Tip speeds of Turbine vs ducted fan blades.

Tip speeds of Turbine vs ducted fan blades.
View related threads: (in this forum | in all forums)

Logged in as Guest

Users viewing this topic: none

Printable Version

All Forums >> RC Airplanes >> Aerodynamics >> Tip speeds of Turbine vs ducted fan blades.

Page: [1] 2 next > >>

Login

Message

<< Older Topic Newer Topic >>

Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 12:06:59 AM

Troy-RCU

Posts: 511
Joined: 1/18/2002
From: Oceanside, CA, USA
Status: offline

I would think that a ducted fan blade tip would have conventional rules apply when it comes to hitting mach 1.0 and that efficiency goes down. BUT, I was having a discussion with a co-worker who has done some home made turbine engine work and he seemed to think that maybe the duct itself acts as a diffuser like a turbine engine has just after the inlet fan blades. So, if you were to spin say a 5" ducted fan unit above 55,000RPM and start seeing mach numbers, would you expect to be robbed of power from the tip speeds or are there other factors that would keep the thrust cranking?? When I hear that the model turbines are spinning at 100K+ RPM, I wonder if they are reaching transonic speeds? I understand there are a lot of other variables but I just wonder if conventional rules apply in both prop and ducted fan applications?

_____________________________

Can you feel that Buddy, HUH, HUH, HUH!!!??
I Have EXORSIZED the electrons, this plane is clear.

Post #: 1

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 12:17:55 AM

ArmedZagi

Posts: 606
Joined: 5/18/2005
From: McAlester, OK, USA
Status: offline

I really don't know, and am curious to hear from some one who does.

If I had to make bet I'd say that the ducted fan would suffer a performance drop after the tip hit mach 1 while the turbine doesn't suffer any adverse affects from it. My reasoning is, in a ducted fan the blades produce the thrust, just as a propeller does. Whereas in a turbine, the blades dont actually produce the thrust, they compress the air where it is mixed with fuel, ignited, and the expanding gases produce the thrust.

Could be wrong and likely am, but thats my guess.

< Message edited by ArmedZagi -- 6/21/2007 12:18:16 AM >

_____________________________

-I love the smell of glow fuel in the morning

(in reply to Troy-RCU)

Post #: 2

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 10:21:39 AM

pimmnz

Posts: 552
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: offline

All airspeeds inside gas turbines are subsonic, bear in mind that the speed of sound is also temperature dependent.
Evan, WB#12.

(in reply to ArmedZagi)

Post #: 3

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 12:52:18 PM

Sport_Pilot

Posts: 7828
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline

Turbine fans are basically ducted fans, the same rules apply to both. The turbine engine must diffuse the incoming air to reduce airspeed, but at least the slower ducted fans must do the opposite.

< Message edited by Sport_Pilot -- 6/21/2007 12:55:29 PM >

_____________________________

“I Saw Elvis at 1000 Feet” John Force

(in reply to pimmnz)

Post #: 4

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 12:56:48 PM

j.duncker

Posts: 1220
Joined: 6/22/2004
From: Where I park the RV, AK, USA
Status: offline

quote:

All airspeeds inside gas turbines are subsonic, bear in mind that the speed of sound is also temperature dependent

I am not sure about this statement. If this is the case how can a gas turbine powered aircraft go supersonic? Is it to do with the temperature of the exhaust gas.

_____________________________

The dreamers of the day are dangerous men, for they may act their dreams with open eyes, to make it possible.

(in reply to pimmnz)

Post #: 5

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 12:57:33 PM

Sport_Pilot

Posts: 7828
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline

quote:

Whereas in a turbine, the blades dont actually produce the thrust, they compress the air where it is mixed with fuel, ignited, and the expanding gases produce the thrust.

Only very old pure jet engines do that. Today much if not most of the thrust is made by the much larger high bypass fans.

