"Fat Boy" 12/118 thrust engine

[bournaVee]

hi John,
wow.. glad to see the engine running.
reading through your thread now cuz ive missed out a lot

[racket]

LOL.......plenty to read ;-)

[smithy1]

Mar 3, 2017 15:03:15 GMT -5 turboron said:

John/Smithy, I ordered a readout and a probe. However, I noted that the probe reads Iron. Does it also work on Aluminum?

Thanks, Ron

Hi Ron,

I've had it running on a small aluminium turbo impeller which I had spinning in my lathe and it was working fine during testing, I was using an airgap of ~0.5mm..if you have grief you might be able to source and more suitable item off fleabay, just takes a little "searching" to find what you're after.

Cheers,
Smithy.

[turboron]

Smithy, I did some research and found that there is a simple correction factor on detection distance for materials other the iron. The correction factor for aluminum is 0.5. Since the iron maximum detection distance was 2mm the maximum detection distance is 1mm for aluminum. This is consistent with your 0.5 mm.

Thanks, Ron

[smithy1]

Mar 5, 2017 14:23:37 GMT -5 turboron said:

Smithy, I did some research and found that there is a simple correction factor on detection distance for materials other the iron. The correction factor for aluminum is 0.5. Since the iron maximum detection distance was 2mm the maximum detection distance is 1mm for aluminum. This is consistent with your 0.5 mm.

Thanks, Ron

Yeah..I suspect the "density" of the metal has an effect on signal...nonetheless...it works fine at 0.5mm...I did try ~1.0mm but it was a bit hit 'n miss as the impeller is curved in the section I was using. I think a little testing will see you sorted.

Cheers,
Smithy.

[racket]

Hi Smithy

Thats good , I'm running ~0.015" inducer clearance, so as long as the pickup is flush with the shroud it'll be OK .

I'll be pulling the engine and disassembling it again to try and find a solution to why its not working and while its apart I'll machine a threaded hole between my discharge ports in the comp cover snout for the pickup .

Cheers
John

[racket]

Hi Guys

A bit more of an update on the last test,the latest concerning "number" is the static pressure measured at the entrance to the diffusing exhaust ,the video indicates minus 35 kpa - ~4.5 psi below ambient

Now my big problem with this engine is the turbine exducer flow area , its rather marginal despite the rather brutal cutback .

With such a large reduction in static pressure at the exducer the density is getting pretty low , even though the velocity has increased due to the greater pressure drop across the wheel , but velocity increases fairly slowly once we get into the >1,500 ft/sec range whereas the density drop is more linear .

Heres my thinking on whats happening , ................once past my 2.0 PR and heading towards a 2.5 PR , the turbine stage NGV is starting to choke with gas velocities of ~2,100 ft/sec , and because of my ~33 degree NGV angle theres ~1,100 ft/sec of radial velocity going straight down into the turbine wheel .

This entry gas has a pretty low static pressure , but because of the sub atmospheric static pressure at the entrance to the diffusing exhaust , the pressure ratio across the wheel is artificially increased , but because of the big drop in the density at the exducer the actual mass flow isn't increasing because the exducer is starting to choke up , this is then putting "backpressure " on the engine exacerbating problems because of increased T I Ts .

The engine is going to be dismantled and a static pressure probe installed just downstream of the NGV to measure the static pressure entering the wheel to find out if and when the NGV chokes.

The diffusing exhaust will have to go if my calculations are correct .

Cheers
John

[jetjeff]

Hi John,

Thinking out loud again,,,any way to fabricate a NGV with adjustable blade angles?

Regards

Jeff

[turboron]

John, have you looked at the Critical Ratio. Once the nozzle is choked the volumetric flow can not be increased. The mass flow can be increased by raising the upstream pressure but the downstream pressure has no effect. The Critical Ratio can be calculated. You will find the calculation in textbooks like Zucrow for example. It is 2.4 for air.

[racket]

Hi Jeff

Unfortunately it'd be impossible with my engine design.

I don't feel the actual NGV is a problem , it has to have the angle it does to effectively process the projected flows , there'd be some possible advantages at part load but at the moment they're relatively OK, its this mid to high power thats my problem, variable vanes would be a big help in getting some trends , a useful tuning aid if I had them :-)

Cheers
John

[racket]

Hi Ron

Yep , I ran into a choked exducer with my TV84 engine >17 years ago , it gave me a lot of grief whilst trying to fathom out why a change in jetnozzle size seemed to have little effect on the engine parameters despite changing scroll A/Rs etc etc .

Once I'd clipped the exducer the engine behaved as the text books predicted a jet engine should when turb stage "nozzles" were changed.

At present I'm not sure if the "backpressure" being produced downstream of the NGV is limiting its ability to actual reach Critical Ratio .

The sub atmospheric pressure I'm reading could actually be substantially lower still at the exducer throats as theres probably some diffusion of the gases between the exit of the wheel and my static pressure pickup point due to the increase in flow area downstream of the blades and hub , its impossible for me to get a reading closer to the wheel.

I'll get the engine out and apart this morning to have a look inside and hopefully find some answers :-)

Cheers
John

[racket]

Hi Guys

Engine opened up and all seems OK inside .

I've mounted the tach pickup between a couple of the surge port discharge holes , I would have prefered a different position but there weren't many options that I could actual machine






Theres a threaded hole for radial adjustment of pickup depth and a "bracket" to make sure it doesn't come loose .

I'll remove the diffuser from the exhaust today and hopefully check the fuel manifold flows for evenness between injectors , the "gas test" looks OK but I want to try fuel to make sure theres no "crap" floating around in the manifold.

Cheers
John

[Chuks]

Hi John,

Great job as always on your engines

I know you will find out a solution to this

problem I fail to understand.


Lol.I would think of piping those holes around the compressor housing for free stage NGV/turbine cooling.

[racket]

Hi Chuksy

Yep , theres plenty of discharge air available , though at pretty low pressure , probably OK for freepower wheel cooling on the downstream side .

It is a bit of a mystery why the engine is behaving as it is , my current thinking is the diffusing exhaust might be the problem , they can be a big benefit under "normal" circumstances , but because I'm not running "normal" configurations it might not be doing what it should be doing .

When a diffuser( megaphone) is fitted to the end of an IC engines open pipe exhaust as on a 4 stroke race bike for instance , it "complicates" the engines behaviour compared to a simple straight pipe, and because I've got exceptionally high gas speeds going into and out of my turbine wheel due to the NGV and exducer blade angles the extra static pressure depression caused by the diffuser cone is "artificially??" reducing gas density to the point where the mass flow that can exit the exducer even when at sonic velocity is reduced , the increase in velocity as a result of the diffuser doesn't compensate for the density drop , density drop is linear whereas the velocity increase has a square root in its calculation , it needs a lot more pressure drop to get ever decreasing amounts of velocity increase once up near sonic .

I'm also considering the use of a standard turbine wheel with more exducer gas deflection to increase the turbines power , but will need a large exducer radial clearance to "bypass" extra gases as the exducer will be choked at a lower mass flow than at present.

LOL, my heads been aching trying to work this one out , hopefully I'll get there eventually :-)

Cheers
John

[turboron]

John, could you go to a G trim turbine to get the power increase?

Thanks, Ron