# Wrong pressure results in the shell side of a shell and coil heat exchanger

When separately simulating each side of a shell and coil heat exchanger, physically correct results are obtained, however, when combining both sides together to simulate the whole system, wrong pressure distribution in the shell side is obtained after convergence!

My questions are:

- How could the presence of the 2 domains together affect results?
- What can be done to fix this?

Summary of used settings:

- All solid and fluid bodies share topology (Conformal mesh)
- Solver: Pressure based - Coupled
- Working fluid in shell: air
- Working fluid in coils: thermal oil (density 850 kg/m^3, viscosity 590e-06 kg/m-s)
- Turbulence model: SST
- Both fluids mass-flow-inlet and pressure-outlet BC
- Each fluid is initialized separately
- Default methods and controls

Note that:

- The same mesh and settings are used when simulating each side separately and when simulating both of them together
- Changing to non-conformal mesh or using SIMPLE pressure-velocity coupling didn't help

## Comments

How are you simulating the combined model? Could you please share some screenshots to help us understand? Please embed them directly into your posts.

Thank you.

Attached here you can find 3 JPG files,

In 1f.JPG you can see the simulation of the shell side only.It shows correct static pressure distribution for air (maximum at inlet and minimum at outlet).

2f.JPG shows the results when the shell and coil sides are simulated together. The thermal oil pressure distribution is correct, however, in 2fRescaled.JPG you can notice how the air pressure distribution becomes incorrect!

I used 2 ways to simulate the combined model:

both ways use the same settings defined in the main post and both of them generate the same problem!

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If you check the flow field (other than pressure) is it as expected?

No all the results for the air side (not only pressure) totally get incorrect when simulated with the thermal oil side.

I pasted the images again here so that you are able to see it.

Pressure distribution for the air side only. Pressure distribution for the whole system scaled for thermal oil pressure.

Pressure distribution for the whole system scaled for air side.

I'd like to add some new info that can help fix the problem. Today I tried to change the thermal-oil to air so that both sides are the same fluid and I got correct results, which means that the problem happens only when the 2 materials are not the same.

I guess this is happening due to some sort of averaging that happens between cells of different fluids which is not supposed to happen. I am not sure which fluid property is causing this problem exactly? Is it the large difference in densities between air and thermal-oil? or is it viscosity?

There must be a way to prevent this from happening.

Any advise?

What boundary conditions and density options are you using?

In the images I sent in the above posts the energy equation was turned off and the exact settings were:

For thermal oil:

For air:

I also tried outflow boundary conditions, its results look nicer but still wrong.

for the final simulation, the energy equation will be turned on and the density, viscosity, specific heat, conductivity and radiation absorption coefficient will all be calculated by piece-wise polynomials.

I think to solve my problem something needs to be done to separate the solution of continuity and momentum for the 2 fluids and to let them only solved together for energy equation. Is there a way to do so?Of course temperatures will affect both continuity and momentum by changing all fluid properties but still solutions of these equations must not interfere with one another.

I don't know also if fluent can deal with cases when the residuals for one fluid converge much faster than the other one and if this may cause a problem?So any suggestions?

Hi,

Sorry, I missed your previous response.

To answer your questions - No, you cannot solve the continuity and momentum equations separately for each fluid. And, for the second question - yes, Fluent should be able to deal with different convergence rates. However, you will only see one single curve for residuals when you solve the two fluids problem.

Having said that, this is not a multiphase simulation you are running, right (just clarifying)?

Also, can you show me the convergence plots (residuals and monitor plots) when you run the two simulations together? What is your overall mesh quality (max skewness and minimum orthogonal quality)?

What is the Reynolds number in your problem - both oil and air sides?

Thanks.

Karthik

Yes this is not a multiphase simulation just 2 different working fluids of a heat exchanger.

Oil Reynolds number = 62000

Air Reynolds number is not applicable but the complicated geometry will force the flow to get turbulent or transitional.

