The Physical Mechanism Of The Supercritical CO₂ Leakage Process In Small Scale Laboratory Conditions

To meet challenges in the current energy supply and the environment, how to utilize the fossil fuels cleanly and efficiently has been widely studied in a global scale. Cabon capture and storage (CCS) is thought to be one of most promising technology to solve the current trouble on lack of industrialization ability and also the bridge to the low-carbon economy.Carbon capture, transportation and storage are the main three parts in CCS and the efficiency and safety are the two key issues in this technology, espically in the transportation of the CO2 part. For the safety concern of the transportation, the leakage behaviour of CO2 should be investigated which has been conducted in UK and South Korea, but it is still in start-up stage in China. In this paper, we mainly focus on the investigation of the behaviour of the supercritical CO2 in small scale leakage and the physical mechanism of highly pressurized jet process.Pipeline transportation is one of the most important ways in CO2 transportation. CO2 is always highly pressurized when transporting. It is proved to be most efficient to transport CO2 in supercritical phase in pipeline as the supercritical CO2 has a low viscosity and high diffusivity like gas, but a high density like liquid. The accident release of CO2 in supercritical phase is more dangerous. In the leakage process, the choked flow appears inside the pipeline which has significant influence on the efficiency of the transportation; the barrel shock expansion and Mach disk show a strong leakage jet in the near field of the leakage; the dry ice bank grows at the leakage hole due to the Joule-Thomson effect and the high pressurized jet outside leakage hole proves to be destructive effect on the pipeline and environment around. Small scale leakage process of supercritical CO2 is selected as the main subject in this paper. It is investigated based on related fundamentals such as the gas dynamics, heat transfer, fluid mechanics, and safety theory, but also combined with experimental study and computational fluid dynamics. The results show the main physical phenomena in the leakage process of supercritical CO2 and the influence it brings on environment. Moreover it reveals the dynamical mechanism in these physical behaviour with comparison to the CFD results. At last it provides some theoretical and technical guidance for the highly pressurized fluid leakage detction and analysis. Some findings of the research are summerised as follows:In the theoretical part, some traditional models of highly pressurized jet were introduced and modified with characteristics of supercritical CO2. Through combining with the typical accidental release model, a three-part release of supercritical CO2 model is established. As the fluid parameters inside the leakage nozzle could hardly be measured with considerable accuracy, an indirect method is proposed to measure the strength of the choked flow with the local Nusselt number. In the near field of the leakage jet, a calculation model is proposed and modified to investigate the relationship between the leakage pressure and leakage mass outflow rate which could explain the development of the dry ice bank phenomena and barrel shock expansion. In the far field of the leakage jet, the thermal field and flow fied are investigated combined with the characteristics of fluid in the highly pressurized jet.In expericmental part, an independent experiment platform is designed and built called "Supercritical CO2 pipeline transportation and leakage simulation platform" Based on this experiment platform, the leakage experiment was conducted with different of leakage initial pressure and different sizes of leakage nozzle. The first series of the leakage experiment are mainly focused on different phases of the CO? at the same leakage nozzle. The leakage pressure, mass outflow rate, leakage velocity and the variation of the temperature were measured in validation and improvement of the leakage model. The second series of the leakage experiment are mainly focused on the different leakage sizes of leakage nozzle with supercritical CO2. The variation of the jet and the relationship between the highly pressurized jet and the environment around are investigated. The detailed data are provided for the analysis of the physical phenomena in superctical CO2 leakage process and the detection of the leakage along the pipeline.In numerical simulation part, a two-step Lax-Wendroffed program is written to simulate the axisymmetric jet and free diffusion process of the supercritical CO2. To investigate the fluid details of supercritical CO2 in the free diffusion process through changing the leakage nozzle size and initial leakage velocity. The Mach disk is simulated and studied by adding artificial viscosity. The velocity, pressure and temperature data are obtained in comparison with the experiment data.Finally combined the theory and experiment investigation, the physical mechanism of the supercritical CO2 leakage process is analysed. Through the comparison with numerical simulation, the highly pressurized leakage model is well validated and modified.