[AR] Re: Estimating Coefficient of Discharge (Cd)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 15 Sep 2014 20:34:16 -0400
On Mon, Sep 15, 2014 at 02:38:38PM -0700, Graham Sortino wrote:
>Additionally, if anyone has any perspective on my other question
>regarding why/how cavitation occurs in a liquid flow I'd really
>appreciate it. I suspect this is a basic concept that I'm not getting
>but I had never consider this as a factor before since I assumed my
>liquid flows would always be above the vapor pressure (hence
>cavitation should not occur?).
When fluid gets moving fast, the pressure goes down, per Bernoulli's
law. Pressure energy is getting converted into kinetic energy. At
the output end of the injector orifice, there's no pressure (beyond
ambient), just velocity.
This will be less of a factor with a running engine, since unlike your
water tests, that will have chamber pressure at the output end, not
just atmospheric pressure. (On the other hand, it'll also have heat,
which will raise the vapor pressure.)
Furthermore, at the entrance to the injector hole, you have fluid
going around a corner, and when that happens there's generally a zone
of lower pressure behind the corner. (Though this is mitigated
somewhat by the fact that the fluid is converging into the hole, and
might be mitigated further if those orifices happen to have a smooth
radius at their entrance, rather than sharp corners.)
--
Norman Yarvin http://yarchive.net/blog
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