热特性参数可变时抛物线型截面环肋传热的最优化研究

肋片传热的最优化的传统处理方法是当执行一定的传热任务时,具有最少的肋片材料消耗(投资).最少重量的冷却肋片是具有抛物线型的截面.本文应用不变嵌入原理研究了当热特性参数可变时抛物线型截面环肋传热的最优几何尺寸,同时还对两个主要的物理参数,即导热系数变化参数α和放热系数变化指数m对最优几何尺寸的影响进行研究.所得结果对工程设计具有现实指导意义.     

变热特性参数条件下肋片温度周期性变化的传热研究*

本文研究了变热特性参数下,根部温度作周期性变化的肋片传热情况.应用摄动法求解控制微分方程;并且采用打靶法和叠加原理进行数值计算,求解过程是嵌进的、非迭代的.对某种形状的肋片而言,当肋片根部温度作周期性变化时,其传热过程受以下几个参数的影响:E──导热系数的温度系数;N──肋片传热的特性参数;ε──温度波动的幅度参数;B──温度波动的频率;以及对流系数的变化模式等.文中给出了这些参数变化时对肋片的温度分布及热流率、肋效率等的影响情况.所得结果,不但具有理论价值,而且对工程设计也有现实指导意义.     

无力磁场和Beltrami流

场矢量处处平行其旋度的管式矢量场具有复杂的拓扑结构．而且通常表现出混饨行为．本文提供了具有常数比例因子的这类矢量场在三种基本坐标系内的解析解,并指出完全导电流体内的一个Beltrami流能维持一个定常无力磁场,如果磁场矢量处处平行速度．     

基于参数化水平集法的材料非线性子结构拓扑优化

针对利用传统水平集法进行非线性结构拓扑优化计算过程复杂及计算效率低等问题,将参数化水平集方法引入材料非线性结构拓扑优化中。通过全局径向基函数插值初始水平集函数,建立了以插值系数为设计变量、结构的应变能最小为目标函数、材料用量为约束条件的材料非线性结构拓扑优化模型,利用有限元分析对材料非线性结构建立平衡方程,并用迭代法求解。同时,采用子结构法划分设计区域为若干个子区域,将全自由度平衡方程的求解分解为缩减的平衡方程和多个子结构内部位移的求解,减小了计算成本。算例表明,这种处理非线性关系的方法可以在保证数值稳定的同时提高计算效率,得到边界清晰、结构合理的拓扑优化构形。     

横肋管内流场的计算方法

利用Usawa-惩罚算法,本文计算了横肋管内的流场。其主要思想是将Usawa算法与惩罚算法相结合(参见[5]),以改进Usawa算法的收敛性。该方法在步长因子的选取上比Usawa算法有更大的余地。我们利用此方法得到的结果与文献[4]中给出结果符合很好。同时,本文讨论了Usawa-惩罚算法的收敛性条件,并讨论原问题的解的存在性和唯一性。一、引言横肋管在工业上广泛用于换热管,以强化传热,这种管内流场的计算,对了解传热强化的机理,改进设计方法有重要意义,因而引起研究者的重视[1]～[4],但还缺乏较好的计算方法。我们在本文中将[5]中的Usawa算法与惩罚算法相结合,以改进Usawa算法的收敛性。同时,我们对该算法的收敛性条件作了研究,从而在理论上证明了横肋管内     

开广义序同态的刻划及拓扑分子格的特征与权

王国俊先生于〔１〕中借助闭元成功地引入了开广义序同态的概念。本文给出了开广义序同态的一个刻划并对其是否增加拓扑分子格的权与特征等问题进行了研究，得到了积拓扑分子格的特征与权与因子拓扑分子格的特征与权间的关系，从而部分回答了ＬＦ拓扑中关于特征的两个公开问题。     

数列极限的教学反思

教学反思是有效地将理论与实践相结合的教学方法,它可以通过实现过程的再思考提高对理论知识的认识,从而促进教师提高教学水平.本文从数列极限这一节的课堂教学出发,通过对教学理念、教学设计、教学过程、教学反馈这四个方面进行反思,有助于重构数列极限的教学设计,提高课堂教学效率.     

有限元软件中拓扑功能对空心剪力墙的优化设计

利用ANSYS有限元软件中拓扑功能对空心剪力墙进行模拟分析,在保证结构安全的前提下可以大大降低工程制造成本,进一步验证了拓扑功能对结构优化设计的重要作用.     

