23 Replies Latest reply on Mar 17, 2015 10:27 AM by Amit Katz

    Closed loop system

    Alberto Canals

      Hi,

       

      I'm trying to do a flow simulation on a closed loop system. Until now, I had worked with open systems to get used with flow simulations.

       

      For this simulation, the inputs are what in an open system would be the outputs: outputs and inputs are the same. I guess that I should define some initial conditions of the fluid (temperature, mass flow,...), but I don't know how to define that given that for this problem I don't have "Lids" as I use to have.

       

      I'll start with a simulation without gravity, and once I know how to do this simulations I'll include gravity and apply a difference of pressure between A and B (see picture)

       

      There is a picture for doing this question more understandable:

       

      Thank you!Tuberías.jpg

        • Re: Closed loop system
          Amit Katz

          I don't have a lot of experience with this type of simulation, but I would start with setting wall boundary conditions on the inner surfaces of your fluid container. You can define temperate, heat exchange coefficients, etc... You will probably need to slice your model up into seperate solid bodies in order to isolate sections of your inner walls.

          • Re: Closed loop system
            Jared Conway

            I don't know what the question here is.

             

            Initial conditions are defined in the general settings, you don't need to do anything differently.

              • Re: Closed loop system
                Alberto Canals

                In fact, I don't know how to define the input/output flux, given that I don't have any Lid.

                  • Re: Closed loop system
                    Chris Michalski

                    How is your pressure difference created? by a pump?  If so, you could place a solid surface somewhere between A and B to create the pressure rise necessary. 

                    If I remember correctly (it has been a while since I performed loops in a flow simulation) you would want to setup goals on your (fake) exit surface and use those to define the conditions for your inlet surface.  In this way you way say that the temperature, mass flow and velocity through surface B is the same as that through surface A but with a pressure rise of (X).

                      • Re: Closed loop system
                        Alberto Canals

                        Thank you Chris, I was thinking on somethink like this.

                         

                        However, I don't still know how to put the inputs. When I have to select the input face of the flux I don't have any Lid, and I can't create anyone. I want, for example, 5L of a fluid in circulation in this closed system. How to define that?

                         

                         

                        Do you have any suggestion?

                          • Re: Closed loop system
                            Chris Michalski

                            Does the internal volume of your tube equal 5L?  Because if you are going to have 5L of liquid and the rest as gas then the liquid and gas will not move.  Flow defines the mesh at the start of a simulation and if a certain mesh cell contains a liquid then it always has to remain a liquid.  The same with gases.  So Flow will not let you have liquid move to fill a location that was originally full of a gas.

                             

                            In order to make the boundaries for inlet and outlet conditions you would need to block off the inside of the tube.  One way would be to make a thin cylinder with the same diameter as the ID of your tube.  With that constrained in place it will make solid surfaces to which you can apply the pressure differential and volumetric/mass flow rate conditions.

                              • Re: Closed loop system
                                Alberto Canals

                                I tried to do what you said, but I don't still have good results.

                                 

                                What I want to see is how the temperature of the fluid change in function of the time.

                                 

                                I put in the first message the file with my model, maybe some of you knows how to fix the problem! The fluid stops when it arrives to the input lid

                                 

                                 

                                Thank you in advance for your time!

                                  • Re: Closed loop system
                                    Chris Michalski

                                    I can try to setup a quick example tomorrow or next week, but I have a length transient simulation running currently.

                                    If I recall correctly when I have done something similar I used surface goals on the outlet, then base the inlet conditions on those goals through equations (so that inlet temp = outlet temp, inlet pressure = outlet pressure + pump head, etc).  But I would have to work with it to remember exactly how to connect them.

                                    • Re: Closed loop system
                                      Jared Conway

                                      I still don't understand the physics of this problem

                                       

                                      if this is a closed system there are no inlets and outlets

                                       

                                      ok no problem

                                       

                                      if fluid circulates, how does it circulate? is there a pump in it? if that is the case, use an internal fan, that will circulate the fluid

                                       

                                      make sure that the region is not excluded from the simulation

                                       

                                      I think until you better define the problem it is really hard to help. you're talking about wanting to apply an inlet and outlet but I think you realize there is none in this case. and then you're talking about gas and fluid. is it a gas or is it a liquid?

                                        • Re: Closed loop system
                                          Alberto Canals

                                          Hi Jared,

                                           

                                          I'm sorry if I haven't been clear enough. I want to have a fluid (gas probably) circulating through the closed pipe. There is a temperature gradient from the top of the pipe (400K, for example) to the pipe underground (250K). I would apply some pressure differences between two points in order to have the gas circulating.

                                           

                                          What I want to see is the evolution of the temperature with the time, taking into account that there will be some heat exchange between the pipe and the underground. I don't want to consider convection with the ambient. I want also to see the evolution of the temperature of the underground (sand, for example) at one point near to the pipe underground (not visible in the design that I attached).


                                          Thank you in advance for your time.

                                  • Re: Closed loop system
                                    Alberto Canals

                                    I've seen another similar post and you had some information about something similar to my problem.

                                    Modeling Closed Loop Heat Exchanger


                                    Do you have some more information?

                                     

                                    Thank you!!

                              • Re: Closed loop system
                                Amit Katz

                                Wouldn't the volume of fluid in your system be defined by the volume of your container?

                                 

                                And what is "Lind"? I am not familiar with that term.