Turboprop PJ experiment update

[Mark]

This topic made me think of the very large slow turning prop of the gossamer condor, a meager ~ 1/2 horsepower at best. I don't know which one this is or if it is a prototype.
Mark
http://www.achievement.org/achievers/ma ... c0-041.jpg

[Fricke]

Hi John!

How is the FPPJ (FreePowerPulseJet) going along?

Can You recommend some good books on designing turbinewheels?
I´m rebuilding/redesigning my kart and thinking of going the route with a freepower turbine...

Cheers
Fredrik

[racketmotorman]

Hi Fredrik
The PJ's coming along nicely , just finished putting some paint on the heat shielding , just a matter of screwing the panels on and she'll be ready for a testrun with the "new" plastic reeds .
Its a bit of a bummer having to develop the basic PJ rather than just making a standard design and getting on with the freepower testing .

The Kamps and Schreckling books are pretty good but are more interested in reaction turbines that are designed for a particular rpm .
With our freepower turbines , I feel its more important to get the "stall" characteristics right as its at low rpm where we experience the greatest "power loss" and the greatest reduction in performance , theres generally more than enough "top end" power :D

I haven't come across a book thats entirely devoted to just turbine design , but I should imagine they're out there , maybe something written in the 1950-60s when they didn't expect you to know everything before reading the book .

One of the thing I'm very interested in finding out with my home made impulse turbine is just how well it converts gas energy into shaftpower at stall rpm compared with the Allison reaction turbine I used in the bike and the turbocharger radial inflow turbine used in the kart .
The inlet angles on my homemade turbine are designed for a maximum of 15,000 rpm compared with the 30,000 rpm of the bikes even though theres not a lot of difference in size .

The design flow area of an impulse turbines nozzle guide vanes outlet is the same area as the basic pure thrust engines jet nozzle , the gases coming out of those NGV's will be at the same velocity as out of the jet nozzle .

Cheers
John

[Stuart]

Hi Fredrik
The PJ's coming along nicely , just finished putting some paint on the heat shielding , just a matter of screwing the panels on and she'll be ready for a testrun with the "new" plastic reeds .
Its a bit of a bummer having to develop the basic PJ rather than just making a standard design and getting on with the freepower testing .


Cheers
John

Wow, I just found this thread. I was gone from the forum for about a year, but you've given me new inspiration. I have my own unique design, sort of a pressure jet, that would really take to your turbine concept.

The reason you can increase efficiency is that thrust is the product of mas flow times velocity, while the energy needed to created that thrust per time is related to mas flow time .5 times V^2.

That is, You can increase thrust either by increasing exhaust velocity (typical p-jet and cousins), or by increasing mass flow (slow moving prop). Velocity increases though cost energy by the square while mass flow only goes up linearly.

[racketmotorman]

Hi Stuart
Yep, lotsa energy made available by efficiently slowing those exhaust gases .
Cheers
John

[Stuart]

Hi Stuart
Yep, lotsa energy made available by efficiently slowing those exhaust gases .
Cheers
John

The engine I've designed (maybe patentable) is a continuous flow pressure jet (sorta). It now runs (anemically), but it runs! (eureka moment here) and there is plenty of room for tweaking, so hope springs eternal.

Unfortunately, I don't have welding equipment nor time to make a proper stator and turbine. I was thinking of using a standard small squirrel cage blower as an approximation to a turbine, just to see if I can get anything usuful out on the cheap. I think the turbo prop extension to these devices makes a whole lot of sense.

I think my machine would be greatly improved by a Borda bell inlet instead of the flat entrance I have now. Also a proper flame holder. Since the power curve should increase by the square of the velocity for my design, I should know by the end of the weekend whether I can goose my system up, or whether it is just a nice space heater. Then on to turbines..

[racketmotorman]

Hi Stewart
Just make sure your "turbine " doesn't restrict the flow , if you don't have static pressure that can be reduced into velocity thru a normal stator setup , you'll need to keep the flow areas "large".
LOL.....I hope that "squirrel cage" is a steel one not an alloy job :D
Cheers
John

[Eric]

Stuart,
I hope you arent talking about a "howling mode" engine.

I know a bunch of us here have made engines that will essentially act as a gas powered rijke tube. I even had one that I could make self sustain just by putting the air probe at a specific spot in the intake, but would function as a normal pulsejet when the air was provided in a different spot.

This effect seems to happen a lot to me with the linear version of the "advanced FWE".

It would only suck air in through the intake and only expell it through the exhaust (neglecting what little was the result of resonant sound production). Also the most horrible noise I have ever heard. Pulsejets dont really produce noise at the resonant frequency of the pipe, but this beast produced pure constant tones, and as loud as a pulsejet. It was like 10,000,000 teachers squeeking chalk or scratching fingernails on a chalkboard.

The resonance from the tube itself was loud, but the pumping action that a normal pulsejet experiences just wasnt there, the combustion was constant as in a turbojet, and the heated air excited the pipe into its natual resonant frequency at that temperature, which was quite high. Once that mode starts the flame front will stay at a certian point within the engine, and the convective action will pump air quite forcefully, though not as strong as normal pulsejet operation.

