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Fluid flow out of a tank design

  1. Mar 23, 2008 #1
    Hi Everyone,

    First time posting here...pretty cool forum. Anyway, here is what I am looking at in nonspecific terms.

    I need to design a tank (non-specific aluminum material that is holding water at room temperature) that is approximately 2.5 feet (must be round) in diameter and 1.5 feet tall (holds about 50 gallons). There must be a hole in the bottom of the tank no bigger than 1.5 inches with a tube leading out of this hole that extends 1.25 inches below the bottom of the tank. The center of this hole must also be 7 inches in towards the center radius from the outside of the tank.

    What is the fastest way to get 35 gallons out of this tank?

    Should i put a tube with a chamfer on the side? A cone? A tube on the inside of the tank with a cone? Just a tube on the inside leading to the 1.5 inch hole with a chamfer?

    Basically, in nonspecific number terms how would you modify the inside of this tank to get the water out as fast as possible?

    Thanks for all the help.
  2. jcsd
  3. Mar 23, 2008 #2
    Sorry, I guess I should add that the top is open to the atmosphere at say 1 atm. And for a few more specifics, say the tank at its highest point is 6 feet in the air and is dumping straight down onto the ground
    Last edited: Mar 23, 2008
  4. Mar 24, 2008 #3
    in what ways are you allowed to modify the tank?
  5. Mar 24, 2008 #4


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    Run some calcs to determine which variables effect the flowrate using Bernoulli's equation (allow for friction losses as well with the Darcy-Weisbach equation). Typically, the outlet type with the lowest Cd will be the fastest.

    Hope this helps.

  6. Mar 24, 2008 #5

    I guess I don't really know where to start since we I have a fair amount of options. Since the drain hole is off-center, isn't the water going to spin off center of the main tank? Meaning the spin will be oblong or some other weird shape? (Or will it look something like a tornado, where the at the top of the tank the spin is in the middle but at the bottom the spin is at the center of the drain hole?) So that is why I bring up the question of putting a cylinder of larger diameter on the inside of the tank that is over the drain hole. So maybe like 4 inches or something in diameter. Won't this get the fluid spinning faster more directly over the drain hole?

    Also to answer an earlier question, I am pretty free to modify the inside of the tank. I can't move any holes or resize anything, but I can add stuff over the hole or add stuff to the tank, make some sort of cone shape over the hole, etc, etc.
  7. Mar 24, 2008 #6


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    This link and the attachment should help you get started with the equations...


    You can use a vortex breaker over the hole. It's basically just a plate (or similar) with legs, kind of like a table, that disrupts the fluid flow and keeps a vortex from forming.

    Hope that helps.


    Attached Files:

  8. Mar 24, 2008 #7
    Thanks for the info...I had never heard of a vortex breaker...looks interesting. I think this has brought more questions than answers, which isn't a bad thing :)

    Anyway, after doing a bit of searching, it seems like there would be some sort of pressure loss and/or friction factor when the fluid flows through the breaker, no? AOr is the friction created by the spinning fluid out way the friction caused by the vortex breaker?

    This brings me to my next question, if one could get the fluid spinning down the drain tube as fast as possible without getting a vortex to form would that be the fastest way to drain the tank?

    Or should the fluid flow in a steady state line straight out the drain hole?

    Lastly, after looking at the link you gave me, it assumes steady state flow. Can I not assume that since there is the possiblity of a vortex forming...and is there an equation to figure out at what height, pressure, time, etc that vortex does form?
  9. Mar 25, 2008 #8


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    Yes, there will be some friction and thus pressure loss as the fluid goes through the vortex breaker, but it will be negligible.

    Since this is a gravity feed tank, it will drain really close to the predicted rate by using the previous equations. However, a vortex contains a lot of rotational energy that gets dissipated as heat due to the viscosity of the fluid. Theoretically, this would slow the flow-rate due to frictional losses and possibly vortex shedding.

    Yes there are ways to determine this, however, I don't know off of the top of my head. You should look through some online journals as this is a well researched area. Vortices are extremely complicated phenomenon. Sarpkaya is a relatively well known author on the subject.

    Personally, I don't think it is worth the effort to try and model or determine where and if a vortex would occur. If you are worried that it will cause a problem in your system then just use a breaker. Typically one would use these in gas separators to prevent gas from becoming entrained in the liquid, not due to flow-rate problems (which should be negligible).

    Hope that helps.

  10. Mar 25, 2008 #9
    So, one can safely assume the fluid will spin as it goes down the drain, due to any number of reasons...lots of which I have seen discussed on this forum. With that said, in your opinion, would it be better to put some sort of baffling in the tank to prevent the spinning of the water at all? Or just try to use a Vortex Breaker and be done with it...(also, will the vortex breaker stop the spinning of the draining fluid, or just stop a vortex from forming?)
  11. Mar 25, 2008 #10


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    I wouldn't use baffles, and I wouldn't be concerned about the fluid spinning. If you need a vortex breaker then you should use it, but like I said before, I've only used them in separators to prevent gas from becoming entrained in the liquid.

    Hope that helps.

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