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**

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

- Measure the internal diameter (D) of the straws (they should all be the same in a given set).
- Build a horizontal pipe with piezometers at the start and immediately before and after each bend (see figure below)
- 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).
- 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.
- Have students measure the height of water in each of the piezometer tubes and the length of each pipe section.
- Have students use the measuring cup to capture a known volume of water over a measured time and calculate the volume flow rate
- Calculate the average velocity in the pipe (U=Q/A) and Reynolds number (Re=UD/ν).
- Calculate the head loss h
_{lp}along each section of pipe based on the change in piezometer height measurements. - Calculate the head loss around the bend (h
_{lB}) based on the difference in piezometer heights. - Calculate the friction factor for the pipe (f=h
_{lp}D2g/U^{2}L) (based on the Darcy-Weisbach equation) - Calculate the loss coefficients for the various bends (K
_{l}=h_{lB}2g/U) - Compare the pipe friction factor (f) and local loss coefficient (K
_{l}) 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 180^{o} 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 180^{o} 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.