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Single Phase simulation of an ejector

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Old   March 7, 2016, 13:04
Default Single Phase simulation of an ejector
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Hi -

I am trying to replicate a technical paper which validated experimental results with a single phase R245fa ejector. I am having a lot of issues getting the solution to converge. I am using the NIST real gas module in Fluent for R245fas simulations. Summary of the model:

2d, planar, density, k-w with SST. The motive nozzle inlet is single phase vapor with a pressure of 4 atm and temperature of 363.15 K. The suction inlet (outlet of the evaporator in a vapor compression cycle) of 0.6 atm and 303.15. The pressure outlet condition of the ejector is 0.1 atm (303.15K).

I am using the implicit Roe scheme (least squares method) second order terms for flow and turbulence. I have lowered the Courant number to 1 (since solution diverges if I go higher than 5) and the "under-relaxation" factors are 0.5.

Currently I don't have mass flow rate convergence at the ejector outlet even after 3700 iterations. Any help is greatly appreciated!!!
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Old   March 8, 2016, 01:03
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3700 iterations is nothing (not very many).

Complex models are generally prone to divergence and even when they don't diverge, can converge slowly.

In general it's very hard to get a real gas simulation to converge because of non-linearities. You need to have a very good initial guess. You also need to tune the urf's a lot.

Are you using a targeted mass flow rate? Instead, use a fixed pressure inlet and fixed pressure outlet. This helps a lot.

I hope you've already tried starting with a slightly simpler simulation (constant properties, or only temperature dependent properties) before going to the full blown NIST real gas.

Using a predefined lookup table also speeds up the computation tremendously if you need to shave some compute hours.
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Old   March 8, 2016, 09:37
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In general it's very hard to get a real gas simulation to converge because of non-linearities. You need to have a very good initial guess. You also need to tune the urf's a lot.

That's a good point. I had that difficulty initially. What I did was to try and solve the problem in 1-D, and used the results from that in my 2-D model. Convergence hasn't improved though (though to your first point, 3700 is not high. Currently, it is at 9000 iterations, and continuity, momentum and energy are still at 10^-1. MFR is oscillating between reversed flow from the outlets and back.

Are you using a targeted mass flow rate? Instead, use a fixed pressure inlet and fixed pressure outlet. This helps a lot.
I am employing a fixed pressure inlet and outlet conditions. I tried the targeted MFR earlier to see if convergence was better, but made it worse.

I hope you've already tried starting with a slightly simpler simulation (constant properties, or only temperature dependent properties) before going to the full blown NIST real gas.
Actually, I tried this with air, with no solution convergence issues. This is great advice, I tried to short myself and probably wasted time. I will try an ideal gas model right away, and post back here.

Using a predefined lookup table also speeds up the computation tremendously if you need to shave some compute hours.
I do use a pre-defined lookup table as advised in the real-gas information via ANSYS.

Thanks LuckyTran!
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Old   May 21, 2016, 12:59
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Dear Lexicon

Does it work with you?
if yes kindly advice with any useful tips since i'm working on 2d-axisymmetry modeling using NIST real gas for R134a and i'm not getting any convergence
also i'm getting REFPROP_error (203) from function: tprho (density)
[TPRHO error 203] vapor iteration has not converged

if anyone can help i will be thankful

Best Regards
Moh'd
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