| OCR Text |
Show - 3- applications; steamers for enhanced oil recovery; and steam superheaters for waste-~ energy systems. In the residential and commercial areas the surface combustor-heater concept can provide the basis for development of highly compact, efficient ultra-low pollutant emissions water heaters, air heaters, thermal fluid heaters, and space heating systems. The purpose of this paper is to describe a mathematical model to simulate heat transfer in a porous bed and to predict conduction-convection-radiation heat transfer in a model surface combustor-heater. There are a very large number of parameters governing the system such as the tube diameter and spacing, number of tube rows, material, porosity and particle diameter of the bed, thermophysical and radiative properties of the bed and the tubes, operating conditions, etc., hence it is not possible to be comprehensive. Some typical results are presented in the paper and the salient features of the system are discussed. ANALYSIS Physical Model and Assumptions A schematic diagram of the physical system considered is shown schematically in Fig.I. A mixture of natural gas and air is introduced into the combustor-heater through a cooled distributor plate to prevent back-firing, and the mixture is burned inside the porous matrix. The chemical energy generated during the process of combustion is released in the gas. The combustion products heat the bed which is capable of emitting, absorbing and scattering thermal radiation. Heat transfer from the bed to the tubes is by conduction and radiation, and heat transfer from the combustion products to the tubes is only by convection as the opacity of the products of combustion (primarily CO2 and H20) is considered to be negligible as a result of the very small interstitial distances in the bed. The purpose of the analysis is to predict the thermal performance of the surface combustor-heater by determining the fraction of the heat released by the combustion which is transferred to the tube surfaces. This objective will be achieved by predicting the temperature distribution of the solid particles and the gases in the bed. The process of combustion is not modeled, and the heat of combustion is assumed to be released in the gas and is replaced by a volumetric heat source. In possible heater designs there could be several rows of tubes in the heater and a large number of tubes in a row through which a working fluid is circulated (Jasionowski et aI., IDS7), mathematical modeling of the entire heater does not appear to be practical. Therefore, the heater will be assumed to consist of numerous but identical vertical sections (modules), and only one section is modeled. The vertical boundaries of the module are located on the planes passing midway between two adjacent rows of tubes which are arranged horizontally (Fig.I). In the figure only a single, square tube is indicated, but in possible heater designs there may be a large number of tubes in a row and a number of rows. In the analysis a square instead of a circular duct is considered, because it is computationally simpler. The transport of mass, momentum and energy is considered to be tw~dimensional. As a first approximation, flow through the porous medium is assumed to be tw~ dimensional and governed by the Forchheimer and Brinkman modified Darcy equations |
