A few questions about a/f ratios and injection/ignition

[unclematt]

Hi all, I am new to the site after reading and enjoying the posts for many years.

I am designing a gas generator to power remote nozzles/jets to generate thrust. I don't want to go into too much detail just yet, but need to figure out some things like what size and configuration of nozzles would be most efficient and produce the most thrust, given a supply of air to the combustor (gas generator) of up to 250 lbs./min at 145 psi. However, the nozzle's exhaust cannot exceed a few inches in diameter. The normal air delivery numbers would probably be lower, but that is the upper range. I need to feed 4 nozzles/jets, and produce as much thrust as possible, while burning the least amount of fuel (of course). What kind of air to fuel ratios would be acceptable to those goals at say 150 to 175 lbs/min of air at 100 psi? I am also open to fuel suggestions.

I am also trying to learn about fuel injection and ignition methods for gas turbines. I have seen spark plugs that are employed, but is the spark only used for startup, or run continuously while the jet is in operation? Do the injectors consist of simple tubes that deliver fuel, or are injectors employed similar to automotive injectors? What pressures are common to aviation injection system for small turbines? Is the fuel heated in the delivery tubes before injection? Would recent advances in automotive fuel injection be beneficial in a jet (fuel heated to a super-critical state)?

If you have any input or answers, let me know.

[unclematt]

Okay, I can tell the suspense is killing many of you, so here are more details. Take a look at patent #3120274 at google patents. It describes a setup to power any kind of propellor, from a ducted fan to a helicopter blade. It incorporates a compressor in the center hub of the prop, with the compressed air travelling down the blades to small jets either in the blades, or the blade tips.

Now then, I came across mention of two new products by turbonetics and procharger. The turbos/superchargers employ large centrifugal impellors that can deliver up to 250 lbs./min at up to 145 psi. Check them out here:

http://www.ls1tech.com/forums/forced-in ... p-pri.html

http://www.turboneticsinc.com/tnx_turbochargers

Talk about monstrous! If any of you know of lightweight impellors that are close to this size, or maybe even larger, please let me know. I would love to find a used one for testing purposes!

Now imagine one of these impellors rotating at the center of a prop, feeding an annular combustion chamber around it, and the resulting high pressure, high temp combustion gases passing down the blades to exhaust out of nozzles at the blade tips. The impellor is mechanically linked to the rotation of the prop, with a ratio built in that provides an impellor speed 10 to 12 times greater than prop speed, or whatever is appropriate for specific applications. With air delivery rates as high as 4.1 lbs/sec. you could generate a lot of thrust and rotational torque. And I have some ideas about how to deal with the subsequent high noise levels and recovering some of the energy in the exhaust gas once expended out of the nozzle.

So let me know what you think of this. Though it is not my idea originally, I think the idea in the patent could be implemented fairly easily, and provide a prop that literally can be bolted to just about anything, creating no torque effects on the aircraft, and is self contained (taking up no internal volume of the aircraft at all). Perfect for DARPA's new personal VTOL aircraft request.

To determine viability, I need to figure out exactly how much propulsive thrust can be generated by small nozzles being fed gases at the temperature , pressure, and volume this impellor could provide. Any help on that front would be appreciated.
I am already working with formulas and information from several papers from the 50's and 60's, when blade tips jets were all the rage.

[unclematt]

While doing research about this concept online, I came across an article by Ken Flanigan, Chairman and CEO of Advanced
Aerospace Technologies. In the article about his company's compound heli-jet, he makes the following claim:

"The Tip-jet Tilt-rotor also shares the advantage of multiple-flow thrust
augmentation. This can make them more efficient than their shaft
driven counterparts. The calculated thrust augmentation for the tiltrotor
pictured in this article is 11.8. This means that for every pound
of thrust used by the tip-jets, the prop-rotors will produce 11.8 pounds
of lift or thrust."

Anyone have an opinion one way or the other about this statement? If one were dealing with a regular size prop, could you still expect a similar effect (though it may be lessened)?

You can check out the author's company website here:

http://www.advaerotech.com/Home/tabid/36/Default.aspx

[racketmotorman]

Hi Unclematt

All of these methods have been tried and found wanting over the years , thats why we see the type of turbine engines we do .

Spark plugs are for initial ignition only and can be deactivated once the fire alight .

Fuel injection can either be high pressure ( ~500 psi) or low pressure with evaporators .

Where did you get the 145 psi air pressure from , you'd need a titanium compressor with tip speeds of ~2,000 ft/sec to achieve that , alloy comps generally top out at ~45psi .

4 lbs.sec will only produce ~250-300 lbs of thrust but >400 shp , a comp that size will be spinning ~20 times prop rpm

4 lbs/sec at 145 psi - 10.8 PR will require a turbine producing ~800 horsepower minimum to drive the compressor wheel .

Theres too many problems with this design ...............unless you've built a "normal" gas turbine and experienced its complexity , you'll have no comprehension of the difficulties of the one

More research will soon throw light on its shortcomings .

