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Post by madpatty on Jun 19, 2016 12:09:40 GMT -5
Hi Chris,
Let me share my way of understanding these things.
1. Turbines need a PR across them to produce power.
2. Load on turbine is increased by increasing the static pressure in the exit plane of the turbine. This is done by putting an exhaust nozzle or a free power turbine.
That means higher the Total(at entry) to Static(at outlet)PR across the turbine least the load on the turbine.
3. As the Total PR across the turbine is fixed(at a given entry PR into the turbine) so you can change that total PR into fully dynamic(at the exit of converging nozzle) or to fully static component(as in case of a diffuser).
4. According to me the available potential energy of the flow is told by its static pressure component(though you can convert the kinetic energy into potential energy by using a diffuser) And this is the reason why diffusers are used For shaft turbines.
Correct me if i am wrong.
Cheers.
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Post by madpatty on Jun 19, 2016 12:16:16 GMT -5
Chris.
One thing that has confused is when you said that in free jet condition Total Pressure has to be equal to ambient.
This can be only true when there is no velocity in the fluid.
That is
Total Pressure = Static Pressure = Ambient Pressure
(Velocity = 0 or dynamic pressure = 0)
Am i right?
Cheers. Patty
PS:- For free jet i would rather prefer to say that static pressure has to be equal to ambient.
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Post by racket on Jun 19, 2016 16:57:22 GMT -5
Hi Patty
In your case where the measurement is being taken near the end of an "open" pipe theres not going to be much total pressure to measure .
Years ago during my TV84 turbo engine development I tried all sorts of ways to measure static/total pressures in the jetpipe to get an idea of mass flows but with varying degrees of success , its very difficult considering the state of the gas flow in our jetpipes as the gases exiting our turbine exducers is a mixture of speeds/directions/pressures etc etc.
I ended up using flow straightening vanes at the entrance of a long diffusing jet pipe that reduced gas velocity to fairly low levels prior to entering the jet nozzle , pressure readings were taken just prior to the gases entering the jet nozzle , total length of jetpipe with nozzle was ~550 mm
If you don't have a jet nozzle applying "backpressure" on the engine then your temperatures will be low , along with your gas velocities making for little total pressure to be measured , you may only have <1 psi at the exit of your pipe where you are taking the readings, the slight flickering of the needle around 0 psi is understandable due to the unsteady non axial gas flow .
Cheers John
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Post by madpatty on Jun 19, 2016 21:52:53 GMT -5
Hi Racket,
Here are some numbers:-
T2 = 159 degrees celsius P2= 20 Psi TOT= 686 degrees celsius
T(ambient) = 36.6 degrees celsius P(ambient) = 14.6 psi
If I take turbine stage efficiency to be in the range 70-75 % then i should expect at least 1.23-1.26 (>3 psi) Total PR at the turbine exhaust.
I am confused where this PR is going.
Should I be expecting this much total pressure in the exhaust?
Thanks. Patty
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Post by racket on Jun 19, 2016 23:20:49 GMT -5
Hi Patty
I still can't reconcile your TOT of 686 C with an "open" exhaust pipe unless there are serious efficiency problems somewhere in the system .
Even using a comp efficiency of 70% and a turb effic of 65% there should still be ~2.5 psi of dynamic pressure in your jetpipe , I'm assuming your jet pipe ID is the same as your turb wheel exducer diameter.
As I said to you in an earlier email , check your gauge operation , you only need a couple of metres of small bore plastic tubing and some water to check it at those 2-3 psi range, the several hundred feet per second exhaust pipe velocity should produce at least a couple of psi .
When doing my TV84 experiments I was producing ~3-4 psi of total jetpipe pressure at 25 psi P2 with TOTs <600 C .
Cheers John
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Post by madpatty on Jun 20, 2016 0:02:10 GMT -5
Hi Racket,
Same here. I also can't understand why the efficiency numbers are matching up.
I am using 3 high class fluke and omega calibrated thermocouples meauring T2. All show reading with <5 degrees. So i am pretth sure compressor efficiency figure is accurate.
TOT is also measured by calibrated fluke unit and fluke thermocouple.
For your information. I am using a bigger turbine wheel now(as per your recommendation) to power my bigger 83mm compressor wheel.
