Applying heat transfer boundary conditions

Shomaz ul Haq · November 26, 2015, 07:50

Thanks a lot Mr. Ghorrocks. Actually there is no volumetric heat source inside the Hot water fluid domain. I thought without that no heat transfer takes place but I was wrong. I just want to study basic heat transfer due to temperature difference between hot and cold fluid. Also you mentioned "heat content". How can I include "heat content" and will it allow heat transfer to take place or is it altogether irrelevant to heat transfer. Another thing what is non-overlap% reported in out file. The detail for Fluid Fluid Interface is as follows:

Discretization type = GGI
Intersection type = Direct
Non-overlap area fraction on side 1 = 3.56E-04
Non-overlap area fraction on side 2 = 1.58E-02

Would be grateful for explanation. God bless you. Thanks.

ghorrocks · November 26, 2015, 16:44

Non-overlap area fraction is the amount of the interface which has not intersected. In your case the % is low (max 1.5%) so most of the interface correctly intersects. I would still have a close look at this to check it is OK.

Regarding heat transfer:
You should apply the flow conditions to match what you are trying to model. Likewise, you should measure the performance on the simulation using a parameter which is of use in your situation. I do not know what you are modelling so cannot say.

But I can guess that most heat exchangers work with a hot fluid coming in at a known temperature and a cold fluid coming in at a known temperature, and you want to know how much heat comes out of the hot fluid and how much goes into the cold fluid. In this case I would use massflowAve of temperature at the inlets and outlets to get the heat gained and lost.

Shomaz ul Haq · November 27, 2015, 10:27

Hi all. Thanks a lot Glenn. I am attaching pics of my double pipe counter flow heat exchanger (2nd image). Why don't I see a change of temperature colors along the length of the pipe as in the first image of a similar tutorial setup in ANSYS Fluent even when I changed the inlet speeds from 5 m/s at the hot and cold inlets to ~.8-.9 m/s (I thought decreasing the speeds will increase the contact time and make the heat transfer more enhanced/prominent along the length of the pipe). Also I will apply your recommendation of using massflowAve at the inlets and outlet boundary conditions (but I converted my outlet boundary conditions from outlet to opening type because of a message of wall being placed and suggestion of conversion from outlet to opening type in out file). Should I change boundary conditions at outlet back to Outlet type? Would be grateful for review and recommendation. God bless you. Thanks.

Shomaz ul Haq · November 27, 2015, 11:47

Hi all. Dear Glenn, I have a few questions.

First of all to insert a massflowAve(T)@ expression at an outlet do I need to define mass flow rate (instead of Average Static Pressure or Normal Speed) in Option in Mass and Momentum in Boundary Details in Boundary:Coldwateroutlet (but for that I need to change boundary condition at outlet from Opening to Outlet? Should I do that? Similarly should I do that for Boundary:Hotwateroutlet)?

Second inserting a massflowAve(T)@ expression for inlets would mean I am not defining static temperatures at the inlets which is usually the norm and like you suggested for a heat exchanger problem. Also for this to happen do I need to define Mass Flow Rate in Option in Mass and Momentum in Boundary Details in Boundary:Coldwaterinlet and similarly for Boundary:HotwaterInlet?

Third irrespective of inlets or oulets do I need to insert massFlowAve(T)@ expression as "Static Temperature" in Option in Heat Transfer in Boundary Details in Boundary?

Would be grateful for reply. God bless you. Thanks.

ghorrocks · November 28, 2015, 04:48

I suspect the heat exchanger in image 2 is not working very effectively. All the heat transfer takes place at the entrance and nothing much happens after that. You need to get the flow rate and temperature difference in a proper operating range for the heat exchanger to function correctly.

Second Post: You apply the boundary conditions to match the flow conditions. Then you look at the best way of monitoring it. You could also use a massFlowInt(T) or cp*T to give an input/output heat flux.