Cheers
John

[unclematt]

Hi Unclematt

All of these methods have been tried and found wanting over the years , thats why we see the type of turbine engines we do .

Spark plugs are for initial ignition only and can be deactivated once the fire alight .

Fuel injection can either be high pressure ( ~500 psi) or low pressure with evaporators .

Where did you get the 145 psi air pressure from , you'd need a titanium compressor with tip speeds of ~2,000 ft/sec to achieve that , alloy comps generally top out at ~45psi .

4 lbs.sec will only produce ~250-300 lbs of thrust but >400 shp , a comp that size will be spinning ~20 times prop rpm

4 lbs/sec at 145 psi - 10.8 PR will require a turbine producing ~800 horsepower minimum to drive the compressor wheel .

Theres too many problems with this design ...............unless you've built a "normal" gas turbine and experienced its complexity , you'll have no comprehension of the difficulties of the one

More research will soon throw light on its shortcomings .

Cheers
John

Hi John, and thanks for responding. I was beginning to feel like a leper. Here is where I got my 145 psi from:

http://turboneticsinc.com/forum/news/11 ... 122-a.html

I also found a great paper written in 2008 about this very topic, and will have to read it all before I can comment more. The problem could very well be that enough torque/hp is not produced to turn the impeller of the size I am suggesting be used. I am waiting to get specifications from turbonetics about their 122mm impeller, as well as answers to many questions, and then I will be able to post a more informed opinion.

It does seem to me that if you could provide the gas for tip jets from a compressor/combustor within the prop hub itself, that could have a variety of uses and benefits. You avoid all the ducting associated with a typical hot cycle tip-jet setup. So no transmission, no ducting, just a fuel and electrical line to the mounting point, as well as any lines/linkages required for collective.

I am also looking into supercritical fuel applications with gas turbines, and if there are any benefits to derive.

[racketmotorman]

Hi

LOL.......Turbonetics do a lot of "turd polishing" , they'll talk up anything .

If you checkout centrif comps for aero engines , eg the RR 250 heli engines , their titanium comps ( and we need Ti comps due to the high temps of compression degrading alloy comps once past a PR of ~5-6 :1 ) only run to ~ 9.2:1 (120 psi boost) on their C47 engines flowing ~6 lbs/sec , and this is state of the art certified stuff produced by Rolls Royce not a turbo company talking up its products .

Turbochargers generally have Trim numbers in the ~50 region , the RR comps are sub 40 Trim , they have to be otherwise there are inlet problems from shock losses from the M>1 relative airspeeds .

I'm sorry I didn't get back to you sooner , I only pop in occasionally to check the Forum as I do most of my "work" over on the Yahoo DIY Gas Turbines Site .

I'll look forward to the comments you receive back from Turbonetics re their 122

Cheers
John

[unclematt]

Hey John,
So lets say we forget turbonetics for the moment. Do you know of any cent comps that could provide 4 lbs/sec? Perhaps something used? How expensive are larger aero comps? even Ti? And that 4lbs/sec number is just arbitrary. I have yet to crunch the numbers on how much air it would take to produce around 100 lbs of thrust from one tip-jet.

And I am still curious about the multi-flow thrust augmentation claims that I quoted earlier. If you could produce around 400 pounds of tip-jet thrust total, perhaps utilizing 4 blades with 4 tip-jets, according to those claims that would translate to a much higher level of static thrust for the whole system. He claimed over 11:1, but lets say conservatively that was reduced to only 5:1. That is still over 2000 pounds of thrust for one prop . Put one of those on each side of a VTOL, and you would be in business.

If used in a VTOL context, you burn at stoich fuel to air ratios for vert takeoff at full power, and then reduce that dramatically and burn at much leaner ratios once at cruise with the props facing forward.

Of course all of this depends on configuring the impeller type and size, flame tube/combustor, blade geometry, collective pitch design, and tip-jet construction (including nozzles with entrainment thrust augmentation) so that they all work together to produce higher levels of efficiency and power than have previously been managed. That is all. lol

I have a few more entertaining concepts to share if the math proves that this may be somewhate feasable, including a shroud to further increase efficiency and perhaps tame some of the noise. If not, its a fun thought experiment. However, I would love to build a prototype hub with no blades, only round pipes extending outward to the tip jets. That way I could determine the HP and torque the syetem could produce without worrying too much about controlling thrust, and building too much ,before feasability is determined.

[racketmotorman]

Hi

I'm still uncertain how this "contraption" is supposed to work , what is powering the compressor ??

There are a number of "problems" I foresee with the setup..................may I suggest you purchase a couple of small and relatively inexpensive books to give yourself some grounding in the fundamentals before you progress any further

Thomas Kamps book ............. https://shop.traplet.com/product.aspx?c=183
and Kurt Schrecklings ............. https://shop.traplet.com/product.aspx?c=187

You'll find that your specific fuel consumption will be horrendous when compared to a piston engine .

