Experimental investigations of coax swirl injectors for hydrocarbons

The design of injector elements is a tedious task in a rocket engine development program. The injector affects the combustion efficiency as well as the distribution of the heat release within the chamber and thereby influences the design of the cooling system of a liquid rocket engine. Furthermore, the distribution of heat release and the acoustics of the injector play an important role for the prevention of high frequency combustion instabilities. In order to investigate basic design principles for injectors in oxidizer rich staged combustion cycle engines, the Institute for Flight Propulsion at Technische Universität München and EADS Astrium Space Transportation, Munich, have set up a test facility to conduct hot fire experiments in a single-element subscale rocket combustor. The setup allows investigating the influence of injector design variations on the combustion efficiency, the distribution of heat release and of heat transfer to the water cooled chamber wall. Spray tests were used to characterize the pressure losses in the injector in a wide operating range. They were conducted with kerosene and gaseous nitrogen in a special spray test chamber, using back-light videography for the optical investigation of the spray. The hot fire experiments were conducted using gaseous oxygen, GOX, and kerosene Jet A-1 as propellants. The mixture ratio was varied from 2.4 to 3.8 with chamber pressures ranging from 3.5 MPa to 8.0 MPa. The combustion chamber is equipped with several static pressure measurements as well as dynamic pressure sensors to assess combustion stability. The heat flux to the wall is calculated from temperature and pressure signals recorded in the water cooling system. The article presents detailed information gained with three injector design principles. Based on a reference design derived from coax swirl injectors used in existent LOX/kerosene main stage engines, two alternative solutions have been investigated in detail. One replaces the orifice at the inlet of the oxidizer post by a conical constriction at the outlet of the recess. The other enhances mixing and reaction of the propellants by means of swirl caps attached to the oxidizer post. From the recorded test data, a fingerprint of each element has been derived which allows for a tailored design of injector head assemblies of large scale engines. Copyright IAF/IAA. All rights reserved.