Why larger restrictor outlet provides more mass flow?

[RTAF]

I simulated my restrictor designs (40mm and 30mm diameter) with the same boundary conditions.

I read Gas Dynamics about Converging-Diverging Nozzle and compressible fluid but I can't find the answer that why larger restrictor outlet and lower outlet velocity provide more mass flow?


these are the results.
https://www.dropbox.com/s/hsxb...el4hd/sim.png?v=0mcn (https://www.dropbox.com/s/hsxbtlyd8xel4hd/sim.png?v=0mcn)

[VFR750R]

Looks good to me. They converge in mass flow in your data and more than likely you just aren't going to fully choked flow otherwise they will converge completely.

The point of the diffuser is not to increase choked flow (which is not possible), but to allow the engine to get to choked flow at the highest absolute exit pressure (least work), or at the lowest rpm possible. And that better pressure recovery on exit results in higher mass flow anywhere below choked flow. The intent is to make the restrictor invisible to the engine until choked flow occurs.

[RTAF]

Thanks VFR750R, I like this bike.

Could you explain why mass flow rate is affected by restrictor exit pressure?

Is it about density?

[RTAF]

These are CFD results of velocity along the flow axis through the restrictor:
https://www.dropbox.com/s/p96b...2898827_o.jpg?v=0mcn (https://www.dropbox.com/s/p96bivlgdduchac/977635_10201248877333143_1642898827_o.jpg?v=0mcn)

I wonder why throat velocity of Model 12 is higher than Model 9, Nozzles are the same.

[VFR750R]

Again, I'm pretty sure you are not seeing choked flow in your analysis, so the velocity is not maxed out. The definition of choked flow would be when the air speed is equal to the speed of sound.

Find sonic flow calculations for an orifice in a fluids book; once you look at the calculation, you'll understand what affects it and what the sensitivities are.

You can also calculate what pressure differential it takes to achieve choked flow. This will help you answer your own question regarding exit pressure. You need to be careful with your boundary conditions.

It seems like you are doing the analysis without understanding choked (sonic) flow, what causes it, and what the entrance and exit geometries do to the mass flow. You need to do some research and read some published papers/books on the subject, I think this will help you a lot more than I can on the forum.