Turbine wheel is 76 x 67 mm.
Earlier it was 70 x 58 mm.
Lol, Strangely temperatures have risen after i shifted to larger turbine wheel.
I don't know where the problem is now.
Cheers. Patty
PS- i will be testing with water manometer at exhaust today.
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Post by racket on Jun 20, 2016 0:58:22 GMT -5
Hi Patty
OK , bigger turb wheel which should be better , BUT , has the rest of the engine bits been modified to match the projected extra flow through the engine due to the turb exducer now being 54% larger than the comp inducer rather than the previous 15% bigger ??
If things like comp diffuser , flametube hole area and NGV throat area haven't been changed to accommodate the potential for a greater flow then efficiencies will suffer.
The new turb exducer is 33% bigger in area than the old one this is a substantial increase and could be allowing the comp to flow in the high flow low efficiency region of the map unless the NGV throat area is sized to prevent it .
Your T2 indicates effic at ~70% which is rather low for only a 2.4 PR , I'd have expected it at closer to 75% , UNLESS , its flowing in the high flow low effic region to the right hand side of its map ...............this could account for an increase in your TOT
Cheers John
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Post by madpatty on Jun 20, 2016 1:07:01 GMT -5
Hi Racket,
I have recently shifted to vaneless diffuser and that too an 8" diameter diffuser.
With pinch from 5mm compressor exducer tip height to 4mm at about 10mm radially from the tip.
It was only after i shifted to the vaneless diffuser that my compressor stage efficiency has crossed 70% mark.
At certain PRs like at 2 PR the efficiency even now touches 80% and rarerly 82% like at 10-12 psi P2.
Earlier with the vaned diffuser the efficiency (everything else same) was in the mid 60s and even lesser.
NGV area is according to flow of 0.3 kg/s which the compressor should flow now(as the diffuser is same as OEM).
What else?
Thanks.
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Post by madpatty on Jun 20, 2016 1:09:35 GMT -5
And:-
Flametube hole area is in same 3:2:5 of the compressor inducer.
Moreoevr NGVs are CNC milled out of solid steel chunk so no crappy machining this time.
Cheers.
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Post by finiteparts on Jun 20, 2016 1:53:02 GMT -5
Hi Chris, Let me share my way of understanding these things. 1. Turbines need a PR across them to produce power. 2. Load on turbine is increased by increasing the static pressure in the exit plane of the turbine. This is done by putting an exhaust nozzle or a free power turbine. That means higher the Total(at entry) to Static(at outlet)PR across the turbine least the load on the turbine. 3. As the Total PR across the turbine is fixed(at a given entry PR into the turbine) so you can change that total PR into fully dynamic(at the exit of converging nozzle) or to fully static component(as in case of a diffuser). 4. According to me the available potential energy of the flow is told by its static pressure component(though you can convert the kinetic energy into potential energy by using a diffuser) And this is the reason why diffusers are used For shaft turbines. Correct me if i am wrong. Cheers. Hi Patty, I think we are on a similar page, but let me rephrase things to make them more clear and see if you agree. 1) Turbines need an enthalpy change across them to produce power. This is usually accomplished via a temperature and pressure drop across the turbine stage. 2) Increasing the back pressure on the turbine stage (i.e. reducing the pressure ratio (PR) across the turbine stage) reduces the enthalpy change across the turbine stage and thus the power that the turbine stage is capable of producing for the same inlet gas temperature. If the inlet gas temperature is increased, the required enthalpy change across the turbine stage can be recovered. So a larger total to static PR increases the turbines ability to produce power. 3) The total to total PR is fixed by the turbine inlet total pressure and ambient conditions. If you have sufficient PR across the turbine stage, such that you have residual dynamic pressure, yes, you can either recover that with a diffuser into a higher exit static pressure or with a nozzle into a lower static pressure. Neither of those will be "fully" converted into potential or kinetic energies. 