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Post by stoffe64 on May 27, 2016 14:28:21 GMT -5
No no,you can not use oxygen to start With,it would be extremely dangerous With a huge explosion!!
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Post by madpatty on May 27, 2016 19:40:16 GMT -5
Hi.
Yup me too thinking the same.
But can we use CO2 or an inert gas like argon?
Thanks.
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Post by racket on May 27, 2016 20:36:16 GMT -5
Hi Patty
I've used oxygen , though its an expensive option , why can't you get industrial air cylinders , they come in big cylinders like oxygen but less costly .
Co2 used onto the turbine wheel is an option , but only a cold turbine wheel .
Cheers John
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Post by madpatty on May 27, 2016 23:34:01 GMT -5
Hi Racket,
O2 is not at all expensive here...its the easiest qnd least expensive for me.. I can get a big cylinder refiled for 3 bucks.
Parents being doctors i even got a spare cylinder at home.
What was your experience with oxygen?
I qm thinking of injecting(impingement) on compressor wheel.
Don't want anything on turbine side. Can it be done on compressor side?
Thanks.
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Post by racket on May 28, 2016 0:34:20 GMT -5
Yes , I impinged on my TV84 compressor wheel .
You'll need to take the usual safety precautions with high pressure oxygen lines/fittings as well as being very careful about any sort of "fuel" coming in contact with the oxygen other than within the flametube
The amount of high pressure oxygen required to spin up the compressor wheel isn't a huge amount compared to the compressor wheels flow of air , it soon gets "diluted" .
Some interesting scuba tank numbers ...........
1 liter = 0.03666 cu ft Volume of gas at atmospheric pressure = (cylinder volume) x (cylinder pressure) / (atmospheric pressure) Air pressure is 14.7 psi From here the common aluminum cylinder with an internal capacity of 11.1 liters filled at 2900 psi (200 bar) will contain 11.1 * 0.03666 * 3000 / 14.7 = 80.15 cu ft of air - the common "aluminum-80"
Now that equates to a tad over 6 pounds of air , lets assume thats sufficient for 1 minutes worth of "impingement" , this is at the rate of 0.1 lbs/sec , on a TV94 sized comp wheel the mass flow at self sustain is >0.7 lbs/sec of air .
High pressure impingement is very fast at spooling up an engine , just make sure theres no "restrictor" in the cylinder valving .
Cheers John
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Post by madpatty on May 29, 2016 20:26:34 GMT -5
Hi Racket,
During my recent spoolups using leaf blower there is something strange happening.
Whenever during the startup the things heat up the rotor gets stuck totally for some time.
It is contacting something as things expand but after a few seconds it starts rotating itself and then engine starts.
But that initial contact is very tight and rotor is totally jammed for those initial seconds.
I have tried to look at both compressor and turbine housing but was not able to find any rubbing marks.
Can you point at something?
Thanks.
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Post by racket on May 29, 2016 21:42:07 GMT -5
It'll be your turbine wheel as the blades heat much quicker than the "thick" cast iron scroll, as the scroll heats and expands the clearance is restored and it starts to rotate again
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Post by stoffe64 on May 30, 2016 1:24:17 GMT -5
Hi John and patty, i have been thinking about this With oxygen as startgas,as most, atleast mechanics know is that oxygen and any oil,grease, fuel or even debries from metal flakes can cause an ignition, theese bearings that we use sometimes isnt the most tight and oil can enter compressor housing OR turbinehousing, IF you then inject oxygen you would get ignition ok in The turbine it would not be so drastic as on the compressor side....that was my concern Cheers stephan
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Post by racket on May 30, 2016 21:01:24 GMT -5
Hi Stephan
Yep , I go along with your concerns, oxygen needs to be respected.
The one positive with our engines is the high airspeeds that tend to inhibit combustion other than within the flametube.
With high pressure impingement , theres probably only a few seconds at most when there'll be an overly oxygen rich mixture of gases going into the engine, the quickly accelerating rotor will soon be pumping huge quantities of air and diluting it.