The mesh quality cannot be the reason behind this problem since the solution converges when each fluid is simulated separately using the same boundary conditions, but anyway here are the values just for your info.

Mesh aspect ratio: Max: 98, Average: 5, Standard Deviation: 9

Mesh skewness: Max: 0.99, Average: 0.27, Standard Deviation 0.14

Mesh orthogonal quality: Min: 9.9e-4, Average: 0.7, Standard Deviation: 0.14

I don't have residuals plot because I'm using HPC nodes. Next post you can find a copy of residuals of last few iterations instead. Note in this report the "e-mass" is a monitor for the air outlet mass flow rate (should reach -0.0113kg/s) and "o-vel" is the oil outlet velocity (correct value around 1.15m/s). Notice also how reverse flow at the air outlet insists (this doesn't happen when simulating air separately).

iter continuity x-velocity y-velocity z-velocity k omega intermit retheta e-mass o-vel time/iter

2372 9.2141e-03 2.7577e-05 2.8067e-05 3.6201e-05 4.2041e-03 8.1038e-04 7.7550e-03 1.5072e-05 -1.1524e-02 1.1625e+00 0:01:05 28

Reversed flow on 119 faces (49.0% area) of pressure-outlet 7.

2373 1.1360e-02 3.7969e-05 3.8537e-05 4.9860e-05 3.2498e-03 6.5447e-04 1.1625e-02 1.1547e-05 -1.1577e-02 1.1626e+00 0:01:03 27

Reversed flow on 119 faces (49.0% area) of pressure-outlet 7.

2374 1.0697e-02 3.2648e-05 3.2890e-05 4.2381e-05 3.7730e-03 6.1632e-04 7.7845e-03 9.1525e-06 -1.1573e-02 1.1626e+00 0:01:00 26

Reversed flow on 119 faces (49.1% area) of pressure-outlet 7.

2375 1.1190e-02 3.8597e-05 3.9147e-05 5.4134e-05 3.2822e-03 6.9063e-04 1.2302e-02 1.5363e-05 -1.1655e-02 1.1627e+00 0:00:57 25

Reversed flow on 119 faces (49.1% area) of pressure-outlet 7.

2376 1.1125e-02 3.5669e-05 3.5944e-05 4.5598e-05 3.8605e-03 6.4259e-04 7.6449e-03 1.0010e-05 -1.1688e-02 1.1627e+00 0:00:55 24

Reversed flow on 119 faces (48.8% area) of pressure-outlet 7.

2377 1.1335e-02 3.9644e-05 4.0187e-05 5.6082e-05 3.2271e-03 6.7429e-04 1.2151e-02 1.5464e-05 -1.1785e-02 1.1628e+00 0:00:53 23

Reversed flow on 119 faces (48.8% area) of pressure-outlet 7.

2378 1.1341e-02 3.6850e-05 3.7169e-05 4.7515e-05 3.8157e-03 6.4426e-04 7.5501e-03 1.0847e-05 -1.1787e-02 1.1628e+00 0:00:50 22

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

2379 1.1491e-02 4.0092e-05 4.0776e-05 5.7011e-05 3.2002e-03 6.6529e-04 1.2317e-02 1.4415e-05 -1.1920e-02 1.1629e+00 0:00:48 21

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

2380 1.1453e-02 3.7606e-05 3.7874e-05 4.8981e-05 3.8741e-03 6.6408e-04 7.7514e-03 1.1709e-05 -1.1692e-02 1.1627e+00 0:00:47 20

Reversed flow on 120 faces (49.5% area) of pressure-outlet 7.

2381 8.5417e-03 1.9685e-05 2.0956e-05 2.6071e-05 2.1750e-03 4.7657e-04 1.2825e-02 2.1264e-05 -1.1982e-02 1.1627e+00 0:00:45 19

Reversed flow on 118 faces (48.3% area) of pressure-outlet 7.

