why I can only use SIMPLE of 1 order?
 

I'm computing a combustion chamber of an engine which has two subsonic air inlets, a fuel inlet and a nozzle.

I set 1 the air inlets as pressure inlets with totol pressure of about 950000 Pa; 2 the fuel inlet as mass flow inlet with mass flux of 0.09kg/s; 3 the nozzle as pressure outlet with 26000Pa.

I use ideal-gas model, K-E turbulence model with standard wall function. And I used tets mesh, no boundary grid.

Only with SIMPLE method of one order, I got a converged result. But when I switch to two order simple method, it didn't converge.

I don't know why? Any one give some advices?