I'd also like to mention sonic vibrations can have an effect on injectors. > -------- Original Message -------- > Subject: [AR] Re: Estimating Coefficient of Discharge (Cd) > From: Ben Brockert <wikkit@xxxxxxxxx> > Date: Sun, September 14, 2014 10:32 am > To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx> > > > With commercial orifices it's likely to be either some crud that got lodged > in the bigger one or cavitation. > > As the liquid flows through the orifice it trades static pressure for > dynamic pressure. If the static pressure drops below the vapor pressure of > the liquid, part or all of the flow will cavitate. Once that happens the > relation between inlet pressure, outlet pressure, and flow rate gets more > complicated than the default Cv formula. > > Cavitation is a really interesting part of propulsion system design that > often gets skipped over. For example you could build an injector that was > also a cavitating Venturi and it would then be impossible for any pressure > changes (instability) in the chamber to affect the flow rate, thus > preventing injector instability modes. The drawback is that you have to > have at least 30% pressure drop over that sort of injector, which is enough > that a normal non-cavitating injector would be unlikely to have injector > coupled instability anyway. > > Ben. > > On Saturday, September 13, 2014, Graham Sortino <dmarc-noreply@xxxxxxxxxxxxx> > wrote: > > > Thanks Dave, that is a fair point and I should have responded to that > > earlier. They are single hole orifices machined by Okeef ( > > http://www.okcc.com/PDF/NPT%20connections.pdf). I'm suspect they are not > > perfectly 0.023 or 0.035" but I've used Okeef orifices for some time and I > > generally find them to be quite accurate. My hunch is that orifices > > themselves aren't the problem. I was just a bit surprised by such a large > > difference in Cd for not much of a change in orifice diameter. > > > > > > > > On Saturday, September 13, 2014 4:42 PM, David Weinshenker < > > daze39@xxxxxxxxxxxxx > > <javascript:_e(%7B%7D,'cvml','daze39@xxxxxxxxxxxxx');>> wrote: > > > > > > Graham Sortino (Redacted sender gnsortino@xxxxxxxxx > > <javascript:_e(%7B%7D,'cvml','gnsortino@xxxxxxxxx');> for DMARC) wrote: > > > Apologies... yes that was a typo. The orifice diameters are 0.023 and > > > 0.035 respectively. > > > > Ah - how did you measure the diameters then? I'd be even more inclined > > to suspect either or both of my previous suspicions, with such small > > diameters... small burrs etc. can make a significant difference, and > > there's the question of how exactly one has achieved the design diameter > > in actual practice. > > > > Were these tests done with single holes or with arrays of similar holes? > > > > > > -dave w > > > > > > > >