STUDY OF MIXED-CONVECTION HEAT TRANSFER FROM AN IMPINGING JET TO A SOLID WALL USING A FINITE-ELEMENT METHOD—APPLICATION TO COOKTOP MODELING

The mixed-convection flow from a hot vertical impinging jet on a colder horizontal disc has been studied. The geometry is analogous to a conventional burning gas cooktop. A numerical simulation of the system has been carried out using the finite-element method to study the dependence of fluid flow and heat transfer on the geometric, thermal, and fluid flow parameters. Results show that heat transfer efficiency versus several parameters such as inlet velocity magnitude and flue gas temperature has an optimum value, in which heat transfer efficiency is maximum. With thermal conductivity of the solid wall, velocity angle, and solid wall diameter heat transfer efficiency has increasing behavior. Finally, with solid wall height and solid wall thickness, heat transfer efficiency has diminishing behavior.     

Impedance measurements on oxide films on aluminium obtained by pulsed tensions

We have performed this study on oxide films sealed or not in boiling water. The films are first obtained on type 1050 A aluminium substrat by pulsed tensions anodizing technique, in a sulfuric acid solution. Afterwards the, Electrochemical Impedance Spectroscopy (EIS) is employed to appreciate the films behaviour in a neutral solution of 3.5% K₂SO₄, in which the interface processes interest only the ageing phenomenon of the oxide films and not their corrosion. We have also attempted a correlation between pulse parameters of anodization and the electrical parameters characterizing these films. The sealing influence on ageing has been studied as well. For all films, ageing is appreciated using impedance diagrams evolution versus time. The results show: – the existence of two capacitive loops confirming the presence of two oxide layers characteristic of oxide films obtained in a sulfuric acid medium. The first loop, at high frequencies, is related to the external porous layer and the second one, at lower freqencies, is related to the internal barrier layer. – the thickness of the barrier layer varies between 25 and 40 nm in relation with the electrical pulse parameters. – the sealing acts favorably against anodic oxide films ageing.     

Implementation of Geometrical Domain Decomposition Method for Solution of Axisymmetric Transient Inverse Heat Conduction Problems

The objective of this article is to study the performance of iterative parameter and function estimation techniques to solve simultaneously two unknown functions (quadratic in time, and linear in time and space) using transient inverse heat conduction method in conjunction with a geometrical domain decomposition approach, in cylindrical coordinates. For geometrical decomposition of physical domain, a multi-block method has been used. The numerical scheme for the solution of the governing partial differential equations is the finite element method. The results of the present study for a configuration composed of two joined disks with different heights are compared to those of exact heat source and temperature boundary condition using inverse analysis. Good agreement between the estimated results and exact functions has been observed for parameter estimation techniques in contrast to those of function estimation approach. In summary, the results show that the function estimation technique is sensitive to the location of measurement points, but is useful to estimate unknown functions without a priori knowledge of the functions' spatial and/or temporal distributions. However, the function estimation technique suffers from a drawback: its implementation and data extraction are less straightforward than parameter estimation method. Finally, it is shown that the use of geometrical domain decomposition offers the possibility of developing a robust inverse analysis code for general purpose heat conduction problems.     

Numerical simulation and performance optimization of a high capacity pulse tube cryocooler

Recent developments of superconductive industries require cryocoolers with cooling power higher than 1 W in the 70–80 K temperature range. High capacity pulse tube cryocoolers assure the cooling power required for operation of superconducting devices. This paper presents numerical simulation of a high capacity pulse tube cryocooler, intended to provide more than 200 W cooling power at 80 K. In this respect the behavior of the cryocooler is explained by applying the mass and energy balance equations to different components of the cryocooler. Nodal analysis technique is employed to simulate the tube section behavior numerically. To perform the system optimization the influence of key operating parameters on cryocooler cooling capacity and coefficient of performance is studied. The proposed model reports the optimum cooling capacity of 244 W at 80 K cold end temperature at frequency of 50 Hz with 3.5 kW net power delivered to the gas.     