Shomaz ul Haq · November 28, 2015, 13:28

Thanks a lot Glenn. I followed your adice and played around with boundary conditions. First I changed the speeds of the inlets but nothing happened. Then I changed the temperature of inlets, again nothing happened. Then I changed the "Speed" Option in Mass and Momentum in Boundary Details in Boundary:Coldwaterinlet (and Boundary:Hotwateroutlet) to "Mass Flow Rate" with a value of 25 kg/s and then in CFD-Post I saw some change of temperature along the length of the pipe but now I am faced with the problem of no convergence of heat transfer and mass and momentum graphs. How to solve this? Would be grateful. God bless you. Thanks.

ghorrocks · November 28, 2015, 20:25

The convergence problem is an FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

Can you post attachments as image files or text files? Word files are a pain to view on some machines.

I recommend you do a simple hand calculation on your heat exchanger before you start CFD work. That way you can find a sensible operating point to start at quickly. There are several methods to approximately size a heat exchanger in heat transfer textbooks. I recommend you use these methods to get a starting point.

Shomaz ul Haq · November 29, 2015, 06:57

Thanks a lot for the feedback Glenn. Is the change of temperature along the length of the pipe correct in your opinion? Basically I am a Mechatronics Engineer by profession and also I have studied a full semester course in heat transfer and another one in thermodynamics in my Bachelors of Engineering. Now I am doing Masters and had taken a summer course on CFD and now am applying CFD code ANSYS for my thesis. But I need your help brother. Usually while designing a heat exchanger on paper/theoretically Mass Flow Rate is chosen/given in problems, but here in CFX-Pre there are six options including "Normal Speed" in Mass and Momentum in Boundary Details in Boundary:Inlet. Why are results so different when I use speed as compared to when I use Mass Flow Rate. Also please find an attached jpeg file for easier view. Sorry about loading word files earlier. God bless you. Please forgive me. Would be grateful. Thanks.

ghorrocks · November 29, 2015, 18:15

Mass flow rate and velocity are the same boundary condition. Are you sure you are specifying them correctly?

I have no idea what the correct temperature change along the pipe is as I do not know what you are modelling. And even if you told me I do not have time to work it out for you - you will have to figure that out for yourself. But I will try to help you as much as I can.

shadnia · December 17, 2015, 19:39

Hi Glenn
I did switch to CFX, so I already made my geometry along with meshing by CFX. However when I run the model, it does not even start. It says:"In Analysis 'Flow Analysis 1': No domains have been defined."
Any help will be appreciated

ghorrocks · December 17, 2015, 20:21

The error sounds obvious - define a domain.

Have a look at the CFX tutorials for examples of how to set things up.

shadnia · December 17, 2015, 22:27

Quote:

Originally Posted by ghorrocks

The error sounds obvious - define a domain.

Have a look at the CFX tutorials for examples of how to set things up.

As I see at following links which show two examples of CFX on youtube, after making geometry and meshing, when you click on setup, your geometry pups up and you can go on and proceed your modeling. However in my case, when I click on setup, my geometry does not pup up.
Please note that all manuals I found, have no example to look at.
Thanks for your help.

https://www.youtube.com/watch?v=Xu8rJGOFWok

https://www.youtube.com/watch?v=ymGQIA_Rv3Q

ghorrocks · December 18, 2015, 02:10

Try the CFX tutorials which come with CFX, under the help/documentation. They come with all the files required to run them.

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November 26, 2015, 07:50		#21
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Thanks a lot Mr. Ghorrocks. Actually there is no volumetric heat source inside the Hot water fluid domain. I thought without that no heat transfer takes place but I was wrong. I just want to study basic heat transfer due to temperature difference between hot and cold fluid. Also you mentioned "heat content". How can I include "heat content" and will it allow heat transfer to take place or is it altogether irrelevant to heat transfer. Another thing what is non-overlap% reported in out file. The detail for Fluid Fluid Interface is as follows: Discretization type = GGI Intersection type = Direct Non-overlap area fraction on side 1 = 3.56E-04 Non-overlap area fraction on side 2 = 1.58E-02 Would be grateful for explanation. God bless you. Thanks.

