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.

Monday, 4 April 2016

Measuring windage.

Windage is defined in the dictionary as "the air resistance of a moving object ... such as a rotating machine part". For a pelton therefore it is the resistance encountered by the cups of the runner as they turn through the air in the casing. What is in the casing though is not just air but the more dense medium of water droplets suspended in air. There is a veritable fog of spray created by the water jets striking the cups.  

Because windage is a resistance to rotation, it causes power to be lost from the power available for generating electricity.  The loss is unavoidable, though it can be mitigated by designing the pelton casing so that spent water and spray are kept away from the rotating runner.   In small turbines like the Powerspout this is almost impossible to do.

Normally, power lost to windage is very difficult to measure and can only be estimated mathematically. But in a 'Eureka moment' recently I realised that two features of my installation provide a unique opportunity to actually measure windage: the turbine is situated outside so removal of the transparent glazing wouldn't matter, and secondly I have a record of power output captured continuously and visible on-line*.

So this afternoon the experiment was done: at a precisely noted time, the glazing was removed whilst the turbine was operating; the turbine was left to run for just under 15 mins with the glazing off and then, without stopping the turbine, the glazing was put back on.

Here are some pictures and below them, the resultant power trace captured from the on-line record:

The power trace unequivocally shows a jump in power output coinciding precisely with when the glazing was removed and replaced.  There can be no doubt about its authenticity.  The actual gain in power amounts to 20 watts (~705 to ~725 watts).

One can speculate that the total power lost to windage will actually be double this because, ...on the assumption a jet strikes the splitter ridges dead centre and is split exactly in half, ...there will still be the windage loss arising from the half bouncing back from the bulkhead.  So it's fair to say that windage loss probably totals 40 watts, which is just under 6% of the power output at the flow used for the experiment.

The flow used was 2.78 l/s.  So in the 15 minutes of the experiment 2.5 tons of water sprayed out (2,500 litres): not an experiment for those whose Powerspouts are housed in smart enclosures !

It was fun to do and pleasing to get the result I sort of expected.  My thanks to Paul Jones, my neighbour, for helping with it and being the one who got wet.

*You can view the actual record here but scrolling back to 4th April 2016 will be tedious for people who read this even a few days after the day of the experiment.


Anonymous said...

Bill, Are you measuring windage? Or does the improved reading relate to the unrestricted outflow of the turbine. Most of the discharge appears to come from a very small area centred close to where the jet impinges on the wheel. Does the guard pose some flow restriction from this point? Jim.

Bill said...

Jim, -your point is a good one: it is probably a jump too far to assume that the power gained by removing the glazing is due only to reducing windage. The discharge performance of the turbine, which I understand to mean the effectiveness of water leaving the runner, and also leaving the close vicinity of the runner, will both have been improved. Put another way, the pelton will have felt itself to be less choked by its exhaust flow with the glazing off. So yes, you are quite right, and it is probably the combination of improved discharge performance and, to a smaller extent, less windage which together explain the surprisingly high figure for power gained that I got. Bill

Hugh said...

hi Jim and Bill,
We are probably quibbling over semantics here but I will offer my opinion. I do not believe that the constriction of the spray as it bounces back onto the runner has any effect on the flow. The flow is purely governed by the pressure difference between inside the pipe and out in the atmosphere. What happens after that is not going to feed back into the discharge from the nozzle.

Having said that, we are not exactly looking at wind, but I do believe we are looking at drag on the runner caused by spray and it's hard to suggest a better category of loss to describe it other than windage.

Michael at PowerSpout made these comments:
"Yes we are aware of this, the main problem is that to avoid this you need a large or no casing, so it practice it is very hard to solve if you want a small turbine that will fit in a box to move about the world.
The case was designed to fit in a DHL carton, then DHL allowed us double the volume, so if we were to design a case again we would add 50-100mm front and back for more exhaust flow and bounce-back clearance. The cost to do this change on a large mould would be prohibitive at this stage.
Some companies have the PLT running in the horizontal plain and the upper bulkhead curved to guide the exhaust flow around - but much falls back in.
So yes, in your case a front extension may give you an extra 40W, if you wish to implement this you will need a longer shaft, so as to get some distance from the rear bulk head. It may then be best on twin jets as your bearing will be a long way away."

I (Hugh) have built several casings for vertical shaft peltons (running in horizontal plane) and when I designed them so the spray followed the casing outward rather than bouncing back down onto the runner it did improve performance.


Hugh said...

Whilst on the subject, Bill, I wonder if you have considered using a draft effect to reduce air pressure in the casing and get more power. That would be right up your street.

Close off the sump and use a draft tube below to suck the pressure down. You need to take care the waste water does not back up and flood the runner due to air being removed by the waste. You need a small vent but sized to allow some suction to develop. This can give more power due to greater velocity and reduced windage. A tiny bit. It's routine on Crossflow turbines.