Practical 2
Objective:
To find out how different factors affect the flow rate of an air-lift pump.
Equipment used:
Procedure for Experiment 1:
First, we fixed the U-shaped tube such that the distance from the base of the jug to the U-shaped tube was b cm. Next, we slotted the PVC tube into the U-shape tube such that a cm of the PVC tube was inside the U-shape tube. The PVC tube and U-shape tube are set up as shown in the figure below.
For this experiment, the value of a will be varied, starting off with a = 2, and b will be fixed at 10cm. After that, the pump was turned on for 1 minute and we recorded the volume of the water collected to find the flowrate of the water. We then repeated the experiment for a = 4, a = 6, a = 4, a = 8, and a = 10.
Procedure for Experiment 2:
The set up of Experiment 2 was the same as Experiment 1. However, we fixed the value of a = 2, and varied the value of b. First, we started off with b = 12, turned on the pump for 1 minute, and recorded the volume of the water collected to find the flowrate of the water. We then repeated the experiment for b = 14, b = 16, b = 18, and b = 20.
Carrying out the experiments:
Due to COVID restrictions, only Trisyia and her family members were doing the experiment, while the rest of us had to watch and take down values for the tables.
The experimental set up was fairly simple, with the PVC tube being attached to the U-shaped Tube through a homemade small segment of the hose. The PVC tube was then attached to the air pump which pumps out the air. The pump was set to high power to allow for the strongest air release from the air pump to allow for the greatest flowrate.
After all the set-up has been done, we proceeded on with the experiment by letting the a be equal to 2 cm and b be at a constant 10cm throughout experiment 1. In order to measure the flowrate, we had to find the volume and the time taken. We decided to set the time taken constant at 60 seconds for both experiments for all runs and we would measure the volume of water that is dispensed by the end of the 60 seconds. We will use those 2 values to find the flowrate. We also measured X after every change in value of a. X is the distance from the surface of the water to the top of the air outlet tube.
The experiment proceeded as seen below. There was very little water flowing out so it was slightly difficult to measure some runs as the equipment that we have was not able to measure such small amount so we had to improvise and find other ways to measure the smaller amount of liquid.
As we proceeded with the experiment, we encountered a major difficulty. The flowrate of the water eventually stopped flowing at a certain value of a. When a = 6cm there was no more flow rate. Since we suspected something was wrong, we decided to stop experiment 1 and proceeded to do experiment 2 first before we go back to troubleshoot experiment 1.
Experiment 2 have the exact same set-up as experiment 1 with the only changing factor being that a is now constant instead of b and instead of measuring X, Y was measured. Y is the distance from the surface of the water to the tip of the U-shaped tube that is submerged in water. Experiment 2 was much more smooth sailing compared to 1 as the flowrate seemed to be stronger compared to experiment 1. The way we measured flowrate remained the same with the time taken constant at 60 seconds while we measure the volume collected during that period of 60 seconds. Below is how we did experiment 2.
After a while, we also had the same problem with Experiment 2, where there was no water flowing out anymore and that was when b = 16cm. We then realised that there was going to be no flow of water after a certain point, as the pump could not pump enough air to allow for the water to flow all the way to the other point of the tube.
After experiment 2 was done, we went back to re-do experiment 1 again. This round, experiment 1 was much better compared to the first try that we did initially. We finally had a more accurate representation of the flowrate and there was still water flowing when a is equal to 6cm. There was no more water being dispensed at a = 10cm therefore we concluded that past 8cm, there will be no more water flowing. With that, we ended the experiment and do the questions on the report.
The tables below are the data that was collected for experiment 1.
Data collection table for Experiment 1:
|
a
(cm) |
X
(cm) |
Flowrate
(ml/s) |
Average
Flowrate (ml/s) |
||
|
Run
1 |
Run
2 |
Run
3 |
|||
|
2 |
11 |
2.58 |
2.58 |
2.92 |
2.69 |
|
4 |
9 |
2.08 |
2.58 |
2.42 |
2.36 |
|
6 |
7 |
1.25 |
1.25 |
1.67 |
1.39 |
|
8 |
5 |
0.83 |
1.00 |
1.00 |
0.94 |
|
10 |
3 |
0.00 |
0.00 |
0.00 |
0.00 |
Data collection table for Experiment 2:
|
b
(cm) |
Y
(cm) |
Flowrate
(ml/s) |
Average
Flowrate (ml/s) |
||
|
Run
1 |
Run
2 |
Run
3 |
|||
|
10* |
13.1 |
3.42 |
3.50 |
3.42 |
3.45 |
|
12 |
11.1 |
1.50 |
2.08 |
1.83 |
1.80 |
|
14 |
9.1 |
1.00 |
1.00 |
0.50 |
0.83 |
|
16 |
7.1 |
0.02 |
0.00 |
0.00 |
0.02 |
|
18 |
5.1 |
|
|
|
0.00 |
|
20 |
3.1 |
|
|
|
0.00 |
Questions & answers
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.
