The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Friday, 20 January 2017

After waiting...

After waiting a month for rain, I've given up waiting any longer, - yesterday I got on with measuring values for the discharge co-efficient of my nozzles.  This was the job left hanging at the end of the last diary entry and is part of my aim to refine the accuracy of knowing what flow the jets deliver.

At this time of year I had hoped that rain would have been increasing the flow and that I could have done the measurements at each increase in nozzle size with little inconvenience; there being no rain, no increase in flow and no call to increase nozzles, all I could do was measure the smaller nozzles one after the other in a single blitz of measurement.

It made for a lot of exercise up and down the hill between turbine and header tank, - and for an interesting record in the on-line power trace:




The results of the experiment are presented in the following graph; on it I have also indicated the flow each nozzle delivered, as determined by measurement:




I was only able to measure the seven smallest nozzles, identified by Roman numerals I to VII; apart from one outlier in the measurement of nozzle IV (which I am at a loss to explain**), the others came in with discharge coefficients scattered around the 0.85 value; there was no obvious trend for Cd to increase or decrease with change in orifice size; I conclude from this that Cd is determined only by the design of the nozzle, - design that includes such features as its angle of taper, its length and the smoothness of the inner surface over which water is flowing. Whether Cd changes with head was something I couldn't determine with this experiment conducted as it was at one site.

Thus far so good: - from now on I will take 0.85 to be the discharge coefficient.  In time there will be opportunity to take measurements for the larger nozzles VIII to XII and when available these will add weight to the picture so far obtained.

Does all this cause the earth to shift on its axis of rotation ? - no but it's still worth investigating !

**later I discovered this nozzle was badly cut explaining why it was different.

Addendum dated 2 Feb 2017
Today copious rain has made it possible to measure the Cd and flow of the remaining 5 nozzles; the results are shown in the revised chart below; overall the Cd of EcoInnovation nozzles on my site seems to be 0.85 ± 0.01.



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