Combustion-Driven Instabilities in Liquid-Fired Swirl Combustors

摘要

Water injection into the recirculation zone of turbulent diffusion flames is an often applied technology for the reduction of flame temperatures and hence, of thermal NO_x emissions. On the other hand, water injection reduced ignition stability and may also cause stability problems due to the appearance of combustion-driven oscillations at a critical water-to-oil mass flow ratio. Combustion-driven instabilities are characterized by the formation of periodic oscillations of the global heat release rate of the flame, of the flue gas temperature and of the static pressure in the combustion chamber. These periodically unsteady operation conditions lead to a different pollutant emission pattern in comparison with steady-state operation and may also cause damage to the combustion chamber, so that the operation range of the combustion process has to be restricted in order to avoid unstable modes. The knowledge of the complex interactions between the turbulent flow field of the fuel/air/water-vapour mixture and the chemical reaction rate of the unsteady flame is an essential requirement for the understanding of the physical mechanism of combustion instability, In this work, the influence of various operation conditions (thermal load, air equivalence ratio, air preheat temperature, mass flow ratio and jet angle of the oil nozzle) on the onset of periodic flame oscillations is investigated and possible feedback mechanisms are discussed. At a constant thermal load, air equivalence ratio, air preheat temperature and a critical water to oil mass flow ratio the instantaneous change from the steady-state (oscillation-free) mode to the periodic unsteady mode of operation took place and the amplitude of the pressure oscillations showed a maximum. Furthermore, the change from steady to unsteady operation mode can also be observed by variation only of the air preheat temperature (without water injection) at a constant thermal load and air equivalence ratio. Therefore, the processes for the formation of self-sustained pressure oscillations under variation of air preheat temperature and water mass flow rate seem to be identical and to depend mainly on fluid-dynamic conditions in the stabilization zone of the swirl flame.