空气横掠错列翅片管束流动与传热的数值研究

应用SIMPLE算法和k-ε模型对空气横掠错列翅片管束在湍流奈件下的流动和传热性能进行了数值模拟。对翅片管束在流体的不同入口流速情况下的换热和阻力特性进行了比较，得到了湍流条件下的努塞耳准则数和欧拉准则数与雷诺准则数之间的关系式，得出努塞耳数随着入口流速的增加而增加，但欧拉数随着入口流速的增加而下降。     

Heat transfer characteristics of cryogenic helium gas through a miniature tube with a large temperature difference

Heat transfer with a large temperature difference between the inlet and outlet flows is often encountered in many cryogenic systems. The heat transfer characteristics of cryogenic gas with temperature-dependent thermophysical properties (TDTP) are different from those in the ambient condition with constant thermophysical properties. In this paper, a combined experimental and numerical study has been conducted to determine the heat transfer characteristics of cryogenic helium gas with temperature-dependent thermophysical properties in a miniature tube. In order to validate the theoretical model, an experimental investigation was conducted. Utilizing the validated model with the FLUENT software, the temperature distribution and velocity profile in a miniature tube have been calculated, and a correlation predicting the temperature effect on the Nusselt number determined.     

Discretization methods with analytical solutions for a convection–reaction equation with higher-order discretizations

We introduce an improved second-order discretization method for the convection–reaction equation by combining analytical and numerical solutions. The method is derived from Godunov's scheme, see [S.K. Godunov, Difference methods for the numerical calculations of discontinuous solutions of the equations of fluid dynamics, Mat. Sb. 47 (1959), pp. 271–306] and [R.J. LeVeque, Finite Volume Methods for Hyperbolic Problems, Cambridge Texts in Applied Mathematics, Cambridge University Press, 2002.], and uses analytical solutions to solve the one-dimensional convection-reaction equation. We can also generalize the second-order methods for discontinuous solutions, because of the analytical test functions. One-dimensional solutions are used in the higher-dimensional solution of the numerical method.

The method is based on the flux-based characteristic methods and is an attractive alternative to the classical higher-order total variation diminishing methods, see [A. Harten, High resolution schemes for hyperbolic conservation laws, J. Comput. Phys. 49 (1993), pp. 357–393.]. In this article, we will focus on the derivation of analytical solutions embedded into a finite volume method, for general and special solutions of the characteristic methods.

For the analytical solution, we use the Laplace transformation to reduce the equation to an ordinary differential equation. With general initial conditions, e.g. spline functions, the Laplace transformation is accomplished with the help of numerical methods. The proposed discretization method skips the classical error between the convection and reaction equation by using the operator-splitting method.

At the end of the article, we illustrate the higher-order method for different benchmark problems. Finally, the method is shown to produce realistic results.     

含内热源管内填充多孔介质的强迫对流传热解析解

基于非局部热平衡假设,研究了管内填充多孔介质的强迫对流传热问题.在附于多孔介质圆柱的外表面施加恒定热流,采用Darcy动量方程描述多孔介质内流体的流动.为考虑工程实际中可能的物理或化学发热源,将多孔介质液相和固相内热源引入传热模型中,获得了液相和固相温度分布的解析解,同时推导了表征传热性能的Nusselt数显式表达式.计算结果表明:计算解析解与文献[10]推导的解析解吻合良好,固相内热源强度愈高,液相和固相温差愈大,Nusselt数愈小,传热性能愈差.此外,还解释了管壁处液相和固相温度梯度出现分岔的原因.     

An experimental comparison of water based alumina and silica nanofluids heat transfer in laminar flow regime

Experimental investigations were carried out to determine the Al2O3/water and SiO2/water nanofluids flowing through a circular tube. convective heat transfer performance and pressure drop of Measurements show that the addition of small amounts of nano-sized Al2O3 particles to the base fluid increases heat transfer coefficients considerably, while the result for the silica nanofluids contradicts with the alumina nanofluids and this leads to some interesting results. In the case of alumina nanofluids, an average increase of 16% in convective heat transfer coefficient is observed with an average penalty of 28% in pressure drop. Moreover, flow resistance increases significantly compared to the base fluid even at very low concentrations of nanofluids. Finally, measured heat transfer coefficients are compared with predicted ones from the correlation of Shah under the same conditions.     

Lyapunov characterization of forced oscillations

This paper develops a Lyapunov approach to the analysis of input-output characteristics for systems under the excitation of a class of oscillatory inputs. Apart from sinusoidal signals, the class of oscillatory inputs include multi-tone signals and periodic signals which can be described as the output of an autonomous system. The Lyapunov approach is developed for linear systems, homogeneous systems (differential inclusions) and nonlinear systems (differential inclusions), respectively. In particular, it is established that the steady-state gain can be arbitrarily closely characterized with Lyapunov functions if the output response converges exponentially to the steady-state. Other output measures that will be characterized include the peak of the transient response and the convergence rate. Tools based on linear matrix inequalities (LMIs) are developed for the numerical analysis of linear differential inclusions (LDIs). This paper's results can be readily applied to the evaluation of frequency responses of general nonlinear and uncertain systems by restricting the inputs to sinusoidal signals. Guided by the numerical result for a second order LDI, an interesting phenomenon is observed that the peak of the frequency response can be strictly larger than the L₂ gain.     

Implementation and control of asymmetric thermal environment in two-dimensional rectangular enclosure

Laminar natural convection in an enclosure divided by an adiabatic partition on its bottom with two unequal discrete heat sources was investigated numerically. The effect of the partition on the flow structure and heat transfer characteristics in enclosure under asymmetric heating condition was studied. The parameters are the Rayleigh number (1 × 104 ≤Ra≤1 × 106) and the height of partition (0≤h/H≤0. 70). The streamlines are produced for various Rayleigh numbers. The results reflected by variations of the average Nusselt number in terms of the height of partition illustrates the convection heat transfer in the enclosure. The role of the partition is to weaken or cut off the heat removal rate from the strong heat source to the weak heat source. It is analyzed that the optimum height of the partition to break the linkage between the strong and weak heat source increases with increasing Rayleigh number.     

模拟界面换热系数和密度对热溶质对流影响

为了能够更好地控制凝固过程热和溶质的传输,以二元合金为例,研究了在单边散热条件下,不同界面换热系数和密度对热溶质对流的影响.研究表明:当材料密度一定时,随着界面换热系数的提高,沿散热方向上的温度梯度增大,热溶质对流的效果增强;当界面换热系数为1 000 W/(m2·K)时,在凝固初期铸件底部出现涡流现象;在界面换热系数不变的情况下,随着材料密度的增加,铸件内液体金属的热溶质对流效应减弱.     

Pb-Sn合金侧向凝固过程A偏析的数值模拟

基于二元系凝固过程热溶质的传输行为,建立了描述A偏析形成及演化的数学模型,给出了固相分数与温度场及浓度场的耦合关系.先用已有的实验结果验证了模型的正确性,然后模拟计算了Pb-Sn合金侧向凝固过程A偏析的形成及演化过程,并研究了浮力数对A偏析形成位置及偏析程度的影响.结果表明,在糊状区中双扩散对流引起的密度变化,导致局部流动,形成偏析通道;为了维持偏析通道中局部液体的流动,枝晶间的液体通过糊状区从液相区得到补充.在相同的凝固条件下,浮力数越小,A偏析的形成时间愈迟,偏析的程度也越小.     

大型变压器风冷却系统的自动控制

介绍了以可编程控制器为核心的大型变压器的风冷控制系统的主要运行方式及控制系统结构、控制要求．实现了风冷机组运行过程中的自动控制。