Improvement and experimental validation of a multi-zone model for combustion and NO emissions in CNG fueled spark ignition engine

This article reports the experimental and theoretical results for a spark ignition engine working with compressed natural gas as a fuel. The theoretical part of this work uses a zero-dimensional, multi-zone combustion model in order to predict nitric oxide (NO) emission in a spark ignition (SI) engine. The basic concept of the model is the division of the burned gas into several distinct zones for taking into account the temperature stratification of the burned mixture during combustion. This is especially important for accurate NO emissions predictions, since NO formation is strongly temperature dependent. During combustion, 12 products are obtained by chemical equilibrium via Gibbs energy minimization method and nitric oxide formation is calculated from chemical kinetic by the extended Zeldovich mechanism. The burning rate required as input to the model is expressed as a Wiebe function, fitted to experimentally derived burn rates. The model is validated against experimental data from a four-cylinder, four-stroke, SI gas engine (EF7) running with CNG fuel. The calculated values for pressure and nitric oxide emissions show good agreement with the experimental data. The superiority of the multizone model over its two-zone counterpart is demonstrated in view of its more realistic in-cylinder NO emissions predictions when compared to the available experimental data.     

Structured Multiblock Grid Solution of Two-Dimensional Transient Inverse Heat Conduction Problems in Cartesian Coordinate System

ABSTRACT

In this study a structured multiblock grid is used to solve two-dimensional transient inverse heat conduction problems. The multiblock method is implemented for geometric decomposition of the physical domain into regions with blocked interfaces. The finite-element method is employed for direct solution of the transient heat conduction equation in a Cartesian coordinate system. Inverse algorithms used in this research are iterative Levenberg-Marquardt and adjoint conjugate gradient techniques for parameter and function estimations. The measured transient temperature data needed in the inverse solution are given by exact or noisy data. Simultaneous estimation of unknown linear/nonlinear time-varying strengths of two heat sources in two joined surfaces with equal and different heights is obtained for the solution of the inverse problems, and the results of the present study for unknown heat source functions are compared to those of exact functions. This study is an attempt to challenge the goal of combining a multiblock technique with inverse analysis methods. In fact, the structured multiblock grid is capable of providing accurate solutions of inverse heat conduction problems (IHCPs) in industrial configurations, including composite structures. In addition, the multiblock IHCP solver is suitable to estimate unknown parameters and functions in these structures.     

Second law based modeling to optimum design of high capacity pulse tube refrigerators

The optimum design of a high capacity double inlet pulse tube refrigerator based on second law of thermodynamics has been presented in this paper. Second law is applied to calculate the work loss in the regenerator and to optimize the cryocooler performance. To investigate the behavior of the pulse tube refrigerator, mass and energy balance equations are applied to several control volumes of the cryocooler cycle. A complete system of conservation equations is employed to solve the regenerator analytically. The proposed model reports the cooling capacity of 110 W at 80 K cold end temperature at frequency of 50 Hz, orifice conductance of 0.4 and double inlet coefficient of 0.6, with 2.4 kW net power delivered to the gas. In this case, the entropy generation in the gas phase is dominant which is contributing more than 85% of the total lost work in the regenerator. The optimum thermal efficiency of 99.1% was achieved at a proper mesh number. However, the second law efficiency is reported to have an inverse behavior at this mesh number.     

Transient natural convection from a horizontal cylinder confined between vertical walls—a finite element solution

A general finite element computer code is developed for transient two dimensional natural convection heat transfer in a laminar regime. This code is used to study heat transfer from a horizontal isothermal cylinder confined between two vertical adiabatic walls. Results are presented for Ra = 1000, Pr = 0.7, and for different wall spacing to cylinder diameter ratios. The time variations of the local and average Nusselt numbers along with some transient and steady state velocity and temperature fields are demonstrated. The steady state results predict the existence of an optimum wall distance for the maximum average Nusselt number. The steady state and the transient Nusselt numbers are compared with the available experimental or theoretical results for a few cases. Their agreement is very good.