convection heat transfer coefficient

ehsanizad · February 6, 2014, 15:29

Hi

how can I obtain convection heat transfer coefficient in channel for constant temperature wall BC using CFX. To calculate h in internal flow we need to have bulk temperature at even cross section but

it seems CFX cannot predict bulk temperature.

ghorrocks · February 6, 2014, 17:53

There is an expert parameter so the h is referenced to a bulk temperature you define. Search the forum to find it.

Alternately you can use the wall heat flux variable available in the post processor and your own reference temperature and work it out. You could define it as a variable and have it available for post processing.

singer1812 · February 6, 2014, 18:23

Glenn's second suggestion is often much more useful. The problem with the expert parameter is that the bulk temperature has to be constant (not an expression as to pull a different bulk from different locations). Using his latter suggest allows bulk temperature to be pulled from CFX in a manner consistent with text books.

ehsanizad · February 7, 2014, 02:51

Thanks for your reply
Please explain more

ehsanizad · February 7, 2014, 03:03

h=q''/(Twall-Tbulk),

Tbulk will vary along the channel length

ghorrocks · February 7, 2014, 04:53

You can define Tbulk to be anything you like, and it does not need to constant. If you want it to vary along the channel length then make Tbulk a function of position and calculate h from that.

ehsanizad · February 7, 2014, 05:36

How can I do that in CFX?

best

ghorrocks · February 7, 2014, 06:06

Define a new variable in CFD-Post, defined as CEL expression Wall Heat Transfer/(T-Tbulk). Then you can define Tbulk as a constant (eg 280[K]), or as a function of distance (Tbulk = 300[K] + 10[K/m]*x).

If you do not know how to do CFD-Post variables have a look in the CFX tutorials.

February 6, 2014, 15:29	convection heat transfer coefficient	#1
ehsanizad New Member ehsan izadpanah Join Date: Dec 2013 Posts: 4 Rep Power: 12	Hi how can I obtain convection heat transfer coefficient in channel for constant temperature wall BC using CFX. To calculate h in internal flow we need to have bulk temperature at even cross section but it seems CFX cannot predict bulk temperature.

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February 6, 2014, 17:53		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	There is an expert parameter so the h is referenced to a bulk temperature you define. Search the forum to find it. Alternately you can use the wall heat flux variable available in the post processor and your own reference temperature and work it out. You could define it as a variable and have it available for post processing.

February 6, 2014, 18:23		#3
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Glenn's second suggestion is often much more useful. The problem with the expert parameter is that the bulk temperature has to be constant (not an expression as to pull a different bulk from different locations). Using his latter suggest allows bulk temperature to be pulled from CFX in a manner consistent with text books.

February 7, 2014, 02:51		#4
ehsanizad New Member ehsan izadpanah Join Date: Dec 2013 Posts: 4 Rep Power: 12	Thanks for your reply Please explain more

February 7, 2014, 03:03		#5
ehsanizad New Member ehsan izadpanah Join Date: Dec 2013 Posts: 4 Rep Power: 12	h=q''/(Twall-Tbulk), Tbulk will vary along the channel length

February 7, 2014, 04:53		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You can define Tbulk to be anything you like, and it does not need to constant. If you want it to vary along the channel length then make Tbulk a function of position and calculate h from that.

February 7, 2014, 05:36		#7
ehsanizad New Member ehsan izadpanah Join Date: Dec 2013 Posts: 4 Rep Power: 12	How can I do that in CFX? best

February 7, 2014, 06:06		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Define a new variable in CFD-Post, defined as CEL expression Wall Heat Transfer/(T-Tbulk). Then you can define Tbulk as a constant (eg 280[K]), or as a function of distance (Tbulk = 300[K] + 10[K/m]*x). If you do not know how to do CFD-Post variables have a look in the CFX tutorials.