4) The available potential energy of the flow is told by the static pressure relative to the total pressure. The reason that diffusers are used for shaft power engines is because they allow for a more efficient recovery of the dynamic pressure, still in the turbine exit gases, into static pressure. Reducing the losses in the exhaust duct allows the static pressure at the discharge plane of the turbine wheel to be reduced to the lowest possible level. This low static pressure at the turbine exit plane allows for a larger total to static PR across the turbine stage and thus more work for a given inlet temperature and flowrate. As for the free jet conditions, the main thing to remember here is that the dynamic pressure represents the decrease in pressure due to the velocity of the gas, not an increase. If we just look at an ideal case of an isentropic nozzle, we would see that there is no change in total temperature or pressure across the nozzle because if you solve the one dimensional energy equation, there is no shaft work or heat transfer. Since the flow enters the nozzle from the turbine discharge, it has a certain amount of static pressure and dynamic pressure as it comes in and these can still be converted between each other. The free jet condition tells us that the jet boundary has to equal the ambient pressure and we just determined from the 1-d energy equation that the total pressure remains constant through the nozzle...so let's say that somehow the static pressure in the jet equaled the ambient. As that jet expands and slows down, the dynamic pressure is being converted to more static pressure. If this happened, the static pressure of the jet would be higher than ambient and thus cause more outflow. As the jet "slowed" down further the self generating static pressure would balloon. This just doesn't make sense. If you read about experiments with subsonic free jets, you will see that the core is sub-ambient. So in the real world, the total pressure will actually drop because there are losses in all systems. The second law tells us that the total pressure ratio across any component has to be less than 1 due to entropy production. Additionally, the above arguments make the assumption of the Mach number being much less than 0.3 so that incompressible flow closely approximates what we are trying to measure. I state this because above 0.3, the total and static pressures are determined via the mach number due to the density changes and the simple pitot equations that are based on Bernoulli's equation do not accurately predict the dynamic pressure or velocity. I hope that helps, Chris
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Post by madpatty on Jun 20, 2016 3:48:00 GMT -5
Hi Chris,
When talking of the free jet condition, obviously you can't impart velocity to fluid at atmospheric condition without decreasing its static pressure(below atmospheric pressure) or else by imparting some energy to the fluid.
But i am confused as to what happens at the turbine exhaust plane.
Suppose the turbine exducer is directly exhausting to the atmospheric.
Ideally the static pressure should be equal to atmospheric at that plane(atleast ideally) and the total PR should be all dynamic....isn't it ?
Now talking of the balloning thing.
Yes that makes sense because the flow has got some total enthalpy( the total PR at the exhaust) which should remain with the fluid unless there are any losses.
If there is any static pressure higher than the atmospheric pressure then it will keep expanding until static pressure becomes equal to atmospheric pressure and the velocity component remains with the fluid unless there is any loss.
Cheers.
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Post by racket on Jun 20, 2016 4:15:47 GMT -5
Hi Patty
Could you give me a bit more info about your NGV throat area as well as the turbine wheels inducer tip height .
Cheers John
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Post by madpatty on Jun 20, 2016 4:27:06 GMT -5
Hi Racket,
15 vanes. 21 degrees to tangent.
5.33mm throat width.
12 mm tip and vane height.
Cheers.
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Post by racket on Jun 20, 2016 4:55:00 GMT -5
Hi Patty
5.33 X 12 X 15 = 960 sq mms = 1.49 sq inches divide by 1.1 to account for "boundary" effects gives us an effective flow area of 1.35 sq inches = 0.009375 sq ft .
lets assume you have your 0.3 kgs/sec - 0.66 lbs/sec , and with a throat temp of say 700 C and a choked PR of 2.36 X 0.95 = 2.242 divided by say 1.8 = 1.25 going into the turb stage with a density of ~35 cu ft/lb or ~24 cu ft/sec .
24 divided by 0.009375 means we need a gas velocity of 2560 ft/sec ............impossible , it'll be closer to 1900 ft/sec or ~35% less , so a mass flow of only 0.49 lbs/sec .
This might put it over the surge line .
You need more NGV throat area .
How did you arrive at your throat area ?
Cheers John
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Post by madpatty on Jun 20, 2016 5:10:39 GMT -5
Hi Racket,
The design point was 0.3 Kg/s at 3 PR(full power) and 900 degrees celsius TIT.
At 2.37 PR i expect the flow to be only 0.23-0.24 Kg/s mass flow.
Cheers.
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