[AR] Re: Estimating Coefficient of Discharge (Cd)
- From: "Monroe L. King Jr." <monroe@xxxxxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sun, 14 Sep 2014 16:49:16 -0700
What about a supercavitating injector?
> -------- 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
> >
> >
> >
> >
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