November 26, 2015, 16:44		#22
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Non-overlap area fraction is the amount of the interface which has not intersected. In your case the % is low (max 1.5%) so most of the interface correctly intersects. I would still have a close look at this to check it is OK. Regarding heat transfer: You should apply the flow conditions to match what you are trying to model. Likewise, you should measure the performance on the simulation using a parameter which is of use in your situation. I do not know what you are modelling so cannot say. But I can guess that most heat exchangers work with a hot fluid coming in at a known temperature and a cold fluid coming in at a known temperature, and you want to know how much heat comes out of the hot fluid and how much goes into the cold fluid. In this case I would use massflowAve of temperature at the inlets and outlets to get the heat gained and lost.

November 27, 2015, 11:47		#24
Shomaz ul Haq Senior Member Shomaz ul Haq Join Date: Aug 2015 Location: Islamabad, Pakistan Posts: 205 Rep Power: 12	Hi all. Dear Glenn, I have a few questions. First of all to insert a massflowAve(T)@ expression at an outlet do I need to define mass flow rate (instead of Average Static Pressure or Normal Speed) in Option in Mass and Momentum in Boundary Details in Boundary:Coldwateroutlet (but for that I need to change boundary condition at outlet from Opening to Outlet? Should I do that? Similarly should I do that for Boundary:Hotwateroutlet)? Second inserting a massflowAve(T)@ expression for inlets would mean I am not defining static temperatures at the inlets which is usually the norm and like you suggested for a heat exchanger problem. Also for this to happen do I need to define Mass Flow Rate in Option in Mass and Momentum in Boundary Details in Boundary:Coldwaterinlet and similarly for Boundary:HotwaterInlet? Third irrespective of inlets or oulets do I need to insert massFlowAve(T)@ expression as "Static Temperature" in Option in Heat Transfer in Boundary Details in Boundary? Would be grateful for reply. God bless you. Thanks.

November 28, 2015, 04:48		#25
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	I suspect the heat exchanger in image 2 is not working very effectively. All the heat transfer takes place at the entrance and nothing much happens after that. You need to get the flow rate and temperature difference in a proper operating range for the heat exchanger to function correctly. Second Post: You apply the boundary conditions to match the flow conditions. Then you look at the best way of monitoring it. You could also use a massFlowInt(T) or cp*T to give an input/output heat flux.

November 28, 2015, 20:25		#27
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The convergence problem is an FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria Can you post attachments as image files or text files? Word files are a pain to view on some machines. I recommend you do a simple hand calculation on your heat exchanger before you start CFD work. That way you can find a sensible operating point to start at quickly. There are several methods to approximately size a heat exchanger in heat transfer textbooks. I recommend you use these methods to get a starting point.

November 29, 2015, 18:15		#29
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Mass flow rate and velocity are the same boundary condition. Are you sure you are specifying them correctly? I have no idea what the correct temperature change along the pipe is as I do not know what you are modelling. And even if you told me I do not have time to work it out for you - you will have to figure that out for yourself. But I will try to help you as much as I can.

December 17, 2015, 19:39		#30
shadnia New Member R-Sh Join Date: Oct 2014 Location: USA Posts: 23 Rep Power: 12	Hi Glenn I did switch to CFX, so I already made my geometry along with meshing by CFX. However when I run the model, it does not even start. It says:"In Analysis 'Flow Analysis 1': No domains have been defined." Any help will be appreciated

December 17, 2015, 20:21		#31
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The error sounds obvious - define a domain. Have a look at the CFX tutorials for examples of how to set things up.

December 18, 2015, 02:10		#33
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Try the CFX tutorials which come with CFX, under the help/documentation. They come with all the files required to run them.