Bulk temperature expression gives value below inlet temperature for half the pipe

Shomaz ul Haq · June 18, 2017, 17:21

Dear all,

Hello. Hope all are well. I have a question. One that is answered by many posts but I really couldn't find my answer there unfortunately after two weeks. My bulk temperature expression for pipe is

(Inlet Temperature)+((Wall Heat Flux*Pi*Diameter*Z)/(Density*Pi*Radius^2*Velocity*Specific Heat Capacity at Constant Pressure))

where my
Diameter=0.02 m
Inlet Temperature=293 K
Specific Heat Capacity=4048.42 J/kg/K
Velocity=0.022 m/s
Wall Heat Flux=1000 W/m^2

My bulk temperature gives a value less than 20 C by a certain fixed amount for half the length of the pipe and greater than that for the rest half of the pipe. I have started my flow from 0.5 m to -0.5 m.

Is my expression correct? I found it in a reference posted by a senior in this forum. I think so but please check.
My heat transfer coefficient values are coming about to be in the range of 100-150. That is very low as compared to literature for laminar pipe flow at similar Re numbers and ~10-20 mm diameters. When I check default heat transfer coefficient variable values they are from 6000-7000 which I know is correct and due to calculation based on Adjacent Wall Temperature chosen by cfx and is mesh dependent giving a large value as my mesh is relatively refined. But I don't want to use those values as I can't compare them to literature because of their non-applicability. The even stranger thing is that Nu number calculated using the same htc is around 4.364 at the end of the pipe validating my model. So does that mean that my calculation for bulk temperature is wrong? Confused!

Would be grateful if someone could help me out. Hope to hear from someone soon. Thanks.

evcelica · June 19, 2017, 16:47

Make sure your expressions are not pre-defined variables like Velocity, Density, Wall Heat Flux. Your z value should be (z-InletZ).

Shomaz ul Haq · June 19, 2017, 18:58

Although none of my expressions are any variables, my expressions does contain some variables like Wall Heat Flux in expression for h and so on. I think that is inevitable. Did you mean I shouldn't use any variables in my expressions? Didn't understand your advice. Thanks.

Shomaz ul Haq · June 21, 2017, 03:40

Dear all,

Hello. Waiting for your help. Thanks.

June 18, 2017, 17:21	Bulk temperature expression gives value below inlet temperature for half the pipe	#1
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 11	Dear all, Hello. Hope all are well. I have a question. One that is answered by many posts but I really couldn't find my answer there unfortunately after two weeks. My bulk temperature expression for pipe is (Inlet Temperature)+((Wall Heat FluxPiDiameterZ)/(DensityPiRadius^2Velocity*Specific Heat Capacity at Constant Pressure)) where my Diameter=0.02 m Inlet Temperature=293 K Specific Heat Capacity=4048.42 J/kg/K Velocity=0.022 m/s Wall Heat Flux=1000 W/m^2 My bulk temperature gives a value less than 20 C by a certain fixed amount for half the length of the pipe and greater than that for the rest half of the pipe. I have started my flow from 0.5 m to -0.5 m. Is my expression correct? I found it in a reference posted by a senior in this forum. I think so but please check. My heat transfer coefficient values are coming about to be in the range of 100-150. That is very low as compared to literature for laminar pipe flow at similar Re numbers and ~10-20 mm diameters. When I check default heat transfer coefficient variable values they are from 6000-7000 which I know is correct and due to calculation based on Adjacent Wall Temperature chosen by cfx and is mesh dependent giving a large value as my mesh is relatively refined. But I don't want to use those values as I can't compare them to literature because of their non-applicability. The even stranger thing is that Nu number calculated using the same htc is around 4.364 at the end of the pipe validating my model. So does that mean that my calculation for bulk temperature is wrong? Confused! Would be grateful if someone could help me out. Hope to hear from someone soon. Thanks.

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June 19, 2017, 16:47		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	Make sure your expressions are not pre-defined variables like Velocity, Density, Wall Heat Flux. Your z value should be (z-InletZ).

June 19, 2017, 18:58		#3
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 11	Although none of my expressions are any variables, my expressions does contain some variables like Wall Heat Flux in expression for h and so on. I think that is inevitable. Did you mean I shouldn't use any variables in my expressions? Didn't understand your advice. Thanks.

June 21, 2017, 03:40		#4
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 11	Dear all, Hello. Waiting for your help. Thanks.