GT3788VA Variable nozzle turbojet build

[racket]

Hi Mitch

Flames out the compressor says SURGE .................open up those NGV vanes

Cheers
John

[mitch]

Thanks for the idea, ill try that. However, it ran previously with the vanes fully closed!!!

[racket]

Hi Mitch

In that case , I don't know :-(

Have you fitted a jet nozzle for this test run ??

Cheers
John

[mitch]

nope. this was just gonna be the first run using the vanes in a position other than fully closed, and with the new oil pump and evaporation system. I isolated the problem to the turbine housing though, I pushed air through the compressor housing and wheel and through the combustion chamber ( all separate) and found that they flowed fine. I also noticed while testing the flow of the turbine housing that my VNT actuator is "T"ing oil just fine inside the bearing core, but the unison ring and guide vanes are not moving at all. They seem to be seized up. I'll have to pull the turbine housing apart tomorrow and figure out whats wrong with it.

[racket]

Hi Mitch

You'll probably find the vanes closed , thats why I recommended that you lock the vanes in a fixed position , variable vanes can be a blessing but also a pain in the butt if they don't behave themselves .

Hope its an easy fix :-)

Cheers
John

[mitch]

I figured it out, the VNT system was seized up so the vanes were stuck closed. I re clocked the turbo and tightened everything back up, this time with the vanes fully open. I ran it on propane and kerosene to a little over 5 lbs of boost (hard to tell cause the gauge fluctuated so much). Needs some tweaking as the turbine wheel looked a little hotter than I wanted it to, and the oil temp was higher than I would like it to be ( I dont have a gauge to read temp installed yet, just based on the feeling of the temp of the lines). Should my turbine wheel be slightly red on the edges? or not red at all?

[racket]

Hi Mitch

If the vanes are fully open your temperatures will be high , close them up a tad, say at 75% open , and the temps should drop, you need to get rid of that "red" from the turbine wheel especially at such low P2s and nil backpressure.

You'll need to fit some sort of T O T gauge on the engine to "tune" it , the vanes may need to be closed even more to get temperatures right .

Without a jet nozzle at 5psi "boost" P2 your T O T temps should be in the 400 - 500 C - 750 - 930 F range

Cheers
John

[mitch]

Thanks for the input, I'll have to mess with vane position a little more, as well as I will open up my flame tube a bit more too. At one point I had the entire turbine wheel glowing red... Time to invest in a EGT sensor to measure tot like you said! I was spooling the turbo with too low of airflow, and too high of gaseous fuel pressures I think. One thing that is amazing to me is how cool the combustion chamber is after running the engine

[racket]

Hi Mitch

If the flametube is made to Jetspecs 30/20/50% ratios , don't change it .

If the vanes are too open the compressor will be flowing well to the choke side of the map with poor efficiency , this then requires more horsepower from the turbine wheel which it finds hard to produce because the vanes are too wide apart making it difficult to produce high gas speeds into the turbine wheel , the only way to compensate is with higher temperatures ............reduce the vane gap and higher gas speeds are the result as well the mass flow is reduced back to more efficient compression............everyone is happy :-)

Cheers
John

[mitch]

I Don't think the flametube is made to those ratios any more. I think its time to make a new flame tube and combustion chamber! it should be pretty close to the correct ratios, but its not spot on. and that makes sense with the vane positioning causing increased EG temps

[mitch]

quick question for anyone reading this, when using jetspecs to find flametube specs, should I use the compressor wheel inducer size (63.5mm) or the compressor wheel exducer size(88mm)? Everything I have seen refers to this turbo as an 88mm turbo, but I am not sure which measurement to use. I'm going to be making a new combustion chamber and flametube, as my current setup uses just a modified combustion chamber and flametube from a smaller 40 mm turbojet.

[nersut]

Sept 10, 2014 9:12:31 GMT -5 mitch said:

quick question for anyone reading this, when using jetspecs to find flametube specs, should I use the compressor wheel inducer size (63.5mm) or the compressor wheel exducer size(88mm)? Everything I have seen refers to this turbo as an 88mm turbo, but I am not sure which measurement to use. I'm going to be making a new combustion chamber and flametube, as my current setup uses just a modified combustion chamber and flametube from a smaller 40 mm turbojet.

Hi Mitch

You should use the compressor inducer diameter (63,5 mm) to calculate your flametube using JetSpecs.
The compressor exducer diameter is "only" used to calculate the max safe rpm for the turbo.


Cheers
Erik

[mitch]

Thanks for clarifying nersut. According to jet specs, I'm gonna need a TON of holes in this flametube...

[racket]

Hi Mitch

Make sure you have some bigger holes in the Primary zone , not all small ones , a lot of very small holes only keeps the wall cool and doesn't allow air to penetrate deep into the flametube for combustion , Jetspecs gives 30 X 0.25" dia holes for the Primary zone in a 5" flametube , drill out 4 of those 1/4" holes to 3/8" , have them equally spaced around the flametube so their discharge can hit each other to create turbulence in the middle of the flametube , the little bit of extra primary zone hole area won't cause any problems.

Cheers
John

[mitch]

Thanks for the input racket. For this new combustion chamber, I plan to have the air inlet tube enter the combustion chamber further up from the base as you had suggested. Where should the tube enter? in the center of the secondary dilution zone?