Quantitative Infrared Absorption Measurements of Vapor-Phase Fuel in a Supersonic Flow

This work presents progress in fuel distribution measurements within a supersonic crossflow using filtered infrared imaging. It outlines an approach to overcome the unique challenges associated with infrared absorption-based measurements of liquid fuels in supersonic flows typical of scramjet engines. Heavy hydrocarbon injected into flows at flight enthalpy are subject to both light scattering in the liquid phase as well as significant emission in the vapor phase, complicating absorption-based measurements. Therefore, smaller chain hydrocarbons, with higher volatility, are proposed as fuel surrogates to emulate the mixing and vaporization distance of heavier fuels in a reduced flow temperature. Two potential surrogates are evaluated analytically using a single-droplet model and experimentally through shadowgraph imaging. Subsequently, n-heptane was selected to study the mixing characteristics of two injection schemes, namely injection from integrated orifices on both sides of a strut mounted on the combustor wall (integrated-injection) and flush-wall injection upstream of the strut (guided-injection). Both schemes yielded a concentrated, but gradually spreading plume, lifted from the lower wall, and with local path averaged equivalence ratios as high as 0.8. Enlarging the guided orifice diameter from 0.60 to 0.77 mm altered this distribution to a bimodal shape with high near-wall distribution. These insights can aid the selection of suitable injection strategies for liquid fueled scramjet combustors.