Experimental Study On Combustion Instability Of RP-3 Jet Fuel Components In An LPP Model Combustor

With the development of aero engine technology and increasing concern on the environmental issues,lean premixed pre-vaporized(LPP)combustion technology is widely used in aero engine industry to achieve low pollutant emissions while maintaining high fuel efficiency.However,under this operating mode,the combustion chamber is prone to the onset of combustion instability,which can dramatically degrade the reliability of the engine.It has been shown that the physico-chemical properties and composition of the fuel can have significant impacts on combustion instability characteristics of the combustor,however,gaseous fuels were commonly employed in most of the previous studies,which gave rise to the question that whether the proposed mechanisms were applicable to aero engines operated with liquid fuels.Therefore,the present study aims to study the effect of liquid fuel component variations on the combustion instability characteristics and flame-flow field interaction mechanisms during stable-unstable transition in an LPP gas turbine model combustor.Test fuels involved liquid RP-3 jet fuel,iso-octane,methylcyclohexane(MCH)and n-dodecane.In the first part of this paper,the stability maps of these four fuels were experimentally determined under a wide range of equivalence ratios and inlet air temperatures.Results suggested that the combustor shifted from unstable to stable combustion with the equivalence ratio decreasing from 1 to lean blow out(LBO)limit,and this trend was similar for all test fuels.However,the LBO limit and the boundaries between stable and unstable transition were distinct for different fuels.The instability characteristics of different fuels were then examined under the same operating conditions.It was suggested that the chemical properties of the fuels showed notable influences on the dominant frequencies and amplitudes of the pressure oscillations,and heat release rate fluctuations.Specifically,the dominant instability frequency increased with the increasing of flame adiabatic temperature,and the ignition delay time of the fuels determined the phase lag between incoming pressure oscillations and heat release rate fluctuation under comparable flow conditions.To gain insights into the mechanisms associated with the instability differences induced by fuel variations,high-speed OH*chemiluminescence imaging and planar Particle Image Velocimetry(PIV)measurements were conducted with iso-octane and MCH flames,aiming to obtain a detailed characterization of the flame and flow field dynamics.For these two cases,it was indicated that the dominant instability modes of both flame and flow field during unstable combustion were characterized by longitudinal oscillations.On the other hand,the inner recirculation zone(IRZ)of MCH flame was narrower than that of MCH flame,and it was located further to the wall but closer to the dump plane of the combustor.Such differences in the flow field further led to the fact that MCH flame was stabilized more closely to the dump plane and featured more intense reaction intensity.Further Proper Orthogonal Decomposition(POD)analysis revealed that the 1st mode of POD contributed mainly to the differences existed in the flame responses subject to similar acoustic oscillations.Finally,to explore the underlying mechanisms responsible for the excitation of limit-cycle stabilities from stable combustion,the flame and flow dynamics during spontaneously intermittent transition between stable and thermo-acoustically unstable were addressed.OH*chemiluminescence imaging and PIV measurements revealed that significant differences existed during the transition,which were associated with the formation of processing vortex core(PVC)and flame element in the outer recirculation zone(ORZ)of the combustor.