Desktop pipe flow and pipe network demonstrations

In class demonstrations on pipe flow head loss and pipe networks have been a challenge but I think I have found a way using kid’s straw construction kits (e.g. 1, 2, & 3). These kits can be used to build any number of different pipe networks. The T connectors can also be used to add in piezometer tubes to measure the local static pressure in the straw. This, in turn, can be used for measuring head loss along the system.

Equipment needed

  1. A desktop constant head tank or other steady water supply.
  2. As many straw construction kits as you desire.
  3. Tape measure and calipers to measure the straw dimensions.
  4. Measuring cylinder and stopwatch to measure flow rate
  5. Imagination

Photograph of the equipment needed including the desktop constant head tank system, straw ‘pipe fittings’, calipers, tape measure, stop watch, and measuring cylinder. In the experiment the water was collected in a plastic cup and then transferred to the cylinder for measuring.

Example demonstration: Head loss along a pipe and local losses in bends

An easy use of these straws is to measure the head loss in a pipe and around bends. You will need all the equipment listed above.

Demonstration

  1. Measure the internal diameter (D) of the straws (they should all be the same in a given set).
  2. Build a horizontal pipe with piezometers at the start and immediately before and after each bend (see figure below)
  3. Connect the start of the pipe to the upper constant head tank and have the end drain into the overflow tank (see here for details of the constant head tank).
  4. Fill the constant head tank and turn on the pump so that water flows along the pipe and also recirculates within the constant head tank system.
  5. Have students measure the height of water in each of the piezometer tubes and the length of each pipe section.
  6. Have students use the measuring cup to capture a known volume of water over a measured time and calculate the volume flow rate
  7. Calculate the average velocity in the pipe (U=Q/A) and Reynolds number (Re=UD/ν).
  8. Calculate the head loss hlp along each section of pipe based on the change in piezometer height measurements.
  9. Calculate the head loss around the bend (hlB) based on the difference in piezometer heights.
  10. Calculate the friction factor for the pipe (f=hlpD2g/U2L) (based on the Darcy-Weisbach equation)
  11. Calculate the loss coefficients for the various bends (Kl=hlB2g/U)
  12. Compare the pipe friction factor (f) and local loss coefficient (Kl) to standard values.

figure 2. (Left) Photograph of the pipe flow setup. The head loss was measured from the piezometer just downstream from the inlet to the tube just upstream of the bend. The outlet is pointed upward to reduce the total head difference along the pipe and to ensure that the piezometer tubes filled up to a height above the red solid T fittings.  (Right) photograph of a T fitting used to insert a piezometer tube into the pipe system.

Discussion

When I did this test (see photograph above), I got a flow rate of 1.75 ml/s with a straw diameter of 4.4 mm. This led to a mean velocity of 1.15 cm/s and a Reynolds number of 506 (laminar). I measured the head loss over 660 mm length of pipe to be 11 mm leading to a calculated friction factor of f=0.109. This is quite close to the theoretical value of f=64/Re=0.126. The head loss around the 180o bend was 0.6 mm which led to a calculated local loss coefficient of 0.89 which is within the range of values quoted for 180o bends. It is fiddly trying to get the system level with the piezometer tubes vertical. I would suggest using a more stable platform than piles of books. That said, the results were encouraging.


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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Video of “A $19 desktop constant head tank”

Here is a link to a video from the “A $19 desktop constant head tank” post. The video shows the water being pumped from the lower tank up into the constant head tank, overflowing into the funnel and draining back into the lower tank. The outflow is bent upward to prevent water flowing out, but can be connected to tubing to provide constant flow rate over prolonged periods of time.


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

A $19 desktop constant head tank

I have been planning on doing some pipe flow demonstrations in class using straws and rubber connectors. However, to do that I need a constant head tank that will drive the flow. Ideally the entire system would sit on a desktop so that no special mounting would need to be brought into the classroom. I am still working on the pipe flow demonstration but I thought that the budget desktop constant head tank design may be helpful to share.

Equipment

You will need:

  1. A small aquarium pump $8 
  2. A funnel $1
  3. 2 plastic tubs with straight sides (I used plastic shoe boxes) $4
  4. A small tube of silicone sealant $3
  5. A binder clip $1
  6. A short length of tubing to connect to the pump $1
  7. A hard plastic straw $1

20180530_140256

Figure 1: Materials needed for construction (binder clip missing).

