Same results for different ambient temperature in ANSYS CFX

jinuvarghese · July 17, 2018, 05:58

Hi all,

Please help me to resolve an issue that I got with the same results for two different conditions for CHT analysis in ANSYS CFX.

Initailly, i did a CHT analysis in cfx with the ambient temperature as 105C. Then i repeated the analysis with the same setup with the ambient temperature as 20C, but i ended up getting the same temperature rise exactly the same as that of first one.

My setup consists of a closed enclosure (top and bottom cover), inside it both the PCB and ICs are placed, inside air is also modeled. Cooling channel is provided on top cover, diesel at -6.11 c is flowing through it.
Mass flow rate of diesel is 0.0019Kg/s (laminar flow)
Outside temperature of top and bottom cover is set as ambient temperature.
is this is the correct way to define the operating conditions?

kindly help me resolving this.

Opaque · July 17, 2018, 21:02

If you refer to outside temperature, I must assume you are using a Heat Transfer Coefficient boundary condition; therefore, you have not specified which value for the HTC you are using.

The solution will be different if the heat flux through those boundaries is meaningful, if it is nearly zero, I would expect the same solution.

jinuvarghese · July 18, 2018, 01:15

Thanks for your reply.
Yes it is heat transfer coefficient boundary condition.
I dont have any airflow on outside so i defined h value as 0.01W/m2K.
Enclosure material is aluminium.

evcelica · July 18, 2018, 18:23

0.01 W/m^2/K is way too low of a coefficient. Natural convection would usually be more like 5 W/m^2/K.

This low of a heat transfer coefficient means is basically does nothing, so yeah, I'd expect the same result regardless of ambient temperature setting.

jinuvarghese · July 19, 2018, 01:46

Thanks to Opaque and evcelica.
When I changed the heat transfer coefficient to 5W/m^2K it is working,getting different results for different ambient temperature.
Thanks...

July 17, 2018, 05:58	Same results for different ambient temperature in ANSYS CFX	#1
jinuvarghese New Member Jinu Varghese Join Date: Aug 2016 Posts: 8 Rep Power: 10	Hi all, Please help me to resolve an issue that I got with the same results for two different conditions for CHT analysis in ANSYS CFX. Initailly, i did a CHT analysis in cfx with the ambient temperature as 105C. Then i repeated the analysis with the same setup with the ambient temperature as 20C, but i ended up getting the same temperature rise exactly the same as that of first one. My setup consists of a closed enclosure (top and bottom cover), inside it both the PCB and ICs are placed, inside air is also modeled. Cooling channel is provided on top cover, diesel at -6.11 c is flowing through it. Mass flow rate of diesel is 0.0019Kg/s (laminar flow) Outside temperature of top and bottom cover is set as ambient temperature. is this is the correct way to define the operating conditions? kindly help me resolving this.

July 17, 2018, 21:02		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	If you refer to outside temperature, I must assume you are using a Heat Transfer Coefficient boundary condition; therefore, you have not specified which value for the HTC you are using. The solution will be different if the heat flux through those boundaries is meaningful, if it is nearly zero, I would expect the same solution. jinuvarghese likes this.

July 18, 2018, 18:23		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	0.01 W/m^2/K is way too low of a coefficient. Natural convection would usually be more like 5 W/m^2/K. This low of a heat transfer coefficient means is basically does nothing, so yeah, I'd expect the same result regardless of ambient temperature setting. jinuvarghese likes this.

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July 18, 2018, 01:15		#3
jinuvarghese New Member Jinu Varghese Join Date: Aug 2016 Posts: 8 Rep Power: 10	Thanks for your reply. Yes it is heat transfer coefficient boundary condition. I dont have any airflow on outside so i defined h value as 0.01W/m2K. Enclosure material is aluminium.

July 19, 2018, 01:46		#5
jinuvarghese New Member Jinu Varghese Join Date: Aug 2016 Posts: 8 Rep Power: 10	Thanks to Opaque and evcelica. When I changed the heat transfer coefficient to 5W/m^2K it is working,getting different results for different ambient temperature. Thanks...