Member postings for Mike Tilby

Thanks Werner, for posting the details of your condensor arrangement. I think I shall make something similar for my tests.

It is interesting to see the data from Turbine Guy and Werner.

Werner, why do you think there is a discrepancy in your temperature / pressure? As far as I can see, the saturation temperature for steam at 4.0 Bar gauge pressure is 152 deg. C. The temperature you measured (148 deg C) was only slightly below this. Do you not think it is within experimental error? Or is there another problem that I have over-looked?

Did you calibrate your thermocouple-based measurement against a known thermometer or are you relying on theoretical voltage outputs from the thermocouple and assuming an accurate amplification factor in your circuitry? The temperature of your exhaust steam is also slightly below the expected temperature of 100 deg C and that is why I ask the question. If both measurements were 4 degrees too low then would not the inlet temperature be as expected after applying a correction?

Mike

Hello Werner

That certainly does look a good test set-up. I hope you have better luck next time.

I like your use of a condenser coil to measure steam consumption. Is it just a coil of copper pipe? Does it work well? What diameter and length do you use?

What type of thermocouple do you use? Are they encased or bare ended?

Regards, Mike

Hi Werner

From my understanding, increase in entropy of the steam as it flows through the pipe will be due to friction and that will give a rise in thermal energy content of the steam which is basically random motion of the water molecules. In a perfectly efficient nozzle there is no increase in entropy as the steam accelerates and all the available thermal energy is converted to kinetic energy as the molecules tend to move more in the direction of bulk flow.

When you say “airplane wing effect” are you referring to loss of pressure in air as it accelerates as it passes over the upper surface of a wing? If steam pressure drops as it flows along the pipe then the steam will have a larger specific volume (i.e. volume occupied per kg). Since mass flow rate is constant that means the volume flow rate must increase and so the velocity must increase. That will require energy and so the temperature of the steam will decrease. A small acceleration of low velocity steam will require very little energy since kinetic energy is propotional to velocity squared.

If I understand you correctly, then you think the velocity increases quite a lot in your pipe. Does that mean the pipe is very narrow?

But whatever loss occurs in the pipe, is it not the steam condition at the nozzle entrance that is important? As Turbine-guy said, if you attach a pressure gauge just before the nozzle, then you will know the pressure and temperature at the point where it matters.

What is the internal diameter of your supply pipe? Is it very small? Knowing the temperature and pressure at the nozzle entrance would allow you to calculate the velocity in the pipe and hence the kinetic energy in the steam. However, it is usual for any pressure drop along the main supply pipe to be small because the pipes are generally much larger in cross-sectional area than the narrow point of the nozzle. By the time steam reaches the narrow point of the nozzle it has expanded to a lower pressure and so the volume flow rate will have increased a lot. This and the small size of the nozzle means a high steam velocity. In the supply pipe the pressure is high so the volume flow rate is small. This fact plus the large cross-section of the pipe means the velocity is low.

I'll be interested to hear if you agree with any of this.

Regards, Mike

Hello Werner, I would think that the drop in pressure at the entrance to your nozzles would result from friction along the pipe and from loss of heat by convection from the outside of the pipe. Of course both these effects will depend on diameter and length and nature of the surfaces of the pipe. Drop in pressure will mean a lower saturation temperature and friction will increase the heat content of the steam. So these effects will act to keep the steam dry. However, loss of heat by convection will tend to lower the superheat and/or increase wetness of the steam. It seems to me that the only way to be certain of the steam condition would be to ensure the steam is initially superheated to an extent that it remains superheated when it reaches the thermocouple at the nozzle entrance. If its temperature drops to the saturation temperature you will not know how wet the steam is since its temperature will not decrease further until it has essentially all condensed. I guess the only way to be sure of the pressure at the nozzle entrance is to have a pressure gauge attached at that point and then you will be able to look up the saturation temperature. (On my boiler I have a thermocouple at the entrance to the nozzle and I'm currently making some connections for pressure gauges before and after the first nozzle of the turbine).

No doubt I've over-looked something in the above waffle so I am ready to be corrected.

Mike

Hi, can I chip in on this very interesting thread which has some impressive photos.