周向加肋非圆柱壳谐振分析的一个新矩阵方法

基于一类柱壳谐振控制方程呈一阶常微分矩阵方程形式以及傅立叶级数展开,提出了一种新矩阵方法,求解两端简支具有环肋加强非圆柱壳在谐外压作用下的稳态响应．该方法和以往同类方法相比,有两个突出的优点:1) 矩阵微分方程的解采用齐次扩容精细积分法替代龙格-库塔法,提高了精度;其中传递矩阵能实现计算机精确计算．2) 环肋作用力借助Dirac-δ函数和三角级数逼近可以解析求出;除法向作用力外,还考虑了切向作用力．通过数值计算,还研究了外激励频率对壳体位移和应力的影响规律．对比有限元分析与其它方法的计算结果,表明了该方法的正确性和有效性．     

具有拟不变测度的交换拓扑群上的正定函数

<正> §1.引言如所周知,拓扑群上的正定函数是研究拓扑群的酉表示的重要工具.对于交换的、具有平移不变测度(即 Haar 测度)的拓扑群,例如局部紧的拓扑群,群上的连续正定函数可以用连续特征标关于对偶群上某个测度的积分表示.对于不具有平移不变测度的交换拓扑群,并不是每个连续正定函数都可以用上述积分表示.因而对于不具有平移不变的交换拓扑群,对群上的连续正定函数较难研究,关于抽象调和分析中其余的问题也有类似情     

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.     

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.     

带鳍通道性能的遗传算法优化

与光滑通道相比,带鳍通道有更高的传热系数,附加的鳍,极大地增强了通道的传热.然而,传热的增强又与压降的升高相关联,这又导致泵动力需求的增加,因此应该寻求对该系统的优化设计.该文的主要目的是,通过如下方式来精确地确定鳍的位置和尺寸:利用遗传算法实现最小压降时达到最优传热.鳍的每种布局作为问题(遗传算法中的一个个体)的一个解.通常,首先随机地产生一个初始种群,然后该算法在所有这些解中搜索,利用布局函数迭代出新解,最后得到鳍的优化设计.     

Topologically complete representations of inverse semigroups

A representation of an inverse semigroup by means of partial open homeomorphisms of a topological T₀-space is called topologically complete if the domains of these partial homeomorphisms form a base of the topology. It is shown how to construct topologically complete representations on the base of a ternary relation satisfying some elementary axioms. This result makes it possible to obtain a pseudo-elementary axiomatization for inverse semigroups that have faithful topologically complete representations in T₁, T₂ and T₃-spaces. A topology is introduced on any antigroup; this topology is a concomitant of the algebraic structure and every topologically complete representation is continuous with respect to this topology.     

A fast running numerical model based on the implementation of volume forces for prediction of pressure drop in a fin tube heat exchanger

Numerical based design of geometrical structures is common when studying systems involving heat exchangers, a central component in several fields, such as industrial, vehicle and household systems. The geometrical structure of heat exchangers is generally comprised by closely placed fins and tube bundles. The creation of a mesh grid for a geometrically compact heat exchanger will result in a dense structure, which is not feasible for personal computer usage. Hence, volume forces were created based on Direct Numerical Simulations (DNS) on a Flow Representative Volume (FRV) of a tube fin heat exchanger in an internal duct system of a heat pump tumble dryer. A relation of the volume averaged velocity and the volume averaged force was established in two different FRV models with a finite element simulation in COMSOL. This relation was subsequently used to create flow resistance coefficients based on volume averaged expressions of fluid velocity and volume forces. These flow resistance coefficients were implemented in two respective porous models, which represent the entire heat exchanger except the interior arrangements of fins and tube bundles. Hence, the computation time was reduced thanks to the absence of a dense mesh grid. Experimental results of the entire heat exchanger showed good agreement with the second porous model in terms of pressure drop and volume flow rate.     

Mixed convection heat transfer by nanofluids in a cavity with two oscillating flexible fins: A fluid–structure interaction approach

This study investigated the effects of two oscillating fins on the heat transfer rate and flow characteristics of a nanofluid inside a square enclosure. Both fins were attached to the hot wall and both fins oscillated at the same frequencies and amplitudes. The finite element method implemented in the arbitrary Lagrangian–Eulerian (ALE) technique was used to solve the equations describing the interactions and movements of the nanofluid and fins. Comparisons of our results and those reported in previous studies demonstrated that the modeling and numerical investigations were valid and reliable. The results showed that the increase in the heat transfer rate was due to the oscillation of the fins. In addition, the increasing trend in the heat transfer rate due to the oscillating fins decreased as the ratio of the thermal conductivity of the fins relative to the nanofluid increased. Increasing the thermal conductivity and viscosity parameters enhanced and weakened the heat transfer rate, respectively.     

Topologically Complete Representations of Inverse Semigroups

A representation of an inverse semigroup by means of partial open homeomorphisms of a topological T ₀ -space is called topologically complete if the domains of these partial homeomorphisms form a base of the topology. It is shown how to construct topologically complete representations on the base of a ternary relation satisfying some elementary axioms. This result makes it possible to obtain a pseudo-elementary axiomatization for inverse semigroups that have faithful topologically complete representations in T ₁ ,T ₂ and T ₃ -spaces. A topology is introduced on any antigroup; this topology is a concomitant of the algebraic structure and every topologically complete representation is continuous with respect to this topology.