Boiling transition characteristics during the liquid oxygen chill down process in an exit-contracted pipe

In a low-pressure liquid rocket engine, there is a pipeline-injector component downstream of the main valve, which could not be precooled. During the start up process of the rocket engine, the chamber pressure is usually lower than the saturation pressure of the cryogenic propellant. The cryogenic propellant in the pipeline rapidly boils, generating two-phase flow and triggering unstable combustion. This study is aimed to clarify the liquid oxygen chill down process of the pipeline-injector component and provide theoretical support for shortening the two-phase flow stage as well as improving the performance and stability of the liquid rocket engine startup process.

The pipeline-injector component was simplified into an exit-contracted pipe, and the liquid oxygen chill down experiments were conducted with two mass fluxes on the rocket engine test platform. The two mass fluxes were respectively 3,750 kg·m^-2·s^-1 and 1800 kg·m^-2·s^-1, which was controlled by the Venturi tube. And the pressure was controlled by changing the area of the throttling hole. Fourteen T-type thermocouples were used to measure the outer wall temperature at seven sections of the pipeline during the chill down process. An insertion-type temperature sensor and a pressure sensor were respectively used to measure the temperature and pressure of the fluid inside the pipe. Before the experiment began, the main pipeline had been precooled. Moreover, during the experiment, the test pipe was insulated by polyurethane foaming.

During the experiments, the outer wall temperature of the test pipe was measured, which could be processed to obtain the inner wall temperature and heat flux of the pipe. Curves were plotted to depict the development and variation of the inner wall temperature during the chill down process . By comparing with the boiling curve, the Leidenfrost point and the critical heat flux could be determined. As a result, the flow pattern development diagrams were plotted to analyze and discuss the fluid field inside the pipe during the chill down process. In addition, a correlation was proposed to predict the heat transfer coefficient of the Leidenfrost point.

As the pressure and the mass flux rate increase, the cooling process speeds up. Under low-pressure conditions, the inner wall temperature curve can be divided into three segments, including the linearly decreasing segment, the rapidly decreasing segment and the slowly decreasing segment. Under high-pressure conditions, the inner wall temperature curve is divided into two segments, namely the acceleratingly decreasing segment and the slowly decreasing segment. There are three liquid rewetting patterns during the chill down process, including Ⅰ, Ⅱ, and Ⅲ, which are controlled by the quenching fronts at the inlet and the outlet, the quenching fronts in the middle, and the high pressure filling-in of the liquid, respectively. While the rewetting patterns at the front 1/4 of the pipe are always Ⅰ for the experimental conditions, the rewetting patterns at the other sections of the pipe change with increasing pressure. For the middle and the rear sections, when the pressure is lower than 1.181 MPa, the rewetting patterns are Ⅰ or Ⅱ. And when the pressure is equal to or higher than 1.181 MPa, the rewetting patterns of these sections transform into Ⅰ at low mass flux and Ⅲ at high mass flux. With an error of less than 34%, certain correlation is employed to predict the h_LFP for 4 measurement points at the 0.15 and 0.30 cross-sections.