Effect of Fin-Guided Fuel Injection on Supersonic Mixing and Combustion

Abstract

Rapid mixing and combustion is a key challenge in supersonic combustors due to the extremely short flow residence time and the effect of compressibility. Mixing enhancement is therefore desirable to ensure timely mixing, reaction, and heat release. Fin-guided fuel injection is one approach that can be optimized for propulsion performance consideration. The present investigation examined the mixing and combustion characteristics of using this alternative fuel injection method to evaluate its performance in comparison to conventional transverse wall injection.

This study was conducted in two parts: (1) fuel-air mixing experiments in a non-reacting Mach 2.2 flow with a test section Reynolds number of 1.15&times10<super>6</super>, and (2) combustion experiments using a high-enthalpy, vitiated air flow with a Mach 2.0 condition at the isolator inlet and Reynolds number of 1.14&times 10<super>5</super>. The non-reacting mixing study used either helium or ethanol, while the combustion study used either hydrogen or ethylene as fuel for each experiment. The mixing behavior of the gaseous and liquid jets was studied using schlieren and a laser sheet technique while quantitative assessments were made from pressure measurements. Similarly, the physical mechanisms in the reacting flow experiments were analyzed using schlieren visualizations while pressure measurements and chemiluminescence emission data were used for performance evaluation.

The fuel-air mixing study highlighted possible tradeoffs between mixing enhancement and the stagnation pressure loss stemming from fuel jet-induced shocks. Since the fin was designed to weaken the oblique shock strength while shielding the fuel jet penetrating into the core airflow, it not only resulted in better mixing but also improved the pressure recovery. For gaseous fuel, fin-guided injection improved jet penetration by 100 to 200% for a momentum ratio between 0.15 and 0.03. It also resulted in 64 to 85% additional pressure recovery of the injection shock loss.

Combustion experiments revealed that the fin could be used to extend the upper limit of supersonic combustion mode in the present configuration, from an equivalence ratio of 0.04 to 0.12, by preventing thermal choking caused by concentrated heat release near the baseline flame holder. This could be advantageous for certain systems by reducing the thermal protection requirements. However, the fin also made the wall cavity flame holder less effective by increasing fuel penetration away from the bottom wall. The net effects on propulsion system performance will ultimately depend on whether ramjet or scramjet mode is preferred for a given operation.