh which is adaptively aligned by a special mesh deformation method. The advantage of this approach is that no expensive remeshing has to be performed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

带尖角的障碍声波散射区域的反演

对带尖角的障碍声波散射区域进行了反演,其前提条件是整体场满足奇次Dirichlet边界条件．在用Nystrom方法解正问题的过程中,由于采用等距网格积分给尖角处带来很差的收敛性,这是因为双层位势的积分算子的核在尖角处有Mellin型奇性,不再是紧算子;为此采用梯度网格,数值例子表明该处理方法的有效可靠性．     

A domain decomposition preconditioner for steady groundwater flow in porous media

In this paper an algorithm is presented based on the additive Schwarz method for steady groundwater flow in a porous medium. The subproblems in the algorithm correspond to the problem on a coarse grid and some overlapping subdomains. It will be shown that the rate of convergence is independent of the mesh parameters and discontinuities of the coefficients, and depends on the overlap ratio.     

Finite Volume Methods for Multi-Symplectic PDES

We investigate the application of a cell-vertex finite volume discretization to multi-symplectic PDEs. The investigated discretization reduces to the Preissman box scheme when used on a rectangular grid. Concerning arbitrary quadrilateral grids, we show that only methods with parallelogram-like finite volume cells lead to a multi-symplectic discretization; i.e., to a method that preserves a discrete conservation law of symplecticity. One of the advantages of finite volume methods is that they can be easily adjusted to variable meshes. But, although the implementation of moving mesh finite volume methods for multi-symplectic PDEs is rather straightforward, the restriction to parallelogram-like cells implies that only meshes moving with a constant speed are multi-symplectic. To overcome this restriction, we suggest the implementation of reversible moving mesh methods based on a semi-Lagrangian approach. Numerical experiments are presented for a one dimensional dispersive shallow-water system.     

三维横向流体诱发直管振动的数值模拟

基于有限体积法和有限元法,结合动网格控制技术,建立了横向流体作用下三维弹性直管流致振动计算的数值模型,实现了计算结构动力学与计算流体力学之间的联合仿真．首先,通过对刚性管的静止绕流计算,研究了网格离散方式和不同湍流模型对圆柱类结构静止绕流流场特征的影响和预测能力,得到了适用于双向耦合分析的CFD模型;其次,利用基于双向流固耦合方法的流致振动模型,计算并分析了流体力与结构位移间的相位关系,指出流体力与位移间的相位差是由流体力引起的,同时对双向耦合和单向耦合进行了比较分析;最后通过对直管流致振动的数值计算,联合管表面压力、尾流区时均速度、分离角等时均量,分析了尾流区的流场特征.