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I'm trying to design a miniature ducted fan engine for a small project, but I'm not too sure how much contraction should happen between the fan and the end of the engine, if at all.

Engines like this or this look like they have very little contraction at all, but why not use the pressure created by the fan and contract the space making the output velocity higher?

This is the same sort of principle behind the bypass air of a turbo fan engine and I don't see why not to use it.

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    $\begingroup$ If you mean a nozzle, then unlike the turbofans you asked about before, ducted fans typically don't need nozzles, they are propellers in a shroud. $\endgroup$
    – user14897
    Mar 16, 2018 at 20:06
  • $\begingroup$ I do mean the nozzle, yeah. Even if they don't need them would it increase thrust? $\endgroup$ Mar 17, 2018 at 17:05
  • $\begingroup$ You can change the question to target that point if you like. $\endgroup$
    – user14897
    Mar 17, 2018 at 19:26
  • $\begingroup$ I edited it. Not sure if it will get an answer at this point since it's an older post. $\endgroup$ Mar 19, 2018 at 20:45
  • $\begingroup$ I’d suggest this paper as a starting place. proceedings.asmedigitalcollection.asme.org/data/Conferences/… $\endgroup$ Mar 20, 2018 at 9:57

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but why not use the pressure created by the fan and contract the space making the output velocity higher?

That is exactly how the pressure energy added by the fan is converted into thrust. Thrust, after all, is the product of the speed increase between free stream and exit flow, mass flow and nozzle area.

That real jet engines don't show so much contraction is due to the expansion of the core gas flow from heating. If you don't add heat, your exit area should be quite a bit smaller than the entry area.

If by "normal ducted fan" you mean one where the duct is simply a cylinder of constant diameter, then yes, a properly designed nozzle will create more thrust.

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  • $\begingroup$ Thank you for the answer. Is there some source you can send me to for how to properly design the nozzle for this? $\endgroup$ Mar 20, 2018 at 5:04
  • $\begingroup$ @TrapAlcubierreDrive: Uh - no, I'm afraid. Maybe there is something, but I don't know it. You should be able to calculate the speed increase and from that the area required for equal mass flow. Keep in mind that the growing boundary layer will increase the area slightly in flow direction. Ideally, half of the contraction happens ahead of the fan and the other half behind it. $\endgroup$ Mar 20, 2018 at 18:13
  • $\begingroup$ @PeterKämpf: Since the efficiency of a ducted fan is 2*w0/(w9-w0) where w0 is the speed of the ambient air and w9 is the exit velocity of the ducted fan, increasing the exit speed with a nozzle would lower the efficiency. To your point that turbofan bypasses don't have a nozzle because of the hot exhaust would mean that something like the airbus e Fan should have strongly converging path, which it doesn't. media.wired.com/photos/5a5c7c7ea92f71125d0f9801/16:9/… $\endgroup$
    – user33651
    Feb 10, 2020 at 13:59
  • $\begingroup$ @hph304j The e-Fan does not add much energy to the flow - what do you expect from electric propulsion? The jet contraction is much higher with the energies needed to power an airliner, so that comparison is not valid. Next, take a straight cylinder and accelerate the flow inside. Look at the efficiency. Repeat with a properly converging cylinder. Of course does some contraction improve efficiency and with it thrust. $\endgroup$ Feb 10, 2020 at 17:00
  • $\begingroup$ @PeterKämpf: If you could please provide me with some sources for your last statement about the straight and converging cylinder, I would highly appreciate it. $\endgroup$
    – user33651
    Feb 10, 2020 at 19:09
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Depending what you mean by a smaller unit. For some time now I have been studying and testing 'electric ducted fans' where gas velocities are less than 100m/s and RE less than 150k. I have manufactured my own and tested numerous units from various sources. I am a retired production engineer with the best part of 30 years dedicated to gas turbines including pure jet, low and high bypass, shaft engines both for flight and static. These little horrors have left me at times baffled. Nothing I have read pans out. First measuring gas velocities from the entry to the exit, not once have I found a significant change in gas velocity after the entry to the fan. Just decay. They do not operate in any way like a full size high bypass fan. It appears only Guy Lussacs gas laws can be applied. Any nozzle other than the vane section reduces thrust for a given power input. More power higher rpm, creating a greater reduction in pressure at the entry to the fan compensates. So far, I have found, only the shape of the entry to the duct can improve static thrust for a given power input. I realise now that I have years of work to provide a comprehensive guide to small low velocity fans. The next step is to build a small dynamometer to ensure I know the power input to the fan and then a small wind tunnel. There is much more I could say but at this stage all I will say is that they are pressure difference propulsive units with very little momentum change. I have thousands of measurements of thrust, gas velocities, power requirements, pressure measurements, etc. Finally good luck in designing a minature high bypass unit. I imagine you are trying to produce a minature gas turbine driving your fan. There is a good deal out there to help you but the bypass fan may not operate in quite the way you expect.

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It all depends upon the configuration of the diffuser for the fan but the answer is not necessarily. For a typical high bypass turbofan on a transonic aircraft application, it will use a fixed geometry diffuser (nozzle) for both fan and gas core exhaust with the nacelle. P&W and GE engine’s have opted to use a separate cool section and hot section diffusers. Rolls Royce seems to prefer a single diffuser which combines both the fan and gas core exhaust prior to exiting the nozzle for noise reduction reasons. Both designs are very efficient for their appropriate applications.

High bypass turbofans have exhaust flow speeds in the high subsonic and transonic regions but do not require a fixed or variable convergent-divergent diffuser for supersonic gas flow. The reasons are primarily that it is 1) less fuel efficient for a subsonic application and 2) supersonic exhaust flow is extremely noisy and limits the airports where the jet could fly into due to noise restrictions.

Your exact design for your particular diffuser is going to depend on the configuration of the engine as well as the installation constraints. You’re also going to have to calculate the impulse the engine needs to provide and this is going to determine the exit speed of the gas through the diffuser. It’s going to be a little more involved than simply copying an existing design and making something that looks similar.

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  • $\begingroup$ Thanks for the response. For a small ducted fan engine, like the one I'm making, is the fan not enough to compress the air? Is a diffuser really necessary for it? I might be totally confused as to what you're referring to with the diffuser because I just googled the part schematic of a jet engine and it seems like they are stationary blades that further compress the air. You say "fixed geometry diffuser (nozzle)" though, so are you saying the nozzle and the diffuser are the same thing? $\endgroup$ Mar 20, 2018 at 5:07
  • $\begingroup$ Nozzle and diffuser are interchangeable terms. And yes, a diffuser is an integral part of the system, as it converts the enthalpy of the airflow added by the fan into kinetic energy, creating the reactive thrust. $\endgroup$ Mar 20, 2018 at 9:55

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