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Free stream temperature in tube wall

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Old   November 15, 2012, 08:55
Default Free stream temperature in tube wall
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sa
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Hi.
I have a question. I am trying to solve the mixed convectionin a Chemical vapor deposition reactor. The reactor has top and bottom plates and inlet and outlet tubes. The reactor is heated by top smaller rectangular plate embodying two heating rod. the top plate is at 250 oc and the bottom plate is at 150 oc. The temperature at top and bottom plate is controlled by the Omnic temperature controller and the heater built in. The temperature controller did a good job to remain constant temperatures of top and bottom plate.As for the tube walls ,the fluid temperature which enters the pipe( tube wall) increases from 26 C up to about 154 C while passing through the pipe. Considering the fact that the length of the pipe is only about 6 mm, this temperature difference seems really huge (128 C). when I got to Ansys Fluent, boundary condition, in tube wall zone,it shows me thermal condition ( heat flux, temperature, convection, radiation,mixed). So because the temperature is not constant at the tube wall and there is not radiation, and the heat flux is not constant, so I chose convection.
When I went to convection, it asked me to get heat transfer coefficient, free stream temperature and heat generation rate.
My question is that I don't know how to get free stream temperature and what this temperature means.Would you please give me some advice.
the other thing is that I put the outlet pressure in "outflow" in Boundary condition. because I was not sure about the pressure. is it OK? I wanted to send my model picture here but I don't know how to do it.
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Old   November 15, 2012, 13:18
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Lucky Tran
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Is your rod embedded in the flow-field? If so, then you do not need to specify any boundary conditions there but rather an interface condition. A picture will help us understand.

Outflow boundary will work.

The convection boundary condition is for when you are imposing or specify the actual convection heat transfer occurring on that wall. Hence you need to specify the heat transfer coefficient and freestream temperature (just write down Newton's law of cooling where htc is defined and it will be immediately apparent). You do not want to be using this boundary condition unless you know the htc beforehand. If you are trying to simulate the flow and then arrive at the htc, you need be using a wall temperature or heat flux type boundary condition instead. Note that the "options" provided in fluent are for constants temperature or heat flux, but you can also specify profiles with some/lots of additional work.
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Old   November 19, 2012, 08:32
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Thank you so much for your response. Actually The rods are placed on the top plate and the fluid is flowing from tube wall and then comes to reactor that hast top and bottom plate. So the fluid is between top and bottom plate. So the rod is not embedded in the flow-field. As for tube wall, actually the temperature of the tube wall is not constant,because of this, I put this in convection boundary condition. So in this case, how can I get stream temperature? by the way, the fluid is flowing in x direction and if want to get the velocity in z( the opposite of gravity) direction, how can I get it? As for the picture, I would like to send it but I don't know how can I do it?

Thanks a gain





Quote:
Originally Posted by LuckyTran View Post
Is your rod embedded in the flow-field? If so, then you do not need to specify any boundary conditions there but rather an interface condition. A picture will help us understand.

Outflow boundary will work.

The convection boundary condition is for when you are imposing or specify the actual convection heat transfer occurring on that wall. Hence you need to specify the heat transfer coefficient and freestream temperature (just write down Newton's law of cooling where htc is defined and it will be immediately apparent). You do not want to be using this boundary condition unless you know the htc beforehand. If you are trying to simulate the flow and then arrive at the htc, you need be using a wall temperature or heat flux type boundary condition instead. Note that the "options" provided in fluent are for constants temperature or heat flux, but you can also specify profiles with some/lots of additional work.
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