The thermal behavior of vertically-operated near-adiabatic in-situ combustion tubes

Abstract

A commonly used approach for studying in-situ combustion processes has employed vertically-operated adiabatic combustion tubes. The data obtained from such apparatus are sometimes subject to interpretation problems because of convective circulations in the annulus, which are induced by the operation of the guard heaters. The main objective of this study was to systematically investigate the operational domain and impact of these convective heat transfers, and, in general, to provide a comprehensive framework for interpreting such experimental data. A microcomputer-based system was developed to automate the operation of the guard heaters and to provide a record of the power outputs of each heater with time. The automated system was used to gather data from five combustion experiments in which operating pressure, injected oxygen concentrations, water/oxygen ratio, and the type of gas in the annulus were varied. The experimental results showed that thermal energy from the heaters, as they responded to the combustion front, was transported upward by convection in the annulus. This energy elevated temperatures in the core behind the front. The tendency for annular convection to occur was found to increase with operating pressure, the on-time of the heaters, and with the Rayleigh number of the gas used in the annulus. Also, as the severity of convection increased there was (a) a reduction in the thermal efficiency of the heaters, and (b) an overall increase in heat loss from the combustion zone which required higher oxygen fluxes in order to avoid declining peak temperatures and to improve oxygen utilization. An analysis of the thermophysical properties of the annulus gases used explained these experimental observations, and demonstrated that gases of lower thermal conductivity may not necessarily reduce heat loss from the combustion tube.