Laminar flow with combustion in inert porous media

This work presents one-dimensional numerical results for combustion of an air/methane mixture in inert porous media using laminar and radiation models. Comparisons with experimental data are reported. The burner is composed by a preheating section followed by a combustion region. Macroscopic equations for mass, momentum and energy are obtained based on the volume average concept. Distinct energy equations are considered for the porous burner and the flowing gas. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to relax the entire equation set. Inlet velocity, excess air, porosity and solid-to-fluid thermal conductivity ratio were varied in order to investigate their effect on temperature profiles. Results indicate that higher inlet velocities result in higher gas temperatures, following a similar trend observed in the experimental data used for comparisons. Burning of mixtures close to the stoichiometric conditions also increased temperatures, as expected. Increasing the thermal conductivity of the preheating section reduced peak temperature in the combustion region. The use of porous material with very high thermal conductivity on the combustion region did not affect significantly temperature levels in the combustion section.     

Melting of PCM around a horizontal cylinder with constant surface temperature

A numerical study of melting of phase change material (PCM) around a horizontal circular cylinder of constant wall temperature and in the presence of the natural convection in the melt region is presented. A two dimensional mathematical model is formulated in terms of primitive variables and a coordinate transformation technique is used to fix the moving front. The finite volume approach is used to discretize the system of governing equations to obtain a system of linear algebraic equations. An implicit scheme is used for the momentum and energy equations and an explicit scheme for the energy balance at the interface. The numerical predictions were compared with available results to establish the validity of the model and additional results are obtained to demonstrate the effects of Rayleigh and Stefan numbers as well as the wall temperature on the time for complete fusion and total melt volume.     

Turbulent Flow in Wavy Channels Simulated with Nonlinear Models and a New Implicit Formulation

This work examines the performance of linear and nonlinear eddy-viscosity models when used to predict the turbulent flow in periodically sinusoidal-wave channels. Two geometries are investigated, namely a converging-diverging channel and a channel with concave-convex walls. The numerical method employed for the discretization of the equations is the control-volume method in a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm is used for correcting the pressure field. The classical wall function and a low Reynolds model are used to describe the flow near the wall. Comparisons between those two approaches using linear and nonlinear turbulence models are done. Here, a new implicit numerical treatment is proposed for the nonlinear diffusion terms of the momentum equations in order to increase the robustness. Results show that by decomposing and treating terms as presented, solutions using nonlinear models and the high Reynolds wall treatment, which combine accuracy and economy, are more stable and easier to be obtained.     

Numerical simulation of a nonconventional alternator connected to arectifier

A numerical simulation of a nonconventional alternator connected to a rectifier is proposed. To take into account the interference between the magnetic and electric circuits, the magnetic equations solved with the help of the finite-element method are coupled with the electric equations. The Newton-Raphson algorithm is used to consider the different nonlinearities (magnetic materials and semiconductors). The complete differential equations system is solved with an iterative procedure. With the proposed model, the alternator is modeled for different operating points. The numerical results are compared to the experimental ones, showing the reliability of the proposed numerical model     

Numerical solution of the phase change problem around a horizontal cylinder in the presence of natural convection in the melt region

This paper deals with the numerical study of melting of phase change material around a horizontal circular cylinder in the presence of the natural convection in the melt phase. A two dimensional unsteady mathematical model has been formulated in terms of primitive variables and a coordinate transformation technique has been used to fix the moving front. The finite volume approach was used to discretize the system of governing equations, boundary and initial conditions and obtain a system of linear algebraic equations. In the numerical solution an implicit scheme was used for the momentum and energy equations and an explicit scheme for the energy balance at the interface. The numerical predictions were compared with available results to establish the validity of the model and the numerical approach.     

Grid Generation and Numerical Analysis of Multi-Stream Flow in the Complex Channel with a Forced Mixer Lobe

The co-located grid, SIMPLEC and Chen-Kim modified k - E turbulence model are applied to investigate numerically the multi-stream flow and temperature fields in the complex channel with a forced mixer lobe at room temperature and at elevated temperature. The body-fitted coordinate grids are generated respectively in sub-domains according to the shapes of the channel by solving Poisson's equations to compose the whole grid of the domain. The large viscosity, linear and simultaneous under-relaxation factors are used to solve the coupling of fluid and solid. The solid grid is complemented at the upper inlet of the secondary flow to keep the same node number at the inlet and at double-wall sub-domains. The numerical results and experimental data show good agreement at room temperature. It is illustrated that the cooling air ejected into the slot between the double plates decreases the temperature of the wall.     

Numerical study of an annular gas turbine combustor with dump diffuser

INTRODUCTIONThemainfunctionofagasturbinediffusersystemistoreduceinletvelocityoftheflametubeandtoachieveminimaltotalpressurelossforimprovedcombustionprocesses.Inmoderngasturbinecombustors,thedumpdiffuserhasbeenwidelyused,becausethesuddenexpansionatpre...     

