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I'm trying to make a simple Venturi tee which will have air flow through it while sucking some water from down below (like paint sprayers do). I want the device to pull the water to a height of 1 meter.

That means that I need 0.1 atmospheres (104 Pascals). The dynamic pressure equation states that $P_{a}=\frac12\rho v^2$, meaning that $v=2P_a/\rho$. The density of air is $1.22 kg/m^3$, $P_a$ is 104 so $v=128\ \mathrm{m/s}$.

I experimented a bit with my air pump and found out that with a nozzle of ~5 mm in diameter I can fill a 10 litre bag with air in ~7 sec. That should mean that the velocity is around 60 m/s. With those results I took a hose of 16 mm in diameter, stuck my pump nozzle inside (from one side) , made a small hole juxatoposed to the nozzle from above (maybe 3-5 mm diameter) and stuck a straw in it. When I turn the apparatus on, hovewer, the straw (placed in a bottle of water) lifts the liquid for no more than several millimeters.

Why does my Venturi pump not work?

enter image description here

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    $\begingroup$ Two questions: (1) do I understand your calculations correctly: you calculated v=128 m/s to be the necessary speed but you only achieve 60? (2) Why do you increase the diameter after the nozzle? Thereby you are increasing the pressure again (because you increase the flow area) $\endgroup$
    – rul30
    Commented May 7, 2015 at 18:49
  • $\begingroup$ (1) Indeed 128m/s (according to my calculations) should lift the water 1 meter high, therefore I reasoned that the 60m/s I managed to achieve should at least have noticeable lifting, not the meager results I've got. (2) Huh. To be honest I thought that the fast air flow is still very close to the straw orifice, and I used a 16 mm hose so that the water have enough space to flow. You think I should narrow the exit pipe? $\endgroup$
    – dactylo
    Commented May 7, 2015 at 19:29
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    $\begingroup$ Yepp @dactylo as a first very crude assumption: the speed and the pressure can be calculated using your equation for the dynamic pressure. Since (another assumption, neglecting the water) the mass flow needs to be constant. The pressure will drop. $\endgroup$
    – rul30
    Commented May 7, 2015 at 19:33
  • $\begingroup$ Thank you very much I will try it...However...now that I think about it, when I tested a 16-18 mm nozzle on my pump it filled the 10 litre bag in 1-2 sec, meaning that the air speed was around 35 m/s. Shouldn't that still generate a noticable pull (of close to 10cm)? If so, then shouldn't my current installation have done so as well? Or there is some sort of multiplication going on here (the smaller nozzle restricts the airflow to the larger nozzle/pipe)? This question is just out of curiosity. $\endgroup$
    – dactylo
    Commented May 7, 2015 at 20:03
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    $\begingroup$ The pressure in the pipe will be lower because the mass flow through the smaller nozzle is smaller than the one with the larger nozzle. $\endgroup$
    – rul30
    Commented May 7, 2015 at 21:39

2 Answers 2

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The problem is that the airspeed drops as soon as the airflow leaves the nozzle. It is not the same as a water-jet expelling into air (in that case the density of the water is much greater than that of air so the water holds its velocity much better). The low-pressure air will suck in air from around it in the wider pipe and thus become turbulent around the outside edges of the cylindrical stream within a very short distance as it leaves the nozzle, and slow right down. The airflow will still be fast in the center of the cylindrical stream, but not at the edges where your intake is. You need to position the intake of your straw inside the nozzle, at its narrowest point. Modified version of the diagram from the original post

I have drawn a zoomed-in part of your diagram and shown an extension to the nozzle in blue, also an extension to your intake in orange so that it opens inside the fastest part of the airflow. If you can't modify it in this way, at the very least place the intake right up as close as possible to the exit-point of the nozzle, but ideally you want the intake to come in just before the air exits the nozzle. Additionally, make the 'straw' and intake about half the diameter of the nozzle exit, as it will lift the water much more easily.

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  • $\begingroup$ Thank you both very much! Yesterday I did those changes advised, it certainly did help a lot and I'm very grateful for the explanation (I imagined any vaccum would be directed toward the water, and didn't think of any air turbulance)! The water climbed up the straw much more noticebaly but now... well, I'll post the question here. I'm not quite sure how to calculate the expected results because the best suction I attained was about 4 litres an hour (Trying several setups where I inclined the straw in all directions and finaly assesed adding a 2mm nozzle imgur.com/qpfTwHV). Th $\endgroup$
    – dactylo
    Commented May 13, 2015 at 18:12
  • $\begingroup$ The air speed is around 40m/s (this is another question I want to know, do pumps work consistantly at the same air speed regardless of nozzle size? I've tried with 2mm, 7mm, 10mm, 16mm and some other head, but they all seem to hover around that figure of 40m/s ), and the intake straw is less than 50% of the nozzle (10mm vs 3-5 mm). What formula should I use here? Thank you very much!!! $\endgroup$
    – dactylo
    Commented May 13, 2015 at 18:13
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enter image description here

Your device must be improved. The straw must be inserted into a horizontal tube, which means that the mouth of the straw should be inserted into the airflow sprayed from the nozzle, as shown by the red curve. Due to the obstruction of the straw, the airflow will first bend upwards and then bend forward. This will create a pressure below atmospheric pressure at the mouth of the straw, causing water to be drawn up. This is similar to the upper surface of a wing, where the airflow first bends upwards and then downwards and backwards, resulting in pressure below atmospheric pressure.

@jhabbott What jhabbott said is also based on reason, which utilizes the contraction section inside the nozzle to bend the airflow upwards, thereby generating a pressure below atmospheric pressure at the mouth of the straw. Your water tank is connected to atmospheric pressure, so the straw must generate a pressure lower than atmospheric pressure in order to suck up water.

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