choked flow pressure drop
choked flow pressure drop
(OP)
How can dp be approximated at near choked flow in vicinity of silencer?
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choked flow pressure drop
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RE: choked flow pressure drop
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: choked flow pressure drop
Katmar has a very good point--sonic velocity is incompatible with the need for a silencer. It is usually more economic to dump your pressure in stages and deal with smaller units of noise than to take it all at once and try to silence sonic flow.
David
RE: choked flow pressure drop
Here is a URL for free online calculators dealing with compressible flow components that you might find useful:
http://members.aol.com/engware/calc5.htm
Thanks,
Gordan
http://engware.i-dentity.com
RE: choked flow pressure drop
RE: choked flow pressure drop
The flow prediction and pressure distribution defined in the asme sect VIII appendix is based on the bechtel Liao method, but it does not directly recognize the possible restriction caused by the silencer. In my opinion, the problem would be solved as follows:
a) confirm the net open area of the silencer is much larger than the exhaust pipe between relief valve and silencer.
b) initially assume the system will provide a pressure at the relief valve elbow lower than required for full relief valve flow rating.
c) for that design flow , work backwards thru the silencer for the min required pressure entering the silencer. Then confirm that the the velocity of the fluid entering the silencer / exiting exhaust pipe is below sonic , ie mach number < 1.0.
d) for a mach no < 1.0 at the exit of the exhaust pipe, calculate the pressure drop in the exhaust pipe by breaking the pipe into 100 smaller segments ( ie divide total fL/d by 100) and incrementally work backwards to the min required pressure at the inlet of the exhaust pipe. exit of releif valve elbow.
e) confirm this elbow exhaust press is lower than required for full relief valve rating.
f) if the velocity calculated from step (c) is above mach no of 1.0, then there is a shock wave at the inlet to the silencer. Increase the pressure at the outlet of the exhaust pipe ( upstream of the shock wave) until that mach number = 1.0, then use the bechtel Liao method ( fanno curves) as outlined in sect VIII to compute pressures thru the piping system.
g) step d can alternately be computed using the fanno curves, but I'll leave that for a student homework assignment
RE: choked flow pressure drop
I doubt a mfgr would give that information for high Mach numbers.
Regards
RE: choked flow pressure drop
The fanno relationships are used in the formulation of the sect VIII curves- the assumption is a constant area along the length of each component, in particular, the exhaust stack . However, the sect VIII curves limit were limited to low fL/d values- to extend this correlation to longer exhaust pipes one needs to directly use the fanno equations for frictionally choked flow .
RE: choked flow pressure drop
Thanks
Gregg
RE: choked flow pressure drop
See B31.1 nonmand app II chart II-1
RE: choked flow pressure drop
Gregg
RE: choked flow pressure drop
RE: choked flow pressure drop
It really is a shame how much esoteric arithmetic is included and how much useful knowledge is omitted.
David
RE: choked flow pressure drop
Other choked flow situations where compressible fluid mechanics are used include:
a) relief/safety valve - flow thru the relief orifice
b) flow thru relief valve exhaust stack
c) reheater /LP steam dump to condenser - flow thru sparger holes into condenser steamspace
d) choked flow across valves,including turbine bypass valves, vent valves, turbine control valves during startup ops
e) drain flow to condenser drip inlets
f) inst air to pnuematic actuators during fast valve stroking
No rockets or jets or missiles involved in powerplants ( unless they are located in the mideast) , but most college profs that write books on compressible fluid mechanics have only been funded thru defense projects, so they only write about defense devices.
RE: choked flow pressure drop
RE: choked flow pressure drop
The pressure ratio (Downstream Pressure / Upstream Pressure)of a choked flow condition is almost equal to 0.5. You may try to test this by plotting a Fanno line graph and asses what the pressure ratio is at the maximum value of the curve. In plotting the fanno line you need to use very accurate enthalpy values to avoid large errors in the calculation of the flow velocity
Philip Oosthuizen
Company info:
SteinMuller Engineering Services
http://www.steinmuller.co.za/