Venting Analysis
Venting Analysis
(OP)
Hi all,
I have to perform a venting analysis to determine how many holes I have to introduce in my waveguide assembly so that there is not a big pressure differential when the assembly is going through vacuum (it is space application).
I have one article dealing with Venting Analysis, 'Venting of Space Shuttle Payloads' by Mironer and Regan but unfortunately the differential equations they provide does not make lots of sense and it cannot be numerically integrated since really unstable.
I am wondering if someone else has already performed a venting analysis and what was his reference.
Franck
I have to perform a venting analysis to determine how many holes I have to introduce in my waveguide assembly so that there is not a big pressure differential when the assembly is going through vacuum (it is space application).
I have one article dealing with Venting Analysis, 'Venting of Space Shuttle Payloads' by Mironer and Regan but unfortunately the differential equations they provide does not make lots of sense and it cannot be numerically integrated since really unstable.
I am wondering if someone else has already performed a venting analysis and what was his reference.
Franck





RE: Venting Analysis
I take it one issue is the varying vent pipe outlet pressure. The outlet pressure drops as altitude increases, and this pressure drop will happen quickly. The second issue is the possibility that the outlet pressure drops to the point that a shock wave forms at the vent outlet. If for example, the outlet pressure is a perfect vacuum, then virtually any significant flow will result in a shock forming at the outlet.
Anderson Greenwood manufactures relief valves and came up with their own analysis where there is a shock wave created at the pipe outlet. So that paper is one possibility, and I can send you a copy if you'd like, or call AG directly. Still, I wouldn't swear by it since I've also done analysis using programs that come up with considerably different results.
I have to believe there are numerous packaged programs available, though they may have a problem with the outlet conditions. I've often created my own itterative programs for something like this using Excel, and just your typical equations for compressible gas flow straight from Crane Paper #410.
RE: Venting Analysis
By any chance do you have the name of some programs that you used?
RE: Venting Analysis
In addition boundary/initial conditions--for example is the intitial pressure atmospheric / temp20C and then linerally (or otherwise) the outside pressure is brought to space/vacuum conditions over some finite time., etc.
If initially atmospheric, then perfect/ideal gas conditions are a good assumption.
RE: Venting Analysis
RE: Venting Analysis
Why are you using dif-eq's? Why can't you use algebraic formulas and use numerous iterations? I have to assume you know outlet pressure (ie: atmospheric pressure) as a function of time, so doing iterative time steps on a spread sheet and ensuring your analysis is conservative should be sufficient to ensure success.
On vent lines where a tube is open to atmosphere, I've only ever assumed a resistance coefficient, not a discharge coefficient. The resistance coefficient given by Crane is 1 if memory serves.
One other thought, is the vent line open with a single opening, or is it open to a T? If it's open with a single opening there is a small thrust created. This has to be reacted by the tube supports, and may even create thrust loads which have to be corrected for if they are high enough.
RE: Venting Analysis
1)The internal fixed volume of the wave guide to be adiabatic. Therefore in that volume pressure is related to specific volume as PV^k =constant or p/(m^k)=constant.
2) Mass rate of flow is simply orifice with a Cd or piping with a resistance. The flow is therefore dependent upon the upstream pressure of the wave guide, back pressure (if not choked) and flow resistance.
Coupling 1 and 2 together will result in a differential equation which can be numerically integrated.
I am curious if any of the above is included in the paper authored by Mironer and Regan?
RE: Venting Analysis
RE: Venting Analysis
I'd appreciate the paper.
sailoday28@yahoo.com