Oklahoma Willy Viper VW

[Richard OConnell]

I believe this was the post you were talking about?

Sept 18, 2013 17:25:27 GMT -5 racket said:

Hi Eric

When theres a vena contracta the airflow is reduced by about one third , so even if the turbine stage was correctly sized for a fully flowing compressor with bellmouth inlet , it would become about 30% oversized with the reduced flow caused by the vena contracta .

If the turbine stage is oversized it becomes difficult to get the required gas velocities to power the compressor wheel because the restricted mass flow can't fill those now oversized turb stage passageways , to overcome this problem its necessary to decrease the density of the gasses by increasing their temperatures , this then allows the gasses to fill those passageways and restore gas velocities to that required to power the compressor.

The compressor will be running at reduced efficiency due to the reduced inlet flow being "off design" with the rest of the compressor wheel design, the reduced efficiency will require relatively more power further exacerbating the situation . ..................the engine gets seriously "out of tune" :-(

The 77mm turb exducer is ~45% greater in area than the 64mm comp inducer , this is already at the upper limits , ideally it needs to be closer to ~25% greater , so you'll need to be careful as Anders suggested , its easy to run in the choke region of the compressor map where tip speeds are excessive for the pressure being produced.

Otherwise, the turbo should make a nice engine :-)

Cheers
John

[gidge348]

Hello Perry,

I am no expert on after burners, so please let me know if I have this wrong.

I see in one of your posts you mention using the "hot streak" to light the AB, and I have seen your you tube post of testing it www.youtube.com/watch?v=DUoyPJzzCwI&list=UU75U7ncxDtlUgJEGqiST-Ng.

It seem to be an additional squirt of fuel into the combustor exit to give a flame line through the HP turbine? This looks fairly scary to me with the chance of turbine overheating and bad things happening.

AB's that I have seen in the past seem to have their own igniter box and plug that ignite into the AB flame holder, but I can't see any igniter plugs on your engine? Does a manual or something show lighting the AB with the "Hot Streak"?

Once you get it bolted down and at full noise if throttle movement does not increase rpm or (only a slight increase) but the temp increases, my bet would be as others have said, possibly Vena Contracta or more likely the FOD guard or a combination of both. Most FOD guards I have seen are a basket shape as opposed to flat. Someone a lot smarter than I am came up with that idea

Definitely a beautiful engine, and I look forward to seeing more vids...

Cheers
Ian...

[Richard OConnell]

Hey gidge, the hot streak setup is exactly that, and it is a pretty popular way of lighting the AB for hobbyists. Its not too bad if you can get your timings down, a small squirt of fuel for about a quarter of a second to get the AB lit is all you need. Any longer and your jet of fuel turns into a cutting torch and can cause serious damage to the turbine.

[racket]

Hi Perry

Man , thats one pretty engine :-)

If you want to run the afterburner you'll be needing a much larger jetnozzle than the 13.7" dia one (specs on your Web Site) , the 13.7" one is barely big enough for non afterburning use with a 53 lbs/sec mass flow , with a thrust of ~3360 lbs that works out at ~63.4 lbs/lb , this needs a velocity of ~2,040 ft/sec and an exhaust gas temp ~600 C .

To produce 5,500 lbs of thrust the velocity will need to go up to ~3,340 ft/sec an ~65% increase ,I don't think this is possible , generally a 40-45% increase in thrust is the norm requiring a doubling of absolute temperature in the jetpipe .

To obtain a 65% increase in thrust the gas temp will need to be ~2.7 times higher ( sq root 2.7 = 1.65) , meaning your gas density will be the same 2.7 times less dense , currently its at ~40 cu ft/lb , so it'll go out to ~110 cu ft/lb , to get that thru a jet nozzle at only 65% faster velocity the nozzle would need to be increased considerably .

13.7" = 147.4 sq ins

147.4 X 2.7 =398 sq ins

398 sq ins divided by 1.65 = 241 sq ins or a diameter of 17.52 " dia .

Hope this helps explain why even a slight increase in jetpipe temperature from a hot streak will cause an "off design" situation that will change your rpm .

Cheers
John

[cursorkeys]

Mar 25, 2014 9:56:33 GMT -5 Richard OConnell said:

I believe this was the post you were talking about?

Sept 18, 2013 17:25:27 GMT -5 racket said:

Hi Eric

When theres a vena contracta the airflow is reduced by about one third , so even if the turbine stage was correctly sized for a fully flowing compressor with bellmouth inlet , it would become about 30% oversized with the reduced flow caused by the vena contracta .

If the turbine stage is oversized it becomes difficult to get the required gas velocities to power the compressor wheel because the restricted mass flow can't fill those now oversized turb stage passageways , to overcome this problem its necessary to decrease the density of the gasses by increasing their temperatures , this then allows the gasses to fill those passageways and restore gas velocities to that required to power the compressor.

