Balance of Heat Transfer

AKHILESH S L · November 16, 2013, 04:28

i am modelling a heat exchanger of concentric tube type with counter flow. temperature difference is obtaining for the two fluids. but when calculating the total Q (Q=m*Cp*delta(t)) it is not equal for the two fluids. the intermediate metal is aluminum. what may be problem. i calculated the heat absorbed by the metal but is very low. also during steady state analysis it wont be a problem right? i have not modeled any convection, radiation or conduction. so there is no chance of any other heat transfer losses. please help me in resolving this issue. outer walls are given as adiabatic only and heat transfer interface is created for the two layers between the to fluids and the wall.

ghorrocks · November 16, 2013, 06:11

Sounds like a simple inaccurate simulation to me: FAQ - http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Particularly read the bit about convergence tolerance sensitivity

AKHILESH S L · November 16, 2013, 07:22

thank you Glenn for your response. but the thing is i am getting correct results and correct energy balance when the dimensions of my outer fluid layer is increasing. in my real problem the outer fluid layer thickness is 0.3mm. but when i change this to atleast 1m i am getting accurate value. one more thing my convergence criteria is given 1e-6 . it is too low na? can you show me a accurate converging lines figure in the solving figure? is the lines should be in parallel itself? my lines are not parallel but inclined one.

ghorrocks · November 17, 2013, 08:05

Sorry, I do not understand your last post. What is the thickness of the outer fluid layer you wish to model?

The documentation has a good description on what convergence means, and how to judge it.

AKHILESH S L · November 22, 2013, 00:47

in actual problem the outer layer thickness is 0.3 mm . but when modelling with this low thickness energy balance is not obtaining. when modelling with atleast 1 mm energy balance is obtaining. what may be problem? meshing is not proper i think so. while modeling fluid-solid interface inflation layer thickness is a problem i don't know how to give proper inflation in the interface. please help me in this problem

ghorrocks · November 22, 2013, 05:31

Try the geometry and meshing forum for questions on meshing.

If you are not getting the energy to balance then converge tighter. You should use imbalances as part of your convergence criterion for CHT simulations like this.

November 16, 2013, 04:28	Balance of Heat Transfer	#1
AKHILESH S L New Member AKHILESH S L Join Date: Sep 2013 Location: Trivandrum Posts: 29 Rep Power: 13	i am modelling a heat exchanger of concentric tube type with counter flow. temperature difference is obtaining for the two fluids. but when calculating the total Q (Q=mCpdelta(t)) it is not equal for the two fluids. the intermediate metal is aluminum. what may be problem. i calculated the heat absorbed by the metal but is very low. also during steady state analysis it wont be a problem right? i have not modeled any convection, radiation or conduction. so there is no chance of any other heat transfer losses. please help me in resolving this issue. outer walls are given as adiabatic only and heat transfer interface is created for the two layers between the to fluids and the wall.

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November 16, 2013, 06:11		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Sounds like a simple inaccurate simulation to me: FAQ - http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F Particularly read the bit about convergence tolerance sensitivity

November 16, 2013, 07:22		#3
AKHILESH S L New Member AKHILESH S L Join Date: Sep 2013 Location: Trivandrum Posts: 29 Rep Power: 13	thank you Glenn for your response. but the thing is i am getting correct results and correct energy balance when the dimensions of my outer fluid layer is increasing. in my real problem the outer fluid layer thickness is 0.3mm. but when i change this to atleast 1m i am getting accurate value. one more thing my convergence criteria is given 1e-6 . it is too low na? can you show me a accurate converging lines figure in the solving figure? is the lines should be in parallel itself? my lines are not parallel but inclined one.

November 17, 2013, 08:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Sorry, I do not understand your last post. What is the thickness of the outer fluid layer you wish to model? The documentation has a good description on what convergence means, and how to judge it.

November 22, 2013, 00:47		#5
AKHILESH S L New Member AKHILESH S L Join Date: Sep 2013 Location: Trivandrum Posts: 29 Rep Power: 13	in actual problem the outer layer thickness is 0.3 mm . but when modelling with this low thickness energy balance is not obtaining. when modelling with atleast 1 mm energy balance is obtaining. what may be problem? meshing is not proper i think so. while modeling fluid-solid interface inflation layer thickness is a problem i don't know how to give proper inflation in the interface. please help me in this problem

November 22, 2013, 05:31		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Try the geometry and meshing forum for questions on meshing. If you are not getting the energy to balance then converge tighter. You should use imbalances as part of your convergence criterion for CHT simulations like this.