Spontaneous Ignition Mechanism of Pressurized Hydrogen due to Burst Conditions and Obstacle Configurations

Abstract

Hydrogen gaining more attention as a next generation clean energy carrier. However as its volumetric efficiency is low, in order to be used for this purpose, it needs to be compressed. However, high-pressure hydrogen presents potential hazards because of its strong flammability. According to past research, over 60% of hydrogen related accidents have been reported that the ignition sources have yet to be identified. In this thesis, both the experimental and numerical studies have been conducted to clarify the mechanism of spontaneous ignition due to a sudden release of high-pressure hydrogen through tubes. Experimental model has been designed to simulate the release of hydrogen through a tube by bursting the diaphragm installed between a reservoir chamber and an extension tube. The numerical simulation integrates rigorously formulated numerical models. Because of the phenomena included such as strong shock, expansion waves, turbulence, chemical reaction, the models should be capable of shock-capturing, contact-resolving, entropy-satisfying, enthalpy-preserving, and conservation-satisfying. Using these approaches, burst pressure range is extended up to 40 MPa which is not tested before. Results from the experiment of relationship between tube length and burst pressure shows a general tendency that the propensity of spontaneous ignition is proportional to the burst pressure and extension tube length. Study on a flow structure, a flame development and propagation inside the axisymmetric tube, which is impossible to visualize, is performed using computational code. The results show that flat shaped complete flame has been quickly developed in smaller inner diameter tube and the influence of a disturbance due to a diaphragm burst remains longer when the tube inner diameter is large. Another simulation tries to show the effect of a diaphragm shape depending on the burst pressure. The results show that the ignition mechanism is closely connected with flow formation inside the tube, which is strongly affected by the burst conditions. It suggests that the possibility that the ignition occurs can be higher if there are any factors that result in mixing, such as multi-dimensional shock interactions or any disturbances from the shape of the disk. However, this trend is shown when the burst pressure is low, as it can be seen from that the mixing process can differ sensitively with the shape of pressure boundary. On the other hand, if the burst pressure is high, the ignition feature is less sensitive with the shape of pressure boundary. In company with the study on the phenomena inside the tube, the effect of an obstacle near the tube exit is studied using flat plate with varying height and distance. Although the plate could generate stagnation region in front of it, any secondary reaction is not observed. The mixing in the recirculation at the edge of the wall is also not able to make the heated-air and hydrogen react. But the obstacle could decrease the flame stabilizing time. The results emphasize the importance of flame generation inside the tube.