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high pressure jet modeling

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Old   May 4, 2008, 20:57
Default high pressure jet modeling
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Rufas
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I would like to run a rough simulation of a high pressure hot gas pipe pinhole leak into a low pressure environment. It would be something like the pressure in the pipe is 900 F at 800 psi and the external environment would be -100 F at 0.004 psi, the gas would be combustion products, but for now air would suffice. I am really looking at the geometry as well as the temperature distribution of the plume. Since I am new to FloWorks I have been looking at their examples but they really haven't been much use to me other than introducing the basics. So my first question is, would FloWorks be able to handle this extreme environment? I have set up some simple geometries that I think should work, but either I get extremely odd results or I get a solver error. This probably means that my model or boundary conditions are flawed, so does anyone have any suggestions on how to set something like this up or know of any online resources that I could look at. I would think that is should be like modeling a rocket nozzle from the pressure chamber through the nozzle and out into the atmosphere far enough that you can look at the pressure and temperature distribution in the plume, I've done this in Fluent FloWizard but I don't have access to that anymore. If the extremes are too much, I would be willing to decrease the pressures and temperatures, but the pressure ratio needs to be fairly high because that is what I am looking at. Any assistance would be appreciated.
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Old   May 8, 2008, 12:29
Default Re: high pressure jet modeling
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John Parry
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Hi Rufas,

Here's what i've been able to find out: First at all, you should use should use "High Mach Number" option. Secondly, we have limitation for Mach number: Mach number should be less than 10. In considered case it is in average more than 12. In the jet it can reach much more value. To solve such task, I would reduce outer pressure step by step. I would begin from Mach around 5. Solve this, look at what Mach number obtained and then slowly reduce the outer pressure, using the previous results as initial data or continue the calculation.
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Old   May 10, 2008, 18:08
Default Re: high pressure jet modeling
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Rufas
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Thanks for the suggestion John, I will definitely try that. Any suggestions on how to set up a model? Right now I have an ideal pipe with a small hole in it and the inlet flow conditions are known. The nominal outlet flow conditions are known, but a leak will probably alter that a little. I have a large volume surrounding the pipe that I used to define atmospheric pressure, it is offset far enough that the gas jet pressure distribution should settle down to atmospheric conditions well before it reaches the boundary, right now I exhausting to STP. I'm not sure if this is the best way to model the open atmosphere because it makes the model quite large, the last run I did took about a week to compute and the results were really odd. Also, during the run I kept getting a high mach number warning even though I relaxed the flow conditions, is this why my run went to pot? Thanks again for the help.
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Old   May 19, 2008, 07:14
Default Re: high pressure jet modeling
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Kelly Cordell-Morris
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Hi Rufas,

I would suggest using point goals within the small flow to check for convergence - I would use pressure /or velocity goals.

You could utilise the automatic grid refinement also in this case to refine the grid around the leak to properly capture the jet.

I would also add an initial mesh to the surface of the small hole to refine the grid in the leak area.

I would set up the model as a closed tube (ie lids on either end of the tube to add inlet/outlet conditions to), with the small hole for my leak. I would solve the case as an external flow, and set the computational domain so that the edges of the domain abut the outside surfaces of lids of the tube. I would extend the domain in the direction of the jet.

If only my inlet conditions are known I would set those (preferably as a mass flow rate condition) and set the outlet as a pressure (atmospheric or whatever is the condition you have for your model).

I'm not sure what fluid you are using? I'm presuming air? But you can set the high mach flow condition in the set up wizard - when you select air at the bottom of the dialogue box you can set special flow conditions - one of these is high mach number, just select it in the tick box to switch it on.

Hope this helps, Kelly Engineer - Flomerics UK

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