The Study Of Low-Frequency Combustion Instability In Air-Heater Based On Droplet Evaporation

In the present thesis, the low-frequency combustion instability in liquid rocket engine is investigated with numerical simulations,theoretical analyses and experimental studies. The dynamic response characteristics of droplet evaporation to pressure oscillation are considered as the key exciting factor for low-frequency combustion instability. The main research contents and results are outlined as follows:(1) Evaporation process is simulated at micro-level with molecular dynamics simulation. The study is focused on the dynamic response of evaporation process and gas-liquid interface to pressure variation. The studies show that the thickness of gas-liquid interface is directly proportional to ambient pressure and temperature, and the evaporation rate increases clearly with the rapid drop of ambient pressure.(2) A test-facility is constructed for studying the dynamic response characteristics of droplet evaporation to low-frequency pressure oscillation. The results indicate that droplet evaporation rate can respond rapidly to low-frequency pressure oscillation, and owns the same frequency but different phase of pressure oscillation, and the maximum value of droplet evaporation rate appears in the latter period of pressure decline.(3) The concepts of saturation vapor boundary layer and converted saturation vapor boundary layer are put forward, and the quasi-steady model of droplet evaporation is established respectively under static and convection environment with the low-frequency pressure oscillation. By solving the model equations, a systematic study of the dynamic response characteristics of the droplet evaporation process to pressure oscillation is carried out. The results show that the amplitude, frequency, average pressure of pressure oscillation, ambient temperature and droplet velocity have significant impacts on the dynamic response characteristics of droplet evaporation rate.(4) Based on the time lag of combustion and droplet evapreationis, the dynamics model is established for a rocket-based air heater system. The numerical simulations are performed for the low-frequency combustion instablilities of the air/oxygen/alcohol tripropellant air heater. The effects of propellant mixture ratio, injection pressure drop, and the propellent supply pipe system on the low-frequency combustion instability are studied. The results show that the dynamic response characteristic of droplet evaporation to pressure oscillation is a major factor to inspire combustion instability.(5) Experiments are performed to study the low-frequency combustion instability in the air/oxygen/alcohol tripropellant air heater. The effects of propellant mixture ratio, injection pressure drop and the dynamic characteristics of the supply pipe system on the low-frequency combustion instability are studied, and the results of experiment prove that the dynamc response characteristic of droplet evaporation rate to pressure oscillation is the major exciting facor of low-requency combustion instability.