Experiment B10 Pumps and Turbines Procedure

Transcription

1 Experiment B10 Pumps and Turbines Procedure Deliverables: Checked lab notebook, Brief technical memo NOTE: This lab exercise has a time limit. Parts I and II must be completed in the first 90 minutes of the lab. Students who fail to complete it will receive zero credit for Part II. Overview A pump is a device that can add kinetic energy to a fluid. A turbine is a mechanical device that can extract kinetic energy from a moving fluid. In this lab, you will examine the efficiency of a hydraulic pump as a function of its operating parameters. You will then embark on the final project for the course: designing a turbine. Similar to B5 Build-a- Beam, this design project will culminate in a contest at the end of the semester to see whose turbine can extract the most power from an air flow. Part I: Hydraulic Pump Curve Theoretical Background Hydraulic pumps use a variety of different mechanisms to move water. The pump you will use in this lab is a centrifugal pump that uses an impeller to move water. An impeller is basically a small paddle wheel coupled to an electric motor. The rate of work performed on the water by the pump, or hydraulic power, is P hyd = pq, (1) where p is the backpressure and Q is the flow rate. If a pump is 100% efficient, then the power supplied to the pump P IN is completely converted to hydraulic power, such that P IN = P hyd. No pump is ever 100% efficient due to viscous losses in the fluid, and the overall pump efficiency is η pump = P hyd P IN. (2) A hydraulic pump s ability to move a fluid is best quantified by the flow rate Q with units of volume per time. The flow rate of a given pump is not constant. Rather, it depends on the backpressure p of the fluid. This relationship between backpressure and p and flow rate Q is known as a pump curve. Combining Eqs. (1) and (2), we find that Q = η pumpp IN p. (3) Thus, the flow rate decreases with backpressure. For example, it will take much longer to pump water up into a 200 ft. tall water tower than a 100 ft. water tower using the same pump. This is B10 Pumps and Turbines 1 Last Revision: 11/2/18

2 because the hydrostatic backpressure of 200 ft. is greater than 100 ft., which results in a lower flow rate. In this lab, you will examine this effect by using an electric pump to flow water up to various heights, as shown schematically in Figure 1. For the case where water is being pumped up to a height h, the backpressure is simply the hydrostatic pressure p = ρ w gh, (4) where ρ w = 1000 kg/m 3 and g = 9.8 m/s 2. You will measure the flow rate Q by timing how long it takes to fill a 1-liter beaker. Plotting the backpressure p as a function of flow rate Q, calculated via Eq. (4), yields the pump curve. You will also determine the overall pump efficiency η pump as a function of flow rate Q by measuring the electrical power supplied to the pump P IN. Combining Eqs. (1) and (4), the hydraulic power can be written as where h and Q will be measured. P hyd = ρ w ghq, (5) Power Meter h Beaker Lab Stand Pump H 2 0 Figure 1 - A schematic of the experimental setup for measuring the pump curve and efficiency curve. Experimental Procedure You will experimentally measure the pump curve for a small aquarium pump. On the floor, you should have a buck, a beaker, and a dishpan full of water. Be careful not to kick any of this over. Safety First: You must wear nitrile rubber gloves and safety glasses while performing this procedure. Do NOT point any hose at an electrical outlet. The end of any hose must be firmly held in place with your hand or a beaker clamp. B10 Pumps and Turbines 2 Last Revision: 11/2/18