There was no flame ejection from the intake, you could litterally stick your finger in the intake with no ill effects. The exhaust flow was extremely hot and constant, though produced little thrust.

Eric

[Fricke]

I haven't come across a book thats entirely devoted to just turbine design , but I should imagine they're out there , maybe something written in the 1950-60s when they didn't expect you to know everything before reading the book .

Hi John!

I´ll be looking around and see what I can find!

Cheers
Fredrik

[racketmotorman]

Hi Guys
Still playing with the bare PJ engine, but thrust levels aren't improving , 25 pounds was the limit this morning on gasious propane , with flameouts as soon as the petrol was introduced :?

What sort of fuel pressures and atomisation are the norm when running petrol ??
I've got an 11 gph semi solid spraynozzle leftover from the turbine, that I might try installing along with a higher pressure pump to try and improve the fuel distribution over the rather crude device I'm presently using .

Then again it could be a lack/oversupply of fresh air getting in , how does one know if an engine is undervalved/overvalved ??

The one bright spot of this mornings trial was the successful running of the "plastic" reeds , ( ref, two stroke reed thread), otherwise its "back to the drawing board " with plenty of possibilities to explore :-))

Cheers
John

[Jonny69]

Hi John, I know you've probably already done this but don't forget to work out roughly how much fuel the jet is going to consume so you can work out a flow rate from the tank/pump

Back to basics: 2-3lbs fuel per lb thrust per hour = 1.5Kg/lb thrust/hr

say 50lbs thrust

that's 75kg fuel per hour or 1.25Kg per minute

Specific gravity of oil is around 0.9 so that's about 1.4 litres per minute

Or one metric gallon roughly every 3.5 minutes

Might be worth double checking just in case. The higher pressure you can pump the better the fuel will atomise as you will be using a smaller jet. Your flameouts might be due to poor atomisation.

Jon

[racketmotorman]

Hi Jon
Yep , I did some calculations and its a rather large amount :shock:
Thats why I've been a bit curious as to how I've been able to run it on gasious propane and still get 25 lbs of thrust .
The "temporary" propane injector , which was installed only for startups, is a simply bit of 1/4 SS tube fitted thru the front plate , with 4 X 1/16 holes radially displaced , and with the end crimped shut , very crude , but works suprising well with virtually instant startups and a fair degree of throttlability , its just the petrol addition that's giving me some problems.

I'll just have to stop being lazy and make up a decent spray setup.

I'm getting impatient as I want to get the turbine back on her :D

The one concept I can't quite get my head around with these PJs is the fact that they cycle at roughly the same rate all the time , yet its possible to add various amounts of fuel to them with a change in thrust from what I image is the same amount of recharge air per cycle , yet to get combustion we need certain limited air/fuel ratios.
Perhaps I should look at them as more of a 2 stroke diesel engine running at constant rpm with full recharge of air on each stroke but power production limited by the quantity of fuel burnt and the temperatures produced , with an excess of air/oxygen at lower power settings and full combustion only at max power ......hmmmm, but that needs very fine atomisation for fast burn rates at low compression ratios and large combustion spaces .

Liquid propane is the ideal fuel in that case, wide F/A ratio capability and very fast burn rate , but with more complicated tankage and delivery systems .

Do you have any thoughts on how we know when we have the correct recharge rate/valving flow other than measuring thrust and fuelburn rate , from what I've read (which could be suspect) the correct valving flow is rather critical to obtain starting/running , if its a bit out we're in trouble , this fact is troublesome to me because of the lack of "problems" when starting or running up to "half power" , its only this upper power levels that are currently unobtainable :(

Thanks for the input Jon, its all appreciated as theres a huge amount I don't know about these engines and which I'm going to have to workout .


Cheers
John

[Eric]

John,
If I had to guess I would say your valve arangement is very sub-optimal. You have a small valve area, and you also have them facing backwards. If you had a small valve area and had scoops on the intake and faced them into the prop you could get more air intake for the same valve area.

Having them backwards and having a prop blow air past them may make the situation even worse by reducing the air intake even more, not only that but any sucking action would subtract from the forward thrust.

Pulsejets are like the economic law of diminishing returns, the more fuel you give them the less thrust they produce for a given volume of fuel. A well designed engine can run on barely any fuel and produce a decent quantity of thrust, though to produce max thrust the fuel consumption goes to hell.

I have a pretty large valveless that will start and run on a smaller volume of fuel than a little FWE will even run on. It might produce like 3 or 4 pounds thrust and would give it phenominal TSFC at that level, but when you crank it up to about 15 pounds thrust it is nearly as fuel hungry as a lockwood.

Its not an uncommon thing either, Im sure anyone who has made a large lockwood knows that you can get decent thrust from just the proane vapor but to produce max thrust it guzzles liquid fuel.

If you had a valve system at 100%, and had the engine idle at 25 lbs thrust you should be able to inject liquid fuel with no problem, the flameouts definately show that its getting too much fuel for the volume of air.

Eric

[Mike Everman]

John... not only that but any sucking action would subtract from the forward thrust.

Eric, Eric, Eric...

[pezman]

http://web.mit.edu/Edgerton/www/FeynmanSprinkler.html