EFFECT OF SECONDARY INJECTION ON NON–CIRCULAR JETS

Studies on non-circular jets have become a captivating field of interest, since the technique exploits the advantage of the nozzle exit geometry, in terms of mixing characteristics. The supersonic jet mixing can be effectively employed in the injector orifice in a Supersonic Combustion Ramjet (SCRAMJET) engine, looking for shorter combustor lengths that accounts to weight reduction of the engine. This paper puts forward the experimental evaluation of mixing characteristics of supersonic flow through a conical converging-diverging nozzle designed for Mach number 1.8. Circular, blunt square, blunt hexagon and blunt octagon shape exit geometries is reckoned with circular inlet and throat geometries. This is accompanied with an active flow control method (i.e.) secondary injection of air located at 20.42% of the total length of the nozzle from the exit of the nozzle. The impact of the exit geometries and the secondary injection on the potential core is scrutinized by intently studying the pitot pressure decay at the downstream of the nozzle. The experimental study involves the flow of non-reacting fluid (air) as the working fluid and secondary fluid under cold flow conditions. The jet entrainment always occurs at the exit effectively, and mixes with the ambient at a faster rate. The dimensionless pitot pressure is plotted against dimensionless centerline distance under design and off-design conditions. The amplitude of the pressure fluctuations was found to increase at all operating conditions with increasing number of sides in the exit geometry. The shock structures were captured using shadowgraph technique for the above conditions. It is observed that the hexagonal jet with secondary injection at an angle of 90◦ shows 16.79% decrease in supersonic core length among the four exit geometries. The shock diamonds are found to be diminishing at a faster rate in hexagonal jet with secondary injection.