Control of combustion instabilities and emissions using an internally mixed liquid atomizer with a vaporizer

Abstract

Since coupling between the combustor oscillations and the unsteady combustion of the fuel spray drives combustion instabilities in liquid fueled combustors, "disruption" of this coupling by changing the fuel spray properties offers an approach for controlling these instabilities. This paper describes an experimental study that employed such an approach to control combustion instabilities in a liquid ftieled (n-heptane) combustor that used a novel internally mixed liquid fuel injector with a vaporizing element. This injector forms a fuel spray by mixing liquid fuel and a secondary air in a liquid/air mixing tube. The generated spray impinges upon a hot vaporizer plate, on top, as it flows towards the combustor, resulting in partial evaporation of the fuel in the spray. The resulting spray then enters the combustor through a slit between the vaporizer plate and the conical flame holder. Combustion occurs when the spray mixes with the primary air stream that enters the combustor through an annular region between the combustor wall and conical flame holder. The dependence of the characteristics of the instabilities upon the operating conditions of the injector (and, thus, spray properties) and acoustic properties of the setup were studied. The spray properties were varied by changing the flow rate of the secondary air stream into the injector's mixing tube and the combustor's acoustics were varied by changing the length of the primary air supply tube. The characteristics of the flow and combustion processes were studied from measured pressure and radicals radiation data. The results clearly showed that combustion instabilities could be controlled by changing the fuel spray properties and that the amount of attained damping depends on the frequency. In addition, emissions measurements show that the concentrations of CO, SO 2 and unburned hydrocarbons decreased as the size of the fuel droplets in the spray decreased. Interestingly, NO levels decreased as the amplitude of the instability increased.

Suppression of Combustion Instability by Controlling Spray Properties in Liquid-Fueled Combustors

“…However, developing such fuel injector actuators (FIA) is currently difficult. Therefore, there is a need for developing alternate, "slow" active control approaches [9,10] that do not employ continuous, high frequency modulation of the fuel injection rate. In contrast to the "fast" active control, the "slow" active control operates at a rate commensurate with that at which operating conditions change within the combustor.…”