|M.Sc Student||Roizman Alexander|
|Subject||Suppression of Combustion Instability in Gas Turbine|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Yeshayahou Levy|
|Full Thesis text - in Hebrew|
Due to the negative impact on the health of human beings and the environment caused by the pollutions from the gas turbines, the amounts of pollutions from these engines were restricted. Special attention is devoted to the emissions of NOx that might lead to serious health implications. Chemical reaction between oxygen and nitrogen that is responsible for the formation of NOx molecules requires relatively high activation energy. Hence, the reaction starts under relatively high temperatures. Therefore, gas turbine manufacturers are constantly looking for ways to reduce the combustion temperature. One of the established methods is Lean Premixed Combustion. In this method, fuel is supplied in gaseous phase and is mixed with clean air upstream of the combustion chamber, creating a well-mixed flammable mixture. In this case, the combustion temperature is a strong function of equivalence ratio. Combustion temperature drops with equivalence ratio for lean mixtures, and thus the premixed mixture is supplied with fuel/air ratio that is close to the low flammability limit. Gas turbine engines that have this method of NOx reduction, suffer from combustion instabilities that develop in combustion chamber and cause undesired pressure oscillations. These oscillations cause engine control system disruptions, enhanced heat transfer on the combustor liner walls, higher rate of mechanical wear and even engine failure.
Combustion instability in gas turbine combustion chamber causes spontaneous pressure oscillations due to oscillations in local heat fluxes. Heat flux oscillations are caused by the oscillations in equivalence ratio of the mixture, due to the changes in the flow rates. However, the changes in the flow rates themselves happen due to the pressure oscillations, creating a closed loop between the thermal oscillations of combustion process and mechanical pressure oscillations. Pressure oscillations inside the combustion chamber develop when the energy required for their formation overcomes the damping factors.
This research suggests a solution for suppression of combustion instabilities in the combustion chamber with lean premixed mixture supply. The method includes installation of additional parallel combustor (pilot) operated under fuel rich conditions. Its exhaust gases are injected into the main combustion chamber primary zone and as a result of the fuel rich conditions (and uncomplete combustion process), pilot exhaust gases contain unstable and very reactive species - free radicals. Due to their high reactivity, injection of free radicals into the combustion primary zone supports the stability of combustion process. By design, the pilot combustor uses part of the fuel that is otherwise supplied to the main combustion chamber, thus total fuel rate remains constant.
Performed experiments show that injection of free radicals into the main combustor primary zone significantly affects the combustion. Operation of the pilot combustor allows reduction of the low flammability limit. During the experiments, main combustor chamber was forced to operate under unstable conditions by mounting an orifice at its exhaust. Two types of combustion instabilities were received: a) Low Frequency Instability. B) Mid Frequency Instability. Experiments show that operation of the pilot combustor under fuel rich conditions significantly suppresses pressure oscillations caused by both types of combustion instabilities.