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Solar heater flow rate, natural convection convergence

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Old   March 23, 2017, 12:24
Default Solar heater flow rate, natural convection convergence
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Thomas
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Hello there,

I find this forum very helpful and I will try to contribute more in the future.
Meanwhile, I have a simple solar water heater problem and I'd appreciate if anyone could help.

It's a natural convection flow inside an inclined rectangular cavity (see the attached picture for the result and boundary conditions).

I have doubts in my solution for the following 2 reasons:
1) the solution did not properly converge after 8000 iterations (see picture). Gradually increasing gravity and reducing under-relaxation factors as suggested somewhere in this forum did not help either.
2) assuming that the achieved convergence is enough to get a rough estimate of the answer, the velocity inside the cavity is relatively high = 0.04 m/s (average), giving a mass flow rate of:

m(dot) = rho*A*v = 1000*0.007*0.04=0.28 kg/s

Thermosyphonic flow in the literature reaches around 0.02 kg/s, which is 10 times less. However, the examples that I refer to are using tubes with different area. If I calculate the velocity of 0.02 kg/s mass flow rate in the tube of 2.5cm diameter, the velocity is around 0.04 m/s, which is like the value that I get.

Therefore, I am sort of right, saying that such mass flow rate is possible, considering a larger area of mine. However, the convergence makes me doubt my solution.

Can I consider my simulation correct (at least within one order of magnitude)?

cfd.jpg
8000.png

Thank you in advance.
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