_____________________________

“I Saw Elvis at 1000 Feet” John Force

(in reply to Sport_Pilot)

Post #: 6

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 1:33:25 PM

Nick C

Posts: 77
Joined: 5/17/2004
From: Bath, UNITED KINGDOM
Status: offline

I agree , velocities inside a gas turbine are subsonic , the nozzle accelerates the gas to Mach 1 at the throat (if their is enough pressure , i.e Pe/Pamb > 1.89 ) and if you have even more pressure then this can expand supersonically, thus driving your supersonic aircraft into that realm !

The mach number is also very much temperature dependant , M = √(Rho R Ts ) , i.e its a function of the static temperature !

Where does the thrust in an engine come from ? there are two methods,
1. Your basic momentum balance , i.e you have much greater momentum exiting the engine than entering and this NET thrust is the available force.
2. Consideration of the forces acting on the engine.... The nozzle itself just experiences a drag force, the forward pressures act on the combustion can, and the compressor section, and the compressor blades. Hence with your modern High bypass engine most of the thrust is being generated by the large fan.

As for the tip speed of a compressor, a very quick calc reveals that the tip speed is about 418 m/s , i.e for ambient air about Mach 1.2 , but thats just the tip speed. The air velocity travelling over the compressor blades , I think would be alot more tricky to establish

Nick

(in reply to Sport_Pilot)

Post #: 7

RE: Tip speeds of Turbine vs ducted fan blades. - 6/21/2007 7:06:47 PM

Troy-RCU

Posts: 511
Joined: 1/18/2002
From: Oceanside, CA, USA
Status: offline

I understand that with ducted fans you also have some benefit of controlling tip vortices which would rob a standard propeller at high RPM's. This means that you could conceivably spin an impeller at higher RPM's than a prop. I just wonder what the limit is before you start hitting the wall (assuming the impellor is designed for those RPM's of course).

_____________________________

Can you feel that Buddy, HUH, HUH, HUH!!!??
I Have EXORSIZED the electrons, this plane is clear.

(in reply to Nick C)

Post #: 8

RE: Tip speeds of Turbine vs ducted fan blades. - 6/22/2007 6:03:30 AM

pimmnz

Posts: 552
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: offline

The limits are the same, it's still an airfoil, a cambered one, moving a mass of air from one side to the other. to do that it still needs to have an AOA...etc, etc, etc. it is a bit more efficient in a duct, and you can vector (Reverse thrust) the stream for braking, but even the big fan jets arn;t doing any more than 3K rpm, some a good deal less, so the tip speeds aren't that high.

(in reply to Troy-RCU)

Post #: 9

RE: Tip speeds of Turbine vs ducted fan blades. - 6/22/2007 9:15:51 AM

Nick C

Posts: 77
Joined: 5/17/2004
From: Bath, UNITED KINGDOM
Status: offline

I agree the limits are still the same regardeless of the tip vortices, your improvement in effieciency comes from the lack of tip vortices constrained by the duct. Its a bit like your aerofoil section in a windtunnel that spans the entire length of the working section, i.e no tip vortices but very steep lift curve slope, but the velocity of the air at the leading edge is still the same.

My last post was for a P80 , i.e , 100000 RPM , nominally Ø0.08mm , = Tip Speed = 418 m/s = Mach 1.2
We run the Grnome Turboshaft engines, used in the wessex heliocpters, I think you boys over the atlantic know them as the GE T58 , anywho, this baby tops out at 26300 , with nominally a stage 1 tip radius of 0.0136 m , = 375 m/s = Mach 1.1
And my Wemotec Ducted fan , Ø90mm , I think its gets up to 32000 RPM , , Tip Speed 150 m/s , Mach 0.44 ,

So the Gas turbines, suggest that a sonic tip speed is accetable, certainly for stage 1 , in the case of an axial turbojet , it may be the case that you "get away" with this initial condition , but as the temperature inside the compressor increases , the mach number will drop for this given rotor speed, and so the rest of the stages are operating subsonically. And its looks like their is a long way to go with my fan...

