Transient ignition on a flat plate

Abstract

In this study, transient ignition on a hot plate is analyzed numerically. In addition to the transient energy and species equations, the transient momentum equation is also considered. A comparison is made between the results of transient velocity field and those of steady velocity field. It is found that the transient velocity field has the effect of increasing the ignition length and ignition time. The effects of wall temperature, Prandtl number and Schmidt number are also studied.     

多孔吸声材料发展现状与展望

吸声降噪在人们日常生活、设备安全以及军事领域具有重要意义，多孔材料是一类重要的吸声材料。介绍了多孔材料的吸声原理、多孔吸声材料的种类及其特性、影响多孔吸声材料吸声性能的因素等。综述了多孔吸声材料的发展现状，并对吸声材料的发展趋势做了展望。     

Mixed isogeometric analysis of strongly coupled diffusion in porous materials

In this paper, we develop a mixed isogeometric analysis approach based on subdivision stabilization to study strongly coupled diffusion in solids in both small and large deformation ranges. Coupling the fluid pressure and the solid deformation, the mixed formulation suffers from numerical instabilities in the incompressible and the nearly incompressible limit due to the violation of the inf‐sup condition. We investigate this issue using subdivision‐stabilized nonuniform rational B‐spline (NURBS) elements, as well as different families of mixed isogeometric analysis techniques, and assess their stability through a numerical inf‐sup test. Furthermore, the validity of the inf‐sup stability test in poromechanics is supported by a mathematical proof concerning the corresponding stability estimate. Finally, two numerical examples involving a rigid strip foundation on saturated soil and a swelling hydrogel structure are presented to validate the stability and to demonstrate the robustness of the proposed approach.     

The heat and moisture transport properties of wet porous media

Existing methods for determining heat and moisture transport properties in porous media are briefly reviewed, and their merits and deficiencies are discussed. Emphasis is placed on research in developing new transient methods undertaken in China during the recent years. An attempt has been made to relate the coefficients in the heat and mass transfer equations with inherent properties of the liquid and matrix and then to predict these coefficients based on limited measurements.Invited paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 988, Gaithersburg, Maryland, U.S.A.     

多孔金属材料制备方法

本文概述了多孔金属材料的典型和特点，着重介绍了该材料的各种制备方法，以期提供一些优化组合生产工艺的线索。     

Thermal Transport Properties of Layered Materials: Identification by a New Numerical Algorithm for Transient Measurements

Transient methods are widely used to determine thermal transport properties. In some situations they can be used for homogeneous media to measure several properties either simultaneously or separately. In this context an analytic model is available and a well-posed inverse problem of parameter identification has to be solved. The examination of composite media is more complicated. The algorithm proposed here allows simultaneous determination of the thermal conductivity and thermal diffusivity of layered dielectrics by transient measurements. It is based on a plane source that acts both as a resistive heater and temperature sensor. For the technique to be successful two essential aspects have to be considered: firstly, the mathematical modeling of the measured data (the forward problem) and secondly, the problem of ill-posedness of the inverse problem. For the proposed measurement configuration, a new fast data analysis algorithm based on an analytic solution for the forward problem is presented. In principle, a numerical solution such as an FEM solution of the heat conduction equation can be used instead of the analytical one, but the computational effort is much greater. The inverse problem is formulated as an output-least-squares problem, which leads to a transcendent algebraic system of equations. The method was successfully tested for different situations.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

Computer model of a porous medium

A computer model has been set up to represent a porous medium. The basis for this model is a two-dimensional square network (100×100) of channels that have randomly assigned widths between the value of zero (closed) and the value of one (open, unrestricted flow). The channel width assignments have been made by a random selection from five different distributions:f(q)=q, f(q)=sinq, f(q)=erf(q),f(q)=1–sinq, andf(q)=1 –erf(q). Diffusion of particles in the network has been studied by a random-walk procedure for each realization of the channel width assignments. The diffusivity is quite sensitive to the distribution of channel widths. The percolation properties of the networks obtained from the three most restrictive distributions have been investigated and the independent, linked clusters within the network have been determined. For cluster sizes that are less than the full width of the network, the network does not percolate and either the flow is not diffusive or the diffusivity is severely reduced. An approximate value for the percolation threshold has been determined in each case and the fractal dimension has been calculated also.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

瞬态高强加热条件下含湿多孔介质传热传质模型研究

提出了瞬态高强加热下含湿多孔介质传热传质的新模型，模型包括的水分种类齐全、水分迁移机制全面，假设条件相对较少，考虑了非Fourier传热效应和非Fick传质效应。新模型通过具体的分区分析得到简化并更加实用。     

一种新型多孔泡沫钨的制备

介绍了一种类网状结构的多孔态泡沫钨及其制备方法。该泡沫钨材料的孔隙分两类:(1)是构成类网状结构且孔径为0.2～1.0mm的主孔;(2)是存在于主孔孔壁和孔棱之上且孔径在几个微米量级的微孔。主孔呈通孔和半通孔结构,并且两类孔隙相互连通,总孔率高于50%。该泡沫钨的制备方法是以有机泡沫材料为基体,采用浸浆干燥烧结法制备而成。其中料浆由钨粉和无毒性有机黏结剂组成,黏度用去离子水调节。     

Transient response of perforated plate matrix heat exchangers

Perforated plate matrix heat exchangers are used in a number of applications such as helium liquefiers, Joule–Thompson cryocoolers operating with pure fluids and mixtures, etc. The time taken for cool down of cryocoolers is very critical in many applications, for example, those used in mobile applications (e.g. missiles). In this paper we study the effect of different parameters on the transient response of perforated plate matrix heat exchangers.     

