Ph.D. Thesis Defense by Jae-Yeon Lee
Thursday, August 14, 2003

(Dr. Ben Zinn, advisor)

"Fast and Slow Active Control of Combustion Instabilities in Liquid-Fueled Combustors"

Abstract

This thesis describes an experimental investigation of novel active control approaches that are employed to suppress combustion instabilities in liquid-fueled combustors. The objectives of this research are to break up the feedback between the combustion process heat release oscillations and the combustor pressure oscillations that drives the instability, and to develop an understanding of the manner in which the active control damps combustion instabilities. To attain these goals, this research investigates two different active control approaches: “fast” and “slow” active control approaches.

A “fast” active controller requires continuous modulation of the fuel injection rate at the frequency of the instability with proper phase and gain. Use of developed optical tools reveals that the “fast” active control system dramatically modifies the characteristics of the combustion process to suppress the instabilities. Specifically, it changes the nearly flat distribution of the phase between pressure and heat release oscillations to obtain a gradually varying phase distribution, thus dividing the combustion zone into regions that alternately damp and drive combustor oscillations. The effects of these driving/damping regions tend to counter one another, which result in significant damping of the unstable oscillations.

In contrast to the “fast” active controllers, a “slow” active controller operates at a rate commensurate with that at which operating conditions change during combustor operation. Consequently, “slow” controllers need infrequent activation in response to changes in engine operating conditions in order to assure stable operation at all times. Using two types of fuel injectors that can produce large controllable variation of fuel spray properties, this study examines the feasibility of using such a control approach to suppress combustion instabilities. It is shown that by changing the spray characteristics it is possible to significantly damp combustion instabilities. Similar to the aforementioned result of the “fast” active control study, “slow” modification of the fuel spray properties also modifies the nearly flat phase distribution during unstable operation to a gradually varying phase distribution, resulting in combustor “stabilization”. Furthermore, deconvolutions of CH*-chemiluminescence images reveals the presence of vortex-flame interaction during unstable operations.

It is shown that the “fast” and “slow” active control approaches suppress combustion instabilities in a different manner. While the former achieves it by “out-of-face” pulsed fuel injections (i.e., the fuel injection rate is also “oscillatory”), the latter does it by controlled modification of the fuel spray properties that tends to decouple the pressure and combustion process heat release oscillations. These studies show, however, that both control approaches successfully suppress combustion instabilities by modifying the temporal and spatial behavior of the combustion process heat release that is responsible for driving the instability.