Leakage through microscale or nanoscale cracks is usually hard to observe, difficult to control, and causes significant economic loss. In the present research, the leakage in a pipe was evaluated by the virtual entropy generation (VEG) method. In virtual entropy generation method, the “measured entropy generation” is forced to follow the “experimental second law of thermodynamics”. Taking the leakage as the source virtual entropy generation, a new pipe leakage evaluation criterion was analytically derived, which indicates that the mass leakage rate should be smaller than the pressure drop rate inside a pipe. A numerical study based on computational fluid dynamics showed the existence of an unrealistic virtual entropy generation at a high mass leakage rate. Finally, the new criterion was used in the evaluation of leakage available in the literature. These results could be useful for leakage control or industry criteria design in the future.

The thesis studies the temperature characteristics of 5kW air natural convectional type proton exchange membrane fuel cell. The three-dimensional mathematical model of the proton exchange membrane fuel cell system is established. Using COMSOL software multiphysics simulation modeling, from the aspect of air natural convection, heat and mass transfer coupling, with natural air cooling characteristics, it gets the numerical solution. The solution is that the battery temperature rises rapidly when the pressure drops. It will reduce the battery performance. Therefore, it is necessary to reasonably control the pressure of air cooling type battery and the temperature range. It has important significance to proton exchange membrane fuel cell efficient thermal management system .