How Fan Shape Affects Turbine Efficiency
To explore different designs for turbines in a geothermal power plant.
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Turbines play a vital role in the generation of electricity from stream in both fossil fuel burning and geothermal power plants. The optimal design for a turbine depends on the materials used and the stream pressure of the source. Turbines must be large enough to "catch" steam without being too heavy to turn effectively in the flow. Another important part of the design is the spindle. Here, the spindle must move as freely as possibly on its axial and friction must be kept to a minimum.
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Students design turbines to explore the generation of mechanical energy by using steam from a modified saucepan of water (a steam generator). Students can control the size and shape of turbine.
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Materials and Equipment
medium size aluminum pie tins, aluminum foil, soup can with both ends removed, stiff wire and coat hangers in 25 cm lengths, a small cork, duct tape, pliers, scissors.
Students will need to wear protective glasses and insulating mitts when working with a steam generator. They should avoid cutting their fingers on any sharp metal edges, such as the soup can or pie tin.
For the stream generator, take a medium sized saucepan and half fill with water. Cover the pan with aluminum foil and cut a hole, and secure the edges to minimize leakage. Cut a hole the diameter of a soup can in the middle. The open-ended can will act to concentrate the steam. Cut a hole the same size in a pie tray and use this to cover the pan. Insert the can into the hole so that it rests in a vertical position. (If you want to add an additional variable then you can cover the can with a layer of foil and vary the number or vents, their sizes, and their spacing).
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- Explain the goal to the students. Describe the steps of the investigation and show them a sub-optimal design for a turbine to give them a general idea of what they are working on.
- Have students design a turbine on paper in their groups. They should consider the sizes and shapes of the blades, their angles, and the diameter of the turbine. Remind them of the diameter of the soup can in the steam generator.
- After reviewing their designs, ask them to make their turbines out of a pie tin, aluminum foil, and duct tape. They will need a pair of scissors. A ruler is useful to help bend straight edges into the pie tin and foil. A pencil is useful for curling the foil if necessary.
- Ask students to mark one of their turbine blades so the number of revolutions it makes can be counted.
- Students will need to hang the turbine on the wire so that is sits vertically and can spin freely above the can. Have them bend the wire into a U-shape, with a flat bottom. One arm should be 5 cm longer than the other. The bottom of the "U" should be half the diameter of the can. They can use duct tape on the outside of the can to fix the longer arm so that the "U" shape is upside down.
- Have students pierce a small hole in the center of each turbine with a push pin. For each test, they slide the turbine onto the wire. After, a small cork is pushed onto the end of the wire below the turbine to keep the turbine in place. The turbine must rest on the cork in a horizontal position to allow it to spin freely.
- Before each trial have the students discuss what they think are the merits of their designs.
- Run the models for each group. The students will measure the number of revolutions of each turbine in a fixed amount of time. Ask students to calculate the speed of each turbine.
- Once all the groups have tested their turbines have them complete the following:
- Discuss what makes a good turbine design.
Good designs usually include some of the following characteristics:
- simple shapes typically make the best fins in a turbine.
- curved faces allow the steam to be caught like a sail enhancing movement.
- fins are best if they are set at an angle to the air flow like those of a cooling fan.
- the turbine must rotate easily on the but should not rock as it rotates.
- many smaller blades are typically better than fewer larger blades.
- the ratio between blade length to the space between blades is important.
- Consider how you would change the experimentation for a second trial.
Students might consider that controlling the distribution of the steam might provide good results. Students could be given another pie dish and asked to cut holes in different places to explore the effects of adjusting the orientation of the steam source.
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