An experimental and numerical study on heat transfer enhancement for gas heat exchangers fitted with porous media

The present experimental and numerical work investigates the effect of metallic porous materials, inserted in a pipe, on the rate of heat transfer. The pipe is subjected to a constant and uniform heat flux. The effects of porosity, porous material diameter and thermal conductivity as well as Reynolds number on the heat transfer rate and pressure drop are investigated. The results are compared with the clear flow case where no porous material was used. The results obtained lead to the conclusion that higher heat transfer rates can be achieved using porous inserts at the expense of a reasonable pressure drop. Also, it is shown that for an accurate simulation of heat transfer when a porous insert is employed its effective thermal conductivity should be carefully evaluated.     

Convection heat transfer from tube banks in crossflow: Analytical approach

The main objective of this analytical study is to investigate heat transfer from tube banks in crossflow under isothermal boundary condition. For this purpose, a control volume is selected from the fourth row of a tube as a typical cell to study the heat transfer from an in-line or staggered arrangement. An integral method of boundary layer analysis is employed to derive closed form expressions for the calculation of average heat transfer from the tubes of a bank, that can be used for a wide range of parameters including longitudinal pitch, transverse pitch, Reynolds and Prandtl numbers. The models for in-line and staggered arrangements are applicable for use over a wide range of parameters when determining heat transfer from tube banks.     

基于TMS320F28335的电力系统稳定器的研究

目前国内普遍采用以电功率作为输入信号的单输入电力系统稳定器（PSS）,然而在原动机功率发生变化时, PSS本身不能区分系统波动和原动机功率波动,容易引起“反调”。设计了一种基于TMS320F28335的双输入PSS,发电机的电功率和转子角速度作为该PSS的输入信号。论述了双输入PSS的模型,硬件设计部分主要包括总体结构的设计,软件设计部分包括总体设计及流程图。运用附加双输入PSS励磁调节器进行了阶跃和反调实验,结果表明该PSS能够有效抑制低频振荡,很好地解决“反调”问题。     

Two-dimensional laminar fluid flow and heat transfer in a channel with a built-in heated square cylinder

A numerical investigation was conducted to analyze the unsteady flow field and heat transfer characteristics in a horizontal channel with a built-in heated square cylinder. Hydrodynamic behavior and heat transfer results are obtained by the solution of the complete Navier–Stokes and energy equations using a control volume finite element method (CVFEM) adapted to the staggered grid. The Computation was made for two channel blockage ratios (β=1/4 and 1/8), different Reynolds and Richardson numbers ranging from 62 to 200 and from 0 to 0.1 respectively at Pr=0.71. The flow is found to be unstable when the Richardson number crosses the critical value of 0.13. The results are presented to show the effects of the blockage ratio, the Reynolds and the Richardson numbers on the flow pattern and the heat transfer from the square cylinder. Heat transfer correlation are obtained through forced and mixed convection.     

Simultaneous double-sided milling of flexible plates with high accuracy and high efficiency—Suppression of forced chatter vibration with synchronized single-tooth cutters

This paper presents a new method to finish flexible plates with high accuracy and high productivity. Precision steel plates are finished conventionally by face milling with electro-magnetic chucks. It is difficult to improve flatness of the flexible plates, because they deform to fit the chuck surfaces while clamped. The authors have applied simultaneous double-sided milling to solve this problem, but it causes forced chatter vibration which is not allowed in precision finishing. Thus, the new method is proposed and verified to suppress the forced chatter vibration, in which the both surfaces are finished simultaneously with single tooth milling cutters with synchronization so that the thrust forces are cancelled out on the both sides.     

