1 Reply Latest reply on Sep 4, 2014 4:39 PM by Jared Conway

    flow simulation of gas filling a vacuum vessel. is it possible?

    Francesco Zuddas


      I am having some trouble doing a flow simulation and request some help..
      The system is the following: an 800 liter vessel (initial conditions 0.5 mbar of nitrogen) is injected with nitrogen through a small 4 mm diameter tubing. The tubing has a 2 bar pressure upstream. In front of the nozzle, inside the vessel, are some obstacles and what I wanted to find out is how the injected gas flows around them and diffuses. There is no exit hole (in reality there is but during injection it is sealed off), hence no mass balance but there is an accumulation of mass and increase of pressure. Flow simulation it seems doesn't allow me to design the system with no exit. Do I need a workaround of some sorts? Am I doing something wrong? Am I missing something?

      help is much appreciated. Thx in advance

        • Re: flow simulation of gas filling a vacuum vessel. is it possible?
          Jared Conway

          have you checked the soldiworks KB? there is an article specific about filling and evacuation.


          very involved calculation

          needs to be time dependent


          actually according to the article it says there is info in the help:


          Here is what the Online Help says about it:

          Calculations of pressurized tank evacuation through small orifices or exhaust system may

          require long computation time due to compression and rarefaction waves which appear in the

          fluid region. Sonic and supersonic waves propagate through the fluid region. The code tries

          to simulate the wave movement choosing small time steps for the numerical integration of

          the governing equations. In many cases the characteristic time of the evacuation process

          (te) is much larger then the characteristic time of the compression/rarefaction wave

          movements (tw): te>>tw. At this condition, the global gas parameters inside the tank are

          changed weakly in comparison with the dynamic gas parameters so evacuation process can

          be considered as quasi-stationary.

          Such evacuation process can be calculated with Flow Simulation as a transient task but it

          may be very time consuming. However, you can estimate gas parameters inside the tank

          versus time using the Calculator tool in Flow Simulation by the given initial parameter values

          and the discharge coefficient. The discharge coefficient can be obtained from the calculation

          as a ratio of the real (obtained from the analysis) mass flow rate to the ideal mass flow rate

          ratio for the effective throat area of your exhaust system. To obtain the ideal mass flow rate,

          the value of the pressure and temperature inside the tank is needed. The recommended

          strategy for solving the evacuation tasks is to run an analysis in Flow Simulation until the

          exhaust jet is formed. Next use the gas parameters from the Flow Simulation analysis and

          calculate the discharge coefficient. Finally estimate the gas parameters by using the

          gasdynamic Calculator tool provided in Flow Simulation


          from personal experience, we did a tank evacuation and it was very difficult to get it to solve reasonably


          you should really decide what you're going to gain from the simulation