Ignition cells in transient cases
 

A question on combustion...

I am simulating a diffusion flame and I have to use ignition cells at first to start the combustion. The problem is that while I can achieve the ignition of the flame in some steady state cases I tried out, I can't ignite the flame in transient runs, no matter how long I keep the ignition cells in high teperature. Any ideas of what might be happening? Thank you in advance.