Practical 2

Airlift Pump Challenge

What is an Airlift Pump? How does it work?

An airlift pump is a device that is made used of compressed air to lift water from a well or sump. As we know, the density of water is significantly greater than the density of air. Hence, when compressed air is combined with water in an airlift pump, air that has a lower density than water will rise due to buoyancy. In the meantime, the water is taken in the ascendant airflow and flows in the same direction as the air. Air is fed into the bottom of the riser pump and this air combines with water and form froth. Hence, the air and water mixture which is also called froth has a much lower density than water. Due to the different densities, the water mixture is able to flow out. In conclusion, the main principle of an airlift pump applied is density difference. 

Experimentation

For the experiment, we assigned each other the following roles:

Members	Role	Responsibilities
Zelene	Team Leader	·        Ensured that all the runs for the experiments were done ·        Helped to keep track of the run that was being done
Zhi Wei	Recorder	Wrote down the data collected during the experiment and helped to calculate the flowrate from the volume measured
Uzair	Timekeeper	Helped Experimenter to keep track of time to record the volume of water that was being transferred by the pump
Rydrew	Experimenter	Worked with the given materials to conduct the experiment, does the data collection and experiment setup

The materials we were given to conduct the experiment were:

1. A small air pump (Super Y Classica ^_^)
2. A 99 cm piece of flexible tubing
3. 5 small tube connectors
4. A green U-tube
5. A small metal ball valve

Additional materials used for the experiment were:

1. Super Glue
2. A wooden chopstick
3. A large plastic container
4. A tall cylindrical container (28.5 cm height)

Before the experiment was changed, we had designed the pump by using super glue to attach the flexible tube 4 cm deep into the bottom of the U-tube. We used a shorter piece of about 6 cm to put into the bottom of the U-tube and used a pipe connector provided to attach the tube that will be connected to the pump.

In the photo above, there is a lot of residual super glue that can be seen from the numerous attempts that took to attach the flexible tube to the U-tube. Rydrew made the mistake of buying adjustable super glue instead of a quick-setting one >.< To aid with the setting process, a chopstick was used to keep the tube in place.

After a few hours of waiting (and one night to let the glue fully set), the pump setup was complete!

We were all set! However, literally 9 hours before we met on campus to conduct the experiment with the pump, we were told that practicals can no longer be held on campus -_-

Hence, we were tasked with a different, albeit somewhat similar experiment that can be conducted at home. 

The New Challenge 

Instead of just measuring the flow rate of the pump and the turbidity of the water as a mixture of flour and water were to be used, we instead had to measure the effect of changing two specific lengths. First was the distance between the opening of the flexible tube and the bottom opening of the U-tube (a in Figure 7.1 below) for Experiment 1, and second was the distance between the bottom of the container and the bottom opening of the U-tube (b in Figure 7.1 below) for Experiment 2.

Figure 7.1: Diagram showing the variables being changed for each experiment. 

The distance between the water level and the opening of the flexible tubing was also measured, as X for Experiment 1 and Y for Experiment 2.

As the experiment was to be done at home, there were some additional materials that we needed to procure additional materials, which are listed under the Additional materials used list above. As you probably noticed and are wondering about, why did we need two containers?

And that was where our major problem for conducting the experiment came in... 

The Container Problem

As the Experimenter, Rydrew had to go buy whatever materials needed for the experiment that were not readily available at home, and those happened to be a large enough container and a measuring cup.

However, he failed to consider that the experiment called for a tall container, not just simply a container that was large enough. The picture below shows the container we had planned to use initially:

We quickly found out that this container was not deep enough, as our pump was already struggling to push out the water through the u-tube at length b = 10 cm, which was the first height (it only went higher from there). We were worried that we would not be able to collect enough data points for the experiments and even decided to go as low as b = 8 cm, and decided we needed to send Rydrew on another (safe !) trip out to find a taller container. Fortunately, he was able to find one, pictured below (and beside the old container for comparison):

After ensuring we had the proper setup, we first tested the pump, which we recorded in this short clip:

Once we verified that the pump was working as intended, we carried out both experiments. We varied the value of a from 2 cm to 10 cm, in 2 cm intervals. We did the same for B, but from 10 to 20 cm. The data recorded can be seen in Appendix 1 of the Practical Report section below.

A short clip of the data collection process for one run is shown here:

For Experiment 2, we ran into a problem with having the value of b of 20cm. The pump was no longer able to push enough liquid over the bend of the U-tube and into the measuring cup. A clip of us discussing this is shown here:

And that wraps up the experimentation part of the Air Lift Pump Challenge!

