Prediction and Measurement of Fuel Cell Flammability Safety Limits

Abstract

Leakage of the fuel through a possible crack in the solid electrolyte or in the separator plate of a fuel cell, can cause uncontrolled combustion. An experimental investigation and theoretical prediction are made 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 disk with a small central hole is installed inside the cylinder perpendicular to its axis. 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 CH4. The computed flammability distances for CH4–O2 and CH4–air flames are in reasonable agreement with, though slightly lower than, the corresponding measured quenching distances. This is a result of neglecting thermal radiation in the present combustion model.