Simulation of turbulent impinging jet into a cylindrical chamber with and without a porous layer at the bottom

Turbulent impinging jets on heated surfaces are widely used in industry to modify local heat transfer coefficients. The addition of a porous substrate covering the surface contributes to a better flow distribution, which favors many engineering applications. Motivated by this, this work shows numerical results for a turbulent impinging jet into a cylindrical enclosure with and without a porous layer at the bottom. The macroscopic time-averaged equations for mass and momentum are obtained based on a concept called double decomposition, which considers spatial deviations and temporal fluctuations of flow properties. Turbulence is handled with a macroscopic k–ε model, which uses the same set of equations for both the fluid layer and the porous matrix. The numerical technique employed is the control volume method in conjunction with a boundary-fitted coordinate system. One unique computational grid is used to compute the entire heterogeneous medium. The SIMPLE algorithm is applied to relax the system of algebraic equations. Results indicate that the permeability of the porous layer and the height of the fluid layer significantly affect the flow pattern. The effect of the porous layer thickness was less pronounced in affecting the flow behavior in the fluid layer.     

A Turbulent Impinging Jet on a Plate Covered with a Porous Layer

This work shows numerical results for a turbulent jet impinging against a flat plane covered with a layer of permeable material, which is kept at a higher temperature than that of the incoming fluid. Parameters such as porosity, permeability, thickness, and thermal conductivity of the porous layer are varied in order to analyze their effects on the local distribution of Nu. The macroscopic equations for mass, momentum, and energy are obtained based on volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure–velocity coupling. Results indicate that inclusion of a porous layer decreases the peak in Nu avoiding excessive heating or cooling at the stagnation point. Also found, was that the integral heat flux from the wall is enhanced for certain ranges of values of porosity, layer thickness, and thermal conductivity ratio.     

TWO-DIMENSIONAL PROBLEM OF A MOVING HEATED PUNCH IN GENERALIZED THERMOELASTICITY

The two-dimensional equations of generalized thermoelasticity are solved for the case of a heated punch moving across the surface of a semi-infinite thermoelastic half-space subject to appropriate boundary conditions. The exponential Fourier transform with respect to one space variable in a coordinate system moving with the punch is applied. The resulting equations are solved and numerical results are given. The results are compared with those obtained by Roberts for the coupled thermoelastic case.     

NATURAL CONVECTION FROM A BURIED PIPE WITH EXTERNAL BAFFLES

Numerical solutions are presented for the natural convection heat transfer from a pipe with two baffles attached along its surface buried beneath a semi-infinite, saturated, porous medium. The surface of the medium is assumed to be permeable. The governing equations for Darcy flow are solved using finite differences. The complicated geometry is handled through the use of a body-fitted curvilinear coordinate system. Results are presented for three baffle lengths and a range of burial depths and Rayleigh numbers. The numerical simulations indicate that substantial energy savings can be realized if baffles are used. The results obtained in terms of the Nusselt number for the case of no-baffle are in excellent agreement with analytical and experimental results available in the literature. A simple correlation for Nu has been developed as a function of Ra, pipe burial depth h/R, and baffle length l/R.     

APPLICATION OF GDQ METHOD FOR THE STUDY OF NATURAL CONVECTION IN HORIZONTAL ECCENTRIC ANNULI

A generalized differential-integral quadrature (GDQ) discretization technique developed by one of the authors was used to solve a natural convection problem in a body-fit coordinate system in their primitive variables form. A special treatment of the boundary conditions to satisfy the continuity and momentum equations along the boundaries with the implementation of the GDQ method was investigated. Comparisons with the experimental and numerical results of other investigators are presented and discussed. In contrast with the existing published results, this highly accurate method was able to reveal extremely weak net circulation around the inner eccentric cylinder that was not found previously by other investigators. This net circulation has its maximum value when the inclination angle of eccentricity is in the horizontal position.     

基于可压缩SIMPLE算法的叶栅通道湍流流场的数值模拟

开发了二维任意曲线坐标系下、采用同位网格布局、能计算跨音速可压缩流动的SIMPLE方法计算程序。为提高精度,采用QUICK格式、CUI格式在不同的网格分布下计算。为了验证本文方法对低速到跨音速范围内的粘性流动的数值分析效果,对几个典型算例进行了数值试验,计算结果与文献和实验数据吻合较好。     

低散热发动机对流散热场的数值分析

本文利用在非交错网格上求解非正交曲线上标系下的Ｎ－Ｓ方程的方法，对低散热发动机对流散热场进行了数值模拟，得到了不同瑞利（Ｒａｙｌｅｉｇｈ）数下的散热通道内流场和温度场的数值解。计算结果表明，通过冷却通道中空气自然对流换热方式所散失的热量很小。     

