# Fixed Wall Temperature Pipe With Odd Results

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 June 13, 2015, 14:15 Fixed Wall Temperature Pipe With Odd Results #1 Member   Kevin Hoopes Join Date: Oct 2010 Posts: 40 Rep Power: 7 I am simulating air flow through a pipe. I am setting the wall temperature at 20 [K] less than the inlet temperature. My issue is that the massflow average temperature steadies out near the end of the pipe, and has the logarithmic shape axial distribution that I anticipated, but it steadies out at about 0.5 [K] above the wall temperature, I thought that it would approach the wall temperature and the delta T would approach 0. Is massflow average the correct temperature to use? Also, my mach number is ~0.05, should I use thermal or total energy, or does it mater? The flow is laminar. RE based on pipe diameter of about 400. Thanks, Kevin

 June 13, 2015, 15:39 #2 Senior Member   Join Date: Jun 2009 Posts: 290 Rep Power: 9 May I ask what is odd in the results ? The fluid will reach the wall temperature at the "infinite length" of the pipe. Is the pipe long enough for the fluid to reach uniform temperature across the section of the pipe. Otherwise, there is a radial profile, and the mass flow average will never be Twall. Hope the above helps, sircorp likes this.

June 14, 2015, 23:31
#3
Member

Kevin Hoopes
Join Date: Oct 2010
Posts: 40
Rep Power: 7
Quote:
 Originally Posted by Opaque May I ask what is odd in the results ? The fluid will reach the wall temperature at the "infinite length" of the pipe. Is the pipe long enough for the fluid to reach uniform temperature across the section of the pipe. Otherwise, there is a radial profile, and the mass flow average will never be Twall. Hope the above helps,
I think what I do not like about the results is that the temperature difference between the wall and the massflow average temperature does not get to zero, but the axial temperature gradient does get to zero. So there is still a 0.5K dt between wall and fluid, but there is no temperature drop in the fluid.

If I am understanding you correctly you are saying maybe it just needs more time to get closer to the wall temp, but I think what I am seeing is that the temperature is not changing anymore.

This leads me to get a zero htc near the end of the tube since I have a finite delta t (from mass flow average) and a vanishingly small q (from enthalpy change). Is this the correct approach to calculate htc?

Kevin

 June 15, 2015, 03:54 #4 Senior Member   Thomas MADELEINE Join Date: Oct 2014 Posts: 122 Rep Power: 2 how do you get these result ? (with a plane, a curve ?) For me supposing you have the average temperature over a small area (massFlowAve), and you get that area closer and closer to the wall it will get closer and closer to your wall temperature. But your fluid is only at wall temperature at the first integration point of your first cell (in wall direction) so if you want to see the exact wall temperature I will not use an massFlowAve function but directly the temperature (along a radius by example).

 June 15, 2015, 05:56 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,809 Rep Power: 85 Are you looking at the total temperature? That will be higher that the static temperature, and the static temperature should match the wall temperature. sircorp likes this.

June 15, 2015, 07:58
#6
Member

Shane
Join Date: Oct 2009
Posts: 52
Rep Power: 7
Quote:
 Originally Posted by khoopes I am simulating air flow through a pipe. I am setting the wall temperature at 20 [K] less than the inlet temperature. My issue is that the massflow average temperature steadies out near the end of the pipe, and has the logarithmic shape axial distribution that I anticipated, but it steadies out at about 0.5 [K] above the wall temperature, I thought that it would approach the wall temperature and the delta T would approach 0. Is massflow average the correct temperature to use? Also, my mach number is ~0.05, should I use thermal or total energy, or does it mater? The flow is laminar. RE based on pipe diameter of about 400. Thanks, Kevin
Kevin

Did you try to set facet vector temp rather average ? Also near the end of the pipe refine the mesh(Adapt) & check it if you still get 0.5K difference. As the sample size(data points) increases, average may get better.

Shane

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