Heat loss through wall to surroundings, wall Heat flux

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 December 20, 2017, 11:02 Heat loss through wall to surroundings, wall Heat flux #1 Member   VB Join Date: Jul 2016 Posts: 35 Rep Power: 3 I need to calculate the Heat loss to the surroundings by an Insulation wall. The insulation is solid domain. one side is in contact with the heat source and the other side(ie wall) is exposed to ambient conditions. I use the following two options in CFX-Pre. Why are they both give different values? additionaly I see 'Wall heat Flow' variable in cfx - pre variables, what is that? 1. Heat loss = (areaAve(T)@walls-ambtemp)*htc*surface area of the exposed wall 2. Heat loss = areaInt(Wall Heat Flux)@walls @walls - this refers to the area of Insulation exposed to atmosphere. I want to observe the heat loss during solver run as I need this data for every time step.

 December 20, 2017, 17:30 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,194 Rep Power: 109 Be careful with the heat transfer coefficient. By default it does not use your ambient conditions as the reference condition, but your equation references it against ambient. You need to set an expert parameter for the HTC to be referenced to a defined temperature.

December 20, 2017, 18:29
#3
Member

VB
Join Date: Jul 2016
Posts: 35
Rep Power: 3
Quote:
 Originally Posted by ghorrocks Be careful with the heat transfer coefficient. By default it does not use your ambient conditions as the reference condition, but your equation references it against ambient. You need to set an expert parameter for the HTC to be referenced to a defined temperature.
Thank you for the response.
In the above equation: amptemp= 25 degC. htc = 10 W m-2 K-1. I am not taking system variables. I defined above two as values using expressions. In this case why am I getting difference values for the above two formulas? and also any info. on Wall Heat flow available in CFX Pre variables?

 December 20, 2017, 18:43 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,194 Rep Power: 109 Oh, I see. You are defining the HTC as the external wall of a solid domain, not as an internal fluid boundary. How big is the difference between the two heat flows? Note that areaAve(T) will give you the area averaged temperature of the control volumes, but areaInt(Wall Heat Flux) integrates over the integration points which is a finer resolution. For a fine mesh the difference should be very small.

December 20, 2017, 19:36
#5
Member

VB
Join Date: Jul 2016
Posts: 35
Rep Power: 3
Quote:
 Originally Posted by ghorrocks Oh, I see. You are defining the HTC as the external wall of a solid domain, not as an internal fluid boundary. How big is the difference between the two heat flows? Note that areaAve(T) will give you the area averaged temperature of the control volumes, but areaInt(Wall Heat Flux) integrates over the integration points which is a finer resolution. For a fine mesh the difference should be very small.
12832 [J], -21167[J] for 1st formula and 2nd formula. "-" sign for the second formula is because of Heat flux term, like heat is going out. There is significant difference. regarding areAve(T), I use this areaAve(T)@exposed wall. So, will not take the average temperature of the wall/surface area exposed instead of the whole domain?

Which formula gives less error or values close to real?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post karmavatar CFX 20 March 20, 2016 09:44 mpeppels CFX 10 June 16, 2015 15:48 Moinul Haque CFX 4 November 25, 2014 18:30 Yr0gErG FLUENT 3 June 12, 2013 02:12 Joee FLUENT 1 August 21, 2010 12:20

All times are GMT -4. The time now is 21:54.

 Contact Us - CFD Online - Privacy Statement - Top