Big Turbo based engine and Afterburner questions......

[finiteparts]

Matt,

I wanted to clarify something I said..."I am not going to sugar coat it, that turbine housing looks awful."

I only mean that it doesn't look good "aerodynamically", to have the flow entering the NGVs at completely different angles depending on which side the flow goes. Maybe when you get it taken apart, they have designed for this by making the NGVs different angles from one side to the other. I don't want to be the voice of discouragement...and I was afraid that I might not have been clear with that statement.

I am sure that even if they were not optimal, it will make a good running engine.

Good luck!

Chris

[mjb777]

Thanks for all the discussion so far guys!

I've been flat out with work, but have been following the discussion when I can........ Lots to think about, particularly with the combustion chamber. Very interesting stuff, and I plan to really sit down and analyse all that as been said etc prior to committing to a configuration. I am still however biased toward a low pressure/ low power vapoiser fuel delivery system.

Finiteparts, no harm done at all your comments make sense. I would love to squeeze a pile of thrust out of this thing but at the end of the day however realistically I suppose that none of the turbo based engines are really candidates for amazing outputs because they simply aren't designed for this application and I imagine that there are losses all over the place with all of the turning etc that the air and gasses have to follow etc.

I would guess that even though the entry into the NGV is not purely radial, this would have been a manufacturer design consideration, and that inlet gas flow from the combustor duct and elbow will be much tidier than pulsed flow from a slow revving big diesel engine?

Matt.

[mjb777]

Something that I have been thinking about, particularly because of both the low max TIT limit and the large NGV throat area and turbine in my turbo, is water injection.

I found a document online about a study that was done for power station engines, and interestingly it was claimed that up to 10% mass flow increase was achieved. I worked out from the graphs in the study that at my rough max mass flow of about 1.4kg/sec that perhaps I could achieve this 10% mass flow increase, hopefully resulting in more thrust , and hopefully a significant TIT drop, for about 2% water/ mass flow or about 1.68kg water per minute, at full blast.

It also seemed that the 10% mass flow increase meant a 10% P2 pressure increase. I'm guessing that this was a fixed RPM and configuration engine and I have the luxury of being able to size my exhaust nozzle as needed for optimum P2 and TIT.

I've used water injection on boosted piston engines engines, however have only a theory level background with water or water/meth in turbines and I wonder if the reduction in TIT will mean a reduction in thrust that will null out the mass flow increase? The engine in the study was a shaft output engine so I am guessing the mass flow increase was beneficial for power extraction from the power turbine, however my only knowledge of water injection in thrust engines is that it was primarily used for TIT/ EGT margin at takeoff thrust when needed. I'm guessing I can tune water flow and exhaust nozzle area to optimise it all.........

Would the combustor geometry/ holes need to be changed to allow for the 10% higher mass flow, eg. as if the compressor inducer area was 10% larger?

Anyone have any experience with this, particularly with our turbo charger based engines?

Matt.

[racket]

Hi Matt

Low pressure vapouriser it is .........they work OK , just need to source some good Inco tubing .

Our turbo based engines actually do remarkably well when it comes to thrust production , they're up there with the early low pressure ratio centrif comp equipped jet engines of the 1950s , similar pounds of thrust per pound of air per second, our only shortcoming is the size of the hole in the front limiting mass flow ...............a 3 or 3.5 :1 pressure ratio isn't a shortcoming when talking pure thrust engines as having lotsa pressure in the jetpipe doesn't mean a commensurate increase in thrust , theres a "square root" in the equation , it needs ever increasing amounts of pressure to produce ever smaller increases in thrust .

Cheers
John

[racket]

Hi Matt

As for your second posting about water injection , it'll be a good candidate if you have a NGV with excess throat area , you can pump some water in to make steam to full those throats.

A lot of the data on water injection is for large engines where theres both time/distance/space and high compression temperatures and aren't as relevant to us as first sight might indicate .

Generally we have an ~10 C degree drop in "static" air temp going into the comp as the air speeds up to ~500 ft/sec , so to avoid the possibility of ice , inlet water injection is only used in warm to hot condition .

The next obstacle to its "efficient" use at the inlet is time , assuming our 500 ft/sec inlet air speed at full power , this is the same speed as large engine use , but our engines are small so the time available for a droplet to evaporate is miniscule , our distance from injection to comp stage discharge might be 1 foot max , so 2 milliseconds , but we want the water to be evaporated before the comp inducer or within the wheel at the most , this then brings the time frame back to 0.5 -1.0 milliseconds ..........bugger all to evaporate any but the very smallest droplet .

This isn't the case with a large aero engine as the distances can be many feet and due to the higher pressure ratios in the later stages the water will boil rather than evaporate , we can't boil water in our comp wheels , the static air temp simply isn't high enough , its only once in the diffuser that it'll boil , but its too late for most of the benefits .

Also in large engines with their many axial compression stages the benefits are at the latter stages due to the cooler air being much easier to compress than hot air at 200+ C , this then means less power required from the turbine so more energy left over for thrust or shaft horsepower .

Have a look at this Paper ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092063.pdf

and maybe this one as well ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930085365.pdf

and ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092050.pdf

You'll need extremely fine atomisation at the inlet , it needs to be positioned as far from the comp inducer as possible to increase dwell time and evaporation and cooling , plenty of methanol in the mix , or even just straight methanol for max cooling then with flametube water injection to further increase mass flow thru the turb.............lotsa possibilities with that big turbine wheel you have :-)

My rudimentary experiments with water injection on my TV84 back 2000 didn't instill me with confidence of much thrust increase at all , but my turb stage was running choked at the exducer without any extra "steam" ,whereas yours won't be , that extra flow capacity needs to be exploited .

