8 Replies Latest reply on Oct 6, 2011 9:02 PM by Mia Molfino

    2008 FloWorks Thermal Simulation

    Mia Molfino

      I'm using FloWorks from within SolidWorks 2008.  That could be my first problem.  Here's what I have going on:

       

      A plate with fins on one side (the outside) and heat generators (132W total) mounted on the other side (the inside).  A thin metal cover goes over the heat generators, enclosing that side of the plate, but a rubber gasket seals the juncture between the plate and the cover.  I am running an external analysis but not excluding internal spaces because I care about the heat conducted from the heat generators through the air and out the cover.  There will be air flow but I ran it without any at first and things seems to be getting exorbitantly hot when I expected more heat to be conducted away by natural convection.  Then I assigned an initial air flow condition and ended up with negative temperatures well below the ambient temp of 20C, which seems completely wrong.  Has anyone else ever had this experience?  I am currently in the process of testing out an even more simple model to try to wrap my head around what FloWorks is doing.

       

      Any input is welcome.  Thanks!

       

       

      UPDATE:  I added files for a very simplified version of the problem.  The other model was impractical to troubleshoot because it would take some 9 hours to run... which means I should probably adjust the mesh settings right?  Anyway, this one lacks a rubber gasket but I'm not too worried about it.  I noticed that it's the min fluid temperature that is dropping below ambient and not the min solid temp.  This is a little less disconcerting but still doesn't make sense to me.  I have ambient set to 20C and the min fluid temp comes out to be about 14C.  How is this possible?  

        • Re: 2008 FloWorks Thermal Simulation
          Ron Reiners

          Mia,

           

          Can you attach the model for other to review?

          • Re: 2008 FloWorks Thermal Simulation
            Ron Reiners

            Mia,

             

            I used Flow Simulation 2011 on your model. Although I didn't see a 14C temperature in the ambient air I did see an area around the model that was in the 19C-20C range. I didn't see ant setting that looked wrong. I will look at this more as I have time.

             

            Cut Plot 1.jpg

            Regards,

            Ron

              • Re: 2008 FloWorks Thermal Simulation
                Mia Molfino

                Hi Ron,

                 

                I'm thinking I changed around the initial conditions when messing with the simplified model and didn't remember and that's why my min temp came out different than what you got.  I definitely previously got results where the min temp came out to be 19.something. 

                 

                I've run this model now with and without gravity and with various (5, 15, 50, 250 m/s) initial velocity conditions in the -y direction.  I also refined the mesh around the fins.  There's really not much going on here i.e. not much for me to screw up, which is confusing me a bit. 

                 

                Min temps for the analyses stayed slightly above the ambient of 20C until I bumped up the initial veloc condition to 50 m/s.  That yielded a min of 19.5C.  Below ambient but I guess still somewhat believable.  However, when I bumped the initial velocity up to 250m/s, the min fluid temp dipped to 7.7C.  I'm attaching a jpeg screenshot of my cutplots.  I just don't understand how this is feasible.

                 

                Thanks for your help!

                250m_s.jpg 

                  • Re: 2008 FloWorks Thermal Simulation
                    Ron Reiners

                    Mia,

                     

                    I am really concerned about this as this type of modeling is very similar to what I need to do with our products. Have you contacted your VAR or tech support?

                     

                    I am going to run the model that I have at a few different flow velocity's to see what I get.

                     

                    Regards,

                    Ron

                     

                    P.S. Sorry this message is late. I never received an e-mail notice of your response.

                    • Re: 2008 FloWorks Thermal Simulation
                      Ron Reiners

                      Mia,

                       

                      I did some more experiment with this model. I think a lot of what you are seeing has to do with friction near the model and high speed air flow around the object. The cooling efffect could be due to air acceleration around the object. I did some internet research but didn't find any information to back this up and the only person here that might have any knowledge on this subject is out today.

                       

                      I setup some configurations with different velocities and noticed that the temperature plume was basically the same shape in each. As the speed increased, the plume enlarged, and the temperature variation increased. I also noticed from your last message that the velocity is getting very high (250 m/s = 559 mph = 0.75 mach). As an experiment I left the velocity at 250 m/s but turned off the heat source and was seeing similar results to the simulation with 25w added. Try this experiment on you model (high speed and no heat source) and see what you get. Let me know how this works out.

                       

                      Regards,

                      Ron

                        • Re: 2008 FloWorks Thermal Simulation
                          Mia Molfino

                          Hi Ron,

                           

                          Hmm that's definitely interesting...  I noticed on the run with 250m/s (heat source still on) that the min solid temp was 48.8C as compared to a min solid temp of 29C when the initial velocity was 50m/s.  There's definitely the threshold where increasing the air flow won't pull away more heat and you must be right that there's probably some excessive drag and heat friction being created.  I'm still confused as to why you get that plume of sub-ambient temperature air but this won't be the first or last thing I find confusing about fluid dynamics.

                           

                          I ran the experiment you suggested (high speed 250m/s, no heat source) and got a solid temp of 47-48C and fluid temps of 7.8 - 50.8C.  Like you, these results were similar to the ones from the high speed experiment with the heat source which yielded a min solid temp of 48.8C and fluid temps ranging from 7.7-66.7C.  250m_s no heat.jpg

                           

                          I was mostly interested in simulating potential aircraft flight speeds and downdrafts.  Depending on the aircraft, 559mph is something that could be seen.  For this platform, I'm more in the realm of 140mph (61ms), which should hopefully give me pretty sane results... I'm running that one as I type.

                           

                          Thanks,

                          Mia  

                            • Re: 2008 FloWorks Thermal Simulation
                              Ron Reiners

                              Mia,

                               

                              The only thing that I could think of for the sub-ambient temperature would be as the air flow is directed around the body, the regions of the plume farthest away need to either accelerate rapidly or are subject increased and then decreased pressure which affect the fluid temperature. Similar to the contrail from jets at high altitude.

                               

                              Since you are talking about aircraft you might also need to take into account the reduced pressure at altitudes.

                               

                              Regards,

                              Ron

                                • Re: 2008 FloWorks Thermal Simulation
                                  Mia Molfino

                                  Ron,

                                   

                                  Your guess is better than mine, so that explanation works for me.  Thanks for the heads up on the reduced pressure.  I didn't think about that having an effect.  Ran it at worst case of 18000 ft and the difference in pressure produces another 5 deg C rise in temp.

                                   

                                  Thanks for all the help and good info!

                                  Mia