Register now to get rid of these ads!

Technical Fan shroud science. A commonly held thought is that fan blades should recess 50% into the shroud for

Discussion in 'The Hokey Ass Message Board' started by 31Vicky with a hemi, May 6, 2019.

  1. Doublepumper
    Joined: Jun 26, 2016
    Posts: 214

    Doublepumper
    Member

    That's an interesting thought. If a goodly volume of air is being slung off the blade tips, it would seem having the tips outside the shroud would allow more 'fresh' air to be pulled through the shroud. If the tips are inside the shroud it is causing the slung off air to be recycled causing less 'fresh' air to be pulled through the shroud....it would seem. Perhaps that's why many fans have the tips angled back. To enable the fan can be installed deeper into the shroud for safety, but still are able to route the air slung off the fan tips towards the outside of the shroud.....?
     
  2. Here's your answer "31 Vicky... Do you want cooling or HP? Just cut it off so 50% of the fan blade is sticking out for cooling, or remove the shroud for HP! Let's go racing! ;)

     
    Last edited: May 6, 2019
  3. Kramer
    Joined: Mar 19, 2007
    Posts: 908

    Kramer
    Member

    With the fan inside of the shroud, what is going to be easier? Drawing air thru a restriction, the radiator, or recycling some of that air back around to the front of the fan? Just thinking out loud.
     
  4. What the guy in the video says, is without a shroud, at a certain RPM the tips of the fan starts a vortex. The faster the tip speed the bigger the vortex, generating less clean air flow meaning less air moving through the radiator, therefore less cooling effect.

    I would call it cavitation. When it cavitates the load gets lighter and nothing gets pumped. Think of an engine oil pump or a boat propeller. When it cavitates you don't go anywhere and the RPM's go up due to less load.

    If you compare that to a helicopter or a prop airplane, when the prop goes supersonic, it just makes more noise and quits pulling. In hot rod terms - YOU FALL OUT OF THE SKY! :eek:

    '31 Vicky, you just need a bigger fan! :D
     
  5. A fan or propeller and wings have a leading edge and a trailing edge and vary from center out. Any disruption between the two edges creates a disturbance. In this case a cross current discharged from the tip of blades as a result of accumulated pressure from hub out because the root (hub area) has a great angle of attack (pitch) as compared to tip which has a lesser degree of attack (pitch). In other words. The discharged air from the tips which is hot can be resent forward creating stalled air.
     
  6. That’s what I’m worried about here,,,
    Stalled air, cavitation, or something else and actually creating unnecessary cooling issues.

    I know HOW to setup both fans and shrouds and importantly so they work well,,, I’m just not sure about the WHY of that matter.

    I know how to build/make a shroud, most of the time I can get 10 lbs of crap into a 5lb bag too. This setup just doesn’t make any sense to let out of here to run like this.
     
  7. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,251

    s55mercury66
    Member
    from SW Wyoming

    Just a couple of thoughts here, but a fan inside ductwork neccessarily must be enclosed by the ductwork, and I'm thinking is designed to work as efficiently as possible in there. Number two, a fan in a shroud in an exposed location must be completely inside the shroud for safety reasons, think OSHA/MSHA. It looks like you have a V6 shroud on a V8 perhaps? Why not cut the 4 or 5 inches from it, or is the fan so far inside that it will not be effective at all? If this is going to be a budget short track car, could you just cut it away and make a simple hoop and rivet it in place? I fully realize you like to do nice work, but is it really neccessary here? I be lurkin'.
     
  8. I have my theories, but It would be really interesting to play with a smoke machine, a fan blade on a drill or electric motor, and play with the depth of the fan into the shroud, shroud depth, diameter clearance, and shroud opening finish .
     
    '51 Norm and Montana1 like this.
  9. Entirely different airfoil/fan.
     
    winduptoy likes this.
  10. Thanks !
    Fitting work that’s done well enough and works as it should is nice work.

    That shroud has a square to round transition so sectioning 5” out of it will cause problems.
    The fan is actually up in the square part.
     
  11. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,251

    s55mercury66
    Member
    from SW Wyoming

    Gotcha
     
  12. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,251

    s55mercury66
    Member
    from SW Wyoming

    Yes, that was what I was getting at.
    Sorry for the double quote.
     
  13. 41rodderz
    Joined: Sep 27, 2010
    Posts: 2,098

    41rodderz
    Member
    from Oregon

    To be up on the latest why not Google what they are doing in Nascar.
     
    31Vicky with a hemi likes this.
  14. 41rodderz
    Joined: Sep 27, 2010
    Posts: 2,098

    41rodderz
    Member
    from Oregon

    But I do believe one truth, the owner is going to have to test and tune . What works on one engine may not work with same results on another engine.
     
    31Vicky with a hemi likes this.
  15. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 5,504

    scrap metal 48
    Member

    I like and agree with what the Flex-A-Lite article says....
     
  16. Ned Ludd
    Joined: May 15, 2009
    Posts: 3,820

    Ned Ludd
    Member

    Three thoughts come to mind.
    1. With a ducted fan we're looking to deliver air somewhere, be it for ventilation, thrust, or whatever. With a radiator fan we just want to get it through the radiator, and we'd be pleased if we didn't have to deal with it after that.
    2. The air expands as it passes through the radiator. We're dealing with a greater volume of air behind the radiator than in front of it.
    3. There is an engine in the way of where this greater volume of air air has to go.

