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Optimizing the speed and number of blades of the propeller or turbine to capture energy
The optimization of a propeller propulsion, or a wind turbine propeller or turbine, through the choice of the number of blades according to the Speed ​​of rotation.

optimum efficiency of propeller

A quick search of optimum speed, for a given geometry can be executed from the tab "optimize". If you wish, HELICIEL automatically calculates the speed offering the optimum performance of a propeller you have determined the geometry.If you wish, HELICIEL automatically calculates the speed offering the optimum performance of a propeller you have determined the geometry.
But for a more complete analysis should be used "multiple analysis" which offers many views of performance and test options. For example when the propeller is coupled to an electric motor or a generator,the actual rotational speed of the propeller will be the one which balances the engine torque and the load torque.To determine how fast the spin system, it is easier to make a superposition of torque curves, according to the rotational speed with multiple analysis .

 

The propellers of air or water and propellers or turbines capture wind, will be treated equally, for the moment. We assume, as a first step, that we have the choice of the rotational speed of the propeller and we want to get the rotational speed providing the best performance.
Brief summary of the situation:
The apparent wind received by blade elements is composed of the rotational speed and the speed of the fluid (we do not speak here of induced velocity, but of course it must be taken into account) .The efficiency of a propeller is the performance of its elements that can be compared to rotary wing portions. The best performance is achieved if the elements lift / drag ratio , is the highest possible. The best lift / drag ratio is obtained at a specific angle of incidences, which determines the twisting of our blade.

vent apparent helice

Apparent wind vectors for each element in HELICIEL
Our blade is twisted for that, the angle between the chord profiles and the direction of the actual speed, corresponds to the incidence providing the best lift / drag ratio of the profile. Lift profiles represent the effective force that we use.
The lift is perpendicular to the apparent wind,but the force that interests us is the thrust (for propulsion propeller) or the torque (as a wind turbine). If the direction of the lift force is aligned with the force we are interested, we can assume that our performance will be optimum ... By varying the speed of rotation we change the direction of the apparent wind so the lift. We can therefore assume that there is a rotational speed directing optimally lift forces..

portance profils palecouple portance eolienne

Directions Lift - pusher propeller thrust (left) and lift-torque wind turbine (right)

 

A propeller equipped with airfoil blades for high speeds, with a low drag would be optimum. But the speed of rotation is limited by other adverse effect (see above) as the masking effect, which leads us to a monopale theoretical optimum propeller. I say "theoretical" because the balancing rotation must be managed on a mechanical level ...

helice monopale

We could say that the faster you turn , the better the performance of the propellers are, but Up to what limit?
The main causes limiting high speeds are::

passage entre pale

cavitation se formant juste apres le bord d'attaque

torque and drag on a small wind turbine

 

conclusion:
We had assumed that the speed of our propeller we were not imposed, but the factors limiting performance are quickly reached.
One of the key factors limiting the speed of rotation being the interaction between the blades, optimize the speed of our propeller involve a search of the optimum number of blade.

 

A further constraint imposed the rotational speed. This is the machine connected to the shaft.. An alternator, a pump, or an air ventilation motor, has characteristics that the propeller must meet. Thus, the speed and torque can be imposed by the machine. If the difference between the optimum rotation speed of the propeller and the optimum rotation speed of the machine is too large, a speed reducer must be installed. But this will decrease performance, increase the cost, weight, and system maintenance. To adjust the speed and torque of a propeller machine, we can play on the geometric parameters of the blades, but the number of blade can help us change the Optimum speed fairly simple.
The number of the blade increases the torque and reduces the optimum rotation speed. So we can adjust our Optimum speed by increasing or decreasing the number of blade.To maintain the performance we must also change the geometry of our blades to reduce the interaction between the blades. As the number of blade increases the blade load decreases, and the thickness of the blades can be decreased. Because it no longer needs to provide much resistance.

 

The few indications above are tracks, but there is no magic formula to optimize speed and a compromise is always to do.This little article the number of blades of the propeller may also provide some track.

 

Optimization procedure of the speed of rotation based on performance in héliciel:

influence du nombre de pales sur la vitesse de rotation offrant le meilleur rendement

At each change of blade geometry, if the user clicks rebuild HELICIEL rebuilt the optimum twist, for the propeller blades, and updates performance to the operating point imposed. The selected speed determines the twist. But for the reasons we mentioned above, this speed might not be optimum. As we have seen, the parameters optimization of the speed of rotation are numerous and interact in such a way that it is very complex to establish a law of optimization, valid for all propellers.
To find the optimum rotation speed in performance, Héliciel proposes a procedure, of Iterative search. A range of rotation speed is tested, the twists of the propeller are re calculated for each performance and speeds are displayed in a graph. When the maximum efficiency is obtained, the propeller is updated at the optimum rotation speed.. The optimization in terms of performance, gives us the most economical propeller for energy. It is a starting point for the study of the propeller. The layout constraints of our propeller, maybe force us to deviate from the optimum rotation speed, but we will always keep a look at the quality of our propeller through its efficiency.
A second optimization procedure optimizes the number of blades: HELICIEL iterates through rotational speed, the first search maximize the propeller, with a minimum blade,the number of blade is then increased again seek the maximum efficiency. As the maximum yield increases the number of blade is increased. When the maximum efficiency is found, the process of optimizing performance stops, and the propeller is rebuilt with the optimum number of blades, for Optimum speed

 

for a more complete analysis should be used "multiple analysis" which offers many views of performance and test options. For example when the propeller is coupled to an electric motor or a generator,the actual rotational speed of the propeller will be the one which balances the engine torque and the load torque.To determine how fast the spin system, it is easier to make a superposition of torque curves, according to the rotational speed with multiple analysis .