2382 9.3098e-03 2.7927e-05 2.8467e-05 3.6966e-05 4.2165e-03 8.0904e-04 7.7451e-03 1.5115e-05 -1.1850e-02 1.1627e+00 0:00:42 18

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

iter continuity x-velocity y-velocity z-velocity k omega intermit retheta e-mass o-vel time/iter

2383 1.1317e-02 3.7945e-05 3.8587e-05 4.9670e-05 3.2519e-03 6.5387e-04 1.1632e-02 1.1543e-05 -1.1868e-02 1.1628e+00 0:00:40 17

Reversed flow on 119 faces (49.1% area) of pressure-outlet 7.

2384 1.0677e-02 3.2646e-05 3.2879e-05 4.2599e-05 3.7854e-03 6.1643e-04 7.7726e-03 9.4815e-06 -1.1908e-02 1.1628e+00 0:00:37 16

Reversed flow on 119 faces (48.8% area) of pressure-outlet 7.

2385 1.1139e-02 3.8524e-05 3.9000e-05 5.3823e-05 3.2806e-03 6.8996e-04 1.2306e-02 1.5705e-05 -1.2009e-02 1.1629e+00 0:00:35 15

Reversed flow on 118 faces (48.3% area) of pressure-outlet 7.

2386 1.1075e-02 3.5661e-05 3.5888e-05 4.5736e-05 3.8477e-03 6.4128e-04 7.6188e-03 1.0554e-05 -1.2021e-02 1.1629e+00 0:00:33 14

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

2387 1.1263e-02 3.9555e-05 4.0050e-05 5.5978e-05 3.2245e-03 6.7269e-04 1.2153e-02 1.6059e-05 -1.2125e-02 1.1630e+00 0:00:30 13

Reversed flow on 118 faces (48.4% area) of pressure-outlet 7.

2388 1.1269e-02 3.6675e-05 3.6909e-05 4.7405e-05 3.7974e-03 6.4279e-04 7.5514e-03 1.1557e-05 -1.2079e-02 1.1630e+00 0:00:28 12

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

2389 1.1405e-02 3.9754e-05 4.0156e-05 5.6480e-05 3.1460e-03 6.6343e-04 1.2299e-02 1.5096e-05 -1.2068e-02 1.1631e+00 0:00:26 11

Reversed flow on 117 faces (47.8% area) of pressure-outlet 7.

2390 1.1386e-02 3.7066e-05 3.7263e-05 4.8453e-05 3.8204e-03 6.6158e-04 7.7695e-03 1.2374e-05 -1.1992e-02 1.1629e+00 0:00:24 10

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

2391 8.5094e-03 1.9466e-05 2.0625e-05 2.5828e-05 2.1649e-03 4.7615e-04 1.2856e-02 2.2677e-05 -1.2154e-02 1.1629e+00 0:00:21 9

Reversed flow on 116 faces (47.3% area) of pressure-outlet 7.

2392 9.2599e-03 2.7686e-05 2.8141e-05 3.6622e-05 4.1889e-03 8.0928e-04 7.7368e-03 1.5160e-05 -1.2002e-02 1.1629e+00 0:00:19 8

Reversed flow on 118 faces (48.3% area) of pressure-outlet 7.

2393 1.1302e-02 3.7607e-05 3.8045e-05 4.9345e-05 3.1918e-03 6.5214e-04 1.1603e-02 1.2124e-05 -1.1992e-02 1.1630e+00 0:00:16 7

Reversed flow on 119 faces (48.9% area) of pressure-outlet 7.

iter continuity x-velocity y-velocity z-velocity k omega intermit retheta e-mass o-vel time/iter

2394 1.0608e-02 3.2059e-05 3.2246e-05 4.1709e-05 3.6890e-03 6.1313e-04 7.7885e-03 1.0183e-05 -1.1899e-02 1.1630e+00 0:00:14 6

Reversed flow on 118 faces (48.4% area) of pressure-outlet 7.

