Practical 2

 Air Lift Pump Challenge 💨⛽

This practical was suppose to be a competition between groups to see which model of air pump would have highest water flowrate. However, due to the heightened measures in light of the Covid 19 situation, we were tasked to complete a table with given set of values. Hence, our air lift pump challenge was done with our group at the comfort of our home!

So what exactly is an air lift pump? What is it used for?👀

An air lift pump is something that uses air to transport or displace water from an initial position to a final position. This device has a simple mechanism that allows water entering from suction then into the tube mixing water and air molecules, this promotes air molecules to reduce density of water, and bringing it up and to the discharge outlet.

How do we construct it?

Here are the equipment given for the challenge: 

We used extra tape to secure air delivery tube inside of the u-tube in order so that it will not move around when the air is being pumped into it. We also used a small container ( measuring 40ml) to collect our water as it exits the u-tube. As seen in the picture above, that was the hole that was constructed for the air delivery tube to enter.

Making a hole on the U-tube:

 

As seen in the picture above, that was the hole that was constructed for the air delivery tube to enter. To achieve this, Derrick, heated up a screwdriver until it was hot enough to poke through the thin plastic layer of the u-tube and form a hole. So innovative !!

Here's what we did!

Above shows Derrick stopping the pump to find the flow rate which can be found by volume of water transferred divided by time taken 

Derrick checking if the length is correct 😛

Experiment Worksheet 🔬

Experimental Data:

Questions:

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). Draw at least one conclusion from the graph.

Conclusion: As the distance from the surface of the water to the tip of the air outlet tube increases from 7cm to 13cm , the pump flowrate increases exponentially from 0 ml/s to 4.42 ml/s. Less than 7cm, the pump flow rate remained at 0 ml/s. Indicating that minimum length of X required would be more than 7cm.

 

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: As the distance from the surface of the water to the tip of the U-shape increases from 9cm to 15cm, the pump flowrate increases exponentially from 0 ml/s to 4.42 ml/s. When Y is less than 9cm, the pump flowrate remains at 0 ml/s. Indicating that minimum length of Y would be 9cm.

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

Our group has learnt how difficult it is to experiment at home because we do not have some of the apparatus required to experiment with a high level of consistency. For example, we did not have a jug, bucket or pail large that meets the requirements for the experiment. We also did not have a sturdy holder for the U-tube to be kept upright. However, we were still able to make things work, but just with a little less consistency. We have observed that the higher the U-tube is positioned above the base of the tank, the lower the flow rate of the air-lift pump. Similarly, as the height of the air outlet inside the U-tube increases, the flow rate decreases. We have also observed that as the water level of the vessel we are using is decreasing, the flow rate of the air-lift pump tends to decrease as well. Overall, we think that the outcome of the experiment could have been different. Had we conducted it in a conducive environment such as the workshop at the campus, we most probably would have access to the most appropriate set of apparatus for this very experiment.

 

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

 

We used a measuring cup with intervals of 40ml, and based the flowrate on how long it takes the water to hit the 40ml interval. We also used two timers and averaged out the timing to get the most accurate result possible. The reason we used the 40ml mark was due to the limitation of equipment, as well as the slow flowrate. After tunning the test using the different variants in heights, it seemed that 40ml was appropriate as some for some runs, getting a steady flow of water was not possible. One possible reason was due to the lack of proper equipment due to it being a home based practical.

 

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

The volumetric flow rate of the water depends upon the air injection flow rate. Increasing the air flow rate results in increasing water flow rate up to a point, after which the water flow rate levels off or may even decrease with a further increase in air flow rate. For a given pump at a fixed submergence, the overall efficiency of an airlift pump is proportional to the ratio of the water mass flow rate to the air mass flow rate. The reason for this is that with increasing air flow rate, the more water is being carried by the air in the tubing. However, to get the maximum efficiency the air flow rate cannot be too high as well.

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

Pump cavitation cannot happen in an air-lift pump. The air-lift pump consists of an air compressor, which only increases the pressure of air. However, cavitation occurs when the pressure of a liquid entering the pump is low enough for the liquid to turn into vapour. The pump then increases the pressure of the liquid and the vapour in the pump, resulting in the collapse of the vapour bubbles which can deal mechanical damage to the pump. Since the compressor is designed to transport air, it will not cavitate as it does not facilitate any phase change from liquid to vapour bubbles which may collapse and damage the pump. 

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

The most suitable flow regime is the slug flow regime. In contrast, at other flow patterns, particularly in the slug and churn flows, air slugs act as pneumatic pistons and push the trapped water between them up along the pipe. The air slug not only pushes the front water but also drags water behind it due to the suction created by quick movement of slugs. This behaviour of slug flow regime is the reason why it is the most suitable for lifting water in the air-lift pump as it increases the efficiency of water going to the higher reservoir. 

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

 

We are to assume that is is constant, not accounting thefact that may be residual droplets of water in the collection container which may affect the water level. We also must account that evaporation of water may occur due to the long exposure to air due to the experiment taking a long time as there were many runs to complete. Some of the water may also have spilled when collecting, or when pouring it , it may have ended up on the table and not be accounted for.