The maximum variability of the flow occurs due to the trailing vortices following the recirculation loops. Rms velocity for the one-staged system. For the staged system Figure 10 , the greatest fluctuation is localized at the blade that is placed at the bottom of the tank.
These fluctuations can be explained by the suction of the flow from the bottom of the tank. In addition, the maximum values of the turbulent fluctuations are created between the two blades due to the interaction between the blades. The fluctuations generated due to the association of the convex impeller with the flat impeller are narrowed when they are compared to the association of the concave and the flat impeller. This effect reveals that the convex blade is not able to create a large fluctuation on the turbulent flow and local vortices are created.
Rms velocity for the staged system. Figure 11 shows the vorticity generated with the curved blade turbines. According to these results, the bulk region of the tank is presented with the medium vorticity value. For the flat and the concave configurations, the propagation of the vorticity is larger than the convex blade.
The lowest value area is localized in the inferior region of the tank which follows the same direction of the second circulation loop. In fact, it has been noted that the highest recirculation loops are more energetic than the lowest ones. For the convex blade two maximum regions are created presenting the clockwise and the counterclockwise CW-CCW vortex pair at the blade tip.
Vorticity for the one-staged system. For the staged system Figure 12 , the vortical structures are localized at the region between the two blades at the same direction of the discharge flow of each blade, which explains the domination of the trailing vortices at the turbulent flow. Vorticity for the staged system. Figure 13 shows the distribution of the turbulent kinetic energy of the curved blade turbine. The turbulent kinetic energy is dimensionless by the square of the tip velocity.
According to these results, the turbulent kinetic energy is maximum at the blade tip, and it decreases progressively moving away from the blade. As it was found in the previous sections, the convex blade dissipates the highest energy in the flow. For the staged system Figure 14 , the maximum turbulent kinetic energy is localized between the two blades at the same direction as the trailing vortices.
In addition, the turbulent kinetic energy is larger at the association of the concave blade than the convex blade. Dimensionless turbulent kinetic energy distribution of the one-staged system. Dimensionless turbulent kinetic energy distribution of the staged system. In this section, we used the decomposition of the flow basing on the eigenvalues. This method allows to reveal the smallest vortical structure that cannot be seen by the usual mean flow according to its energetic amount by using the dimensionless eigenfunction.
In fact, many vortices are presented with different sizes and shapes. For the one-staged system Figures 15 — 17 , the loop created at the blade tip is the most energetic.
The largest one is obtained from the flat turbine. However, the narrowed loop is created by the concave shape, which can be explained by the axial velocity above the impeller. The clockwise and the counter clockwise CW-CCW vortex pair at the blade tip can be clearly seen at the highest modes that look similar to the development of the trailing vortices of the Rushton turbine [ 18 , 19 ].
The trailing vortices are more extended by using the concave blade. However, the flow reaches the bottom of the tank faster by using the convex form. POD field for the flat blade. POD field for the concave blade. POD field for the convex blade.
For the staged systems Figures 18 — 21 , it can be seen that many vortices are created at the region localized between the two blades. This region represents the interaction between the highest and the lowest impeller. Hence, the flow becomes more energetic that explain the cause of the development of these vortices. The flow reach the free surface as well as bottom of the tank faster by mounting the flat turbine at the top of the tank. The trailing vortices become more energetic by using the combination between the flat blade at the bottom and the convex blade at the top.
The development of the different modes shows that the shoes of the combination is extremely important and can affect the mixing inside the vessel. Consequently, the combination between impellers can lead to affect the final product in terms of homogeneity and the cost in terms of the time mixing and power consumption.
This found contradicts what has been observed in the study of the mean velocity field, as it gives almost the same results. In addition, it proves that the mean flow is not able to show the real behave of the flow.
The objective of this paper is to investigate experimentally the hydrodynamic structure of the curved blade turbine using the particle image velocimetry. Thereby, several results were evaluated which contain velocity field, axial and radial velocity distribution, root mean square velocity, vorticity, and the turbulent kinetics energy.
Two circulation loops were presented. However, the concave configuration produces a larger lowest loop than the other configurations. Hence, the downer region of the tank is more turbulent than the flat and the convex configurations.
The maximum radial velocity generated by the flat blade turbine spreads to reach farther places. This can be explained by the ability of the blade shape to generate training vortices. The convex shape of the blade gives the turbine the ability to move easily within the water and transmit more velocity and energy while not giving it enough capacity to expand much.
In fact, it has been noted that the convex blade is not able to create a large fluctuation on the turbulent flow and local vortices are created. In addition, it has been noted that the fluctuation of the flow is dominated by the trailing vortices more than by the recirculation loops. For the staged system, an oblique flow is created between the two impellers, and turbulent fluctuations are greater at this region due to the interaction between the blades. Licensee IntechOpen.
This chapter is distributed under the terms of the Creative Commons Attribution 3. From here it can either travel along the curved side of the blade, or the relatively flat side.
The wind travelling along the curved side takes longer to reach the end of the blade than that which travels along the flat surface. This process is known as lift. The blades of a turbine are twisted as they extend from the rotor to the blade tip. This increases the force exerted near the tip, as the tip moves much faster than at the rotor. Inside the turbine head known as the nacelle , there is a low speed shaft connected to the rotor. Large-scale turbines typically rotate at 20 rpm, while domestic sized turbines tend to revolve at roughly rpm.
In most large-scale turbines, the low speed shaft is connected to a gearbox. The gearbox increases the rotational speed of the shaft, up to rpm. This is the required rotational speed of most generators to produce acceptable levels of power. The gearbox increases the rotational speed of one gear by connecting to a gear with a smaller radius. For instance, imagine the low speed shaft is attached to a gear with 20 teeth, which is connected to another gear with 10 teeth, which in turn is connected to the high-speed shaft.
If the toothed gear spins once, the gear with 10 teeth will have to turn twice. Therefore the high-speed shaft will be spinning at twice the speed of the low-speed shaft. But lets not get into the details of electromagnetism in this blog…. The generator in wind turbines produces Alternating Current AC electricity. The purpose of this, is so the frequency and phase of the electricity is in line with that supplied by the grid. By converting the variable speed AC current, produced by the generator, to DC current and then back to AC, the electrical signal is converted to a phase and frequency like that produced by the grid.
Most electrical equipment in your home runs on predefined electrical conditions, and an unstable supply can damage your electrical equipment. Turbines have a few nifty extra features that can maximise the amount of power generated. These include the ability to change pitch and yaw. Pitch control - The pitch control system alters the angle of the blades. For wind turbines to be functional they need to be designed and engineered very well and this is the reason why they are so expensive.
Most of the modern day wind turbines are provided with two to three blade propellers that rotate around a horizontal axis. These wind turbine blades convert the wind energy into shaft power known as Torque, which is produced as a result of the deceleration of wind blowing over the wind turbine blades, with help of various techniques.
A lifting force is generated due to the curved shape of the blades just as in case of an aeroplane wing. A low air pressure is created on the side with most curve, while the high pressure created beneath pushes the other side of blade aerofoil.
This results in generation of a lift force that is perpendicular to the air flow direction. The rotor blade also needs to be designed appropriately to generate the right amount of rotor blade thrust and lift to produce the exact amount of deceleration of air.
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