Why are turbine blades twisted?

[socrates_1]

Hi guys, in gas turbines, the blades of the turbine are twisted.I have searched it and as far I understand it has to do with the angle of attack,as the blade has the shape of an aerofoil.Does anyone know why are they twisted?
Why they could not just have an angle without been twisted?
I would appreciate it if you could answer,as they is no much information online(or I can't undestand their explanations).Thank you.

 

[Oryon]

As a pilot and aviation nut, my understanding is that due to the relative speed differences between the tip and root of the blade, to produce an even amount of thrust/compression across the face of the disc, requires a lower angle of attack on the tip vs the root.same reason a propeller is twisted, helicopter rotors do the same i think, and boat screws as well..

I again, I am not an aerodynamics expert, just a pilot, so I could be wrong...

 

[socrates_1]

As a pilot and aviation nut, my understanding is that due to the relative speed differences between the tip and root of the blade, to produce an even amount of thrust/compression across the face of the disc, requires a lower angle of attack on the tip vs the root.same reason a propeller is twisted, helicopter rotors do the same i think, and boat screws as well..

I again, I am not an aerodynamics expert, just a pilot, so I could be wrong...

Thank you for your reply.What you just said is completely correct.I confirmed it through an explanation on a website about the aerodynamics of a wind turbine.
Here is the section of the article which confirms your description:

Twist
The closer to the tip of the blade you get, the faster the blade is moving through the
air and so the greater the apparent wind angle is. Thus the blade needs to be turned
further at the tips than at the root, in other words it must be built with a twist along its
length. Typically the twist is around 0-20° from root to tip. The requirement to twist
the blade has implications on the ease of manufacture.

 

[rock.freak667]

Why they could not just have an angle without been twisted?

From how I understand steam turbines to work, if the blades were just at an angle, they would get the correct nozzle angle to the second set of rotating blades, but you might get less exit velocity of the steam, so less efficiency. By having it twisted, I believe that you would increase the exit velocity to be higher and thus higher efficiency. (There is less pressure drops associated with twisted design as well).

Twisted blades = less drag -> more energy can be converted to work so higher efficiency.

 

[Aero51]

A turbine blade (or any propeller blade) is essentially a rotating wing. Because this wing is rotating, the velocity along the span of the wing will not be constant. Specifically, it will increase from 0 at the hub to the angular velocity times the length of the blade at the very tip. Or:

$V_{tip} = \omega L$

However, this velocity variation is only in the plane of the blade. There is also a velocity component moving perpendicular to the blade or in other words, along the direction of the hub. If you resolve these velocity components you find that the wind direction is actually coming at an angle relative to the blade. This angle is greatest at the tip of the blade.

To generate thrust the blade, being a wing, essentially has to generate lift. The equation for lift is as follows:

$L = C_{L} 1/2 \rho V^{2} S$

L = lift
CL = lift coefficient
$\rho$ = air density
V = wind velocity
S = Blade project surface area

The lift coefficient is the key here, it is almost always experimentally determined and has been found to be proportional to

$C_{L} \propto [ \alpha, M, Re ]$

Where:
Re = reynolds number, the ratio of inertial forces to viscus forces most basically
M = the mach number, ratio of wind velocity to speed of sound
α = angle of attack, the angle of the blade relative to the direction of the wind.

For the most basic cases:
CL varies linearly with $\alpha$
As Re increase the maximum possible CL increases
CL varies minimally with M as M<.3, when M>.3, CL slightly increases as M increases. Crazy things happen when M>1, you can actually get negative lift with the same airfoil shape at lower speeds! I tried to find a more concise explanation of the trend, but I couldnt. I may contribute later on tonight if I can dig up one of my old books.

All else being equal, it is clear that the lift coefficient is the key to changing the blade performance. It is also suffice to say that the angle of attack is the only thing a designer has direct control over given some set of operating conditions. In conclusion, the blades are twisted to change the angle of attack and thus the blade performance.

What are some examples of performance parameters?

If you wish to design a highly efficient blade you would want to minimize the total drag incurred. To do this, you must ensure that the CL/CD (CD being the drag coefficient, analogous to the lift coefficient) is maximized along the span. For every section of a wing, there exists a maximum CL/CD which is proportional too...α!