3 1. Sketch a schematic of the experimental setup in your lab notebook. 2. Examine the aquarium pump. Make sure it is unplugged and pinch the sides to remove the cover. Look at the impeller inside and think about how it works. Replace the cover and make sure the tube is connected to the barbed outflow connector. 3. Make sure the power strip is plugged into the Kill-a-Watt power meter. The power meter should be on the Watt setting. 4. Check to make sure the pump works. With the pump sitting high and dry on the lab bench, plug it into the power strip. Turn on the power strip. The pump should start running, and the power meter should give a reading of around 5 W. Unplug the pump when you are finished. 5. Make a table in your lab notebook containing four columns: power P IN, height h, time Δt, and volume V. 6. Fill up the dishpan with DI water and place it on the floor in front of your lab bench. 7. Find the height where the pump stalls and the backpressure is too great to move water. Do this by holding the end of the tube in your hand above the buck on the floor next to the dishpan. Gradually increase the height of the tube until water stops coming out of the end. 8. Record the stall height and note that the flow rate Q = 0 at this height. This should be included as a data point on your pump curve. 9. Make sure the pump is unplugged. Set up the experiment as shown in Figure 1. The beaker should be in a bucket to collect any stray splashes. The end of the tube should be held in the beaker clamp just above the empty beaker. 10. Begin with a height close to h = 0. Plug in the pump, turn on the power strip, and use a stopwatch to measure the time Δt that it takes to fill the beaker to a volume V. Record the power P IN, height h, time Δt, and volume V in the table in your lab notebook. 11. Empty the beaker back into the dishpan and repeat the measurement 2 more times for the height close to h = 0. CAUTION: Avoid plugging in the pump or turning on the power strip with wet hands! You should only plug it in or turn it on using a dry, gloved hand. 12. Increase the tube height h and repeat the procedure. You should perform the procedure 3 times for at least 8 different heights. 13. Unplug the pump and clean up your lab bench: a. Wipe up any water that may have spilled or splashed. b. Remove the pump from the dishpan, drain any residual water from the tube, dry it off, and place it on the lab bench. c. Empty any water from the beaker or bucket back into the dishpan. B10 Pumps and Turbines 3 Last Revision: 11/2/18

4 Part II: Hydraulic System Design Challenge Using your data from Part I, you will now design and build a system to lift water up to a height of 4 feet. You will be given three pumps, three dishpans, all of the materials from Part I, and whatever amount of tubing you would like. The winner of the challenge will be the group who can fill up a 2-liter beaker to a height of 4 feet above a low-lying reservoir (i.e. a dishpan on the floor) using the least amount of energy. Safety First: Do NOT point any hose at an electrical outlet. The end of every hose must be firmly held in place with your hand or a beaker clamp. Contest rules: You must finish building your system within 30 minutes of the lab ending. If you do not finish by then, you will be disqualified and receive a zero. You may use up to 3 pumps and 3 reservoirs. You may only use the materials provided. In the beginning, only the lowest reservoir can have water in it. All other reservoirs must be dry. Ends of all hoses must be held firmly in place by hand or by beaker clamp. Any container on the bench may be used as a reservoir. All pumps must be plugged into a single power strip, which will be plugged into the Kill-a- Watt power meter on the lab bench. All pumps will be switched on simultaneously at the beginning. The switch on the power strip will be switched on; all pumps will start drawing power; and the timer will start. The 4-foot vertical height will be measured from the initial surface of the water in the lowest reservoir to the final exit tube above the 2-liter beaker. The lab instructor will time how long it takes your system to fill the 2-liter volume. The total energy will be calculated by multiplying the time by the total power measured on Killa-Watt power meter. 2% of your overall course grade depends on how well your system performs relative to the other students in the class. Students whose system consumes the least amount of energy will receive the highest grade. Energy less than one standard deviation below the mean will receive a 2/2. Energy within one standard deviation below the mean will receive a 1.7/2. Energy within one standard deviation above the mean will receive a 1.5/2. Energy more than one standard deviation above the mean will receive a 1/2. Students who fail to complete the challenge in the allotted time will receive a 0/2. Be sure to sketch your design in your lab notebook before you leave. B10 Pumps and Turbines 4 Last Revision: 11/2/18

