The Development of a Methodology to scale Between Cold-Flow and Hot-Fire Evaluations of Gas-Centered Swirl Coaxial Injectors

Abstract : Uni-element cold flow and hot fire evaluations were performed on a variety of gas-centered swirl coaxial injectors. Gaseous oxygen and various liquid hydrocarbons were used in the combustion evaluations, while water and gaseous nitrogen were the simulants for the cold flow experiments. The connections between the two sets of data were examined. The cold flow experiments demonstrated that the mixing efficiency of the various injector designs was highly sensitive to the internal geometry of the injector as well as the scaling methodology used to simulate the hot-fire conditions. When the proper scaling methodology was employed, a correlation which captures the general trend of injector geometry and c* performance between the measured cold-flow mixing efficiency and hot-fire c* performance was observed. This semi-empirical correlation was developed based on a film stripping mechanism that relates the measured c* efficiency of these injectors to the injector geometry and fuel properties. The effects of injector geometry on the injector internal flowfield were ascertained with a combination of cold-flow CFD simulations and experimental measurements. The correlation also implies that fuel properties are secondary to injector geometry effects in determining the performance of various injector configurations. Hot-fire testing of several common hydrocarbon fuels including RP-1, Butane, JP-10, JP-7 and JP-8 confirmed that injector geometric effects dominated performance and demonstrated that c* efficiency in excess of 95% is achievable with all of these fuels. However, the effect of fuel properties does appear to be within the measurement limits of the experiments and a correlating parameter which captures these effects was found.