Steam turbines, spreadsheets, and a few observations.

[Nerd1000]

As a result of ongoing discussions, I've been working on a steam turbine design spreadsheet. This is the latest (probably final, for now) model.

New features compared to the Mk II version posted on the tesla turbine thread:
Condensation within the turbine is now considered, and a target exit steam quality can be set. Generally this would be in the range 0.9 to 1.
Calculation of convergent-divergent nozzle throat sizes.
A method for estimating the partial admission losses on the first stage, this can be used to set the losses parameter to an appropriate value.

Funky issues:
The convergent-divergent nozzle throat calculations can't handle partially condensed steam, and will sometimes specify a throat larger than the outlet if there is condensation in the nozzle. I don't even know what should actually happen when steam is condensing in the gas stream as it exits a supersonic nozzle. Perhaps it would be best to only use subsonic nozzles in condensing stages.

So, some general observations.
Model sized turbines have a big problem: If you want a practical size of turbine, you must use partial admission (not all of the turbine rotor is in front of nozzles). This rules out using reaction turbines, and causes big problems for impulse turbines too because partial admission can dramatically reduce the turbine efficiency. In fact in small turbines it tends to be by far the largest source of losses. The issue is generally worse when the system is operating at higher pressure because the steam is denser. Multiple pressure stages tend to amplify the issue because the losses apply for every stage! Velocity staging can be a bit of a workaround if only because using it tends to result in larger nozzle areas per pressure stage, but generally the efficiency is poor. This issue starts getting much more manageable above a few kW rated output... but that's a lot for a model!

So for anyone hypothetically wanting to power their house with a little steam turbine, not good news. I think the best operating condition for a small scale steam turbine would be as the second stage after a reciprocating expander. Then it can work in low density pre-expanded steam and have a much higher degree of admission- still not full admission, but enough that the efficiency isn't reduced too much. The other thing that helps a lot is strong condenser vacuum, as it allows everything to work at lower pressure and density.

 

[Toymaker]

As a result of ongoing discussions, I've been working on a steam turbine design spreadsheet. This is the latest (probably final, for now) model.

New features compared to the Mk II version posted on the tesla turbine thread:
Condensation within the turbine is now considered, and a target exit steam quality can be set. Generally this would be in the range 0.9 to 1.
Calculation of convergent-divergent nozzle throat sizes.
A method for estimating the partial admission losses on the first stage, this can be used to set the losses parameter to an appropriate value.

Funky issues:
The convergent-divergent nozzle throat calculations can't handle partially condensed steam, and will sometimes specify a throat larger than the outlet if there is condensation in the nozzle. I don't even know what should actually happen when steam is condensing in the gas stream as it exits a supersonic nozzle. Perhaps it would be best to only use subsonic nozzles in condensing stages.

So, some general observations.
Model sized turbines have a big problem: If you want a practical size of turbine, you must use partial admission (not all of the turbine rotor is in front of nozzles). This rules out using reaction turbines, and causes big problems for impulse turbines too because partial admission can dramatically reduce the turbine efficiency. In fact in small turbines it tends to be by far the largest source of losses. The issue is generally worse when the system is operating at higher pressure because the steam is denser. Multiple pressure stages tend to amplify the issue because the losses apply for every stage! Velocity staging can be a bit of a workaround if only because using it tends to result in larger nozzle areas per pressure stage, but generally the efficiency is poor. This issue starts getting much more manageable above a few kW rated output... but that's a lot for a model!

So for anyone hypothetically wanting to power their house with a little steam turbine, not good news. I think the best operating condition for a small scale steam turbine would be as the second stage after a reciprocating expander. Then it can work in low density pre-expanded steam and have a much higher degree of admission- still not full admission, but enough that the efficiency isn't reduced too much. The other thing that helps a lot is strong condenser vacuum, as it allows everything to work at lower pressure and density.

You've clearly put in a great deal of work into your spreadsheet,...are you planning to build a 4 stage axial flow turbine ?

 

[Nerd1000]

You've clearly put in a great deal of work into your spreadsheet,...are you planning to build a 4 stage axial flow turbine ?

Heh no. I just get obsessed with something technical and have to understand it sometimes.

 

[Toymaker]

Model sized turbines have a big problem: If you want a practical size of turbine, you must use partial admission (not all of the turbine rotor is in front of nozzles).

Why must partial admission be used?? The tiny gas turbines made for model airplanes , the few that use axial flow blades in the hot section, use full admission. Why is steam flow different ?

 

[Jasonb]

Long thread on ME about model turbines with plenty of practical examples.

https://www.model-engineer.co.uk/forums/topic/model-turbines/

 

[Nerd1000]

Why must partial admission be used?? The tiny gas turbines made for model airplanes , the few that use axial flow blades in the hot section, use full admission. Why is steam flow different ?

gas turbines develop far more shaft power than steam turbines for a given net output, it would not be unusual for 60% of the power developed by the turbine stages to be used driving the compressor. Also the working fluid is less dense, in large part due to lower pressure ratios. So the volume flow through the turbine is much greater. This is further increased by the fact that they pump lots of extra air just to limit the turbine inlet temperature to something the turbine can withstand. Finally they actually develop quite a bit of power! This 64mm unit develops 2.5 kW at the output: https://www.kingtechturbinesaustralia.com.au/product-page/k30tpg4

I'd guess it's developing at least 3 kW on the gas generator turbine.

A steam engine avoids the large compressor power demand by condensing the steam into water, because this reduces its volume by a huge amount the pump work is greatly reduced and almost all of the power from the engine drives the load. Unfortunately there are no free lunches, so the tradeoff is that all the latent heat used turning water into steam is for the most part unable to be used by the expander. This is why superheating is so great, you basically get to use all of the heat added in the superheater to do work, vs only a fraction of the heat added in the boiler itself.

 

[Toymaker]

it would not be unusual for 60% of the power developed by the turbine stages to be used driving the compressor.

Agreed. In fact, most modern aviation turbines today are twin-spool designs, with a few 3-spool designs. The primary tasks of the core turbine, (often referred to as the gas generator) in a twin-spool engine, is to turn the compressor for the core section and supply enough exhaust gasses to turn the outer spool, which turns the fan and provides most of the thrust.

Also the working fluid is less dense, in large part due to lower pressure ratios. So the volume flow through the turbine is much greater. This is further increased by the fact that they pump lots of extra air just to limit the turbine inlet temperature to something the turbine can withstand. Finally they actually develop quite a bit of power! This 64mm unit develops 2.5 kW at the output: https://www.kingtechturbinesaustralia.com.au/product-page/k30tpg4

I'd guess it's developing at least 3 kW on the gas generator turbine.

A steam engine avoids the large compressor power demand by condensing the steam into water, because this reduces its volume by a huge amount the pump work is greatly reduced and almost all of the power from the engine drives the load. Unfortunately there are no free lunches, so the tradeoff is that all the latent heat used turning water into steam is for the most part unable to be used by the expander. This is why superheating is so great, you basically get to use all of the heat added in the superheater to do work, vs only a fraction of the heat added in the boiler itself.

So, can you link any of your statements above to explain why partial admission must be used?

I ask because the only reason I can think of to use partial admission in small steam turbines is, "to be able to actually machine an ideal CD nozzle configuration"; all full admission nozzles are a compromise between what can be machined in an annular configuration and the ideal CD shape.