Combustion under relight conditions depends highly on the atomizer's performance, both in terms of the atomization quality, and the flow field in the primary zone. Low spray quality results in evaporation-rate limited combustion, whilst flame stability depends on the size and intensity of the recirculation zone. These two areas are relatively unexplored for an aero gas turbine under relight conditions. This study explores the atomizer flow field and spray characteristics under relight conditions to improve sub-idle combustion efficiency prediction within sub-idle whole engine performance models. The results indicate that the atomizer flow field is highly dependent on the flight Mach number during a windmilling event, with a stronger and large recirculation zone at high flight Mach numbers. Similarly the spray characteristics improve with the windmilling flight Mach number, both in terms of the droplet size and uniformity, which affect both combustion efficiency as well as the minimum ignition energy.
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