Control of combustion in a thermally stabilized burner

Abstract

A model has been developed with the assumption of a plug flow reactor that simulates heat transfer originating from the combustion of premixed reactants within a refractory tube. The work depicted in this article successfully implemented a transient one-dimensional coupled model of a plug flow reactor, which finds practical applications in a batch-type heat treatment furnace. All modes of heat transfer, namely conduction, convection, and radiation, were included in the model with single-step global chemistry for combustion of both methane and propane. Effects of variations of parameters such as inlet gas temperature, mass flux, and inlet fuel mass fraction on the gas and tube wall temperature profiles were studied to understand the shift in flame position along the tube length. It is established that the inlet gas temperature or the preheating level is one of the major governing parameters that determines the position of the flame in the tube. Similarly, reduction in mass flowrate shifted the flame location significantly upstream of the tube. It was also observed that the flame temperature exhibits a monotonic decay with depletion in mass flowrate. Results also show that for fixed inlet gas temperature and mass flowrate, the flame location shifts upstream for an increase in inlet fuel mass fraction. Thermally stabilized sustained combustion mode with optimum inlet gas preheating requirement was analysed as a function of mixture stoichiometry and mass flux. Open-loop and closed-loop control schemes were effectively implemented to establish the thermally stabilized scheme of heating and desired flame position within the tube. For open-loop control, with reduction in the mass flowrate (= 0.01 kg/s), an inlet gas temperature as low as 650 K can be maintained for sustaining combustion inside the tube. The joint variation of these inlet parameters has been well predicted by a correlation developed for a particular aspect ratio for high Reynolds number flows. The problem of absence of combustion and its solution towards stable flame position for tubes having smaller ratio of cross-sectional area to surface area have been addressed, which leads to the study for low Reynolds number flows. In the small Reynolds number flow cases, a thermally stabilized scheme has been successfully implemented that eliminates the necessity of preheating the gas prior to its combustion inside the tube.