Development of a novel model for emptying of a self-pressurising nitrous oxide tank

Self-pressurised, or vapour pressurised system is a system that maintains operational pressure in ambient conditions, with no need for additional pressurisation. An example of such system is a tank filled with nitrous oxide, which keeps pressure of 50.35bar at 20°C. Self-pressurised tanks are used in hybrid rocket engines, due to low weight and simple construction. At the same time, accurate prediction of the dynamics of these systems during emptying creates a significant challenge, due to occurrence of near equilibrium two phase flow and rapid changes in pressure and temperature. The authors have developed several numerical models with aim of capturing described phenomena. The development started with basic assumption of lumped parameters approach and using mass and energy balance equations. First model featured two elements, one for the investigated fluid and second for the tank walls, with assumption of fluid in equilibrium, having single pressure and enthalpy. Such approach, while capturing major trends, lacked accuracy. To order to improve it, fluid was divided into two separate elements, one with saturated vapour and the second with the two-phase mixture. This proved to be more useful for predicting mass flow, but still did not offer accurate results. Further development included treating both fluid elements as two phase mixtures, first being mostly vapour and the second being mostly liquid and required additional equation describing tank's internal dynamics. Resultant model allows for significantly higher pressure prediction accuracy than previously and gives better prediction of parameters of fluid flowing out of the investigated tank.