HTC Calculations in a Helical Coil

Thakar · February 3, 2015, 08:00

I have performed hand calculations of HTC and Outlet Temperature in a Helical coil using equations from Heat Atlas(Gnielinski) and have obtained HTC:4739 W/m2k and T_outlet:52C.I am now trying to validate these results with ANSYS CFX.I have read all the posts related to this query and yet left with some questions.

My V_inlet:0.5m/s,T_inlet:10 C,T_wall:100 C,Tbulk for HTC:304K with Laminar flow as the Reynold number is less than the critical in the hand calculations.

My results are varying hugely with Inflation layers and its Growth rate(Eg: 2794 W/m2K and 37 C for 6 layers inflation to 3498 W/m2K and 45 C for 10 layers and 1,4 growth rate for both) and unable to get close to the results abtained from hand calculations.I find these results surprising becase it a laminar flow with Reynolds number:6800.Am I missing anything else?Thanks in advance

Sorry for so many questions...I am a beginner with CFD and CFX.

Opaque · February 3, 2015, 12:13

HTC calculations are based on a temperature difference between the wall and a reference value.

What reference value are you using for CFX ? Read output file for warning message regarding this issue.

How does the reference value used for CFX compares with the used in the hand calculations ?

HTC calculations (by hand or any software) are useless w/o the complementary definition of the reference temperature (named bulk temperature, free stream, etc). The only quantity that is unchanged is the heat flux since it should always be computed as:

heat flux = HTC_for_Tref * (T_wall - T_ref)..

In CFX,

heat_flux = HTC_for_WAT * (T_wall - Wall Adjacent Temperature)

Hope the above helps,

Thakar · February 4, 2015, 04:46

I have accidentally double posted the same question.

From the previous posts reg this topic I understood that by entering an expert parameter(tbulk for HTC) I can force CFX to take a Temp_ref for calculating HTC otherwise it might take just take the wall Adj temperature.I have chosen my tbulk for HTC as the average of Inlet-Outlet that I have obtained from my hand calculations.Therefore T_Delta is (T_wall-Tbulk for HTC).So my HTC will change only with Heat Flux now as T_Delta is constant.

For my hand calculations (Iterations with approx outlet) I use T_Delta as ((T_wall-T_inlet)-(T_wall-T_approx outlet))/ln((T_wall-T_inlet)/(T_wall-T_approx outlet)).

Please correct me If I am making a mistake.

February 3, 2015, 08:00	HTC Calculations in a Helical Coil	#1
Thakar New Member Thakar Join Date: Feb 2015 Posts: 4 Rep Power: 11	I have performed hand calculations of HTC and Outlet Temperature in a Helical coil using equations from Heat Atlas(Gnielinski) and have obtained HTC:4739 W/m2k and T_outlet:52C.I am now trying to validate these results with ANSYS CFX.I have read all the posts related to this query and yet left with some questions. My V_inlet:0.5m/s,T_inlet:10 C,T_wall:100 C,Tbulk for HTC:304K with Laminar flow as the Reynold number is less than the critical in the hand calculations. My results are varying hugely with Inflation layers and its Growth rate(Eg: 2794 W/m2K and 37 C for 6 layers inflation to 3498 W/m2K and 45 C for 10 layers and 1,4 growth rate for both) and unable to get close to the results abtained from hand calculations.I find these results surprising becase it a laminar flow with Reynolds number:6800.Am I missing anything else?Thanks in advance Sorry for so many questions...I am a beginner with CFD and CFX.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
helical coil heat exchanger	sarsam.9296	FLUENT	3	January 14, 2016 03:02
Creating section along the length of helical coil	tshv247	Visualization & Post-Processing	0	July 10, 2014 09:16
Helical coil	E.M.coil	FLUENT	0	May 9, 2014 15:14
[DesignModeler] Best way to model helical coil	ritodip	ANSYS Meshing & Geometry	0	March 27, 2014 00:34
Periodic Boundary for helical coil	nilhas	FLUENT	1	December 28, 2012 11:35

February 3, 2015, 12:13		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	HTC calculations are based on a temperature difference between the wall and a reference value. What reference value are you using for CFX ? Read output file for warning message regarding this issue. How does the reference value used for CFX compares with the used in the hand calculations ? HTC calculations (by hand or any software) are useless w/o the complementary definition of the reference temperature (named bulk temperature, free stream, etc). The only quantity that is unchanged is the heat flux since it should always be computed as: heat flux = HTC_for_Tref * (T_wall - T_ref).. In CFX, heat_flux = HTC_for_WAT * (T_wall - Wall Adjacent Temperature) Hope the above helps,

February 4, 2015, 04:46		#3
Thakar New Member Thakar Join Date: Feb 2015 Posts: 4 Rep Power: 11	I have accidentally double posted the same question. From the previous posts reg this topic I understood that by entering an expert parameter(tbulk for HTC) I can force CFX to take a Temp_ref for calculating HTC otherwise it might take just take the wall Adj temperature.I have chosen my tbulk for HTC as the average of Inlet-Outlet that I have obtained from my hand calculations.Therefore T_Delta is (T_wall-Tbulk for HTC).So my HTC will change only with Heat Flux now as T_Delta is constant. For my hand calculations (Iterations with approx outlet) I use T_Delta as ((T_wall-T_inlet)-(T_wall-T_approx outlet))/ln((T_wall-T_inlet)/(T_wall-T_approx outlet)). Please correct me If I am making a mistake.