Numerical analysis of effect of ignition methods on flame behavior during passing through a sudden contraction near the quenching conditions

The present study numerically investigates the propagation of a laminar premixed propane–air flame in various tubes with a sudden contraction and isothermal cold walls in the stoichiometric mixture. Two different ignition methods are examined: plane and round shaped ignition. These investigations show that for relatively small tubes, the method of ignition does not influence the final flame shape in the tube – it is mushroom-shaped. For wider tubes and the plane ignition method, the flame transforms to a tulip-shaped one. The numerical results show that a flame can enter a smaller diameter tube from a wider one and propagate in a stable manner in all configurations of diameters of both tubes as long as the diameter of the narrow tube is far from the quenching diameter. Decreasing the diameter of the narrow tube, we approach the conditions where the dimension of the wide tube determines if the flame will propagate or quench. This is associated with a temporarily increased ratio of heat losses to thermal energy generation caused by a greater heat loss area. It is also shown that a tulip-shaped flame occurring at large diameters of the wide tube is particularly exposed to quenching during propagation through the sudden contraction.