Stability Limits of High Temperature Air Spray Combustion

Abstract

The objective of this study is to suggest a new method to predict the stability limit of high temperature air spray combustion. For the first stage of this study, behaviors of the high temperature air spray combustion at the stability limit are investigated experimentally. Kerosene was used as fuel and was supplied into the furnace through a fuel-air spray nozzle. Mixtures of air and nitrogen were used as oxidizer and were preheated up to 1023K. The O₂ concentration in the oxidizer was changed from 21 % to 9 %. Heat loss in the furnace was controlled by cooling tubes up to 2.3kW. The stability limit was determined on the basis of CO concentrations measured at the exit of furnace. An increase in heat loss shifts the stability limit to higher preheated temperature and O₂ concentration conditions. When an experimental condition approaches to the stability limit, the temperature of the recirculated burned gas decreases to consequently lead to the lifted flame. A further decrease in the temperature in the recirculated burned gas delays the ignition of the unburned mixture and increases the liftoff height. This fact indicates that the liftoff height is determined by the temperature and O₂ concentration in the recirculated burned gas. At conditions below the stability limit, the liftoff height becomes larger than the position of the core of recirculation vortex. The vortices recirculate both the burned gas and the unburned mixture of vapor fuel and oxidizer. The recirculation of the unburned mixture decreases the temperature in the recirculated gas, thus the ignition does not occur. It has been therefore founded that the use of the temperature and O₂ concentration in recirculated burned gas provides a generalization of the stability limit regardless of the variation of heat loss.