The compressor will be running at reduced efficiency due to the reduced inlet flow being "off design" with the rest of the compressor wheel design, the reduced efficiency will require relatively more power further exacerbating the situation . ..................the engine gets seriously "out of tune" :-(

The 77mm turb exducer is ~45% greater in area than the 64mm comp inducer , this is already at the upper limits , ideally it needs to be closer to ~25% greater , so you'll need to be careful as Anders suggested , its easy to run in the choke region of the compressor map where tip speeds are excessive for the pressure being produced.

Otherwise, the turbo should make a nice engine :-)

Cheers
John

That's the one! Good search skills

It was the first time I'd even heard the term used.

[perry]

Hi all,
thanks for all the comments. The hot streak ignition is widely accepted, at least in the drag racing world, as being a far more reliable ignition source than an electrical solution. The plc on my engine operates the hot streak solonoid for just 0.3 seconds and this is enough to light off the A/B every time.

So a couple of further questions:

My engine has been built primarily for displays and demonstrations and so it is important to me that it looks pretty as well as being functional, however, achieving maximum potential thrust is less important. I would therefore prefer to keep the vertical FOD mesh and not add a bellmouth.... but i can if really necessary and safety is obviously important!

SO, am i right in saying that if i can accelerate the engine to 100% without any rise in EGT then i probably do not have a vena contracta issue? And vice versa, if the engine labours towards the top end and EGTs do increase, then i have a problem. Is that a sensible way to assess the efficiency or otherwise of the engine.... or not.

Racket: Many thanks for the calcs! My nozzle outlet is actually 15 1/2" 13.7 was the original aircraft jet nozzle diameter, i will update my site! From what you say i suspect 15 1/2" is still too small though, but the maths is beyond me. Would you be so kind as to do me a favour and work out what diameter the jet nozzle should be assuming a 45% thrust increase (4872 Lbs)?
Much appreciated

Perry

[racket]

Hi Perry

The vena contracta issue with regards your temperatures could be "clouded" by just how big your jet nozzle is , with an "after burner size" jetnozzle your current "dry" temperatures in the jetpipe should be rather low , but they may be at "normal" levels if the vena contracta is causing a problem.

You actual thrust measurement is probably the only way to know if "two wrongs are making a right" temperature.

A pitot tube in the jetpipe will provide some guidance as you should be producing a jetpipe total pressure of ~13-14 psit so as to produce the theoretical max design velocity/thrust across a choked nozzle.

With a vena contracta happening the mass flow will be reduced and that jetpipe total pressure will be down if temperatures are in "normal" design range.

For a 45% thrust increase the afterburner nozzle would need to be ~16.5" dia

A 45% increase in thrust means a ...... 1.45 X 1.45 = 2.10 times temperature increase over dry operation , so we need to multiply the original dry nozzle area by 2.1 times from its 147.4sq ins to 309.9 sq ins , but as the gases are going 45% faster we divide that new area by 1.45 to get an A/B area of 213.7 sq ins or ~16.5 inch dia .

Hope this helps :-)

Cheers
John

[gidge348]

Mar 25, 2014 22:42:45 GMT -5 Richard OConnell said:

Hey gidge, the hot streak setup is exactly that, and it is a pretty popular way of lighting the AB for hobbyists. Its not too bad if you can get your timings down, a small squirt of fuel for about a quarter of a second to get the AB lit is all you need. Any longer and your jet of fuel turns into a cutting torch and can cause serious damage to the turbine.
..............
The hot streak ignition is widely accepted, at least in the drag racing world, as being a far more reliable ignition source than an electrical solution. The plc on my engine operates the hot streak solonoid for just 0.3 seconds and this is enough to light off the A/B every time.

Hi Richard & Perry, like I said I am no expert on this,

"a bad day is a day when you don't learn something" and I have learnt something today

[perry]

Many thanks Racket, i need to open up the nozzle by around an 1" then ideally.
Cheers

perry

[turbochris]

with that turbine, if the AB nozzle is too small, you may hit surge before you notice the EGT getting too high. I run 2 streak solenoids in series. I also power my timer off the button, so if the timer screws up, I just let go of the button and it cuts the power to it. Hung timers and solenoids will screw you in a heartbeat.

[racket]

Hi Perry

You could leave your nozzle as it is but your fuel flow to the A/B would need to be reduced somewhat to maintain design TOT , the reduced fuel flow will lower the A/B temperature and along with that the thrust level.

You main consideration is TOT , add just sufficient fuel to the A/B to keep the TOT within limits and whatever thrust is produced , is the output .

As Chris advises , watch out for surge , those axial comps don't have much flexibility with flow range , too much fuel into the A/B and she'll protest .

Cheers
John

[perry]

Hi guys,
two years on and i am still building my jet truck, should be finished and on the road / drag strip for June time.

[racket]

Looking good

[perry]

Hi all, thought i would post a few photos of my newly completed Viper 535 powered VW pick up "Oklahoma Willy"

[racket]

Lookin' good :-)