Pipe Flow Heat Transfer

Saima · January 17, 2011, 07:10

Hello,

I am working on a pipe flow(hot air, compressible) for which i have boundary conditions:
Pin = 533 Kpa
Tin = 700 K
mass flow rate: 0.68 kg/sec (outlet)

This pipe is insulated witha another material. I know k =0.70, and outside temperature in 343 K.

Now i have to figure out wat will be the surface temperature after applying this insulation.

I did some basic calculation from heat transfer book (conduction+convection) for pipe in radia direction and found TSur = 400K., its like convection...conduction ...and then convection

But when i tried this in CFX by giving the above boundary conditions plus wall with heat transfer coefficient (h = 1/R: from analytical convection+conduction), it gave me surface temperature 698K.

1. Am i applying the right boundary condition for Heat Transfer on wall?

2. Do i have to do meshing for insulation material as well?

What will be the right way to applying heat transfer boundary condition for hot air inside pipe pluse insulated wall and then natural convection?

Kindly guide me.

Regards,

ghorrocks · January 17, 2011, 16:55

I would model this including the pipe and insulation (at least initially) so you do not have to specify a boundary condition on the inside of the pipe. On the outside of the pipe you can specify a combined convection and radiation boundary - CFX does not support this but are pretty easy to derive and apply.

You may be able to ignore radiation on the inside of the pipe providing the pipe internal temperature is constant enough. If this is the case the radiation will cancel out and can then be ignored.

Saima · January 30, 2011, 02:50

yes, i calculated combined (Cond+Convection) cefficient and i can specify outer temperature (environment) as well.

I am taking SST and total energy model is it ok?

When i did hand calculation it gave me Tsur = 477 K. I took insulation as well.

But after solving problem in CFX it was not give me that temperture, it is around 655K. In CFX i have include insulation but when i am given h comined it has effect of both.

Kindly let me Is this the right way?

Regards.

ghorrocks · January 30, 2011, 04:41

Are you sure your simulation is accurate? See http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Are you sure your hand calculation is accurate?

Saima · January 30, 2011, 08:52

Hello,

I did hand calculation from heat transferbook by taking cylinder in layers (Conduction+Convection) and for sure that are accurate.

Is it possible, i cans end you cfx file by email.

Regards,

ghorrocks · January 30, 2011, 16:41

Quote:

and for sure that are accurate.

Um, right. What did you use for the convection coefficient? How accurate was that?

Please don't send me the files, I don't have time to look at them.

January 17, 2011, 07:10	Pipe Flow Heat Transfer	#1
Saima Senior Member Saima Join Date: Apr 2009 Location: Canada Posts: 185 Rep Power: 16	Hello, I am working on a pipe flow(hot air, compressible) for which i have boundary conditions: Pin = 533 Kpa Tin = 700 K mass flow rate: 0.68 kg/sec (outlet) This pipe is insulated witha another material. I know k =0.70, and outside temperature in 343 K. Now i have to figure out wat will be the surface temperature after applying this insulation. I did some basic calculation from heat transfer book (conduction+convection) for pipe in radia direction and found TSur = 400K., its like convection...conduction ...and then convection But when i tried this in CFX by giving the above boundary conditions plus wall with heat transfer coefficient (h = 1/R: from analytical convection+conduction), it gave me surface temperature 698K. 1. Am i applying the right boundary condition for Heat Transfer on wall? 2. Do i have to do meshing for insulation material as well? What will be the right way to applying heat transfer boundary condition for hot air inside pipe pluse insulated wall and then natural convection? Kindly guide me. Regards, Last edited by Saima; January 17, 2011 at 07:29.

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January 17, 2011, 16:55		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I would model this including the pipe and insulation (at least initially) so you do not have to specify a boundary condition on the inside of the pipe. On the outside of the pipe you can specify a combined convection and radiation boundary - CFX does not support this but are pretty easy to derive and apply. You may be able to ignore radiation on the inside of the pipe providing the pipe internal temperature is constant enough. If this is the case the radiation will cancel out and can then be ignored.

January 30, 2011, 02:50		#3
Saima Senior Member Saima Join Date: Apr 2009 Location: Canada Posts: 185 Rep Power: 16	yes, i calculated combined (Cond+Convection) cefficient and i can specify outer temperature (environment) as well. I am taking SST and total energy model is it ok? When i did hand calculation it gave me Tsur = 477 K. I took insulation as well. But after solving problem in CFX it was not give me that temperture, it is around 655K. In CFX i have include insulation but when i am given h comined it has effect of both. Kindly let me Is this the right way? Regards.

January 30, 2011, 04:41		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Are you sure your simulation is accurate? See http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F Are you sure your hand calculation is accurate?

January 30, 2011, 08:52		#5
Saima Senior Member Saima Join Date: Apr 2009 Location: Canada Posts: 185 Rep Power: 16	Hello, I did hand calculation from heat transferbook by taking cylinder in layers (Conduction+Convection) and for sure that are accurate. Is it possible, i cans end you cfx file by email. Regards,