scholarly journals A theoretical analysis of local thermal equilibrium in fibrous materials

2015 ◽  
Vol 19 (1) ◽  
pp. 69-82
Author(s):  
Mingwei Tian ◽  
Ning Pan ◽  
Lijun Qu ◽  
Xiaoqing Guo ◽  
Guangting Han
Keyword(s):  
Thermal Equilibrium ◽  
Internal Heat ◽  
Constant Heat Flux ◽  
Local Thermal Equilibrium ◽  
Transfer Model ◽  
Fibrous Materials ◽  
Two Phase ◽  
Convective Boundary ◽  
Boundary Problems ◽  
Criterion System

The internal heat exchange between each phase and the Local Thermal Equilibrium (LTE) scenarios in multi-phase fibrous materials are considered in this paper. Based on the two-phase heat transfer model, a criterion is proposed to evaluate the LTE condition, using derived characteristic parameters. Furthermore, the LTE situations in isothermal/adiabatic boundary cases with two different heat sources (constant heat flux and constant temperature) are assessed as special transient cases to test the proposed criterion system, and the influence of such different cases on their LTE status are elucidated. In addition, it is demonstrated that even the convective boundary problems can be generally estimated using this approach. Finally, effects on LTE of the material properties (thermal conductivity, volumetric heat capacity of each phase, sample porosity and pore hydraulic radius) are investigated, illustrated and discussed in our study.

Analysis of Constant-Heat-Flux Heat Transfer in Metal Foam

2007 ◽  
Author(s):  
Nihad Dukhan ◽  
Kuan-Chin Chen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Temperature Profile ◽  
Thermal Equilibrium ◽  
Metal Foam ◽  
Constant Heat Flux ◽  
Local Thermal Equilibrium ◽  
Constant Heat ◽  
Open Cell ◽  
Simplifying Assumptions

The use of open-cell metal foam in contemporary heat exchange technologies is increasing rapidly. The high surface area density of metal foam places them among the best options for heat exchanger core materials. Certain simplifying assumptions for the combined conduction/convection heat transfer analysis in metal foam have not been exploited. Solving the complete, and coupled, fluid flow and heat transfer governing equations numerically is time consuming. A simplified two-dimensional analytical model for the heat transfer in open-cell metal foam block subjected to constant heat flux, and cooled by a low-conductivity fluid, is presented. The model assumes local thermal equilibrium between the solid and fluid phases in the foam, and neglects the conduction in the fluid. The local thermal equilibrium assumption is supported by previous studies performed by other workers. The velocity profile in the foam is taken as non-Darcean slug flow. An approximate solution for the temperature profile in the foam is obtained analytically. The temperature profile decays in what looks like an exponential fashion as the distance from the heat base increases, and increases in the flow direction. The model along with the simplifying assumptions were verified by direct experiment using air and an aluminum foam block heated from above by constant heat flux, for a range of Reynolds numbers. Very good agreement was found between the analytical and the experimental results.

Theoretical Post-Dryout Heat Transfer Model

2018 ◽  
Vol 1 (1) ◽  
pp. 142-150
Author(s):  
Murat Tunc ◽  
Ayse Nur Esen ◽  
Doruk Sen ◽  
Ahmet Karakas
Keyword(s):  
Heat Transfer ◽  
Droplet Diameter ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Operating Pressure ◽  
Vertical Pipe ◽  
Two Phase ◽  
Experimental Values ◽  
Reactor Operating ◽  
Coolant System

A theoretical post-dryout heat transfer model is developed for two-phase dispersed flow, one-dimensional vertical pipe in a post-CHF regime. Because of the presence of average droplet diameter lower bound in a two-phase sparse flow. Droplet diameter is also calculated. Obtained results are compared with experimental values. Experimental data is used two-phase flow steam-water in VVER-1200, reactor coolant system, reactor operating pressure is 16.2 MPa. On heater rod surface, dryout was detected as a result of jumping increase of the heater rod surface temperature. Results obtained display lower droplet dimensions than the experimentally obtained values.

scholarly journals On the Two-phase Fractional Stefan Problem

2020 ◽  
Vol 20 (2) ◽  
pp. 437-458 ◽  
Author(s):  
Félix del Teso ◽  
Jørgen Endal ◽  
Juan Luis Vázquez
Keyword(s):  
Free Boundary ◽  
Stefan Problem ◽  
Free Boundary Problems ◽  
Classical Problem ◽  
Nonlocal Diffusion ◽  
Two Phase ◽  
Boundary Problems ◽  
Numerical Studies ◽  
The One ◽  
Evolution Type

AbstractThe classical Stefan problem is one of the most studied free boundary problems of evolution type. Recently, there has been interest in treating the corresponding free boundary problem with nonlocal diffusion. We start the paper by reviewing the main properties of the classical problem that are of interest to us. Then we introduce the fractional Stefan problem and develop the basic theory. After that we center our attention on selfsimilar solutions, their properties and consequences. We first discuss the results of the one-phase fractional Stefan problem, which have recently been studied by the authors. Finally, we address the theory of the two-phase fractional Stefan problem, which contains the main original contributions of this paper. Rigorous numerical studies support our results and claims.

scholarly journals Tank Design for On-Board Hydrogen Storage in Metal Hydrides

2008 ◽  
Author(s):  
Karelle Couturier ◽  
Farida Joppich ◽  
Antje Wo¨rner ◽  
Rainer Tamme
Keyword(s):  
Heat Transfer ◽  
Metal Hydride ◽  
Storage Tank ◽  
Internal Heat ◽  
Transfer Model ◽  
Cooling Fluid ◽  
Internal Heat Exchanger ◽  
Tank Design ◽  
Technical Solutions ◽  
Charging Dynamics

The aim of this work is to reduce the refueling time of a metal hydride storage tank by improving its design, taking in account the total volumetric and mass capacity of the tank. A heat and mass transfer model is proposed and solved to obtain the charging curve for 1 kg hydrogen in a LaNi5 reference storage tank. Compared to gas transport and reaction kinetics, heat transfer is found to limit the hydrogen charging dynamics of the storage tank. To improve the refueling time, it is found to be necessary to increase first of all the heat transfer inside the metal hydride bed, and subsequently the heat transfer from the metal hydride bed to the cooling fluid. Technical solutions such as the implementation of aluminum foam and/or internal heat exchanger tubes are investigated. By combining both solutions, the refueling time can be reduced from 400 minutes (reference tank) to 15 minutes. The tank volume still meets the DOE targets, but its mass remains a problem. Therefore, new materials with improved gravimetric capacity have to be developed. With this work it is now possible to improve the tank design for newly developed storage materials and to evaluate their potential for technical applications.

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