Spray Flame Characterization of a Dual Injector for Compact Combustion Systems

Abstract

ABSTRACT The scalability of liquid fuel operated jet stabilized combustion system toward scales and mixing timescales relevant for corresponding micro gas turbine systems is impeded by the lack of suitable injection concepts. The high momentum of the combustion air jet can deteriorate the spray quality of conventional injection systems. In combustors with reduced mixing timescales (e.g. MGT and compact aero-engines) this results in elevated emissions and a prompt primary atomization is essential. For this purpose, a canonical confined jet spray burner is developed and equipped with a novel in-house dual-pressure swirl/airblast injection concept. An extensive parameter variation on the jet bulk velocity (80 , equivalence ratio (0.6 , combustion air preheat temperature (500 (K) 800) and premixing length (0.0 (mm) ≤ 48) is conducted to delineate the operational range. The spray process originating a partially premixed and a direct injection case is characterized by means of phase Doppler interferometry and shadowgraphy. The combustion process is described via OH*-chemiluminescence, flame photographs and exhaust gas measurements. The results show that the developed injection system yields Sauter mean diameters predominantly below 25 μm over the entire parameter range due to robust and prompt primary atomization. Moreover, the primary atomization process of the dual injector is sufficient under all conditions to confine secondary atomization close-to the combustor nozzle exit. The flame stabilization and reaction progress are more sensitive to the air preheat temperature than the bulk jet velocity variation as the mixture homogenization is limited by the vaporization process. Moreover, with decreasing reactivity (i.e. equivalence ratio and preheat temperature) the flame stabilization is increasingly dominated by the recirculated hot exhaust gas, indicating a transition from conventional turbulent flame propagation to auto-ignition influenced reaction progress. The resulting NOx and CO emissions approximate the values measured previously in related methane powered combustion systems. Consequently, the developed injection system is promising to expand the flexibility of compact gas turbines to liquid fuels while maintaining low emissions.