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.

Tuesday, 11 August 2015

Energy conversion.

I've had a bit of fun this morning trying to photograph the Powerspout with its front window off. Operating as I am now at pretty well the lowest flow possible (0.56 lps), it was the best opportunity to see, without causing a major flood, what the pattern of water discharge from the runner buckets looks like unobstructed.

The physics underlying the operation of a pelton is, on the surface, simple: the pressure energy of the water in the penstock is converted to kinetic energy in the water of the jet emerging from the nozzle, which is then converted into rotational energy in the turbine shaft by the transference of momentum from water to runner. 

This time last year I posted this picture of a jet without the turbine being in place, - the jet travelled for a distance of 15 metres:




The picture gives a good idea of the energy there is in the jet but today's photographs show how completely that energy is transferred to the shaft by means of the pelton cups:






The point to note is that the energy of the water leaving the runner buckets is now only sufficient to carry it a short distance, about 2 metres, and the direction it travels in is more or less at 90⁰ to the original line of the jet: all the forward momentum has been extracted and transferred to the runner.

I had hoped to get a close-up shot of the jet hitting the splitter ridges of the runner, but insufficient light and too much spray thwarted this.  Nevertheless, it was nice to see the pattern of discharge free from it being blocked by the perspex window.  Though a simple machine, an analysis of what goes on as the pelton cups pass through the jet leads to an appreciation of how complex the pattern of flow actually is.  Something of this complexity can be seen in the three dimensional 'fan' of water which emerges.

It's good to be reminded that things which appear simple at first glance sometimes turn out to be quite complex.

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