Cheers
John

[Viv]

Hi

Tip jets seem to have a magical attraction

Have a look at Foa's book "elements of flight propulsion", this has a chapter on tip jet propulsion of rotors using hot and cold jets from a thermodynamics point of view, the conclusions are not terribly optimistic but do provide a good basis for further study.

Viv

[unclematt]

Hi

I'm still uncertain how this "contraption" is supposed to work , what is powering the compressor ??

There are a number of "problems" I foresee with the setup..................may I suggest you purchase a couple of small and relatively inexpensive books to give yourself some grounding in the fundamentals before you progress any further

Thomas Kamps book ............. https://shop.traplet.com/product.aspx?c=183
and Kurt Schrecklings ............. https://shop.traplet.com/product.aspx?c=187

You'll find that your specific fuel consumption will be horrendous when compared to a piston engine .

Cheers
John

Hey John,

Though they make have valuable information, those books deal with a smaller scale than I want to work on, and I am not planning on using small jet trubines at the tips (if that is why you suggested the first book). What I really need is literature and references dealing with propulsion nozzles and related issues. I also need to figure out how to design and fabricate the annular flame tube and pressure container. I find more online with each passing day.

In my concept the impeller is turned by mechanical means, similar to the method used by the Rotrex supercharger. A cylinder (to which the blades are connected) turns with 3 discs in contact with the inside of the cylinder. The impeller shaft is in contact with the 3 discs in the center. When the cylinder turns the impeller shaft is counter rotated at whatever ratio is built into the size of the 3 discs.

I admit this idea may not be efficient enough, but it still fun to think about, and perhaps apply new methods, such as supercritical fuel states.

[unclematt]

Hi

Tip jets seem to have a magical attraction

Have a look at Foa's book "elements of flight propulsion", this has a chapter on tip jet propulsion of rotors using hot and cold jets from a thermodynamics point of view, the conclusions are not terribly optimistic but do provide a good basis for further study.

Viv

Thanks Viv, there is a huge amount of info on line about tip-jets, and I have been able to download many NASA and researcher studies about them. Again, this may be a dead end, but I have already learned a lot just researching this concept.

[unclematt]

Take a look at this pic:

http://www.grc.nasa.gov/WWW/RT/RT2001/i ... ham-f2.jpg

http://www.grc.nasa.gov/WWW/RT/RT2001/i ... ham-f3.jpg

My idea is very similar, the difference is the impeller and combustor would be in the blue hub (in the first photo) with the gases flowing directly to the blades and out the tip-jet nozzles. So you would remove a lot of the inefficiencies associated with ducting and the rotational rotor hub seal from the ducting to the central hub.

I'm also still curious about air to fuel ratios in jets, so if anyone has any input on that front, fire away. I do plan on getting those books though John, so don't think I am too lazy.

[racketmotorman]

Hi

Don't worry about fuel , turbines will burn just about anything if designed correctly .

Get the books and read them several times , they may be for models but they relate to much larger units as the maths don't change all that much , gas turbines scale up rather well .

Once you've read the books you'll shelve your plans to construct a hub mounted gas producer powering tip jets , if it was a feasible proposition it'd already be in production as the ideas have been well tested

Cheers
John

[unclematt]

Hi

Don't worry about fuel , turbines will burn just about anything if designed correctly .

Get the books and read them several times , they may be for models but they relate to much larger units as the maths don't change all that much , gas turbines scale up rather well .

Once you've read the books you'll shelve your plans to construct a hub mounted gas producer powering tip jets , if it was a feasible proposition it'd already be in production as the ideas have been well tested

Cheers
John

I didn't express a concern about what kind of fuel, but with fuel efficiency, whatever fuel is being used. I understand and concede your point about efficiency levels of past attempts at a tip-jet power plants, but I have yet to see any that are quite like what I am proposing. Yes, the individual concepts have been well demonstrated and proven, but not put together and implemented in quite the way I am suggesting. Of course, if anyone has proof otherwise, I am always open to new information. I also understand that at least one company is pursuing DARPA's latest request for a roadable vtol vehicle in part utilizing tip-jets in a rotor. So I am not the only person doing research on this front. Again, it may well be a dead end, but I haven't gotten to that point yet.

I will check out the books you have suggested. And anyone else with input or information is welcome to contribute to the discussion.

[racketmotorman]

Hi

Fuel efficiency is a product of the expansion ratio , we need a PR of at least 6:1 for it to become acceptable , you won't get that with a turbo , once past a PR of ~4:1 the comp efficiencies drop beyond the point where any further pressure rise doesn't produce any significant benefits even though the comp is capable of producing a higher pressure .

Maximum turbine inlet temperatures will depend on the quality of turbine materials but will be <900 deg C for off the shelf turbo related parts used for aircraft where reliability is required , this TIT would mean an overall air fuel ratio of ~50:1

As for how combustion would cope with the centrifical loads of a spinning motor its anyones guess .

I could probably write several thousand words on why this won't work , but I'll let you find out for yourself )

Cheers
John