4 Replies Latest reply on Jan 8, 2017 7:56 AM by Rob Brakes

    Getting A Pressure Increase When Running Flow Simulation On Spear Valve

    Rob Brakes

      Hi,

       

      I'm working on a university project for my masters and am getting a recurring problem when using flow simulation.

       

      My aim is to calculate the coefficient of performance for a spear valve under different flow rates and spear positions. I've set an inlet flow and outlet pressure of 1atm and asked solidworks to calculate the total inlet pressure, total outlet pressure as well as some other parameters. Each time I run the simulation my total outlet pressure is slightly higher than my total inlet pressure.

       

      Aside from this obviously being impossible I've set equations to calculate the energy loss through heat by taking the mean inlet and outlet temperatures and am consistently getting an energy loss which must come (I think) from turning pressure energy into heat energy.

       

      Has anyone encountered a similar problem before/has any ideas on what I might be doing wrong?

       

      Thanks,

      Rob

        • Re: Getting A Pressure Increase When Running Flow Simulation On Spear Valve
          Bill McEachern

          The loss would be viscous losses  from, what is likely turbulent mixing. Flow does calculate this.

          What's a spear valve exactly? Orifice with a post through it like in an oleo strut? It is suppose to be some sort of low loss valve at all flow rates or something?

          • Re: Getting A Pressure Increase When Running Flow Simulation On Spear Valve
            Bill McEachern

            From Google it appears that this is a variable geometry nozzle discharging into a different phase (liquid exitng the nozzle into a gas volume) typically used in a pelton wheel turbine. this has some complications in getting a good approximation. You might want to post an image of the domain and maybe a cross section through the mesh. I assume your equation goals are simple like total energy in  minus energy out, which should be small (it could go either way as it would be a numerical error level quantity if there is no heat transfer to the surroundings.

              • Re: Getting A Pressure Increase When Running Flow Simulation On Spear Valve
                Rob Brakes

                Hi Bill,

                What you've written is correct. It is used to vary the flow rate of a pelton wheel turbine. I'm aiming to find the total pressure at the inlet and outlet under different operating pressures and different spear positions.

                 

                As I mentioned about, under certain circumstances the total outlet pressure is greater than the inlet pressure. I think you're comment about there being a numerical error would be valid for very low flow rates but under high flow conditions a pressure loss of about 10% is more reasonable.

                 

                I've found so far that running a coarse mesh will give what looks like good result, I can then increase the refinement up to a point when outlet pressure will suddenly jump up to about 1% greater than inlet pressure. Using the spear position below and a pressure of 15bar a global mesh of 5 can be used and the results are as expected, however a mesh of 6 or 7 will give outlet pressures greater than the inlet. If the "channels" box is ticked within a local mesh the same happens (under most circumstances) and as the valve is closed (and the head of the spear begins to interfere with the outlet lid) this problem seems to occur more frequently.

                 

                Sorry my description is so vague. I have really struggled to find any reason/trend which seems to be causing this, any ideas on whats causing this would be much appreciated.

                 

                Rob

                 

                 

                spear valve.png

              • Re: Getting A Pressure Increase When Running Flow Simulation On Spear Valve
                Rob Brakes

                So it turns out that the problem was that the mesh was not refined enough around the spear head.

                 

                Annoyingly the results which were being produced from a fairly coarse mesh were very close to the values I had expected. As I increased the level of the mesh it appeared that the results were becoming less accurate. Increasing it to level 8 and 9 around the spear head then produced the results which I had expected again.