[MFGT]

Hello,

i want to collect some information about the MZ Settings with SAGE, share recommendations and experiences.

The standard example cases for Diesel or Gasoline ("Engine sector - Non-premixed combustion (SAGE)", "SI8 engine - Port fuel injection (SAGE)" and "SI8 engine - Premixed combustion (SAGE)", etc..) have these settings:
Dimension: 2 / Temperature: 5K / Phi: 0.05
Fuel is either DIESEL2 (which evaporates into C7H16), or IC8H18.

The "Engine sector - Dual fuel" case has premixed CH4 combined with DIESEL2:
Dimension: 3 / Temperature: 5K / Phi: 0.05 / C7H16: 0.001

In "Engine sector - Multi-component fuel", DIESEL2 evaporates into three components (Iso-Octane, n-Heptane, Toluene):
Dimension: 4 / Temperature: 5K / Phi: 0.05 / C7H6: 0.001 / C7H16: 0.001

Obviously, adding a further dimension for dual fuel simulations makes sense, as the Spray and premixed fuel are not distributed to the same locations. However, when injecting a multi-component fuel, these components should be distributed the same way, so couldn't I skip the extra dimensions? Or is the assumption that the three components are similarly spread into the domain not valid?

I ran tests in the past and the increase of CPU time with MZ Dimension of 4 has been enormous. I also plan to do some investigation on MZ size, e.g. 2K and Phi 0.025 with my current fuel (Iso-Octane plus n-Heptane).

[jlemoine]

When you have N number of fuel in your domain, you need to have at least 2+(N-1) bins. In the case of the "Engine sector - Dual fuel" that will give you 3 bins (temperature, phi, one fuel), for the "Engine sector - Multi-component fuel" that will give 4 bins (temperature, phi, 2 fuels). The reason for this is the fuel mixture is not uniform across you domain. Let's take the example of two cells at T=900K and phi=1.0, one cell has Iso-octane and the other has n-Heptane. If there are only two bins (phi and temperature) the two bins will be grouped together. If there are three bins (phi, temperature and one fuel) the two cells won't be grouped together. In this case, you only need to have three bins to differentiate the two cells. In the case of "Engine sector - Multi-component fuel", you need 4 bins to differentiate the cells with the same composition for two fuels.

[MFGT]

Hi Jerome,

yes, i understand that. But if a parcel (containing 50% Iso-Octane and 50% n-Heptane) evaporates, does it not approximately lead to the same Iso-Octane and n-Heptane gaseous distribution?

Therefore, there won't be any cells that contain Iso-Octane or n-Heptane only? This way my Phi would always be calculated with my initial fuel 50/50 composition.

Well, the longer i think about it. If the gaseous fuel composition is nearly the same, another MZ dimension wouldnt add much to the computational time, i guess?

[MFGT]

I have ran a test with my RON92 IC8H18/C7H16 Fuel, simulation was at rated power, combustion only -60 up to +40 °CA. I varied MZ dimension and bin sizes, but there is nearly no difference in the result.

02 is baseline with Dimension: 2 / Temperature: 2.5K / Phi: 0.05
02b is with Dimension: 3 / Temperature: 5K / Phi: 0.05 / C7H16: 0.001, increase of 62% CPU time
02c is with Dimension: 2 / Temperature: 2.5K / Phi: 0.05, increase of 8% CPU time
02d is with Dimension: 2 / Temperature: 5K / Phi: 0.025, increase of 6% CPU time
02e is with Dimension: 2 / Temperature: 2K / Phi: 0.02, increase of 30% CPU time

[jlemoine]

We give our best practices as a starting point, this is good that you are able to do two MZ dimensions without compromising your volume averaged results.

[SamWijey]

Tobias,

Different fuels have different vapor pressures specified in liquid.dat and hence will evaporate at different rates. The parcel composition is going to vary when evaporation starts due to the this fact and hence the gas phase composition will also be different.

Even though your test case did not show any difference, I have seen many multi-component fuel cases that will give wrong results if the additional multi-zone bins are not used. If all your fuel components are premixed and in gaseous phase, you do not need additional binning dimensions. But for muli-component fuel spray cases you should use more binning dimensions.

Hence treat our recommendations as accuracy settings which might not necessarily give you the best speed up. Once you achieve the desired accuracy using our recommended settings, you can make additional assumptions (like this one) to see if you can achieve better speed-up without compromising accuracy.

Thanks,

Sameera

[MFGT]

Thanks for your input Sameera. I guess its also caused by that low fraction of n-Heptane. If the two fuels were more different and had similar fractions each, it could make a bigger difference i guess.