[HWilliams79]

I am doing a multi-component gas simulation. One of the gases which I need is C_2 H_6 (ie. ethane gas), but it does not seem to be in the database when I try to add it as a mixture component.

Is there some way to manually add a new component if I look up (or work out) the material properties like the dynamic viscosity and so on?

[FliegenderZirkus]

You can add a new material in Tools -> Material Databases. "Tools" in the simulation tree on the left, not the top menu.

[HWilliams79]

I wonder if I could ask a follow-up question (I could ask as a separate question if necessary). I have used a multi-component physics model to set up three different gases going into a box through three different inlets.

I specify the mass flow rates at the three inlets and then try to specify that only one gas is going into each inlet by for example writing for the species mass fraction [0.3, 0, 0] at the first inlet where the mass fraction of the gas for that inlet is meant to be 0.3 and so on, to specify the relative proportions of the gases.

When I click the green flag on the toolbar to initialize the solution it says

''Sum of mass fractions is zero on [Fluid]''

Could you advise on what might be causing this?

[FliegenderZirkus]

I think there are two things going on. First, check if you have a meaningful initial condition for the mass fractions. Normally you'll find this in the Physical Continuum node. Star doesn't make any assumptions regarding the initial condition and leaves all mass fractions as zero so you have to update them and make sure they add up to one. And second, the mass fractions at your inlets also have to add up to one, so a pure component 1 should be [1.0, 0, 0], not [0.3, 0, 0]. Sometimes you can choose to only enter N-1 mass fractions and then the last one is calculated so that the sum is = 1, but I don't think that's the case here.

[HWilliams79]

In Physics Continuum under Initial Conditions there is a condition ''Species Mass Fraction'' which I think is for the total mass fraction profile, so this also adds up to 1 and I can use this to specify the relative proportions of each gas, for example [0.3 0.3 04], or something like that?

[FliegenderZirkus]

Exactly. Does the error ''Sum of mass fractions is zero on [Fluid]'' still appear when you enter [0.3 0.3 0.4]?

[HWilliams79]

Yes, it seems to be working in the sense that one at least gets a picture for the mixing of concentrations which looks reasonable.

However, I've attached the legend which shows the same value on left to right of the spectrum for some reason.

[FliegenderZirkus]

I'd need more context to have a chance of guessing what may be causing this!

[HWilliams79]

My mistake, it's simply because clicking the green flag sets the initial conditions in place, so the molar concentration is the same everywhere in the geometry. In this case, the molar concentrations observed correspond to the mass fractions set at the inlet.