Visualization of Coolant Liquid Film Dynamics in Hypergolic Bipropellant Thruster

We conduct a high-speed visualization of coolant liquid film dynamics inside a 10N-class bipropellant thruster using monomethylhydrazine and a mixture of nitrogen tetroxide with approximately 3% nitric oxide as propellants at equivalent conditions with the flight model. The direct visualization of the liquid film inside a quartz glass chamber demonstrates the transient dynamics of film flow from ignition to cutoff for the first time. The scenario for the life of the film flow is found as follows: the coolant fuel jet impinges on the chamber wall being the liquid film; the friction force from the chamber wall rapidly decelerates the film as one-tenth of the injection velocity; next, the film moves sheared by the fast combustion gas downstream; and eventually the film completely evaporates by heat transfer from the hot combustion gas. The liquid film presents the typical ripple wave structure, covered by the velocity and thermal boundary layers. The effect of the mixture ratio is significant as the film flow rate increases at a small mixture ratio, leading to the longer film length.