Spray flame characterisation under lean blow-out conditions

The scalability of liquid fuel operated jet stabilised combustion systems towards dimensions and mixing timescales relevant for compact designs (e.g. MGT, compact aero-engines) 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 leading to elevated emissions rendering prompt primary atomization essential. For this purpose, a canonical confined jet spray burner was recently developed and equipped with a novel in-house dual pressure swirl/airblast injection concept. In the current study, the lean blow-out (LBO) sensitivity, a critical design parameter of aero-engines, of the spray burner is delineated across an extensive parameter variation on the jet bulk velocity (80≤uj(m/s)≤160), combustion air preheat temperature (500≤Tj(K)≤800), premixing length (0.0≤lm(mm)≤48) and fuel type (Jet-A1, extra light heating oil and n-dodecane) with distinctly different distillation curves. The spray and combustion process of a partially premixed and direct injection hardware configurations are characterised by means of phase Doppler interferometry (PDI) and OH∗-chemiluminescence imaging during the LBO approach, i.e. equivalence ratio (Φ) reduction. N-dodecane exhibits highest resilience against LBO, followed by Jet-A1 and finally heating oil. The PDI results show that atomization and physical fuel properties, i.e. viscosity, are not the driving cause for the measured LBO sensitivity. To delineate the observed LBO fuel sensitivity, the data analysis is further supported by a leading order Damköhler number scaling to determine progress limiting processes, e.g. vaporisation, mixture formation and auto-ignition based reaction onset. The enhanced auto-ignition propensity of n-dodecane in comparison to the complex fuels leads to differences in the measured flame lift-off height as well as the spatial heat release distribution and fluctuation during the LBO approach. However, the chemical Damköhler numbers (flame and auto-ignition) do not support the measured difference in LBO sensitivity between the complex fuels Jet-A1 and Oil. By contrast, the determined vaporisation timescales align with the measured LBO fuel ranking, suggesting that mixture formation is strongly influenced by the vaporisation process and resulting reactivity stratification assumes a dominant role in LBO sensitivity. Novelty and significance The current work delineates the lean blow-out (LBO) sensitivity in compact liquid fuel operated high momentum jet stabilised combustion systems over a wide operation condition and physical fuel properties. The present study extends the combustor design critical parameter of LBO to such burner concepts with a targeted application in future lean premixed hybrid aero-engines. For this purpose, a canonical single nozzle confined jet spray burner with an in-house dual pressure swirl/airblast injector was recently develop. The excellent atomization performance of the utilised injection system allows an investigation of the LBO sensitivity independent from the atomizer performance and thereby separating physical (namely viscosity and atomization) from fuel chemistry effects. The consistent mixture quality offers further novel insights on the thermochemical conversion process during the LBO approach highlighting the significance of vaporisation in the LBO process itself and of auto-ignition in the combustion process near LBO.