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March 9, 2011, 19:34 
No Slip Wall with Fixed Temperature Boundary Condition

#1 
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Hi,
I am using CFX to model a 3D turbine cascade. My ultimate objective is to compute the heat transfer coefficient distribution at the blade surface. The flow is subsonic compressible so I am using "Total Energy" and "Air Ideal Gas" for the working fluid. I am also using the SST turbulence model, with an Ogrid mesh around the blade, and a small distance of the first node away from the wall in order to obtain a yplus of around 3. Since I would like to compute the heat transfer coefficient distribution at the blade surface, I defined the blade walls, to be no slip and applied a fixed temperature of 296K. However, as I postprocess the simulation and export the variables at the blade loading line, I do not obtain a temperature of 296K at the blade wall. How can that be since I did specify a temperature? I would really appreciate any comments you may have. I would also benefit from the experience of those who did heat transfer calculations in a turbine cascade. Thank you in advance 

March 9, 2011, 19:37 

#2 
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Glenn Horrocks
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If you correctly defined a fixed temperature then they have the temperature you defined. Either you set it up wrong or you are post processing it wrong.


March 9, 2011, 19:44 

#3 
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Dear Glenn,
Thanks for the answer. First of all there are not so many ways of specifying a temperature at the blade. Under "Boundary Details" > "Heat transfer" option, I choose "Temperature" and apply a fixed temperature of 296K. So it seems plausible to me that I am post processing it wrong. I have a question concerning that. What is the "Blade Loading Line" in CFX? You see what I do in CFD post in that I go to " File > Export for the Location, I choose "Blade Loading Line" for the Boundary Data, I choose "Current" then I select all the variable I need (of which the temperature) and I save it to a .csv file. Are you familiar with this procedure? Would you know of any other way for computing variables at the blade surface, at a certain span location? Thanks 

March 10, 2011, 09:54 

#4 
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You're probably exporting conservative values rather than hybrid values. See the CFX doc for the difference.


March 10, 2011, 12:05 

#5 
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Dear Stumpy,
You are correct. I was exporting conservative values! Thank you! 

March 10, 2011, 16:28 
Heat Transfer Coefficient in Compressible Flow 3D turbine cascade

#6 
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Hi,
I am using CFX to model a 3D turbine cascade. My ultimate objective is to compute the heat transfer coefficient distribution at the blade surface. The flow is subsonic compressible so I am using "Total Energy" and "Air Ideal Gas" for the working fluid. CFD Post outputs data for two particular variables. The "wall heat flux (q) " and the "wall heat transfer coefficient (h) ". From the CFX manual, I understand that these two are related by: h = q / (T_wall  T_adjacentwall) These values are very high and not in agreement with experimental data. I tried to compute the wall heat flux myself by using the following: q = k * (Twall  Tadjacent wall) / y where k is the thermal diffusivity Twall is the wall temperature (which I had specified as a boundary condition) Tadjacent wall is the temperature of the first node away from the wall. and y is the distance of the first node away from the wall to the wall itself (this value I defined when I was creating the Ogrid mesh around the airfoil in ICEM CFD) Still I see that the variable are underpredicted compared to experimental data and the trend of heat transfer distribution with streamwise direction on the blade surface is not smooth and fluctuating. I would appreciate any comments and knowledge you can share about computing the heat transfer coefficient in compressible flow. Thank you so in advance. 

March 10, 2011, 17:46 

#7 
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If you are comparing to experimental data then their HTC is likely based on a reference temperature other than Tadjacent wall. You should use the expert parameter "tbulk for htc" to set a reference temperature for the HTC calculation.


March 10, 2011, 18:23 

#8 
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Do you know where I can specify tbulk?


Tags 
blade, fixed temperature, heat transfer coefficient, wall 
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