Nick

(in reply to Troy-RCU)

Post #: 10

RE: Tip speeds of Turbine vs ducted fan blades. - 7/6/2007 8:22:02 PM

morgan4321

Posts: 4
Joined: 4/17/2007
From: Frankfurt, GERMANY
Status: offline

In a turbine engine temperatures are very high, thus the speed of sound is higher than in the outside air. All the calucations above showing tip speeds of mach 1.1 or 1.2 are based on the speed of sound at standard sea level conditions. A turbine engine at high power can run 800-900 C easily, which causes the speed of sound to be much higher. Instead of mach 1.1 or 1.2 you really have around .9.

The large inlet on turbofan engine(like on an airliner) are diffusers that slow the incoming air and heat it up. Many supersonic jet fighters have movable inlets that can change their shape based on speed so the inlets work as designed over a wider range of speed. Engineers go to great lengths to get the intake air just right to make turbine engines run as close to the limit as they can. The limiting factors of turbine engines are speed(limited by sonic conditions at the blades) and temperature(limited by the metal the turbines are made of).

Sonic flow in a turbine will cause stall and flameout and it will probably destroy itsself in the process. A supersonic turbine/propellor doesn't move air, similar to a cavitating boat propellor.

(in reply to Nick C)

Post #: 11

RE: Tip speeds of Turbine vs ducted fan blades. - 7/7/2007 4:40:25 AM

wellss

Posts: 71
Joined: 6/14/2002
From: St. Catharines, ON, CANADA
Status: offline

The speed of sound at 650 C is 609 m / s

(in reply to morgan4321)

Post #: 12

RE: Tip speeds of Turbine vs ducted fan blades. - 7/7/2007 6:27:55 PM

BMatthews

Posts: 9005
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline

I didn't see what I think is the real answer. In a turbine it has already been pointed out that it's more of a compressor than a lift generator. And if I remember my speed of sound physics right the speed of sound rises in denser air so I suspect that has a lot to do with it. Now that would not happen for ducted fans but since someone seems to have done the math to show that a typical fan (5 inch diameter?) needs to turn at 55,000 then it's not an issue since I have't seen any of the larger ducted fan engines hit much above 25000.

Once through the combustion chambers I suspect that much of the gas flow is supersonic. Turbine spools that I've seen tend to have the wedge shaped blades on them while the compressor blades seem to all have the airfoil shapes. Certainly gas flow coming out the tail pipes can often go supersonic as shown by the shock wave diamonds in so many pictures of planes throttling up for takoff.

_____________________________

Witty saying to be plagarized shortly.....

(in reply to wellss)

Post #: 13

RE: Tip speeds of Turbine vs ducted fan blades. - 7/8/2007 10:36:54 AM

morgan4321

Posts: 4
Joined: 4/17/2007
From: Frankfurt, GERMANY
Status: offline

quote:

ORIGINAL: BMatthews
Turbine spools that I've seen tend to have the wedge shaped blades on them while the compressor blades seem to all have the airfoil shapes. Certainly gas flow coming out the tail pipes can often go supersonic as shown by the shock wave diamonds in so many pictures of planes throttling up for takoff.

The cross section shape of turbine vs compressor blades is commonly dictated by the strength of the materials - the hot turbine blades require much higher temperature materials to resist the constant centrifugal force while spinning. The turbine material is what limits how hot you can run a turbine(the hotter it can run the more power it makes). Billions of dollars are spent developing new materials for turbines to let them run hotter and thus get better fuel economy.

A turbine's exhaust is never supersonic except in the case of a turbojet engine while afterburners are operating. In this case the flow becomes supersonic in the long nozzle behind the turbines - the flow through all the turbines is still subsonic.

(in reply to BMatthews)

Post #: 14

RE: Tip speeds of Turbine vs ducted fan blades. - 7/8/2007 6:31:11 PM

pimmnz

Posts: 552
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: offline