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Cold Turbofan Concept (Fluid Flow Concern)

  1. Jul 26, 2011 #1

    I recently joined this forum to get some feedback on this project I'm working and which I have searched for an answer without any luck.

    My goal is to get the most airflow velocity in an outlet of a ducted fan. I'm using a 20,000RMP fan that is pushing air from a ~1.25" inlet to a .20" outlet as seen from the attached picture of which I have built a quick physical model. However, to my dismay, I get little air coming out of the outlet and some coming from the front due most likely to turbulence. I'm using an electric motor set up similar to a turbojet engine such that the fan is in front of the motor. Reversing the polarity of the motor pushes the air towards the 1.25 opening so direction of rotation is not an issue.

    My concern is whether these dimensions are too crude to get any results. Note that this is not for flight but rather obtain the highest velocity in a .2" outlet. I presume that this could be made for flight with a variable outlet. Any insight would help.

    Attached Files:

  2. jcsd
  3. Jul 27, 2011 #2
    You are running your fan in surge http://en.wikipedia.org/wiki/Compressor_stall, you either need to open the duct more or run at a lower mass flow (fan speed). Running this as an aero engine where are you going to get the electricity to run the fan?
  4. Jul 27, 2011 #3
    Thanks for the info. I assume I need to open the duct to more than .2". I have tried lowering the fan speed with a power regulator and see if this reduces surge, but at lower speeds I get the same result linearly. It seems that lowering the speed just cuts down the airflow and I don't get an increase of flow in the outlet as compared to running it at full power. This is not for aero flight. I'm using a 9VDC transformer running at 800ma.
  5. Jul 27, 2011 #4


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    Do you have a fan curve for this fan?
  6. Jul 27, 2011 #5
    Are you referring to the profile curve of the fan blades? I basically built one from scratch as seen in this video
    At full power, air flows backwards and faster than out the small opening. Logically, I would think all air will flow from the back of the small hole and faster similar to a water nozzle.
    Last edited by a moderator: Sep 25, 2014
  7. Jul 28, 2011 #6
    There are many different factors effecting your fan performance here,
    As you reduce your outlet nozzle you are reducing the mass flow of the fan and increasing the pressure behind the fan, I would guess that you start to go into surge very soon after you start reducing your nozzle size. I also think that your flow around the outlet will be very non-laminar also reducing your performance.
    If you have a source of smoke it would be interesting to look at airflows with and without outlet nozzles. Is this purely for interest or do you have some application in mind?
  8. Jul 28, 2011 #7
    Yes, it would be interested to see airflows and turbulence. Unfortunately, I made it from super high density foam in white that when sanded you get a smooth surface. Maybe If I made from clear plastic. Without the tapered section and the small outlet, a lot of air flows as there is no obstacle and as expected. I was considering a blower application using a turbofan such that the air is concentrated in a small hole like my .2" approach. I saw this video too where they use variable nozzles and interested to watch.
    Last edited by a moderator: Sep 25, 2014
  9. Jul 28, 2011 #8


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    You have a HUGE reduction of area here - a factor of about (1.5 / 0.2)2 = more than 50 times.

    As such, the flow you will get through the outlet hole depends how much static pressure the fan can create. It doesn't mean much to say you want "the most airflow" because the higher the pressure, the more flow you will get.

    A good way to get a pressure increase from an axial flow compressor is to put some stator vanes after the fan rotor. If you look at the design of multi-stage axial compressors in jet engines etc, each rotor stage increases the air velocity at more or less constant pressure across the rotor, and the following stator stage reduces the velocity again (and takes out the tangential or whirling component of the velocity) and increase the pressure as it does so.

    To get enough pressure increase for "high" airflow (say to 2x the inlet pressure) you will need a multi-stage compressor.

    Or, you could consider about a radial flow compressor, which can produce a bigger pressure increase per stage than an axial flow.
  10. Jul 28, 2011 #9
    Yes, my reduction area is a lot. It could have been worse if I had put a needle in the center. I'm going to put some stator vanes after the fan rotor and will let you know on the results. I understand that a multi-stage compressor would help but I'm dealing with an electric system. I remember reading something about having two fans going in opposite directions but don't remember why it was done that way. I considered the radial flow compressor with the same motor and different fan. I Just happen to stumble on the turbojet as I can make all those pieces within hours including the turbofan. See picture of fan.

    Attached Files:

  11. Jul 29, 2011 #10
    The fan speed determines a maximum overpressure which limits the air speed at the outlet. A smaller outlet won't improve the speed, but does limit the throughput.

    Nor is a faster outlet desirable! Because thrust depends on throughput*speed increase but power consumption on throughput*0.5*(speed increase)^2 a propeller better accelerates much air a little bit. That's why turbofan have big diameters, even though it's more complicated, and helicopters an even bigger one.
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