Design

The basic idea is that there is a lower reservoir tank that feeds the upper tank via a pump. The upper tank contains a constant height weir overflow, with return to the reservoir tank, and an outlet below the overflow that will have a constant head. Provided the weir length on the overflow is large and the flow rate through the pump is substantially larger than the flow rate out of the constant head outlet then there will always be water flowing over the weir and the head over the weir will be relatively constant. In this budget design the reservoir and upper tank are plastic shoe boxes and the weir overflow is a funnel. Water is pumped using an aquarium pump from the lower to the upper tank and returns through the funnel to the lower tank. The upper tank rests on the lower so that the entire system can sit on a desk.

CH Tank

Construction

  1. Drill a hole in the center of the base of one of the shoe boxes with a diameter equal to that of the middle of the funnel neck.
  2. Drill a hole in the side of the same shoe box with a diameter a fraction smaller than the straw.
  3. Push the straw through the side hole (it should be a tight fit) and then seal around the hole with the silicone sealant on both sides
  4. Place the funnel inside the same shoe box with the neck protruding through the hole and seal around the funnel neck on both sides of the hole. The top of the funnel should be below the rim of the box so that water will flow into the funnel before it overflows out of the box.
  5. Attach the tubing to the aquarium pump and place it in the second shoe box.
  6. Attach the binder clip to the box with the funnel and use it as a mount for the tubing such that the tubing is pointed into the box but not into the funnel
  7. Place the box containing the funnel on top of the box containing the pump with the funnel outlet draining into the lower box.

Figure 2: (a) fully assembled constant head tank system. The yellow straw is the constant head outlet. (b) close up of the upper tank showing the inflow tube mounted (from the pump) through the binder clip and the funnel overflow back into the lower reservoir tank. (c) alternate view of the entire system.

Operation

  1. Block the end of the outlet straw or connect it to the test rig to be used.
  2. Fill the lower box until it is almost overflowing and the upper box until it is about to overflow into the funnel.. This is most easily done by pouring water into the upper tank and allowing it to overflow through the funnel into the lower tank.
  3. Turn on the pump. The water will be pumped into the upper box and drain through the funnel back into the lower box. The head in the upper box will remain essentially constant provided there is water overflowing into the funnel.

Comments

The $19 budget is approximate. You will use only a fraction of the $3 tube of sealant and may need to buy a box of binder clips to get the one you want. The whole thing takes about 15 minutes to assemble provided you have an electric drill with the appropriate drill bits for cutting the holes.

Video of “Compressible vs incompressible flow and conservation of mass”

Below are GIFs of the compressible and incompressible versions of the “Compressible vs incompressible flow and conservation of mass” demonstration. The full videos are linked from the GIF headings.

Compressible flow (air)

Incompressible flow (water)


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Compressible vs incompressible flow and conservation of mass

This is a really simple demonstration of how conservation of volume can be used for incompressible fluids but not for compressible fluids. The demonstration was suggested by Dr. Baburaj of IIT Madras. I teach in a civil engineering department where practically everything is incomopressible and we mostly talk about conservation of volume. The demonstration below is so simple yet so clear.

Equipment

You will need:

  1. Two identical syringes,
  2. A few feet of clear tubing that fits tightly over the end of each syringe,
  3. Some water, and
  4. Food dye (optional)

Demonstration

Compressible flow

  1. Have one syringe (A) with the plunger fully pushed in and the second plunger (B) fully pulled out.
  2. Connect each end of the tube to the syringes
  3. Slowly press the plunger on syringe (B)

Assuming that the syringe plunger’s are a little stiff you should be able to push the plunger on (B) all the way in before the plunger on (A) is pushed all the way out. Mass is conserved because there are no leaks but volume is not conserved as the plungers move different distances on identical syringes. This works better with stiffer syringe plungers.

Incompressible flow

  1. Have one syringe (A) with the plunger fully pushed in and the second plunger (B) fully pulled out and the syringe full of water.
  2. Fill the tube with water (food dye can help with visualization) and connect the tubes in the same way as for the previous version. This is tricky as you want to ensure that there are no air bubbles in the lines.
  3. Slowly push in the plunger on syringe (B). The plunger in syringe (A) should move out at exactly the same speed. you can show this clearly by having the syringes pointing away from each other with the plunger ends next to each other. As you push one in the other should move right next to it.

Analysis

There is no analysis for this demonstration. The gas is compressible so volume is not conserved whereas the liquid is incompressible so volume is conserved. Analysis of the change in pressure in the compressible case and resulting motion of the plungers is complex as you need to know about the friction in the syringe.

Thanks again to  Dr. Baburaj for suggesting the demonstration.


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.