PD-01 Jet Engine

[madpatty]

Hi Racket,

No the needle doesn't bounce around during the run....the guage has pretty low least count (max reading is 30Psi) and it doesn't show more fluctuation than max 0.5 psi in any case.

There is no blockage between the pressure pickup point and the guage....the pressure is picked up from the hole near the diffuser discharge....

Cheers,
Patty

[racket]

Hi Patty

Well , I'm at a loss to know what the problem is :-(

When you did the "string at comp inlet" test, what results did you see ??

Is there any "pulsing" sound at the comp inlet , it should be a consistent sucking sound ?

Cheers
John

[madpatty]

Hi Racket,
There's nothing like that at the compressor inlet....

I want to clear my doubts regarding the surge and choking..
Is it physically impossible that my compressor be flowing on the choke side rather than the surge side....

The compressor map is similar to this one:-

i187.photobucket.com/albums/x281/mice325/Project%20Jatkettu/kompurakartta_zps12cb64b6.jpg

OK...lets say these maps are not valid but still it takes a lot of pressure disturbance to take the current running condition point(that is on the choke side) to the surge side of the map......and even when the gauge is not registering any such drastic effect....i have a compressor impeller in surge and the pressure gauge evn with a greater least count was dancing badly.....

Cheers,
Patty

[racket]

Hi Patty

The compressor stage can surge if theres any sort of "restriction" downstream of the compressor wheel that doesn't allow the wheel to flow within its design range at any particular rpm .

The restriction could be the comp diffuser , the flametube wall hole area , too much fuel creating a "thermal blockage" , too restrictive a NGV throat area , too small a turbine wheel flow area and finally a too tight a jet nozzle , ..............take your pick as to what could be causing your problems .

Try doing some flow calculations for your NGV and turbine wheel ...........the fact that you are experiencing very high turbine temperatures indicates a problem somewhere as the engine doesn't have a "restrictive" jet nozzle attached , with full expansion across the turbine your exhaust temperatures should be <500C , with no "colour" at the turbine wheel whatsoever.

There is one other simple test to do , run the engine and measure the fuel burn rate at a constant setting , when taken in conjunction with your temperatures you should get an idea of mass flow , compare this with the "map" to determine whether you are at the surge or choke limit.

Cheers
John

[racket]

Hi Patty

Looking through your NGV design theres a couple of things that worry me , firstly the taper on the outlet of the vanes is producing what appears to be a diffuser after the throat , also there a lot of metal in those NGV vanes due to the excessive number and length of them which will be producing a lot of frictional losses.

You've worked out the NGV area "correctly" but with all that extra boundary layer you'll probably be needing more throat area than you currently have simply to flow a "normal" range flow , theres no way you'll be able to flow in the choke region of the map .

Theres also a concern with the comp diffuser as I noticed your lube drain extends into two diffuser channels at a point where the flow is still at very high velocity , theres a chance this disturbance could then cause the whole compressor wheels output to become unstable.

Also it appears theres not a large vaneless space between comp tip and diffuser inlet tip , what distance do you have between them ??

Also your P2 pickup point needs to be from a quiet position in the engine , somewhere towards the rear of the engine outer casing at a point where the airflow has had time to slow down , your current position near the diffuser could be "problematic" .


Cheers
John

[madpatty]

Hi Racket,
That taper at the NGV outlet was to get the required throat area and also to get the right aerodynamic tip because i learnt that a tapered tip is better that a flat tip...

The throat area that was calculated was 896 sq.mm and i am running a NGV area of around 1020sq. mm to accomodate all those restrictions.

The diffuser tips start at 100mm diameter where the impeller exducer is at 83 mm diameter.

Regarding the efficiency at which the compressor is operating...should i measure the temperature at the diffuser outlet to calculate at what efficiency the compressor is operating because that is what is important for us ??

Cheers,
Patty

[racket]

Hi Patty

Whatever the reason for the NGV vane shape , it doesn't look good , and if theres an increase in area after the throat its not desirable .

At ~14% greater than design , the NGV throat area certainly won't get you into choke region of comp map .

Vaneless comp space is big enough , but that scavenge line intruding into the diffuser passageway isn't OK , especially at that high speed position .

Unfortunately measuring T2 might not be successful in determining efficiency due to the large discrepancy between P2 and rpm , but it won't hurt to try .

Cheers
John

[finiteparts]

Hey Patty,

John is correct that the slight expansion after the "throat" on the NGVs could potentially become an issue due to wake turbulence or if the flow is choked in the NGVs. If the flow did reach sonic velocities at the NGV throat and then is allowed to expand, there is the possibility for the flow to accelerate to supersonic conditions. If you generate shock waves in the turbine NGV discharge flow, there exists the potential to damage the turbine tips due to fatigue.

Think of it like this...a shock wave is a sudden jump in pressure...so every time the blade tip crosses the shock wave, it experiences a sudden jump in pressure. If you integrate the pressure rise over the area, you get a force that the blade tip experiences each time it passes that NGV passage (like a little ping with a small hammer)....then, if each vane passage is doing the same thing, you are quickly adding stress cycles to the blade tips (they get hit a bunch of times with that little hammer). The end result is that you break off the turbine blade tips due to high cycle fatigue.

This is a well documented problem with some of the diesel engines using VGT turbos as a means of exhaust braking. When they choke down the exhaust flow to backpressure the engine by swinging the NGVs to a very small flow area, they generate shock waves off of the NGV trailing edges and break off the turbine tips. Though...just to be clear, if I had to guess, I would say that you are not choking the flow in the NGVs on your engine due to the low pressure ratios.