水库垂直二维湍流与水温水质耦合模型

将Ｋ－ε湍流模式引入水库水温与水质的垂向二维分布规律研究中，考虑它们之间固有的交互作用，建立了水库垂直二维湍流与水温水质耦合模型。文中采用了微分方程坐标变换技术，数值格式采用具有自动迎风性质的混合有限分析五点格式，所建模型用于红枫湖水库垂直二维水温与ＣＯＤ分布的研究取得了良好的效果。     

Investigation of electrospun nanofibers with an electrified non-Newtonian jet using differential quadrature method

In this paper, the influence of non-Newtonian rheology of nanofibers on electrospinning process is analyzed numerically. A simple and highly accurate numerical method called the Differential Quadrature Method (DQM), is used for solving the governing equations of electrospinning systems. The validity of the results of DQM solution are verified via comparison with experimental data and a good agreement between the present method and the experimental data is observed. The behavior of the elongation, velocity, stress and total force profiles with variation of some physical parameters are discussed in details. The results show that by increasing the values of yield stress, the fluid elongation is reduced significantly. Furthermore, it is found that Differential Quadrature Method can be easily extended to other strongly nonlinear equations and can be found widely applicable in engineering and science.     

Simulation of a Turbulent Impinging Jet into a Layer of Porous Material Using a Two–Energy Equation Model

This article presents numerical results for a turbulent jet impinging against a flat plane covered with a layer of permeable and thermally conducting material. Distinct energy equations are considered for the solid porous material attached to the wall and for the fluid that impinges on it. Parameters such as Reynolds number, porosity, permeability, thickness, and thermal conductivity of the porous layer are varied in order to analyze their effects on the local distribution of Nu. The macroscopic equations for mass, momentum, and energy are obtained based on volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer eliminates the peak in Nu at the stagnation region. For highly porous and highly permeable material, simulations indicate that the integral heat flux from the wall is enhanced when a thermally conducting porous material is attached to the surface.     

Numerical and experimental study of an integrated solar collector with CPC reflectors

The aim of this work is to develop a numerical code able to predict the thermal behavior of a double tank integrated collector storage system (ICS) with compound parabolic concentrator (CPC). The developed numerical model is based on the detailed analysis of the different forms of heat transfers occurring in the ICS system. The balance equations of each element of the system have been established and solved by means of a transient algorithm. A prototype of an ICS device was constructed and experimentally tested outdoors in order to observe the variation of water temperature in the storage tanks. The experimental results are presented and the validity of the model is examined by comparison of the theoretical results with experiments which demonstrates a good agreement. The numerical model is then used to perform theoretical study on the present ICS solar heater. The simulation results of the variation of the thermal efficiency are presented. The results of the yearly parametric study of the effect of the concentrators reflectivity, the absorber emissivity and the use of double glazing on the thermal performance of the ICS system are also presented and discussed. The developed numerical tool within this work can be considered as important for the study of double tanked ICS solar water heater regarding its transient thermal behavior.     

基于细观尺度的混凝土耐久性数值分析

为了从细观尺度对混凝土结构耐久性问题进行数值模拟研究,阐述了混凝土内物质传输机理,建立不同传输过程的数值方程。通过对数值方程的求解,得到混凝土内物质传输过程的扩散系数及有害物质浓度,与相关试验结果对比分析表明,理论求解与试验结果吻合度较高。采用Galerkin有限元法、中心显式差分格式、迎风差分格式、Crank-Nicolson格式对数值方程进行求解,得到数值解的稳定性不同。采用Pe数判别法,且保证Pe数小于1,验证数值方程解的稳定性,其中,Crank-Nicolson格式求解的稳定性最好,表现为无条件稳定,其余3种方法求解的稳定性相同。     

Performance predictions of laminar heat transfer and pressure drop in an in-line flat tube bundle using an adaptive neuro-fuzzy inference system (ANFIS) model

This paper shows how to predict the heat transfer and pressure drop for in-line flat tube configuration in a crossflow, using an adaptive neuro-fuzzy inference system (ANFIS). A numerical study of a 2D steady state and incompressible laminar flow for in-line flat tube configuration in a crossflow is also considered in this study. A finite volume technique and body-fitted coordinate system is used to solve the Navier–Stokes and energy equations. The Reynolds number varies from 10 to 320. Heat transfer and pressure drop results are presented for a tube configuration at transverse pitch and longitudinal pitch. The variation in velocity profile, isotherm contours and streamlines were compared for various configurations. The predicted results for average Nusselt number and dimensionless pressure show a good agreement with available previous work. The accuracy between numerical values and ANFIS model results were obtained with a mean relative error for average Nusselt number, pressure drop less than 1.9% and 2.97% respectively. Therefore, the ANFIS model is capable of predicting the performance of thermal systems in engineering applications, including the model of the tube bundle for heat transfer analysis and pressure drop.