You can ignite the mixture by putting a heat flux.  Try a value of 5E5 on a small segment of the surface.  I did the video in the gif link below using a small segment of the bottom wall with such a flux.  About 1/4 of the way through, the flux is turned off.  The mesh shows where the heat flux is located.  I used the default 2-step mechanism.

 Link to the solution video gif:


And here is where the mesh shows the small part of the surface that has a heat flux to ignite the mixture. The inlet is on the left and the outlet is on the right.