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how to solve gravity in a chimney

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Old   June 28, 2016, 09:58
Default how to solve gravity in a chimney
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Peter Sliacky
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Hello there,

i want to simulate the natural depression in a chimney. The known conditions are the inlet-temperatur @ 553K and the mass-flow @ 0.12g/s. Also the Material Properties are known for heat transfer between fluid and chimney walls. The temperature outside should be 288K. The chimney is 8,6m high.

- mesh is valid
- k-o SST - Turbulennce is set
- pref is 101325Pa
- ro,ref is 1,23Kg/m3 (@288K)
- altitude @ +z = 8,6m, becaus hier is the static and piezo pressure equal
- mass flow inlet with given T and mass flow
- pressure outlet @ 0Pa and 288K
- steady
- coupled flow

so i tried a lot of things to achieve a good solution, but the problem is that i receive always backflows into the inner of the chimney. I read something about to define the boundaries with field function, but i dont know how to do this. It would be great if someone has an answer

best regards and thank you a lot
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Old   June 28, 2016, 10:15
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kevin alun
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if you want to simulate the chimney effect than I don't understand why you would specify the flow rate or are you trying to get the heat transfer from air to wall. What is your goal of the simulation? Maybe you should be running pressure-pressure

Getting backflow is a common thing in a natural convection problem, you might want to simulate the space around the outlet of the chimney and use larger and larger cell sizes as you move away from the exit of the chimney.
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Old   June 28, 2016, 10:49
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Peter Sliacky
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thanks for answering.

the goal of the simualtion:
- depression at inlet
- outlet temperature
@ given parameters temp. and mass flow

Genrally it is an thesis an my prof give me the inputs. Its based on normal households. I have to find out if the depression is big enough and check the temperatur at the outlet because of condensation.

Is it possible without space?
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Old   June 29, 2016, 03:24
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Perhaps try this, for the outlet assign it to mass flow rate and input a negative value, for the inlet assign it as pressure, static zero, during the run you will get the error message reverse flow on pressure boundary but that is OK it is correct, I believe you can turn that warning message off.

You will have to assign a value for temperature on your mass flow rate boundary but I don't think it will matter because the flow is leaving the domain,

for the pressure boundary you also assign a temperature and because the pressure boundary has reverse flow it will use that temperature,
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Old   June 29, 2016, 06:33
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Peter Sliacky
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Hi,

I tried to change Input and Output with your given response. Now there is no backflow at output, but the flow at input does not work correctly, when you think of the pyhsical relations.

So what im now going to do is the following: i want to create space behind output. Therefore i increased the fluid region. I put an extrusion of 2.5m x2.5m x 20m on the output. So now my Ouput is on the top of the extrusion. I defined the other sufraces as walls @ the temperature of the enviroment 288K.
Is it right to increase space in that way?

Thank you a lot helping me
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Old   June 29, 2016, 06:55
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I thought you didn't want to include any space thus I suggested a negative inlet, but if what you described by adding more space for the outlet that is also a good way forward
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Old   June 29, 2016, 07:23
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yeah, first i dont want to, but the results with your given response were not correct. So now im trying to use a greater space @ outlet.

Do you think its ok, how i model space? or how would you define space behind outlet?
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Old   June 29, 2016, 08:02
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If your former outlet is centered where a box sits on top, which is 2.5m by 2.5m which is 20m high that sounds good,
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