Experimental analysis of the spreading of a liquid film on a bipropellant thruster chamber wall

In film cooling approaches, a film of liquid fuel is formed on the chamber wall of bipropellant thrusters to protect the chamber wall from high-temperature combustion gases. To optimize the amount of liquid fuel required to sufficiently cool a chamber wall in this manner without degrading the performance of bipropellant thrusters, the formation process of the liquid film needs to be understood. To this end, in this study, factors affecting the spread of liquid film were experimentally investigated. In particular, experimental apparatus that could reproduce the state of a liquid jet being injected onto a wall to form a liquid film was developed. Water was used as the test liquid because hydrazine-derivative fuels, which are generally used in bipropellant thrusters, are toxic, and the density and surface tension of water are similar to those of hydrazine. The liquid film formation processes were visualized and analyzed by using a still camera. Results indicated that the liquid jet velocity, nozzle diameter, and impingement angle were the key factors affecting the film width, and the maximum film width exhibited a linear relation to the liquid jet velocity component perpendicular to the wall. Considering these results, a general relationship between the key factors and maximum film width was identified, and it was noted that the dimensionless maximum film width could be defined as the product of the Weber number and sine value of the impingement angle. In this manner, the maximum film width can be predicted when deciding injection conditions, which can assist thruster designers during the design process