questions on wall film simulation 2
 

Dear Support,

I have another question on wall film simulation.

We have 2 port designs, A and B, for a GDI engine. In experiments, A has higher soot PN (numner density) than B does. We expected A would also have more piston wall film than B does. But, in simulation, A has less piston wall film than B does.

So my questions are

1. Suppose my logic is problematic, should it be expected that PN correlates with piston wall film, proportionally?

2. Suppose numerical results are wrong, which part shall I pay particular attention to in wall film simulation. I set up my simulation according to the tutorial SI8 case.

Thanks a lot.

Wall films are only one source of SOOT formation. There can be other sources for soot formation in SI engines like combustion of overly rich mixtures. I'm not implying that your hypothesis is wrong. But we need more analysis to figure this out. Here's what I would try :

1. Before looking at any emission species, we need to make sure quantities like, cylinder trapped mass, amount of fuel injected/premixed, LHV of fuel, cylinder pressure during compression, heat release and expansion are all correctly modelled in the simulation.
2. Hence check if the cylinder pressure before spark matches within 1% of the measured data. Intake mass flow rate should match within 2% of the measured data. If not, you have to question your boundary conditions and rectify any uncertainties in them first.
3. check if you are supplying the same amount of fuel as in the test engine. If the LHV of fuel the same as what's in the simulation?
4. Are you getting the correct equivalence ratio as the test engine?
5. What is the octane number of fuel used in the test? Is the fuel surrogate used in the simulation has the same octane number?
6. How confident are you on the wall temperatures? The wall films depend on wall temperatures
7. Are you using correct model settings for spray, film, and combustion? Is your mesh refined sufficiently to capture spray and combustion?
8. Finally what is the chemical mechanism you are using? Has this being validated under the engine conditions of interest?

Thanks,