In regard to the design of nozzles. Although, as has been said already, convergent-divergent nozzles are theoretically required to get maximum velocity from steam expanding to below the critical pressure, there were two reports in Model Engineer where people had assessed steam velocity using an impulse plate where the jet was directed against a plate attached to electronic weighing scales. The first report was by Mr Southworth in relation to designing the turbine for his 5" g loco "Turbo" (Southworth (2000) M.E. 185 (4136): 638 - 640). He experimented with many different shapes of nozzle and concluded that the best result was with a plain convergent nozzle which out-performed convergent-divergent nozzles of various angles and lengths - all with the same throat diameter. His article prompted a letter from Tom Jones and Professor Bill Hall (Jones & Hall (2001) Model Engineer 186 (4138): 65). They reported coming to exactly the same conclusion in their own tests on injector nozzles. It seems possible that in miniature nozzles effects such as increased friction in very small diameter ducts make it difficult for the steam to attain supersonic velocities and if sub-sonic steam flows into the divergent part of the nozzle then it will be slowed down instead of accelerated. In both those studies steam from conventional loco boilers was used. However, for their turbines, Mr Bamford and Prof Chaddock used steam at very high pressures and temperatures from flash boilers, so I guess the convergent-divergent nozzles might have worked O.K. under those circumstances. So it seems to me that the question of nozzle design is very much open to doubt and, of course, Werner has had good results with plain nozzles.

Regarding small diameter end mills, I buy them on Ebay where you can often find packs of 10 0.5mm cutters for less than £10.

Regards, Mike

Thanks for the messages of encouragement. I've just created an album called "Micro-milling contraption".

When I started this project I was a bit surprised that hobby-oriented milling equipment advertised as micro-mills generally seem to have max rpm of perhaps just 2,000 rpm whereas professional "mico-milling" is ideally done at upwards of 40,000. The Proxxon grinder that I use runs at up to 20,000 rpm. I have the impression that this is not really fast enough for cutters as small as 0.5mm diam. but it appeared decent quality and was a lot cheaper and smaller than a spindle made by Kress or other professional brands. So far it has been working O.K.

I'd be interested to hear if anyone has experience of micro-machining with similarly sized milling cutters.

Mike

Hello

(I posted this in the wrong place initially so hopefully I'll get it right this time.)

I've been lurking around this site for quite a while so I guess it is about time I was sociable and stuck my head above the parapet. I'm in Newcastle upon Tyne and my main interest is model steam turbines plus a part finished 3.5"g Caribou that I inherited.

A few years ago I built an electronically controlled flash boiler (inspired by the steam plant built by Ian Gerrard). This gives a known rate of generation of steam at a measured temperature which is starting to prove useful for testing nozzles etc. That project also gave me the electronics confidence to have a go at building an electronically controlled micro-milling contraption which I'm using to machine miniature turbine blades and nozzles. It uses tungsten carbide micro-milling cutters down to 0.5 mm diameter. (I have a few 0.3mm diam. cutters but so far have not dared use them). Currently I'm attempting to build a multistage turbine - which I realise may be a failure. The journey is more important than arriving at the destination (I suppose).

I'd be very interested to hear from or about other people who use similar micro-milling cutters as I've not come across anythng in the magazines.

Mike

Sorry for barging in on this thread. I posted this message in the wrong place. I thought I was starting a new thread and realised my mistake as soon as I had clicked on the button. I've just posted the message again - hopefully as a new thread this time.

Thanks for the response Andrew - I'd best reply in the proper thread.

Mike

Hello

I've been lurking around this site for quite a while so I guess it is about time I was sociable and stuck my head above the parapet. I'm in Newcastle upon Tyne and my main interest is model steam turbines.

A few years ago I built an electronically controlled flash boiler (inspired by the boiler built by Ian Gerrard). This gives a known rate of generation of steam at a measured temperature which is starting to prove useful for testing nozzles etc. That project also gave me the electronics confidence to build an electronically controlled micro-milling contraption which I'm using to machine miniature turbine blades and nozzles. It uses tungsten carbide micro-milling cutters down to 0.5 mm diameter. (I have a few 0.3mm diam. cutters but so far have not dared use them). Currently I'm attempting to build a multistage turbine - which I realise may be a failure. The journey is more important than arriving at the destination (I suppose).

I'd be very interested to hear from or about other people who use similar micro-milling cutters as I've not come across anythng in the magazines.

Mike