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. |
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. |
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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.
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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. |
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. |
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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.
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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. |
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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.
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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. |
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 |
The error sounds obvious - define a domain.
Have a look at the CFX tutorials for examples of how to set things up. |
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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 |
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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