Practical 2

Practical 2: Air-lifting pump challenge

 

Hi guys!! So, for our second practical experiment, we had to make an air-lift pump system. Here are the materials needed to make the air-lift pump system:

 

 

 

We had to measure the flowrate of water while varying the positions of the different tubes that were used, if you see the picture below, there are markings marked out for the different positions each tube should be at different points of the experiment.

 

 

To start off, we labeled everything we needed to use a black marker. So, we labeled the bottled 10cm from the base and in increments of 2cm up till 20cm, and we marked out 2cm increments starting from the base of the green tube up until 10cm. 

 

 

For each variation of the experiment we did, we shifted the inner tube to correspond to the marked points of the green tube as shown above. A blue tack was used to hold up the green tube in place at the desired position. And to record the flow rate of water, we switched on the pump and started the stopwatch simultaneously. We had to use a Nalgene bottle to measure the volume of the pump as it had volume markings on the side of the bottle. To measure the flowrate, we stopped the stopwatch right when the volume of water reached 100ml. This process was then repeated throughout the different variations of the experiment.

 

Short clip on how the experiment was conducted. 

 

Here are the results we obtained from the experiment!

a (cm)	X (cm)	Flowrate (ml/s)			Average Flowrate (ml/s)
		Run 1	Run 2	Run 3
2	14.5	9.29	10.19	8.80	9.43
4	12.5	6.68	6.19	6.92	6.60
6	10.5	4.83	4.86	4.64	4.78
8	8.5	2.19	2.17	2.29	2.22
10	6.5	0.75	0.76	0.76	0.76

 

b (cm)	Y (cm)	Flowrate (ml/s)			Average Flowrate (ml/s)
		Run 1	Run 2	Run 3
10*	16	9.29	10.19	8.80	9.43
12	14	4.68	4.34	4.77	4.60
14	12	2.50	2.62	2.30	2.47
16	10	0.00	0.00	0.00	0.00
18	8	0.00	0.00	0.00	0.00
20	6	0.00	0.00	0.00	0.00

 

Unfortunately, the pump we had could not pump over x=16cm, and hence all the results thereafter indicate no results:( 

 

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.

 

           

From the graph, we can observe that as the distance from the surface of the water to the tip of the air outlet tube represented by x, increases, the pump flow rate also increases. 

 

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.

 

          

 

From the graph, we can observe that as the distance from the surface of the water to the tip of the U-shaped tube that is submerged in the water represented by Y, increases, the pump flow rate also increases.

 

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

 

It is observed that when the values of A increased, the flowrate of water decreased. This can be explained by the inner tube being placed closer to the surface of the water, as the height above the inner tube is less, it prevents the air bubbles released from expanding to larger volume before it collapses on the surface. Whereas, with A being at 2cm, it allowed more travel time of air bubble in the tube, and as it travels up towards the surface, it was allowed to expand more, creating larger difference in buoyancy of the water, causing the water to travel out the tube at a quicker rate.

 

It is also observed that for increase in values of B while A was a constant, resulted in decreasing flow rates of water. This can be explained by the level of water in the u-shaped tube, as the overall, level of water decreases with increasing B, the distance of the surface of water to the outlet increases, making it increasingly harder to pump water out.

 

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

 

(a)      The volume of water is measured by using a water bottle up to 100mL marking indicated on the bottle. At the same time, the time taken for the water level to reach 100mL is recorded for each value of a and b. The stopwatch starts when the first drop of water starts to fall in the bottle.

 

(b)      Repeat step 1 twice more to get 3 timings for each value of a and b.

 

(c)      Calculate the average timing from the 3 timings recorded for each value of a and b.

 

(d)      The equation to find flowrate is Flowrate = Volume/time, where the volume is fixed at 100 mL and the time represents the average time for the respective values of a and b which was calculated.

 

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

 

The liquid flow rate of an air-lift pump directly correlates to the airflow rate. The higher the airflow rate the higher the liquid flow rate from the U-tube.  When air mixes with water in the U-tube, it results in an air-water mixture.  The density of the air-water mixture is lower compared to that of the water.  The difference in density would result in different buoyancies which would cause the air-water mixture to rise naturally in the flow of water. The air bubbles formed by pumping air into the U-tube will then also help to lift/carry the water up the U-tube, delivering the air-water mixture out the other end of the U-tube.

This is illustrated in our experiment. When the air pump (2 different settings available) is kept at the low setting for a= 10cm, with a lower air flow rate the water does not pump out the other end of the U-tube. However, when the pump is changed to the high setting, we were able to obtain an average flowrate of 0.76ml/s.

 

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

 

No, Cavitation cannot occur within an air-lift pump. Cavitation occurs when air bubbles, or voids, form within a fluid because the pressure quickly drops below the vapor pressure. When the bubbles experience higher pressures they collapse, creating small shockwaves.  Since there is no fluid flowing through the pump, only air, it is not possible for cavitation to occur.

 

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

 

              The flow regime that is most suitable for lifting water in an air-lift pump is the turbulent flow regime. When there is a turbulent flow regime, there is a high level of mixing that occurs in the air-water mixture. The high degree of mixing is favorable to the transportation of water in the U-tube. This is because a higher degree of mixing would ensure the density gradient is maintained by the air-water mixture and water in the bucket. The difference in buoyancy would then arise from the difference in density which is the operating principle of an air-lift pump.

 

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

 

During the experiment, an assumption about the water level that must be made is that the water level is constant while water is being pumped out. As the water level decreases, the distance of the water and the outlet increases, making it increasingly harder for water to be pumped out as there is less room for air bubbles to expand as it reaches the surface, thus creating less buoyancy effect throughout the run, hence this creates the effect where the flowrate of water is exponentially decreasing throughout each run.

For ease in the determination of flow rate, the water level is assumed to be kept constant, which in turn results in a uniform flow rate.