# Limiting factor in design of small gas turbine w/ LOW self sustain speed

• jsh111
jsh111
TL;DR Summary
I would like to design (hobbyist) a small heater with self powering blower based on gas turbine principles with a relatively low self sustain speed, and gas velocity based, rather than pressure based. The idea is to have heat and small power generation in disaster situations where biomass or combustible oils are available, but more refined fuels are not
-Gas turbines, as a general rule require very high rotational speeds (tens of thousands of rpm for small tabletop models) in order to reach a self sustaining speed. Gas turbines are also limited to efficiency based on the pressure ratio generated by the compressor.

-Question: What is the limiting factor that demands such high rotational speeds, and what can be done to lower the self sustain rotational speed such that it is more on par with something like a reciprocating engine? I suspect the answer is based on axial pressures intrinsically requiring high rotational speed to produce any kind of significant compression

-A simplified version of Bernoulli's equation establishes that the pressure and kinetic energy of a gas are interchangeable, and in an ideal system remain constant.

-Question: Therefore is it not possible to design a gas turbine where rather than compressing the air to high pressure, we take advantage of faster moving air instead, where heat applied causes the air to move even faster

OR, suppose we have a high flow, low pressure air mover. can we not simply employ a venturi tube arrangement to expand the air, and therefore increase the pressure

-Is there any prior work that focuses on lower speed gas turbines for relatively low amounts of input thermal energy I might study (I am thinking 5-10kw of input energy). The closest so far I have come are Brayton's piston engine designs which operate on the Brayton cycle, but are not gas turbines

jsh111 said:
Question: What is the limiting factor that demands such high rotational speeds, and what can be done to lower the self sustain rotational speed such that it is more on par with something like a reciprocating engine?
The absence of an enclosed combustion chamber with valves that open and close.

The better solution would be to connect the gas turbine to a gear set to reduce the rpm while increasing the torque to the desired level.

jsh111 said:
Question: Therefore is it not possible to design a gas turbine where rather than compressing the air to high pressure, we take advantage of faster moving air instead, where heat applied causes the air to move even faster
A gas turbine is a low-pressure machine, therefore the need for high velocities to get the same power output as other types of engines.

jsh111 said:
suppose we have a high flow, low pressure air mover. can we not simply employ a venturi tube arrangement to expand the air, and therefore increase the pressure
What would be the point of accelerating the air to slow it down right afterward? You will just go back to the original pressure at best.

jsh111 said:
Is there any prior work that focuses on lower speed gas turbines
By design, gas turbines are high-velocity, low-torque, machines. It would be counter-intuitive to work against that. Again, when you want a lower rpm, you use a gear set:

berkeman, Flyboy and jsh111
The issue with small gas turbines is the scale. Most of the turbine engines I know of below, say, 500hp output rely entirely on centrifugal compressors, with some exceptions using a few axial flow stages before the centrifugal stage to boost performance. Small scale axial flow is effectively impossible due to things like excessive tip clearances, as well as some aerodynamic factors that I don’t fully understand, like how the airflow changes with scale.

At such small scales, often referred to as micro turbines, you need such high rotational speed to overcome the losses from larger gaps and the shorter working distances. From my understanding, you still get similar tip velocities as you would in a larger turbine.

Now, contrary to a lot of people’s understanding, gas turbines don’t actually squeeze the air flowing into them, not like they think. You’re accelerating the airflow and reducing the cross-section of the airflow path to maintain ambient pressure. Then as the air leaves the compressor, it hits a diffuser that expands and decelerates the airflow, causing your pressure rise needed for the combustion and turbine sections to work correctly. Turbines can be optimized for maximizing the velocity of the exhaust, as on a turbojet engine, for extracting as much energy from the exhaust as possible and leaving it as close to ambient pressure and velocity as possible, as in a turboshaft, or something in between, like a turbofan or turboprop. You’re looking for more towards the turboshaft arrangement, if I’m understanding correctly. And that’s going to be really tough at a hobbyist/DIYer level. Especially if you’re aiming for low pressures and low speeds. And that’s not getting into the whole mess of fuel selection, designing a metering and delivery system, developing a heat exchanger for cogeneration of heat and power… It’s going to be a massive challenge.

Honestly, for the scale and use case you’re talking about, I would suggest a Stirling cycle. They’re actually pretty easy to make in a garage/DIY shop, albeit with less than ideal efficiency, but you can certainly upgrade it or replace it easily enough. And since they’re effectively an external combustion engine, you can run it on a wide range of fuels, with differing levels of efficiency.

I totally understand the appeal of the gas turbine approach. There’s a definite “rule of cool” aspect to it. But I strongly suspect that you’re better off pursuing a Stirling engine.

Rive, jsh111 and berkeman
jsh111 said:
The idea is to have heat and small power generation in disaster situations where biomass or combustible oils are available, but more refined fuels are not.
For liquid fuels, I would suggest a lower technology, in the form of an old style diesel engine, without the common rail injection that now requires highly refined fuel.

To include solid fuels, take the advice of Flyboy.
Flyboy said:
I totally understand the appeal of the gas turbine approach. There’s a definite “rule of cool” aspect to it. But I strongly suspect that you’re better off pursuing a Stirling engine.

Any solution must be routinely operated to verify functionality and operator training in maintenance and in the preparation of the different possible fuels.

Rive, jsh111 and Flyboy
jsh111 said:
The idea is to have heat and small power generation in disaster situations where biomass or combustible oils are available, but more refined fuels are not
What about the good* old wood gasification paired with gasoline engines?

Dirty, polluting, damaging and dangerous, but has really low requirements and totally DIY without much fuss.
Also plenty of example projects, starting from wood-powered lawnmowers to electricity generation.

* well, actually there is nothing 'good' with it apart from the fact that it works with everything, from dry grass to leftover wood crumblings from the last century...

Small turbines are plagued by issues caused by boundary layers. As they are caused by surface area small diameters have a larger boundary layer ratio than large diameter. With a homemade jet engine using a turbo the turbine has to produce 105 hp to drive the compressor fast enough to cause enough compression of the air. From memory it’s 40 psi at about 65000 rpm burning 6.5 gallons of diesel an hour to provide 56 lbs of thrust or 52 hp if it was coupled to a freepower turbine. Really bad efficiency caused by the work needed to be done by the compressor due to flow issues using smaller diameters and boundary layer issues.

Flyboy

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