Laser-induced incandescence study of diesel soot formation in a rapid compression machine at elevated pressures

Simultaneous laser-induced incandescence (LII) and laser-induced scattering (LIS) were applied to investigate soot formation and distribution in a single cylinder rapid compression machine. The fuel used was a low sulfur reference diesel fuel with 0.04% volume 2-ethylhexyl nitrate. LII images were acquired at time intervals of 1 CA throughout the soot formation period, for a range of injection pressures up to 160 MPa, and in-cylinder pressures (ICP) up to 9 MPa. The data collected shows that although cycle-to-cycle variations in soot production were observed, the LII signal intensities converged to a constant value when sufficient cycles were averaged. The amount of soot produced was not significantly affected by changes in in-cylinder pressure. Soot was observed distributed in definite clusters, which were linked to slugs of fuel caused by oscillations in the injector needle. The highest injection pressure exhibited lower soot productions and more homogeneous soot distributions within the flame. Despite diffusion flames lasting longer with lower injection pressure, it appeared that the extended oxidation time was insufficient to oxidize the excess production of soot. In addition, soot particles were detected closer to the nozzle tip with higher injection pressures. The recording of LII sequences at high temporal resolutions has shown that three distinct phases in soot formation can be observed. First, high soot formation rates are observed before the establishment of the diffusion flame. Second, a reduced soot formation rate is apparent from the start of diffusion flame until the end of injection. Finally, high soot oxidation rates occur after the end of injection and for the duration of the flame.     

Thermal convection for a thermo-dependent yield stress fluid in an axisymmetric horizontal duct

This paper deals with the combined forced and free convection heat transfer of a yield stress fluid in a horizontal duct heated uniformly with a constant heat flux density. It is assumed that (i) the rheological behavior of the fluid can be described by the Herschel–Bulkley model and that the consistency K* varies with temperature T*, as K* = a exp(−bT*); (ii) the variation of the fluid density ρ, with temperature, , is considered important only in the buoyancy term and (iii) the Péclet number is sufficiently large so that it is possible to resort to an asymptotic solution. The aim of this study is to quantify the effect of the rheological properties on the magnitude of the secondary flows induced by the thermo-dependency of K* and ρ. Expressions for local Nusselt number and wall shear stress are given. Finally, to be consistent with the variation along the duct of the axial velocity in the central zone around the axis, due to the thermo-dependency of K* or ρ, the pseudo-plug zone and pseudo-yield surface notions are introduced. Some characteristics of the stresses distribution within the pseudo-plug zone are discussed.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Geometrically nonlinear forced vibrations of the symmetric honeycomb sandwich panels affected by the water

Geometrically nonlinear forced vibrations of the symmetric rectangular honeycomb sandwich panels with the four edges simply supported and one surface affected by the water are investigated in this paper using the homotopy analysis method (HAM). The honeycomb core of hexagonal cells is modeled as a thick layer of orthotropic material whose physical and mechanical properties are determined using the Gibson correlations. The effect of water acting on honeycomb panels can be described as added mass, additional damping and additional stiffness coefficients which are obtained by the semi-analytical fluid pressures. The basic formulation of nonlinear forced vibrations has been developed base on the third-order shear deformation plate theory and Green Lagrange nonlinear strain–displacement relation. The equilibrium equations have been obtained using the Hamilton’s principle. Effects of water velocity, height and height ratio on the nonlinear forced vibration response have been studied for the honeycomb sandwich panels.     

滚动优化PSS参数搜索小干扰稳定极限边界策略研究

当前电力系统具有大区域间弱互联并伴随同步发电机快速高放大倍数励磁系统大量使用等特征,低频振荡问题多发。小干扰稳定域表征了参数空间中所有能够保持系统小扰动稳定性的平衡点的集合,对于电网的安全稳定运行有重要意义。但现有对小干扰稳定域边界的研究总体处于不计及阻尼控制器参数影响或仅考虑固定阻尼控制器参数的影响的状态,稳定域边界趋于保守。为拓展稳定域边界,以常用的PSS为对象,试图通过优化PSS参数达到拓展边界的目的,对电网规划以及运行方式校核等离线分析提供了一定参考,进一步提高了电网运行的安全性和经济性。为此,首先基于系统平衡点建立了以最小阻尼比最大化为目标的多PSS参数协调优化模型,并采用了改进的PGSA优化算法进行求解。在此基础上,形成了一种基于滚动优化的思路,通过逐步外拓来实现小干扰稳定极限边界点搜索策略。最后,基于4机2区算例,通过与常规方法的对比,验证了所提策略拓展稳定域边界的有效性和优越性。