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Project

Study of Cryogenic Transient Flows. The Impact of the Fluid Thermosensitivity on Cavitation

Two phase flows appear in spacecraft operations due to low-pressure or vacuum conditions in the propulsion system. When the launch starts, the propellant is confined in tanks which are isolated by a set of valves. As the spacecraft is ejected from the launch vehicle, the propellant lines are filled and pressurized. This is the so-called priming phase. A bubble flow is produced by the combination of different phenomena such as boiling, due to the chilldown of the propellant line, cavitation, caused by the passage of the fluid through a valve, and fluid hammer. Each of these processes takes place in a highly thermodynamic non-equilibrium environment. The consequence is that validated theoretical models to describe them are not available yet. Further difficulties arise from the experimental point of view due to the cryogenic nature of the propellants and the complexity of a multiphase process, strongly limiting the possible techniques to be used.

The aim of this thesis is the characterization of two sub-processes happening during the priming operation: cavitation and fluid hammer. For this purpose an orifice configuration will be used, which are quite common in rocket propulsion systems. Moreover, with this configuration cavitation can easily be experienced due to the high velocity obtained at the vena contracta. Since cryogenic fluids are thermosensitive, their behavior during cavitation is completely different from that of non-cryogenic fluids. Therefore, Liquid Nitrogen (LN2) seems to be the best choice and is therefore chosen as the working fluid in the experimental setup. In fact, its properties allow less stringent requirements, in terms of temperature, with respect to other cryogenic fluids. It is an easy handling and relative low cost cryogen as well.

Determination of the technical requirements of the experimental set-up and test conditions, for LN2, are the first tasks of the project. Selection of proper measurement techniques is requested as well. The experimental investigation will be carried out by means of pressure, temperature and flow rate sensors and special attention will be given to optical visualizations. The aim of the experiments is to retrieve quantitative information about the thermodynamic properties, flow topology and void fraction during cavitation and fluid hammer. The pressure waves generated by the fluid hammer will also be analyzed, as well as the temperature variations.

Experiments with water, on an isothermal test bench, have already been carried out in order to characterize different orifices. The aim was to define parameters which allow a hydraulic similitude between tests in water and in liquid nitrogen. The new cryogenic test section will be designed to guarantee this similitude

Finally, the results of the cryogenic experimental campaign will be used to create a database. It will help to validate numerical codes, developed to design valves and pipes against damages due to cavitation and fluid hammer.

Date:7 Mar 2016 →  30 Jun 2020
Keywords:cavitation, orifice, cryogenic flows
Disciplines:Control systems, robotics and automation, Design theories and methods, Mechatronics and robotics, Computer theory
Project type:PhD project