8 Replies Latest reply on Jul 10, 2013 8:32 PM by Jared Conway

    “External Fluid Temperature” for Inner Walls?

    Jerry Pham

      Hi,

       

      I have a question on the “Real wall” boundary condition:

       

      Typically containers have both inner and outer walls because the solid material has thickness. It is reasonable to set an inner wall to be “Real”, however in addition to the Heat Transfer Coefficient, the software also asks to specify “External Fluid Temperature”. However for internal fluid, the direct inner wall bounding it has no external fluid, but only the solid material inside the wall.

       

      boundary.jpg

       

      So what does “External Fluid Temperature” mean for inner “Real Walls”?

       

        

      Jerry

        • Re: “External Fluid Temperature” for Inner Walls?
          Jared Conway

          can you post a picture of your geometry?

           

          are you running an internal or external analysis?

           

          are you solving a problem with heat conduction in the solids enabled? is it set to heat conduction in the solids only?

           

          what you're doing with a real wall is overriding what is happening on that wall. so you could set a temperature or roughness which is the most common. with your option, you're setting a convection coefficient on the wall. IE, you're defining the heat leaving that wall. Similar to SolidWorks simulation or a heat conduction in the solids only solution, you're saying the fluid adjacent to that wall is 298K. Remember, flow doesn't know internal vs external wall, it just knows that you've selected a surface.

            • Re: “External Fluid Temperature” for Inner Walls?
              Jerry Pham

              Jared,

               

              So in this case even if I set the "T of External Fluid Temperature" to be even a ridiculously high temperature, such as above the melting point for an metal (2000K) tube, the internal fluid will still regard the external environment as being consisted of

              1. fluid at such (2000K) a temperature.
              2. heat transfer coefficient specified.

              and completely ignores the REAL material composition of the solid wall?

               

              But if "Heat conduction in solids" is also enabled, how will the software continue for solids then? Does it also assume inside the wall material everywhere the temperature is 2000K? Or does it assume that the ideal thin layer adjacent to the inenr fluid is 2000K, and regard that 2000K layer as a constant temperature boundary condition, and solves the temperature within the wall material using solid heat equations?

               

               

              Jerry

                • Re: “External Fluid Temperature” for Inner Walls?
                  Jared Conway

                  "Or does it assume that the ideal thin layer adjacent to the inenr fluid is 2000K, and regard that 2000K layer as a constant temperature boundary condition, and solves the temperature within the wall material using solid heat equations?"

                   

                  this is the closest to what i described.

                   

                  you're telling flow simulation to not perform the convective heat transfer calculation on that face by defining a real wall. but there are caveats depending on options you have selected.

                   

                  my recommendation is to describe your conditions, setup a test model and try the different combinations to see how it comes out. if something doesn't look right, it will be a lot easier to explain with an example.

                    • Re: “External Fluid Temperature” for Inner Walls?
                      Jerry Pham

                      Jared,

                       

                      If I want to simulate the air temperature change in an enclosed container, and in which case the wall temperature cannot be assumed to be constant because it is affected by water and air in the container, what boundary condition should I choose? Could you have a look at question Boundary Condition for Enclosed Air?

                       

                       

                      Jerry

                      • Re: “External Fluid Temperature” for Inner Walls?
                        Jerry Pham

                        Jared,

                         

                        And it seems also possible to specify "Real wall" type without any options selected (all dim). What does this mean?

                         

                        realWall.jpg

                         

                        "Wall temperature" (1st option) and "Heat transfer coefficient" are incompatible and if we try to enable both we will get a prompt saying that the two cannot co-exist. So if we choose "Real wall" but leave all dimmed, how does the software interpret that?

                         

                         

                        Jerry

                          • Re: “External Fluid Temperature” for Inner Walls?
                            Jared Conway

                            correct, you can't create 2 on the same.

                             

                            don't know what would happen with no options selected, would recommend you test and see if it changes your answer. if it doesn't change it (my guess), maybe we could ask the developers to eliminate the ability to do that. if it does change it, we'll have to look at what it does, but all walls are "real walls" in simulation, you're choosing an option that modifies the real wall. like overriding outerwall conditions..etc.

                              • Re: “External Fluid Temperature” for Inner Walls?
                                Jerry Pham

                                Jared,

                                 

                                wallC.jpg

                                 

                                In each flow simulation there is a default outer wall thermal condition which includes a customized Heat Transfer Coefficient. For inner walls which for internal analysis can be left unaltered, what is their default values? The software doesn't provide explicit way to modify them except in "Real wall" options. However,

                                1. "Real wall" s have limitations as discussed above.
                                2. But it is possible that in one project (of internal anslysis) there are different fluid domains with different surfaces, and in general Heat Transfer Coefficient differ between them. To accurately model the different surfaces we need to be able to intput their specified coefficients (which also have realtion to liqiud type, etc.), and it "Real Wall" the only way to do that? But the "T of external fluid" options seems to make it inappropraite for many situations.

                                 

                                 

                                Jerry

                                  • Re: “External Fluid Temperature” for Inner Walls?
                                    Jared Conway

                                    internal or external analysis?

                                    internal cavities included or not?

                                    heat conduction in the solids only enabled or not?

                                     

                                    if internal cavities aren't included, you can't do anything. they aren't included! they are considered adiabatic. (the area is filled with insulator)

                                     

                                    if internal cavities are included, then there is no need to specify a heat transfer coefficient, you're performing a CFD, let the software calculate it. you only need to specify heat transfer coefficients on faces where there is no fluid problem to solve or if you're solving heat conduction in the solids only enabled (or using simulation pro thermal analysis).

                                     

                                    as noted previously, i'd take a step back and focus on the fundamentals. i've been doing this for 10 years and used a real wall maybe twice in hundreds of projects. i think there is a fundamental component that you're missing but i'm not sure what your background is or what your actual problem is. if you can swing it, i'd highly recommend a training class and some one-on-one training. email me at jared@hawkridgesys.com if you'd like to discuss some options.