Operating liquid-fuel airblast injectors in low-pressure test rigs: strategies forscaling down the flow conditions

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For reasons of economy, gas turbine atomizers are often tested at lowerpressures and temperatures than exist in the engine. The spray behaviourdiffers in the rig from that in the engine. For good measurement practiceit is necessary to find rig operating conditions at which the aspect ofthe two phase flow being measured (e.g. patternation, droplet size) issimilar in the rig to that in the engine. We describe a number of strategiesfor finding operating conditions which allow good measurement. Theseconditions reduce the air and fuel flow rates below engine values but conservecertain flow quantities. We define an error function to quantify how wellthe rig conditions match key non-dimensional groups.We consider anexample of a typical aero gas turbine airblast atomizer operated in a rig at323 K. To preserve patternation it is desirable to conserve the Reynolds(Re), Mach(Ma) andStokes (Stk)numbers. A perfect match is not possible but we find a numberof acceptable compromise strategies. The match improves asthe rig pressure increases up to ∼600 kPawhere Re andMa are matchedand Stk is correctto an order of magnitude. Further pressure increase give little improvement. It may be possibleto match Stkby using a different fuel.