If you want to produce a blade that gives you the most amount of lift for a given rotation speed, you want to maximize CL along the span. Again, the maximum value of the lift coefficient is strongly proportional to...α!

In addition, you can create special CL distributions along the span of the blade, which may meet some design criteria.

Hope this helped. Also, check out http://www.desktop.aero/appliedaero/preface/welcome.html for more info.

 

[marellasunny]

For cases for it is not possible to manufacture 'twisted' blades, engineers make use of what is called a 'feathering mechanism'-also to change the angle of attack.

 

[E_Q]

I think aero51's finished this thread really, but I'd like to explain a bit more into it (or perhaps reword it more simply). As he said:

V_tip=ωL (ω is angular velocity, radians/second)

But really, in the context of this question just think of V=ωS, With S being the distance from the hub to the point on the rotor. It's now obvious that the further along the blade, the faster it is spinning. Imagine a spinning roundabout in a play park; if you sit in the middle it doesn't feel that fast but as you move outwards you have to start pulling in noticeably.

Again, there is also the wind moving in the direction of the hub (the actual wind the turbine is trying to harness), which can be assumed reasonably to be constant across the turbine's diameter. Hence resolving velocities the further along the blades from the hub you go, the larger the (spinning) velocity perpendicular to wind direction is, but the wind speed itself is the same; thus the angle of attack (direction of resultant force acting upon the blade) is more deflected from the wind's direction.

Interestingly in the context of wind, before the development of actively pitched turbine blades (blades are rotated to control turbine as it reaches cut-off wind speed), blades were designed to take advantage of the stalling effect of turbulence created by the blades at higher wind speeds. The blade would stall progressively towards the hub, becoming less efficient (reducing angular velocity) and thus maintaining rated power into higher wind speeds.

 

[Baluncore]

Now that we know why turbine blades are twisted we can wonder why helicopter and autogyro blades are not twisted. The blade section is also usually symmetrical.

When flying with straight rotor blades, at any time, only part of the blades length can have an angle of attack that produces significant lift. Under different conditions different parts of the blades generate the lift, while other parts are stalled or have neutral lift.

A helicopter flies with a powered rotor, with air moving down through the rotor. To fly forwards a helicopter leans forwards. An autogyro flies without rotor power, with air moving up through the rotor. To fly forwards an autogiro leans backwards and is pushed along by a normal propeller.

When a helicopter suffers engine failure, the rotor is adjusted to auto-rotate, like an autogyro rotor. That might explain why the helicopter blade section is symmetrical, it is a safety feature, but it cannot be true as the airfoil of a glider needs to be the same way up as a powered aircraft. Nor does it explain why the autogyro has a symmetrical blade section even though the airflow is always in the same direction.

While on the subject of twisted airfoils, optimised propellers and wind turbines blades are quite different. They may have a very similar twist but the airfoil section is inverted because the differential pressure between the faces is reversed.

 

[AeroDoc]

Now that we know why turbine blades are twisted we can wonder why helicopter and autogyro blades are not twisted.

I do not know much about auto-gyros but helicopter blades usually incorporate some twist into it.
"All helicopter rotor blades use some amount of spanwise twist in their shape, although in different amounts. ... twist provides the rotor with several important performance advantages."

Ref: Principles of helicopter aerodynamics, by Gordon Leishman, 3rd edition, pg 121
Link: http://books.google.co.in/books?id=...f Helicopter Aerodynamics 3rd edition&f=false

 

[rcgldr]

Now that we know why turbine blades are twisted we can wonder why helicopter and autogyro blades are not twisted. The blade section is also usually symmetrical.

In forward flight, the ratio of the speed differential between air and rotor is not a linear relation ship to the distance from the axis to a point on a rotor blade. The relative air speed at the forward moving tip is greater than the backward moving tip, but both are at the same radius. Non symmetrical airfoils generate a "downwards" twisting torque, and this would present a problem for the long slender rotor blades themselves as well as a torque load where the blades are mounted onto pins in the cyclic / collective mechanism.