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  • Design propeller for exhaust ventilator

    Discussion in 'The main mechanical design forum' started by GoodCat, Nov 28, 2018.

    1. GoodCat

      GoodCat Well-Known Member EngineeringClicks Expert

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      Hello everyone!

      I need ideas for a new silent blade design.
      1- Re-Design and optimisation of new type of propeller direct drive exhaust ventilator.
      2-Diameter : 213 cm 84"inch with motor 4.0kW
      3- qty blade:5
      4- material : aluminum sheet
      5- propeller RPM : 600 ( note the ventilator are connected with inverter so speed will be 100 to 600
      6- static pressure maximum max ; 0.4''inch of watter (100Pa) normal utilisation ; 0.2'' -50Pa

      The propeller performance at maximum pressure difference should not be less than 20-25m ^ 3 / s
      Propeller is balanced with an accuracy of +/- 1g

      It is necessary to achieve minimal noise. I have tried many ways, but maybe there are more ways that I missed)

      I tried to use a curved blade, but this significantly complicates the production, I tried to change the shape of the profile, but the best option was a profile with a tail with a chord to height ratio of 0.1mm. I tried to use a profile that changes its cross section on the end, but all this complicates manufacturing and allows to get the blades only by stamping from a sheet.

      Now I think in the direction of changing the frequency of the sound wave in the direction of increasing the frequency. Higher frequency sound attenuates at a distance faster than low frequency sound, but so far I have not achieved a good result in this, since such a form greatly reduces the characteristics of the propeller.

      Now it is planned to make a test with a curved blade in the direction of rotation, but even if I can achieve a good result here, most likely I will give up this method since it greatly complicates production.

      Perhaps one of you will tell the right direction to reduce noise without significantly reducing the performance of the propeller

      version with several impeller are not suitable, since they change the direction of rotation of the flow and as a result we get a lot of aerodynamic noise and also reducing the efficiency of the fan as a whole, I do not consider this option. Сhange the input parameters of rotational speed and power of the electric motor is not allowed. Perhaps there are materials that reduce noise, for example, composite materials or polymers? I want to hear any ideas even absurd, maybe this will help me find a solution)

      Best,
      GoodCat
       
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    3. Bill Toulas

      Bill Toulas Well-Known Member EngineeringClicks Expert

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      Well, leaving aside all the "noisy details" such as the engine, the sound baffling elements that you may use, the bearings, etc., you always have to fight the sound of the turbulence created by the propeller. That's always the biggest source of the noise, and you can do very little to damp it down when you have a truly effective and powerful propeller. The best approach would be to calculate how much would you like the performance at nominal speed to be and then figure out the most efficient blade design for this performance level. The most efficient will be the most silent as well. Also, this video may be inspiring to you:

      To my understanding, if the blades incorporate a more "liquified" design, there will be less turbulence for the same amount of air pushed. That said, I would try to pull all edges a bit, making things smoother.
       
    4. GoodCat

      GoodCat Well-Known Member EngineeringClicks Expert

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      «To my understanding, if the blades incorporate a more "liquified" design, there will be less turbulence for the same amount of air pushed»

      No, opposite turbulence will increase. The essence of the blade is the displacement of air masses; its shape is calculated for a certain working geometry; any elastic deviations from the shape will impair its operation. The higher the stiffness of the blade the better.


      The main source of propeller noise is disruption of the flow at the tip of the blade. At the end of the blade, the maximum angular velocity, which leads to aerodynamic noise when the stream slides along the blade at the end.

      A second-time important source of noise is harmonic oscillations at even the number of blades sound waves are combined and amplified.

      The third important source of noise is profile instability during flow. The blade must be rigid because, unlike the wing of the plane, the blade creates swirling flow that act on the blades and make it tremble. Blade vibration breaks airflow causing additional turbulence and also creates noise, and also reduces the lifting force and increases the load on the hub. This means that the use of flexible blades as in the video has an extremely negative impact on the work of the propeller. In quadcopters, the most vulnerable element is the propeller, they constantly break, so these blades are used to break them less often.

      On helicopters, the blades have the ability to deform elastically, but this is not an advantage; this is a feature of contraction of long blades. In terms of strength, the blade can be regarded as a beam with a fixation at one end and an applied load at the other. Elastic deformation always occurs. When the blade elastically flexes along the blade the flow slides resulting in aerodynamic losses. The second problem is the change in the angles of attack of the blade section at different points, which also negatively affects the characteristics of the propeller.

      Blades on the video clearly have more area and other angles of attack in order to compensate for the negative effect of low stiffness, but it will inevitably increase the load on the electric motor.

      The fourth source of noise is the angle of attack of the blade and its profile; the larger the angle and the higher the profile, the greater the noise.

      The most effective form of a profile is a flat profile; its efficiency is within 90%, but such a profile has an extremely low lifting force. As a rule, profiles with a height to stroke ratio of 0.05-0.1 mm are used. The efficiency of such a profile is about 70-80%.



      «The best approach would be to calculate how much would you like the performance at nominal speed to be and then figure out the most efficient blade design for this performance level»

      It is not possible I have already set the desired parameters of the propeller, I can not change them.

      I already have a certain diameter of the propeller and the frequency of its rotation, as well as its parameters. I can not deviate from this

      in the video of the interesting shape of the blade end, I think this is done in order to direct the sliding flow not perpendicular to the axis of rotation, but at an angle, it reduces the noise just as noise decreases with a curved blade. Perhaps if i add a stabilizing shank to this blade, it will become a more noiseless form.
       

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