Practical Report

1.           Plot tube length X versus pump flowrate. (X is the distance from the surface of the water to the tip of the air outlet tube). Draw at least one conclusion from the graph.

 

Conclusion:  According to the graph above, we can conclude that when X which is the distance from the surface of the water to the tip of the air outlet tube increases, the average flow rate of the pump will increase also.

 

2.          Plot tube length Y versus pump flowrate. (Y is the distance from the surface of the water to the tip of the U-shape tube that is submerged in water). Draw at least one conclusion from the graph.

Conclusion:  According to the graph above, we can conclude that when Y which is the distance from the surface of the water to the tip of the U-shape tube increases, the average flowrate of pump will increase also.

3.           Summarise the learning, observations and reflection in about 150 to 200 words.

Answer: As seen from our results, as both X and Y increases, the average flowrate of water also increase. In experiment 1, the increase of the flowrate is relatively gradual, whereas in experiment 2, the increase of the flowrate is abrupt. In experiment 1, the variable that causes X to change is the length of the tip of the tube from the opening of the U-tube (a), causing a change to the height of the surface to the tip of the tube. As for experiment 2, the variable that causes Y to change is the height of the U-tube to the base of the container (b). The shorter the length between the U-tube and the base of the tank, the higher the height between the surface of water to the entrance of the U-tube and the flowrate is higher. A greater distance between the opening of the flexible tube and the water level results in a higher flowrate. This is noticeable in experiment 2 as there is a greater difference in the length between the water level and the opening of the flexible tube, Y compared to X in experiment 1.

 

4.           Explain how you measure the volume of water accurately for the determination of the flowrate? 

Answer: We used a 300ml measuring cup with 50ml graduations and a 14ml small measuring cup with 1ml graduations. When water started to flow from the U shaped tube, we held the 300ml measuring cup below the opening of the U tube to ensure minimal spillage, and if the level of water is below 50ml, we would use the 14ml measuring cup to get the volume of water. Hence, it would be more accurate.

 

5.           How is the liquid flowrate of an air-lift pump related to the air flowrate? Explain your reasoning.

Answer: The liquid flow rate is directly proportional to the air flow rate. For a fixed time interval, a higher flow rate of air would mean a larger volume of air is introduced to the tube that will displace the water. Hence, since water is incompressible, when a larger volume of air is introduced, a larger volume of water will also be displaced. Since flowrate is volume per unit time, increasing the volume of water that is displaced per unit time will result in a higher flowrate.

 

6.           Do you think pump cavitation can happen in an air-lift pump? Explain.

Answer: No. Cavitation occurs in centrifugal pumps that transport liquid. However, since the equipment being used to transfer energy to the water is an air compressor, cavitation will not occur.

 

7.           What is the flow regime that is most suitable for lifting water in an air-lift pump? Explain.

Answer: Turbulent flow.  In turbulent flow, water undergoes irregular fluctuations and is being mixed. This makes it easier for the air from the air lift pump to push the water up. As there are irregular fluctuations, there are spaces for the compressed air to go in and mix with the water. Therefore, the density of the air and water mixture will be lower than the density of water only. Making it easier to rise up and make a loop out of the tube.

 

8.           What is one assumption about the water level that has to be made? Explain.

Answer: The water level in the container did not change and is assumed to be “steady state”. The U-tube is exposed to atmospheric pressure. This means that the water level in the U-tube is the same as the water level in the container. The height of water in the U-tube will affect how much water the pump will push out. Therefore, if the water level in the tube is low, it will need more air to displace the same amount of water out of the tube.

Appendix A

Table 1: Data collected from Experiment 1

a (cm)	X (cm)	Flowrate (ml/s)			Average Flowrate (ml/s)
		Run 1	Run 2	Run 3
2	13	6.33	6	5.83	6.05
4	11	4.17	4.5	3.83	4.17
6	9	2	2	2.17	2.06
8	7	1.07	1.3	1.17	1.18
10	5	0.4	0.5	0.4	0.43

 

Table 2: Data collected from Experiment 2

b (cm)	Y (cm)	Flowrate (ml/s)			Average Flowrate (ml/s)
		Run 1	Run 2	Run 3
10	16.5	12	12.3	12	12.1
12	14.5	4.75	4.63	4.55	4.64
14	12.5	3.67	3.33	3.33	3.44
16	10.5	1.63	1.67	1.47	1.59
18	8.5	0.133	0.167	0.233	0.178
20	6.5	0	0	0	0