Plasma-assisted sensing and control of combustion instabilities in lean high-pressure turbulent flames

Project summary:

The purpose of this project is to reliably detect and mitigate combustion instabilities using a dual-function non-thermal, low-temperature plasma-assisted sensing and control system that a) serves as an onboard diagnostic tool to detect the onset of instabilities via continuous monitoring of flame front ionization and b) dynamically actuates to suppress the combustion instabilities via plasma-enhanced combustion processes. Low-temperature plasma discharges at high-repetition rates (1-100 kHz) can control combustion instabilities via three feasible pathways: thermal, kinetic, and transport. The proposed study aims to investigate these fundamental processes behind plasma-assisted active control of combustion instabilities in an optically accessible swirl-stabilized high-pressure (1-10 bar) combustor operating near lean blowoff limit subjected to self- or external-excitation. Three promising future fuels with diverse low-temperature chemistry--a low-carbon fuel (methane) and two carbon-free fuels (hydrogen and ammonia, either pure or blended [e.g., hydrogen + ammonia])--will be studied. Three distinct low-temperature plasma systems: 1) nanosecond repetitive pulse discharge (NRPD), 2) dielectric barrier discharge (DBD), and 3) radiofrequency (RF) advanced corona system (ACS) will be judiciously employed, varying the reduced electric field strength (E/N) to emulate different flame instability response. A suite of advanced laser diagnostics tools will be utilized to study the fundamental science of plasma actuation of combustion instabilities in engine-relevant conditions.

Plasmas have been well known to promote flame stabilization via heat addition and radical production. Recently a handful of preliminary studies published in the last five years have demonstrated that pressure oscillations due to combustion instabilities can be substantially reduced via strategically discharging plasma in the combustor. However, the underlying mechanisms through which plasma processes interact with the unstable flame dynamics, instability growth/decay modes, and combustor acoustics at lean conditions remain an open scientific question. This project will conduct fundamental research to elucidate the underlying plasma actuation processes to mitigate combustion instabilities at the lean limit. In this study, the onset of combustion instabilities will be detected at the plasma electrode tip by measuring either combustion-generated electrons and ions that induce ionic currents or through a change in tip capacitance caused by the presence of the flame. If combustion instabilities are detected in the form of fluctuating ionic current or capacitance, a feedback control system will immediately actuate plasma discharges to mitigate/suppress instabilities. The mechanisms through which basic plasma processes interact with highly transient instability phenomena in its early phase (e.g., growth period) will be investigated in depth.

A fundamental understanding of the plasma interaction with combustion instabilities is essential to successfully deploy this novel and transformative concept in commercial gas turbine engines. Low operating energy and cost, along with easy implementation and minimal maintenance, make the low-temperature plasma system an ideal candidate for infrequently maintained gas turbine engines operating in ultra-lean conditions.