    All that suggests that a radiator fan really wants to be a hybrid axial-centrifugal fan, which spreads the air in a rough cone around the engine. That argues more for the fan partly or wholly sticking out the back of a shroud than contained within a shroud.
     
    XXL__ likes this.
  17. Blue One
    Joined: Feb 6, 2010
    Posts: 9,498

    Blue One
    ALLIANCE MEMBER
    from Alberta

    So, if the fan is up in the square part, why not remove the round part completely and build something with a round ring to take care of the fan starting from the appropriate place on the square section ?
     
  18. That’s the plan sorta simmering in the back of my brain while I’m clicking things off the to do list.
     
    indyjps and Blue One like this.
  19. indyjps
    Joined: Feb 21, 2007
    Posts: 3,572

    indyjps
    Member

    Customer supplied parts.
    Explain your concern, let them decide if they want to try it - "not on you if it doesnt cool well"
    OR, does the customer want to move forward with a shroud that looks nice and works well, which you can provide.
     
  20. indyjps
    Joined: Feb 21, 2007
    Posts: 3,572

    indyjps
    Member

    Ah, some fluid dynamics. Air has a density and does react as a fluid like water, with less resistance. Been a while for me, so Im not gonna spout off any calculations.

    Thats why all the those yay-hoos spend millions on wind tunnels and flow cylinder heads. Calculating flow of water thru a pipe is the same calculation as air thru a pipe (or cylinder port) with different density of fluid (air or water).

    When I learned that shit, It was like a cartoon image of a light bulb over my head.
     
    Ned Ludd and XXL__ like this.
  21. The Engine Masters video was enlightening... It's also interesting to note that older pre-shroud fans had that little 'flip' on the end of the blades to break up turbulence...
     
    Montana1 likes this.
  22. Peter Berghs
    Joined: Jul 16, 2018
    Posts: 3

    Peter Berghs

    Technically speaking you can't get cavitation in a gas. Cavitation occurs in a liquid when the local pressure droips to the vapor pressure of the liquid thereby causing the creation of a vapor that will reduce liquid flow, create noise and material damage where the bubbles collapse back to liquid.

    A car fan is an axial flow fan and has behaviour different than a centrifugal fan. Look up axial fan shrouding on Google.
     
    Blues4U and indyjps like this.
  23. 392
    Joined: Feb 27, 2007
    Posts: 874

    392
    Member

    I just finished a ammonia condenser fan on a Frick IDC. Each blade is around 48” and it has 5 blades. Pitch is 26.2 and the clearance to cone is 3/8 - 1/2” max and the fan is located inside of cone that is straight about 12” below lip. Now to ride I’ve made 2 shrouds in last 6 months and didn’t like either one. First one was a full aluminum with ring and a 16” fan the half in half out and it didn’t help. Second one I just built a ring with 4 tabs and 1/2 in 1/2 out with 18” fan. Again I didn’t like results. And yes fan is about a inch or so from radiator. I’m going to try 18” fan with full cone and fan inside lip similar to above example on condenser.
     
  24. gene-koning
    Joined: Oct 28, 2016
    Posts: 1,122

    gene-koning
    Member

    31vicky, I don't believe the fan in your application will do much good, and really, it probably doesn't have to.
    The idea of a fan and shroud is to pull more air through the radiator at low vehicle movement, or low motor rpm. On a circle track car both of those conditions are pretty limited.

    One of the concerns about sustained higher vehicle speed is that the fan can actually restrict the air flow through the radiator because the fan acts like a big disc blocking the air flow, especially if the fan is centered in the shroud. With so much of the radiator exposed outside of the shroud on both sides, that would be a non-issue in this case.

    The other concern is how efficiently the fan can pull the air through the radiator, the reason the fan is centered in the shroud opening is because that is the most efficient position for the fan to be in to be able to create the most air flow. With the fan in the square section of the shroud, and almost 5" forward of the most efficient location, the fan will provide little air flow, with the exception of the area directly in front of it, provided the fan is not very far away from the radiator. The air turbulence was an interesting idea, how this fan/shroud would effect the air turbulence is pretty unknown.

    At least this shroud covers the fan, by the rules. That may be its only value. I don't see it causing an overheating issue, but it probably won't help, should one occur either.
    Personally, I would just make a small bracket to attach to the radiator that would cover the top of the fan, unless this is an open front end car. Then I would just cut off the existing shroud behind the fan. Gene
     
  25. But should it be half in or half out ? ;)
     
    Last edited: May 10, 2019
  26. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,312

    Blues4U
    Member
    from So Cal

    Yep, thought the same thing when I read that other post.
     
  27. AngleDrive
    Joined: Mar 9, 2006
    Posts: 586

    AngleDrive
    ALLIANCE MEMBER
    from Florida

    I believe in air it is called turbulence:)
     
    Montana1 likes this.
  28. alchemy
    Joined: Sep 27, 2002
    Posts: 13,547

    alchemy
    Member

  29. AngleDrive
    Joined: Mar 9, 2006
    Posts: 586

    AngleDrive
    ALLIANCE MEMBER
    from Florida

    Isn't that when you put the fan on backwards?:D
     
    milwscruffy likes this.

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.