INTERACTION BETWEEN SPRAYS FROM MULTIPLE COAXIAL AIRBLAST ATOMIZERS

Phase Doppler measurements of size, velocity, liquid flux, and average mass fractions were obtained in sprays produced by three identical co axial airblast atomizers, with their axes placed in a triangular arran gement at distances of two air jet diameters from each other; the arra ngement simulates the spray interaction in the preburner of the space shuttle main engine with water and air respectively replacing the liqu id oxygen and hydrogen of the preburner sprays. Each nozzle comprised a liquid jet with exit diameter of 2.3 mm centred in a 8.95 mm diamete r air stream. Two liquid flowrates were examined, while the air flowra te was kept constant, resulting in Weber number at the exit of the noz zle around 1100, air-to-liquid momentum ratio 8.6 and 38, velocity rat io 24 and 51, mass flowrate ratio 0.35 and 0.75, liquid jet Reynolds n umber 10,000 and 21,000 and air jet Reynolds number around 108,000. Th e air flow characteristics were compared to the flow without liquid in jection. Up to 10 air jet diameters from the nozzle exit, individual s pray characteristics dominated and maximum Sauter mean diameters, typi cally around 150 mu m, and liquid flux were observed on the geometrica l axes of the nozzles. Spray merging was strong in the region between the nozzle axes, where the Sauter mean diameter reduced and the liquid flux and the mean and rms of the fluctuations of the axial velocity o f the droplets and the airflow increased relative to the single spray. Downstream of 25 air jet diameters from the nozzle exit, the multiple sprays merged to a single spray-like flow produced by a nozzle locate d at the centre of the triangular region between the nozzle axes. Redu ction of the liquid flowrate by 50 percent, improved atomization by 25 percent, shortened the axial distance from the nozzles where the indi vidual spray characteristics disappeared by 30 percent and increased t he airflow turbulence by 20 percent. Droplet coalescence was negligibl e for high liquid flowrates, but for reduced liquid flowrates coalesce nce became important and the Sauter mean diameter increased with the a xial distance from the exit by around 15 percent. Spray merging increa sed the airflow turbulence and the local mass fraction distribution of the air in the region between the nozzle axes by around 50 and 40 per cent respectively relative to the single sprays, resulting in a fuel r ich region with increased gas flow turbulence which may influence the ignition process in the preburner of the space shuttle main engine.