Cheers
John

[mjb777]

I could read those NACA/ NASA docs all day long!

It will be very interesting to see what thrust I can achieve with combined water and afterburner augmentation with this turbo.

I've been reading other NACA/ NASA docs re. variable nozzle efficiency and that raises some additional questions. Even though it is very appealing to have a variable nozzle, it may cost me thrust, and at the end of the day I'm aiming for maximum performance at full blast, and not maximum SFC through a range of power settings. It seems variable nozzles are a compromise......

Something else that I wonder from reading that stuff, is if a variable nozzle is used, whether the variable nozzle should be positioning to achieve a limit/ target EGT or P2?

Matt.

[racket]

Hi Matt

When you've fully developed your engine using a plain jet nozzle you'll end up with certain temperatures and pressures in the jetpipe , those parameters won't/shouldn't change if either a freepower turbine stage or an afterburner are fitted downstream in place of that plain jet nozzle .

Your P2 will be set by your rpm/tip speed limits , though this will/can vary with the mass flow for any rpm depending where on the map you are flowing, generally the P2 will drop the further we flow towards the choke/right hand side of the map , and increase towards the surge line. ............for us guys not overly interested in SFC we can compromise a bit and flow towards the choke side if we get a mass flow/thrust benefit without too much of an inefficiency with compression , .............compromises.

Cheers
John

[ripp]

Hi John,

Your statement about P2 and temperature I can not sign!

By change the nozzle or connecting a freepower turbine, the pressure and the temperature change significantly.

Theoretical example: In the case of full-throttle, Anders JU-01 has the following values: 550 ° Celsius and 0.4 par in a straight nozzle (diameter is equal in length). By changing the nozzle or connecting a smaller freepower turbine, the values increase to 750 ° C and to 0.7 bar pressure. that would be the goal. The engine must be develop in this way in order to achieve a low exhaust gas temperature at full throttle

Cheers
Ralph

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[racket]

Hi Ralph

NO NO NO .

We don't want a low gas temperature at full throttle , low temperatures mean little thrust or horsepower , we want temperatures up at the limit of safe turbine survival .

A gas turbine gas producer can't differentiate between a plain jet nozzle, an afterburner, or a freepower turbine , all the gas producer feels is a restriction downstream that produces backpressure.

I think the "translation" of "plain jet nozzle" is the problem , it appears you understand it as a straight parallel walled jetpipe , whereas its meant to be a straight section of pipe with a cone at the end which has a smaller diameter opening than the pipe diameter , I used the word "plain" to differentiate from an "afterburning" jetnozzle which will be considerably larger in diameterto handle the lower density gases .

Cheers
John

[CH3NO2]

Hi Ralph,

Maximum efficiency occurs at maximum temperature and pressure drop.
The most efficient engine conversion of thermal energy to work energy occurs at the greatest temperature and pressure the mechanical system can tolerate.

Tony

[ripp]

Oct 24, 2017 3:30:30 GMT -5 racket said:

Hi Ralph

NO NO NO .

We don't want a low gas temperature at full throttle , low temperatures mean little thrust or horsepower , we want temperatures up at the limit of safe turbine survival .

A gas turbine gas producer can't differentiate between a plain jet nozzle, an afterburner, or a freepower turbine , all the gas producer feels is a restriction downstream that produces backpressure.


Hi Ralph,

Maximum efficiency occurs at maximum temperature and pressure drop.
The most efficient engine conversion of thermal energy to work energy occurs at the greatest temperature and pressure the mechanical system can tolerate.

Tony

I totally agree!

but a jet nozzle, an afterburner, or a freepower turbine produces backpressure.
backpressure increases the pressure in front of the freepower turbine (more power is generated)
but also the temperature increases. so the turbine gas producer must run cool. this statement can be read in the books of Kamps and Schreckling. I have also experienced it while testing my kj66 turbine.
without nozzle the machine ran ~ 550 ° c, with nozzle ~ 670 ° c and more thrust

"a standard problem" page 105

www.lcis.com.tw/paper_store/paper_store/[%E6%A8%A1%E5%9E%8B%E5%96%B7%E6%B0%94%E5%8F%91%E5%8A%A8%E6%9C%BA].Thomas.Kamps.-.Model.Jet.Engines-2014713124110570.pdf








Cheers
Ralph

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[racket]

Hi Ralph

The Kamps engine and other home made engines using turbine wheels constructed from stainless steel HAVE TO run cooler because of the inferior material compared to the Inconel wheels used in our engines, the Kamp engine will have all of its passageways sized for "low temperatures" , its designed for a low temp.

Our Inconel wheeled engines will run cool if they are equipped with just a jet pipe , no nozzle, I've experienced temperatures in the low 500s C , even into the high 400sC at Kamp level of P2.

Its how the engine is designed and with what materials that will determine the temperatures , Kamps wants low temperatures , so he designed accordingly ............the KJ66 is a good engine, its just a "low powered" engine when it comes to thrust per pound of air per second because its designed with a relatively low pressure ratio and temperatures .

When Kamp talks about a "model jet engine with a high exhaust gas temperature is a less capable engine" hes talking about that engines use in a very specific scenario.

He then says in the next sentence " Exactly the opposite applies to full sized aircraft engines" .................our Inconel wheeled engines producing either thrust or shaft horsepower are "full sized engines" capable of running the same sort of turbine temperatures as any uncooled aircraft turbine wheel .

Cheers
John

[enginewhisperer]

What John's saying is that if you optimise the engine performance using a normal jet nozzle, the gas producer temps and pressures will stay the same if you swap to a correctly sized freepower turbine or afterburner setup.

You wan't the engine's performance sweet spot to align with the maximum safe turbine inlet temps for maximum power / thrust.