Reactive shear layer mixing and growth rate effects on afterburning properties for axisymetric rocket engine plumes

Abstract

A semi-empirical model was developed for predicting the after burning ignition location of film cooled rocket engines. The model is based on two characteristic distances, the distance required for turbulent mixing to generate a combustible mixture with the reactive film layer and the distance traveled during the ignition delay. The mixing length is affected by the mass flow, composition of the film cooling layer and the fuel-rich air to fuel ratio required to support combustion. The ignition delay is determined by the composition directly through the auto-ignition reaction time. Both distances are affected by the velocity and temperature of the rocket core and air. This model was experimentally verified over a range of co-flow air velocities using a liquid rocket engine of approximately 440 N thrust, varying amounts of reactive film cooling and compositions of film coolant, and a co-axial annular airflow generator producing airflow at velocities up to nearly 200 m/s. Mean ignition locations experimentally observed were between 3.8 and 9.8 centimeters from the nozzle lip and varied due to the airstream velocity, and film coolant composition and mass flow. All model predictions were within the standard deviation of the experimentally observed ignition points.