Transient diffusion of gas in a highly dispersee sorbing porous solid

The transient diffusion of gas in a porous sorbent is considered on the basis of the “dusty” (or “dust-laden”) gas model. An equation is derived for the diffusion coefficient.     

Size-dependent vibration analysis of a three-layered porous rectangular nano plate with piezo-electromagnetic face sheets subjected to pre loads based on SSDT

ABSTRACT

In the present research vibration of a porous rectangular plate which is located between two piezo-electromagnetic layers based on two variables sinusoidal shear deformation plate theory and according to nonlocal theory is investigated. The plate is resting on Winkler–Pasternak foundation and was subjected to pre loads. The motion equations have been obtained using Hamilton principle and are solved using analytical Navier's solution method. The effects of porosity coefficient, pores distribution, nonlocal parameter, pre load values, foundation constants and geometric size of the plate have been discussed in details. The results can be used to design more efficient sensors and actuators.     

多孔材料在三向载荷作用下的力学模型

为更全面掌握三维网状多孔材料在工程应用中的安全承载问题,分析了各向同性的该类材料在三向载荷(多向载荷)作用下的力学性能.在其结构简化的基础上,提出了该类材料在三向载荷作用下相应的承载力学模型,其中三向载荷中的任何一个载荷都可以任意为拉伸和压缩.运用这一模型,可以判断当该类材料在承受上述载荷条件时是否会发生破坏,其用来判断的材料固有指标即是多孔体的孔率,该指标是多孔材料最基本的参量.     

Density of capillaries and the oxygen diffusion model in the porous silk fibroin film

In order to obtain porous silk fibroin films (PSFFs) fit for the repair of different tissues and organs and design the configuration of the PSFFs more rationally, a model of the oxygen diffusing system of the capillary was built, and also the equations of the model were solved. Moreover, the relationships between the distribution of the oxygen concentration and each affecting factors were discussed, a method was developed to estimate the density of the capillaries in the tissue, and hereby discussed the characteristics of the oxygen diffusion in the tissues around the open capillaries.     

Transient Hot Wire (THW) Method: Uncertainty Assessment

The standard method for measuring thermal transport properties of dielectric solids such as ceramics and refractories is the transient hot wire (THW) technique. In its simplest arrangement, a thin wire is embedded between two sample halves, where it acts simultaneously as a resistive heat source and a thermometer. From its temperature signal, the thermal conductivity and the thermal diffusivity of the dielectric can be derived. Up to now, there is no uncertainty assessment for this technique strictly following the ISO Guide to the Expression of Uncertainty in Measurement. Here we analyze the ISO standard uncertainty of the THW technique in the same way as in a previous paper on the uncertainty of the closely related transient hot strip (THS) technique. The two papers provide a comprehensive comparison of the most important advantages and disadvantages of these two transient techniques. The results obtained here for the uncertainty (5.8% for the thermal conductivity and 30% for the thermal diffusivity) are nearly the same as those for the THS method. Experiments on a Pyrex standard-reference sample confirm the results.     

Transient free convection about a vertical flat plate embedded in a porous medium

The problems of transient free convection in a porous medium adjacent to a vertical semi-infinite flat plate with a step increase in wall temperature and surface heat flux are considered in this paper. By assuming a temperature profile in each case, the governing equation for the boundary layer thickness is obtained by an integral method. These governing equations are first-order partial differential equations of the hyperbolic type that can be solved exactly by the method of characteristics and approximately by the method of integral relations. The results based on the method of characteristics clearly indicate that during the initial stage when the leading edge effect is not being felt, heat is transferred as if by transient 1-dimensional heat conduction. At a later time, depending on the vertical location, the heat transfer characteristics change from transient 1-dimensional heat conduction to steady 2-dimensional convection. The thickness of the boundary layer is shown to be increasing with time until it reaches steady state where its value remains constant thereafter. The growth rate of the boundary layer thickness exhibits a discontinuity at the end of the transient period and the beginning of the steady state period. On the other hand, the results based on the method of integral relations show that the boundary layer thickness grows continuously with time and approaches the steady state value asymptotically; the growth rate of the boundary layer thickness decreases from a finite value to zero continuously as the steady state is approached. Except between the end of the transient period and the beginning of the steady state period, the results based on the method of integral relations are in good agreement with those based on the method of characteristics.     

分层多孔材料吸声结构的性能分析

利用声波在分层介质中的传播方程,给出了由不同种吸声材料复合而成的多层吸声结构吸声系数的递推计算公式。继而以由多孔吸声材料复合而成的双层吸声结构为基础,通过数值仿真,深入分析了内外层材料的厚度、孔隙率、和微孔半径变化对吸声系数的影响。以导出的递推计算公式为基础,通过仿真实验研究了分层吸声结构的参数优化设计。结果表明：由多孔材料复合而成的分层吸声结构具有良好的吸声效果,通过合理配置各层材料结构参数,可以在较宽的频段上达到较为满意的吸声效果,从而为相应产品的开发提供了理论依据     

Transient vibration and sound radiation of a rectangular plate with viscoelastic boundary supports

This work considers transient vibration and sound radiation from an impact‐excited rectangular plate with viscoelastic boundary supports. The approach used is based on the modal strain energy (MSE) method. Vibration of the plate is approximated by a double infinite series in the spatial co‐ordinates. Each term of the series is constructed with vibration modes of beams having the same boundary conditions as the plate, multiplied by a time‐dependent function. Modal loss factor of each mode is obtained by the MSE method. The sound radiation pressure in the time and frequency domain is obtained by numerical integration of the Rayleigh integral. Effects of the viscoelastic boundary supports on the vibration response and the radiated sound pressure of the vibrating plate are also discussed. Copyright © 2001 John Wiley & Sons, Ltd.