5 Part III: The Wind Trainer A turbine is a device that extracts kinetic energy from a moving fluid. We will spend the remainder of the semester designing and building turbines. Your turbines will be tested in a commercially built system known as the Wind Trainer. The Wind Trainer is a small wind tunnel containing a dynamometer, which is used to measure torque τ and angular velocity ω of the spinning turbine. Multiplying torque τ and angular velocity ω yields shaft power P shaft = τω. Similar to B5 - Build-a-Beam, this final project will take the form of a contest to determine who can design and build the turbine that extracts the most power P shaft. Unlike the B5 - Build-a-Beam contest, you will have a virtually unlimited amount of material to work with and 3 weeks to design, test, and redesign your turbine. Really, the only constraints in this project are time and the physical dimensions of the Wind Trainer. (Official contest rules and guidelines for the final written report can be found in the B11 handout.) For Part II of this week s lab, the lab instructor will demonstrate the Wind Trainer system and go over the contest rules. You will then go off in groups of 3 to brainstorm ideas for your turbine design. In particular, you should consider the various parameters you can vary and test. Possible design parameters include, but are not limited to: Blade length Blade width Blade pitch angle Blade shape Number of blades The design space is quite large, and we encourage you to begin with crude prototyping using 1/8 thick balsa wood sheets, 3/16 wooden dowels, and hot glue. Then, after you have found the optimum design parameters, you may attempt more sophisticated fabrication techniques, such as 3D printing. However, keep in mind that time is the biggest constraint here, so your final prototype for the contest may still be made of balsa wood. After the lab instructor has demonstrated the Wind Trainer, you will sit with your group members and devise an initial plan for the design. 1. Measure the dimensions of the wind trainer. 2. Think about which parameters you wish to test. 3. Draw some sketches for how your first set of test blades will look. Re-measure the dimensions of the wind trainer to make sure your blades will fit. 4. Write a brief bill of materials for your initial prototype. Available materials include: 6 long, 3/16 diameter wooden dowels 1/8 thick, 1 x 36 strips of balsa wood 1/8 thick, 2 x 36 strips of balsa wood 1/8 thick, 3 x 36 strips of balsa wood 1/8 thick, 6 x 36 strips of balsa wood Hot glue sticks 5. Present your sketches and bill of materials to the lab instructor before you leave. B10 Pumps and Turbines 5 Last Revision: 11/2/18

6 6. If your plan looks reasonable, the instructor will issue the materials. Tools are available in B14 Fitzpatrick and 212 Stinson-Remick (where you worked on Build-a-beam). 7. Fabricate your blades some time over the next week, so they are ready for testing next week during your regularly scheduled lab meeting. Turbine hubs are available in B14 Fitzpatrick, but they must remain in B14 Fitzpatrick. Plan on mounting your blades in the turbine hubs when you arrive in lab next week. Data Analysis and Deliverables Using LaTeX or MS Word, make the following items and give them concise, intelligent captions. Additionally, write 1 3 paragraphs separate from the caption describing what you did in lab and how it relates to the plot/table. Any relevant equations should go in this paragraph. 1. Make a plot of the pump curve. That is, make a plot of the pressure head in units of feet of water as a function of the flow rate in units of gallons per hour (GPH). (Use the average of the three measurements for each data point.) 2. Make a plot of the pump efficiency curve. That is, make a plot of the overall pump efficiency as a function of the flow rate in units of gallons per hour (GPH). (Use the average of the three measurements for each data point.) 3. Using any software you like, make a professional looking schematic of the hydraulic system you designed in Part II. 4. Fabricate at least one set of turbine blades from balsa wood to test next week in lab. Talking Points Discuss/answer these in your paragraphs. What pressure and flow rate yielded the maximum efficiency? Discuss the rationale behind your design in Part II. Were there any unexpected effects that hindered your design in Part II? B10 Pumps and Turbines 6 Last Revision: 11/2/18

7 Appendix A Equipment Parts I & II 3 SongJoy 132 GPH Aquarium Submersible water Pump2 3 Dishpans Kill-a-Watt power meter Power strip with switch 4 ft. long, 1/2 ID Tygon tubing Stopwatch 1 liter beaker 2 gallon bucket 2 lab stands w/ 3-finger clamps Lab jack scissor lift 2 liter beaker Part III Wind trainer 12 spoke turbine hubs with 3/16 holes 2mm Hex keys 6 long, 3/16 diameter wooden dowels 1/8 thick, 1 x 36 strips of balsa wood 1/8 thick, 2 x 36 strips of balsa wood 1/8 thick, 3 x 36 strips of balsa wood 1/8 thick, 6 x 36 strips of balsa wood Hot glue sticks Hot glue gun Cutting mats Exacto knives Box cutters Straight edge Combination square Sand paper Dremmel tool B10 Pumps and Turbines 7 Last Revision: 11/2/18