Direct Injection of Liquid Nitrogen into Water for a Cryogenic Engine

The cryogenic engine was developed as a non-combustion zero emission engine that converts thermal energy to work by the vaporisation of liquid nitrogen using ambient heat. Its output is dependent on the heat transfer rate between liquid nitrogen and the ambient, where the efficiencies of previous designs were operated in an indirect process and heavily reliant on the performance of air heat exchangers used. The direct injection process was recently introduced to improve the heat transfer rate by directly injecting liquid nitrogen into a warmer fluid. The large temperature difference between two fluids in contact is expected to lead to rapid vaporization of liquid nitrogen, hence improving the performance of a liquid nitrogen engine. However, the feasibility of direct injection is challenged by the lack of an adequate valve system to control the delivery of the cryogen, along with a deep understanding of the heat transfer mechanism involved. To address these challenges, a fundamental study of the direct injection of liquid nitrogen into water at ambient temperature is conducted in this thesis. A static rig is designed to allow for the control of the injection process in order to investigate the influence of the injection parameters on the pressurisation rate. The injection timing and valve movement are controlled by a hydraulic actuator, which allows for synchronized injections at the desired pressure and temperature. Over 400 injections of nitrogen at different thermodynamic conditions were conducted and maximum pressurisation rates of up to 5156 bar/s were recorded for a clearance volume of 5 ml, which is 10 times greater than those found in the literature as a result of the increased injection pressure. Based on the pressurization curve, a two-stage boiling mechanism of liquid nitrogen is inferred that occurs inside the vessel, that is, film boiling and breakup of liquid nitrogen jet into small droplets, and subsequent boiling of liquid nitrogen droplets. To further increase our understanding of these complicated processes involved, a numerical study using CFD software Fluent is conducted to simulate the jetting and boiling of a liquid nitrogen droplet in water. A heat transfer and thermodynamic analysis is conducted and the implications of the results to the engine performance and its development are discussed.