2395 1.1071e-02 3.8132e-05 3.8447e-05 5.3475e-05 3.2071e-03 6.8922e-04 1.2299e-02 1.6369e-05 -1.1815e-02 1.1631e+00 0:00:12 5

Reversed flow on 116 faces (47.3% area) of pressure-outlet 7.

2396 1.1031e-02 3.5161e-05 3.5235e-05 4.4931e-05 3.7846e-03 6.4184e-04 7.6237e-03 1.1207e-05 -1.1687e-02 1.1631e+00 0:00:09 4

Reversed flow on 117 faces (47.8% area) of pressure-outlet 7.

2397 1.1240e-02 3.9228e-05 3.9461e-05 5.5507e-05 3.1630e-03 6.7124e-04 1.2144e-02 1.6575e-05 -1.1602e-02 1.1632e+00 0:00:07 3

Reversed flow on 120 faces (49.3% area) of pressure-outlet 7.

2398 1.1210e-02 3.6306e-05 3.6340e-05 4.6677e-05 3.7583e-03 6.4165e-04 7.5329e-03 1.2121e-05 -1.1521e-02 1.1632e+00 0:00:05 2

Reversed flow on 119 faces (48.8% area) of pressure-outlet 7.

2399 1.1421e-02 3.9725e-05 4.0099e-05 5.6607e-05 3.1535e-03 6.6447e-04 1.2316e-02 1.5506e-05 -1.1465e-02 1.1633e+00 0:00:02 1

Reversed flow on 117 faces (47.8% area) of pressure-outlet 7.

2400 1.1395e-02 3.7083e-05 3.7255e-05 4.8431e-05 3.8384e-03 6.6315e-04 7.7690e-03 1.2790e-05 -1.1793e-02 1.1630e+00 0:00:00 0

A skew of 0.99 and an ortho quality below 0.05 is a problem, you may just have got away with it for the earlier models but running two domains may be enough to cause the problem as the stability functions are now trying to deal with two systems.

But this high skew and low ortho quality values occur only in less than 0.001% of the cells. Does this small % still affect the results that much?!

And if stability functions are the reason, then why do they work fine when both fluids are set to be the same (keeping Reynolds constant as the original case)?! Which means they can deal with 2 systems.

I'd definitely try to improve the overall mesh and see if the two system results improve. As Rob was saying, it is possible that the overall mesh quality might be what is causing the solution to diverge. Please let us know your findings.

Thanks.

Karthik

Is there a way to delete this whole discussion? or delete one of its posts?

Which post would you like us to delete? We can do it for you if you wish to do so.

Thank you.

Karthik

I would like to delete the 2 posts containing screen shots. Both of them were in 21 September. Thank you

Done, note if you're working with a company on a project with an academic licence you MUST review the T&Cs as all work should be publishable.

Thank you Rob. This is required for my master thesis not a company project.

I tried to simulate again with a mesh that doesn't contain inflation layers and has:

Mesh skewness: Max: 0.87, Average: 0.25, Standard Deviation 0.11

Mesh orthogonal quality: Min: 0.127, Average: 0.74, Standard Deviation: 0.107

Of course in this case y+ values are large but I just wanted to see if the same problem happens, and yes it happens unfortunately.

To get closer to identifying the problem I tried the simulation with air and a hypothetical fluid that has the same density as air but the same viscosity as the thermal oil keeping mass flow rate and though Reynolds number the same. Surprisingly, I got a correct result!

which means that the large difference in density(not the difference in viscosity)is probably the source of the problem!Is it advisable to do an extra step in the setup when simulating 2 such systems of large difference in densities?Unfortunately, I didn't find any similar heat exchanger tutorial on the internet that has 2 working fluids of large density differences. It is always either water-water, air-air or water-external air heat exchanger which doesn't apply to my case.

I hope you can help me on that.

Thank you