DESIGN OF THE EVAPORATOR FOR A SPACE ENGINE – FLUIDICS ANALYSIS AND NUMERICAL SIMULATIONS

The following thesis work focuses on the design of an evaporator for a space engine; in particular, fluidics analyses and numerical simulations that drive the design process of this component are reported. The evaporator is part of a new cold gas propulsion system developed by the company T4i. The new engine takes advantage of the gas expansion in the nozzle to create thrust, but the propellant is stored in liquid state in the satellite. A specially designed evaporator is required to obtain the propulsive fluid in the gaseous phase and in order to use fluid for propulsion. The evaporator’s task is to ensure at any time and in any operating condition the complete vaporization of the propellant. Numerical simulations are required to study the evolution of the flow and the evaporator temperature. These simulations are used to understand if the evaporator actually fulfils its functions and to guide the design phase of the component. In the evaporator, the flow enters in liquid phase and exits in gaseous phase; so there is a phase-change process of the fluid. The Dittus-Boelter equation connects the heat exchange coefficient to the flow field, but the equation is only valid if the flow is a mono-phase. In the evaporator, the flow is bi-phase during the vaporization process; the heat exchange coefficient must be calculated based on the Saitoh experimental correlations for the calculation of this coefficient. When the flow is completely vaporized, the flow is assumed as ideal gas and its evolution is described by the Rayleigh flow. The evaporator tube in which the fluid flows is designed with a helical geometry with external revolutions and internal revolutions. This geometry is designed to maximize the use of the power supplied to the evaporator and minimize heat losses to the interfaces. The relationships between the geometric parameters of the project and the vaporization of the fluid is determined by several numerical tests. These correlations are used to improve the geometry of the evaporator so that the vaporization process takes place as quickly as possible. In addition, several initial temperature conditions are analysed in order to study the behaviour of the evaporator under different operating conditions. The component is tested in a sub-orbital flight of the company Virgin Galactic; the evaporator is part of the experiment TRAP (TetRafluoroethAne sPonge). The evaporator will be tested and it will be certified for space use in a micro-gravity environment during the flight.