The wake turbulence from the NG vane trailing edge divergence that your engine has can produce a similar problem, but to a much lesser degree. The pressure difference between the wake and bulk flow is not as severe as what is experienced through a shock wave, but over time even small things can add up. Large industrial turbines have corn-cobbed the compressor rotors due to the wake turbulence from the struts in the inlet housings setting up a forced response on the passing compressor blades. Admittedly, I would give that a negligible chance of your engine ever having any fatigue failure in the turbine tips due to this wake turbulence unless you start running it for hundreds of hours! But I thought that for completeness, it should be stated.

So with all that stated, what am I suggesting that you do about the NGV trailing edge divergence? I say nothing. I think that from the data that you have, you are likely flowing too much airflow through your engine. The only concern that I have is how reliable is you data?

The compressor map you are showing does not look like a Holset map, so I really question making any decisions based on it. I have a Holset map for my H1E that is very close to the size of your compressor. My Holset H1E compressor measures 56mm x 83mm...while your inducer tip diameter is slightly smaller 54mm...this map should be pretty close, just a slightly larger flow shown than what you will experience.



So the first thing is that you are measuring the pressure ratio essentially as a total to static, while you will see that the Holset maps use total to total pressure measurements. If your diffuser does a good job of slowing down the flow, then the static pressure you are measuring is close to the flow total pressure...but, if you don't recover the kinetic energy of the flow sufficiently, then you are using a erroneous measurement for your pressure ratio.

The original compressor map is, like I have said many times, essentially useless.

    1. Because you have modified the diffuser section, your compressor will now stall, surge and choke differently.
    2. It will hopefully recover static pressure more efficiently and so your mass flow, RPM and PR will not be in the same place on the map.
    3. Since the surge to choke margin is reduced when going from a vaneless to a vaned diffuser, the map width will be smaller.
    4. The location of the efficiency islands will be completely different.

Additionally to that, one of the key features on new compressor systems that really adds to the wide operability window of the compressor system is the recirculation system, which is usually not kept...so the map width is narrower even without changing to a vaned diffuser.

Usually, the map moves like this...



the map narrows, the peak pressures go up, the surge and choke lines move to smaller flow rates...all these things can have your estimates of where you are operating as completely off.

The thing you need to do is to measure the inlet mass flow rate and the total pressure at the diffuser discharge. I made a simple mass flow measurement device with a fairly simple pitot tube setup in an old air snorkel (turbo to air cleaner). In retrospect, I would now either add a bell mouthed inlet to this or move the measurement plane back away from the inlet a bit so that I make sure the airflow is uniform and smooth.  As it shows there, there is the potential that I might have had a flow separation at the inlet plane of that snorkel due to the abrupt entry.  I shared a few pictures previously, but here they are again...





The idea was to get the inlet area as large as possible so the flows were slow enough to match the differential pressure gauge that I had at the time, but you could easily build a water manometer if the flow speeds were slow enough. Then, you take your differential pressure measurements to calculate the flow velocity and then with the area of the measurement plane, you can calculate the mass flow. There will be some error, but if the flow speeds are low at the measurement plane, you can get a decent estimate. Then for the total pressure, that is easy, just make a forward facing probe and place it where you have a high confidence that the flow direction will correspond with the probe orientation (or make it turn and adjust to the highest pressure reading).

Good luck!

Chris

[finiteparts]

Just a quick note...my "Dynamic Systems" professor drilled this into us...

"Damped" refers to controlling or restraining something, like a damped oscillation.

"Dampened" refers to making something wet.

When we try to get systems to NOT vibrate themselves to death, we add "dampers". When we add "dampeners", we get systems that rust after they vibrated apart! Ha!

So a dampened gauge wouldn't do much for you!

~ Chris

[enginewhisperer]

well if it's liquid filled maybe dampened is the correct term?

[racket]

Hi Chris

LOL....bloody english language , wheres spell check when you need it

When working on coal fired boiler we "damped" the fire to keep it slowly burning overnight , lucky it wan't dampened , ..............though the guy firing her back up next morning was thankful it had been "damp'ened" ;-) ...............english is such a living and evolving language , but frustrating at times .

Yeh , I think Patty needs to look at his NGV as well as a couple of other constructional issues to get temperatures down and bring rpm and P2 into alignment .

Cheers
John

[madpatty]

Hi Guys,

I did some minor changes in the engine design...reduced NGV throat area to 980 sq. mm and bell-mouthed the compressor inlet.


Ran the engine today and the run was rather fantastic. Engine ran upto 1.5 bar P2 first ever time and TOTs hovering around 720-730 degrees at Max P2.

The Readings are as follows:-

P2(Psi)             RPM

7                     71002
10                   81000
11                   85500
12                   88385
14                   91399
15                   98261

18                   A6764
20                   L1797
22                   L4985


The Last three Tacho readings are above 100000 rpm but the tacho display being of 5 digit reading was displaying these values. Can anybody interpret the numbers please.

cheers,
Patty

[Johansson]

Good job Patty!

[madpatty]

Thanks Johansson.

Making a jet engine is an art as i read somewhere in this forum...and thats exactly what it feels when doing certain improvements and then waiting for the results that follow.
With all you experts around this art learning has become more enjoyable and informative. There's lot that i have learnt and still learning since i first made a turbocharger based jet engine.

[Johansson]

I think the most important lesson to be learned from building engines like ours is that it is better to spend 90% of the time planning and constructing and 10% troubleshooting than the other way around.

I hope some day I reach a 50/50 ratio on my builds. :-)