[AR] Re: Estimating Coefficient of Discharge (Cd)
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Sun, 14 Sep 2014 12:32:10 -0500
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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