Hydraulic similitude assessment for cryogenic cavitation in propellant lines: The case of thick orifice

Abstract

Cryogenic cavitation can be easily encountered in rocket liquid propulsion systems at the very initial phase of the launch due to the propellant passing through orifices and valves. This leads to flow instability, performance degradation, and even damages to the structures. Hence, the design of the propulsion systems cannot overlook cavitation phenomena. Despite the relevance of the topic, their accurate prediction is still hampered by the difficulty in modeling the high non-equilibrium phenomena involved. From an engineering point of view, the main interest is to design such restrictions present in propellant systems and, hence, to predict the maximum flow rate they can reach. This work questions the possibility to design orifices for cryogenic applications based on purely isothermal testing. Therefore, we performed cavitation experiments with both water and liquid nitrogen by considering two orifices hydraulically similar. This implies a geometric similitude in terms of the orifice characteristic ratio (β) and its dimensionless thickness (th). The results show that the hydraulic similitude is not sufficient to predict the orifice behavior for a cryogenic flow correctly. In particular, the level of liquid subcooling (ΔTsub) upstream of the orifice plays an important role since it impacts the minimal value of pressure of the flow downstream the orifice and thus its thermodynamic state.