Experimental investigation of transient injection phenomena in rocket combusters at vacuum with flash evaporation
In the frame of future changes in propellant combinations and ignition methods for cryogenic upper stage engines or for RCS (Reaction and Control System) and OMS (Orbital and Manoeuvring System) engines, fluid flow structures and injection phenomena affecting ignition have to be investigated due to changed injection transients and boundary conditions for the injected propellants under vacuum conditions. One of these phenomena is the so-called flash evaporation, which is a sudden evaporation of a superheated liquid. A liquid becomes superheated if it is injected into an atmosphere with a pressure below its saturation pressure at injection temperature. Because of the sudden evaporation the pressure in the combustion chamber increases continuously until vapour pressure of the fuel is achieved shortly before ignition. The exact condition in the combustion chamber regarding present phase, mixture and temperature is very important for the ignition. It is very important for the probability of a successful ignition in general but also for avoiding unacceptable high pressure peaks.
TP-B4 focuses on the experimental investigation of LOX (liquid oxygen) flash evaporation and its characterising injection parameters. Based on this characterisation of flash evaporation of cryogenic propellants, models can be developed and verified to describe the transient starting process of rocket combustion chambers using cryogenic propellants. With these models it should be possible to run numerical simulations of combustion processes with real injection geometries and propellants. Especially the numerical TP-B5 needs the injection parameters and results of TP-B4 for validation of their models.
Preliminary investigations at an existing test bench at DLR Lampoldshausen with LOX injection into a vacuum chamber showed the limited suitability of this test bench due to its lack of a temperature adjustment system. Hence a new test bench with a specific cryogenic injection and temperature adjustment system is being developed. This test bench will permit a systematical and detailed characterisation of LOX flash evaporation as a function of the injection conditions and geometries. With optical diagnostic techniques like Shadowgraphy, PDA, PIV or Schlieren photography the influence of injection parameters like e.g. the LOX injection temperature and pressure or the chamber pressure on the atomisation and vaporisation processes will be determined.
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