Ideally we should only be using compressed air , but even it has its dangerous side at 200 Bar , due caution required , as with every thing connected with our engines :-)
Cheers John
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Post by madpatty on Jun 17, 2016 9:01:58 GMT -5
Hi Racket,
I needed some advice on how and where to locate the pitot tube inside the jet pipe.
In my case, the jet pipe is almost 8-10 inches long divided into two parts.
First part almost 6 inches long is seamless and welded to the turbine shroud and the remaining portion to which pitot tube is welded is mounted with a flange to the jet pipe.
The pitot tube is located pointing towards the turbine at about 6 inches from the turbine and is located at the central axis of the jet pipe.
I yesterday test run thew engine and took it to 20 psi P2 (~2.38 PR) and the pressure gauge(0-15 psi) wasn't registering any pressure. The needle was just flickering around the 0.
Is there something wrong in how I am doing this.
Cheers. Patty
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Post by racket on Jun 17, 2016 17:00:29 GMT -5
Hi Patty
Fit the pitot tube ~10-15 mm from the wall of the pipe , the central portion of the gas flow is affected by the wheel hub boss
Cheers John
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Post by madpatty on Jun 19, 2016 1:51:49 GMT -5
Hi John,
I positioned the entry of the pitot tube at about 10mm from the wall fo the jet pipe and still no pressure reading.
Took the engine to 2.3 PR.
Does the axial position of the pitot tube entry with respect to the turbine also play a role in this?
In my case, the entry to the pitot tube is located at the furthest end near the exit of the jet pipe(just say an inch inside the jet pipe). Can it effect?
Thanks for your help.
Cheers.
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Post by racket on Jun 19, 2016 3:29:29 GMT -5
Hi Patty
Having the pitot pickup just before the jet nozzles should allow the gases to even out somewhat and give an honest reading .
Could your pressure gauge be reading incorrectly at the lower end of its range ??
You'll probably only be producing a few psi of pressure at the most , it might pay to check it by fitting some plastic tubing to the gauge and fill with water, you should get roughly 1 psi for every 2 feet of water column , so 6 feet of water column should give you a few psi on the gauge.
Once you can verify the accuracy of the gauge at those low pressures we can then look for possible reasons why you don't seem to be producing any jetpipe pressure.
Cheers John
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Post by madpatty on Jun 19, 2016 3:52:51 GMT -5
Hi John,
I should have mentioned that the Jet pipe i am talking of is an open jet exhaust pipe without any convergence. And the pitot entry at the end of the pipe.
But as total pressure should remain same so i think atleast setup is OK.
Isn't it?
Cheers.
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Post by finiteparts on Jun 19, 2016 10:36:56 GMT -5
Hi Patty,
If you recall your fluid dynamics, namely, what was termed the "free jet condition", you will remember that in a subsonic flow, the total pressure has to be equal to ambient. The dynamic pressure due to the jet velocity makes up the difference between the total and static pressure, thus the static pressure is sub-ambient. This is the reason that exhaust diffusers work for gas turbines that are used to produce shaft power. By recovering more of the exhaust energy efficiently, the static pressure at the turbine exit plane can be driven further below ambient and give a larger pressure ratio across the turbine.
If you are trying to back out the exhaust velocity, you need a true "pitot" tube with both a port for total pressure and static pressure measurement. If you are trying to collect exhaust tube static pressures, you can just collect that from a similar tube with the end plugged and a hole drilled in the side.
In the ideal sense, the total pressure through a nozzle will be constant just like in a straight pipe, assuming the flow is subsonic, since no work is done on the fluid. Once the nozzle chokes, the upstream flow has no way to "communicate" with the ambient and thus will be different. But, because nozzles and tubes in the real world have losses and there is "work" done on the boundary layer fluid, there will be a loss in total pressure through the nozzle. The straight pipe has a lower loss in the static pressure sense, so the difference in total pressure through the tube will be very small.
Good luck! Chris
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