Prediction and measurement of fuel cell flammability safety limits

摘要

Leakage of the fuel through a possible crack in the solid electrolyte, or in the separator plate, of a fuel cell, to the oxidant side, can cause uncontrolled combustion. An experimental setup is design to simulate the above conditions. A cylindrical combustion chamber is supplied with an axial jet of methane from one side and air (or pure oxygen) jet from the other side to create a counter flow laminar diffusion flame. A 0.5 mm thick disk, having a 0.5-mm diameter central hole, is installed inside the cylinder perpendicular to its axis. The fuel and oxidant flow rates were kept constant while the distance of the oxidant is changed until a stable flame is established. The distance is then reduced until the flame extincts. The experiment is repeated for different fuel flow rates keeping the oxidant flow rate constant, while recording the flame extinction distance. It was found that the quenching distance, between oxidant nozzle and the disk, is of the order of 1.0 mm when oxygen is the oxidant while for air it is about 15 mm. These distances give an upper limit for the oxidant gap size of the fuel cell, based on safety considerations. Moreover, the measurements show that fuel cells are safer when using air as oxidant rather than pure oxygen. A new computer code is developed capable of simulating flow, heat transfer, and chemical reactions in 2D geometries, similar to the above flammability experiments. The governing laminar equations for momentum, mass and mixture fraction are solved by a numerical iterative algorithm. The computed mixture fraction is used to calculate its dissipation rate. They are then used to access a flamelet library to obtain the local flame properties and/or its possible extinction. This flamelet library is generated separately, based on a 3-step reaction mechanism for CH{sub}4. The computed flammability distances for CH{sub}4-O{sub}2 and CH{sub}4-air flames, are lower than the corresponding measured quenching distances. This is a result of neglecting thermal radiation in the present combustion model.