9 Replies Latest reply on May 13, 2016 8:47 AM by Amit Katz

    Energy balance air to air

    Faiz Muchamad

      HI

      I have a case with energy balance on my internal flow simulation project. In my simulation i want to heat air inside the tube (T = 301 K ) by air flowing outside the tube (T = 994) K. The tube material is carbon steel which the properties i input manually. Then after simulation finish i got the values of average temp outlet from the surface of two outlet lids and for specific heat i got from the book. From these values i calculate the energy balance using formula mdot x cp x dT (where mdot = mass flow rate, cp = specific heat and dT = temp difference) and when i calculate it the energy is not balance. Error for this about 80%.

      Note: -no heat out from the system i use adiabatic as the outerwall boundary

                  -mdot air in tube = 1,1246 kg/s

                  -mdot air out tube =0,03 kg/s

       

      Im very grateful for u all try to help me

      This is my heat exchanger model

        • Re: Energy balance air to air
          Kim KyuHyung

          In my simulation results,

          mdot air in tube = 0.0805kg/s

          mdot air out tube = 0.13kg/s

          Q cold =29848W

          Q hot = 29099W

          Error for this case 2.51%

          try to simulation with more refinement mesh

          111111111.jpg

            • Re: Energy balance air to air
              Faiz Muchamad

              Thanks for your help Kim KyuHyung, your simulation result is excelent, so i try to do what you said for refining the meshing. In solid component i make a local initial mesh on solid area of tube with level 4 with goals only surface bulk temp in 2 outlet no more adding. But when i do that i got my total mesh it is exceed 5 million. I ignore that and i try to run the simulation. Then next problem is the time, It has passed one day and the iteration only reach 19.

               

              what I want to ask is whether your total mesh are more or less with mine? is there any method to get accurate result with less total mesh?

            • Re: Energy balance air to air
              Amit Katz

              Adding to what Kim said, this is what I see when I mesh the project using the saved settings in the archive:

               

              Capture.PNG

               

              You don't have any actual solid surfaces. This is why it's critical to always check your mesh before you run your project.

                • Re: Energy balance air to air
                  Faiz Muchamad

                  Thanks for your help Amit Katz, now I know my mistake in refining solid component like thickness of tubes. like I said to Kim when I refining the mesh the total of mesh exceed 5 million

                   

                  do you have any idea to decrease my mesh? and my goals only surface bulk temp in 2 outlet no more adding is that correct method?

                    • Re: Energy balance air to air
                      Amit Katz

                      Your mesh is large because you are trying to simulate a lot of details, you're going for the flow inside every pipe as well as the flow between the pipes. You could do a general analysis of the flow between pipes, with repeating conditions at the edges of your domain, something like this:

                      Untitled.png

                       

                       

                      This doesn't include edge cases of course, but you could use this to solve for a general heat transfer coefficient by varying the ambient temperature of the free flowing fluid.

                        • Re: Energy balance air to air
                          Faiz Muchamad

                          Sory amit unfortunately i didnt get what you mean with repeating fluid domain edges like the picture you attach. if im not bother you, can you guide me step by step to do that

                           

                          and this is my meshing with error 70%,

                          3.jpg

                           

                          The meshing above reach total mesh about 600 thousand mesh, 

                            • Re: Energy balance air to air
                              Amit Katz

                              I can't provide a guide right now, sorry.

                              If you edit your fluid domain you have the option of setting edges as symmetric or repeating (I see you already set one edge as symmetric so you know how to do that.)

                              What you could try is to set your boundary cutting through two sets of parallel pipes and set both of those edges to symmetry. That will let you simulate just one "slice" of the repeating pattern and save on a lot of cells.

                              Once again this is ignoring the edge cases near the walls of the  exchanger, but I think it's a reasonable approximation if you're short on computational resources.

                      • Re: Energy balance air to air
                        Amit Katz

                        You can set convecting heat transfer coefficient as a surface goal. That's probably important for your simulation.