11 Replies Latest reply on Apr 23, 2015 2:13 PM by Jared Conway

    Two different HTC values in a single anular pipe

    Simone Storace

      Good morning,

      I have a problem / doubt.

      I want to post-process some HTC values (heat transfert coefficient) in a CFD model.

      The CAD is very simple, two concentric pipes in which steam evolves (an image is appended).

      The boundary conditions are:

      - Mass flow inlet = 30 kg/s (steam)

      - Inlet temperature = 735 K

      - Inlet Pressure = 102 absolute bar

      - Outlet Pressure = 101 absolute bar

       

      Results:

      [FACE1] External pipe External face = 100 w/(m2 k) imposed in General Settings

      [FACE2] External pipe Internal face = 5000 w/(m2 k)

      [FACE3] Internal pipe External face = 400 w/(m2 k)

      [FACE4] Internal pipe Internal face = 320 w/(m2 k)

       

      How is it possbile that Face 2 and Face 3 have so different HTC values? They are 2 faces seeing "each other" and the steam-flowing on them is the same (same velocity, same temperature, same density, etc...).

      Obviously there is a bit temperature difference on the two pipes (4/5 kelvin), but I do not think that it could be the cause of theese unusual HTC values.

       

      Thank you

      Greetings,

      Simone

        • Re: Two different HTC values in a single anular pipe
          Jared Conway

          just to be clear, you want 2 and 3 to be exactly the same?

           

          is the surface temperature on them exactly the same?

           

          can you post some screenshots of the results or better yet the file?

            • Re: Two different HTC values in a single anular pipe
              Simone Storace

              Hi Jared,

              the temperature values are respectively:

              Face 2 = 724 K

              Face 3 = 734.9 K

              Face 4 = 735 K

              I expected that the two  HTC values were very close to each other, not exactly the same.

              In my opinion:

              - Nusselt number should be constant because Prandtl and Reynolds have no significant variations (dynamic viscosity in fact is between 2.68E-05 and 2.74E-05 Pa*sec on the opposite surfaces [face 2 and face 3])

              - HTC = (Nu * Fluid conducibility) / (Equivalent Diameter) ---> Nusselt almost constant, Kfluid almost costant, Equivalent Diameter little variations.

              I attach an image with HTC values on th faces 2-3-4.

              The whole model is too heavy to upload, I upload an alternative test with different boundary conditions and different diameter values, but the concept is the same.

              Thanks a lot

              Greetings

              Simone

                • Re: Two different HTC values in a single anular pipe
                  Jared Conway

                  something maybe to note is that HTC is a post-processed value based on the reference temperature

                  but if your solid and fluid temperatures are the same, your reference temp is the same, your HTC should be the same

                   

                  i haven't had a chance to check your sample but your sample shows the exact same issue?

                  • Re: Two different HTC values in a single anular pipe
                    Jared Conway

                    looking at the file now

                    can you describe what conditions you want to test in?

                    i see steam and 10000kPA, is that correct? i don't think that is in the range of the steam model

                    does the problem go away if you use air and steel and 101325Pa and a room temp?

                      • Re: Two different HTC values in a single anular pipe
                        Jared Conway

                        and how long does it take to solve and does it converge/complete?

                          • Re: Two different HTC values in a single anular pipe
                            Simone Storace

                            Hi Jared,

                            I know HTC is a post-processed value based on a reference temperature, but I don't know which temperature is the reference one in SW Simulation.

                            The boundary condition are the real conditions within pipe, 100 abs bar and 735 K (I've listed them in the first post).

                            At this pressure and temperature steam is a supeheated steam, I've understood SW gives a range of values only for the phase transition (as described in the Technical Reference).

                            I don't know if using air or water the problem could be the same or not, but unfortunately I have to use steam.

                             

                            The convergence is quite fast, 1 or 2 hours. Did you run the model yesterday?

                            Can you tell me which is the procedure to set the reference temperature? Is it the "Initial temperature" in "Initial Conditions" (for fluid and solid)??

                             

                            Thanks

                            Greetings

                            Simone

                              • Re: Two different HTC values in a single anular pipe
                                Jared Conway

                                hi, i didn't run the problem because the inputs didn't make sense to me.

                                 

                                regarding your questions, here are the limits for steam:

                                 

                                200.15 K < T < 583.15 K, 10^3 Pa < P < 10^7 Pa.

                                 

                                I think you may be reading too deep into the phase transition comment, I've always been told by the developers this is the range.

                                 

                                so you are outside the range for steam. you may need to come up with your own properties.

                                 

                                regarding reference pressure, if you look in the help, you'll see where it is found, i believe it is under flow simulation tools > results > parameters. by default it is set to your initial temperature.

                                 

                                and regarding using air and steel, I'm only suggesting that as a troubleshooting method.

                                  • Re: Two different HTC values in a single anular pipe
                                    Simone Storace

                                    Hi Jared,

                                    first of all thank you for the quick reply and your suggestions.

                                    Oh, it's a bad news for me, I have to run some cases with supeheated steam (above 583K) and so I must re-check some old calculation now that are out of this range.

                                    Tomorrow I'll try to run a simulation with air and other simulation with steam in a correct range.

                                    If you want, You could run my model with some correct steam conditions and check if the HTC values on faces 2 and 3 are different or not.

                                    I'll do the same thing.

                                    Thanks

                                    Greetings

                                    Simone

                                      • Re: Two different HTC values in a single anular pipe
                                        Jared Conway

                                        i'm also not clear on what the expected outlet conditions are. with your components not merged together the software could get very confused. is there supposed to be an outlet on the inner pipe and the outer pipe?

                                          • Re: Two different HTC values in a single anular pipe
                                            Simone Storace

                                            Hi Jared,

                                            as suggested, I made some test with steam in correct range conditions and another test with air.

                                            Nothing is changed, the HTC values are different within the model.

                                            Example:

                                            OLD calculation with STEAM 100bara and 735K

                                            - [FACE2] External pipe Internal face = 5000 w/(m2 k)

                                            - [FACE3] Internal pipe External face = 400 w/(m2 k)

                                            - [FACE4] Internal pipe Internal face = 320 w/(m2 k)

                                             

                                            New calculation with STEAM 4.75bara and 500K

                                            - [FACE2] External pipe Internal face = 444 w/(m2 k)

                                            - [FACE3] Internal pipe External face = 7.3 w/(m2 k)

                                            - [FACE4] Internal pipe Internal face = 8.5 w/(m2 k)

                                             

                                            New calculation with AIR 4.75bara and 500K

                                            - [FACE2] External pipe Internal face = 503 w/(m2 k)

                                            - [FACE3] Internal pipe External face = 7.5 w/(m2 k)

                                            - [FACE4] Internal pipe Internal face = 9 w/(m2 k)

                                             

                                            I've use, for each project, an inlet velocity boundary conditions of 35m/s.

                                             

                                            Greetings

                                            Simone