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.
3. Summarise the learning, observations and reflection in about 150 to 200 words.
In this experiment we got to see how an air-lift pump works.
As seen from table 1, the flowrate of the water decreases as
the amount of the air tube inside the U-tube increases while the distance
between the u-tube and the bottom of the jug is kept constant at 10 cm.
As seen in table 2, the flowrate decreases as the distance
between the u-tube and the bottom of the jug increases while the amount of the
air tube inside the U-tube is kept constant at 2 cm.
From this, we observed that the pump can pump out more water
when the air tube and the U-tube is deeper in water. Meaning when the distance
between the end of the air tube and the surface of the water level is larger.
We also observed many possible errors there could be from
this experiment, such as the way the tube is held and how the air tube easily
comes out of the U-tube when the pump is on. These errors may have affected how
the water flows and thus make our readings inaccurate. We also did not have
scientific equipment to measure the volumes which could also lead to
inaccuracy.
4. Explain how you measure the volume of water accurately for the determination of the flowrate?
In order to measure the flowrate, the volume of water and the time taken is needed. The time was set to fix at 60 seconds for all values of a in Experiment 1 and b for Experiment 2 for all runs. The volume will be the amount of water dispensed by the pump during the 60 seconds.
Since there was no scientific measuring cylinder to measure the volume of the water collected, there was a need to compromise and use items that can measure volume. Cooking materials that usually is used for measuring volume for baking was used in order to determine an accurate reading of the water that was obtained to determine the flowrate.
5. How is the liquid flowrate of an air-lift pump
related to the air flowrate? Explain your reasoning.
As the air flowrate increases, the liquid flowrate increases until a maximum point then proceeds to decrease. When air the air is pumped into the water, a water-air mixture is formed which has a lower density compared to the water which will result in a buoyancy effect pushing up the water.
When more of the air is being pumped to the water, there will be a greater difference in density which allows for greater flow of the water. As air is continued to be pumped at higher flowrate, more air is introduced which forms bigger air bubbles inside the water-air mixture. With larger air bubbles, there will be more displacement happening which will also increases the liquid flowrate. However, as the air flowrate continue to further increase, bigger and unstable bubble will form eventually leading to an annular flow which will obstruct the water thus decreasing the water flowrate instead.
6. Do you think pump cavitation can happen in an air-lift pump? Explain.
The flow regime that is the most suitable is turbulent flow. This is because turbulent flow allows for a greater mixing between the water and air. This allows for greater air flowrate compared to laminar and transitional flow therefore with turbulent flow, it will also have a greater liquid flowrate as the liquid flowrate will increase with air flowrate.
7. What is the flow regime that is most suitable for lifting water in an air-lift pump? Explain.
The flow regime that is the most suitable is turbulent
flow. This is because turbulent flow allows for a greater mixing between the water and air. This allows for greater air flowrate compared to laminar and
transitional flow therefore with turbulent flow, it will also have a greater
liquid flowrate as the liquid flowrate will increase with air flowrate. This is due to the difference in the density as the turbulent flow causes the water-air mixture to have a lower density compared to the laminar flow and transitional flow.
8. What is one assumption about the water level that has to be made? Explain.
One assumption about the water level that has to be made is that the water level of the water in the jug remains constant throughout the entire experiment. When transferring the water to the measuring cup and back in the jug there is bound to be displacements of the water. Water droplets may also be left in the measuring equipment or the cups. Gradually, this will cause the water level to decrease which may cause the runs to be slightly inaccurate. To reduce the amount of work done to refill the jug every time a new run is carried out, we assumed that the water level is constant throughout the whole experiment.
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