Study of Hydraulic Machines Reaction T urbines

1. Study of Hydraulic Machines Reaction Turbines Department of Civil Engineering Swedish College of Engineering and Technology Wah Cantt.

2. Department of Civil Engineering Reaction Turbines • The other main type of energy-producing hydroturbine is the reaction turbine, which consists of • Fixed guide vanes called stay vanes, • Adjustable guide vanes called wicket gates, • and rotating blades called runner blades Working Mechanism Flow enters tangentially at high pressure, is turned toward the runner by the stay vanes as it moves along the spiral casing or volute, and then passes through the wicket gates with a large tangential velocity component. Momentum is exchanged between the fluid and the runner as the runner rotates, and there is a large pressure drop.

3. Department of Civil Engineering Reaction Turbines There are two main types of reaction turbine - Francis and Kaplan. Francis Turbine • Francis turbine is somewhat similar in geometry to a centrifugal pump, but with the flow in the opposite direction. • The Francis turbine is named in honor of James B. Francis (1815–1892), who developed the design in the 1840s. Kaplan Turbine • In contrast, the Kaplan turbine is somewhat like an axial-flow fan running backward. • If you have ever seen a window fan start spinning in the wrong direction when wind blows hard into the window, you can visualize the basic operating principle of a Kaplan turbine. • The Kaplan turbine is named in honor of its inventor, Viktor Kaplan (1876–1934).

4. Department of Civil Engineering Classification of Reaction Turbines We classify reaction turbines according to the angle that the flow enters the runner. • If the flow enters the runner radially as in Fig., the turbine is called a Francis radial-flow turbine • Inward flow turbines: In such turbines, the water enters the runner at the outer periphery and the flows inwards ( i.e., towards the center of the runner) • Outward flow turbines: In such turbines, the water enters at the center of the runner and then flows outwards (i.e., towards the outer periphery of the runner) • If the flow enters the runner at some angle between radial and axial, the turbine is called a Francis mixed-flow turbine. Francis mixed-flow turbine Francis radial-flow turbine

5. Department of Civil Engineering Classification of Reaction Turbines • If the flow is turned completely axially before entering the runner, the turbine is called an axial-flow turbine. The main difference between Francis mixed-flow and propeller mixed-flow runnersis that the runners of the former have a band that rotates with the runner, while the runners of the latter do not. There are two types of propeller mixed-flow turbines: Kaplan Turbines have adjustable pitch blades, while propeller turbines do not. Propeller mixed flow Propeller axial flow

6. Reaction Turbines • Francis turbines are most suited for heads that lie between the high heads of Pelton wheel turbines and the low heads of Kaplan turbines. • A typical large Francis turbine may have 16 or more runner blades and can achieve a turbine efficiency of 90 to 95 percent. • Compared to the Pelton and Francis turbines, Kaplan turbines and propeller turbines are most suited for low head, high volume flow rate conditions. Their efficiencies rival those of Francis tur-bines and may be as high as 94 percent. • In Francis’ turbine runner, the number of the blades is generally between

7. Classification of Reaction Turbines Francis Reaction Turbines 1. Inward flow turbine 2. Outward flow turbine 1. Kaplan 2. Propeller 1. Kaplan 2. Propeller • Kaplan turbines are called double regulated because the flow rate is controlled in two ways—by turning the wicket gates and by adjusting the pitch on the runner blades. • Propeller turbines are nearly identical to Kaplan turbines except that the blades are fixed (pitch is not adjustable), and the flow rate is regulated only by the wicket gates (single regulated).

8. Department of Civil Engineering Difference b/w an Impulse Turbine and Reaction Turbine Impulse Turbine Reaction Turbine The available energy of the water is not converted from one form to another. The water is guided by guide blades to flow over the moving vanes. The water glides over the moving vanes with pressure energy. The pressure of the flowing water is reduced after gliding over the vanes. • The entire available energy of the water is first converted into kinetic energy. • The water flows through the nozzles and impinges on the buckets which are fixed to the outer periphery of the wheel. • The water impinges on the buckets with kinetic energy • The pressure of the flowing water remains unchanged and is equal to the atmospheric pressure.

9. Department of Civil Engineering Difference b/w an Impulse Turbine and Reaction Turbine Impulse Turbine Reaction Turbine It is essential that the wheel should always full and kept full of water. The water must be admitted over the whole circumference of the wheel. The work is done partly by the change in the velocity head but almost entirely by the change in pressure head. • It is not essential that wheel should run full. Moreover, there should be free access of air between the vanes and the wheel. • The water may be admitted over a part of the circumference or over the whole circumference of the wheel. • The work is done by the change in kinetic energy.

10. Study of Hydraulic Machines Pumps Department of Civil Engineering Swedish College of Engineering and Technology Wah Cantt.

11. Department of Civil Engineering Pumps • The word pump is a general term for any fluid machine that adds energy to a fluid. Some authors call pumps energy absorbing devices since energy is supplied to them, and they transfer most of that energy to the fluid, usually via a rotating shaft. • Fluid machines may also be broadly classified as either positive-displacement machines or dynamic machines, based on the manner in which energy transfer occurs. • In positive-displacement machines, fluid is directed into a closed volume. Energy transfer to the fluid is accomplished by movement of the boundary of the closed volume, causing the volume to expand or con-tract, thereby sucking fluid in or squeezing fluid out, respectively. Your heart is a good example of a positive-displacement pump • In dynamic machines, there is no closed volume; instead, rotating blades supply or extract energy to or from the fluid. For pumps, these rotating blades are called impeller blades, while for turbines, the rotating blades are called runner blades or buckets

12. Department of Civil Engineering Pumps • Some fundamental parameters are used to analyze the performance of a pump. The mass flow rate of fluid through the pump, m, is an obvious primary pump performance parameter. For incompressible flow, it is more common to use volume flow rate rather than mass flow rate. In the turbo-machinery industry, volume flow rate is called capacity and is simply mass flow rate divided by fluid density, • The performance of a pump is characterized additionally by its net head H, defined as the change in Bernoulli head between the inlet and outlet of the pump,

13. Department of Civil Engineering Pumps • For the case in which a liquid is being pumped, the Bernoulli head at the inlet is equivalent to the energy grade line at the inlet, EGLin, obtained by aligning a Pitot probe in the center of the flow as illustrated in Fig. • The energy grade line at the outlet EGLout is obtained in the same manner, as also illustrated in the figure. In the general case, the outlet of the pump may be at a different elevation than the inlet, and its diameter and average speed may not be the same as those at the inlet.

14. Department of Civil Engineering Pumps • Net head is proportional to the useful power actually delivered to the fluid. It is traditional to call this power the water horsepower, even if the fluid being pumped is not water, and even if the power is not measured in units of horsepower. By dimensional reasoning, we must multiply the net head of Eq. by mass flow rate and gravitational acceleration to obtain dimensions of power. Thus, • All pumps suffer from irreversible losses due to friction, internal leakage, flow separation on blade surfaces, turbulent dissipation, etc. Therefore, the mechanical energy supplied to the pump must be larger than W water horsepower. In pump terminology, the external power supplied to the pump is called the brake horsepower, which we abbreviate as bhp. For the typical case of a rotating shaft supplying the brake horsepower, where ωthe rotational speed of the shaft (rad/s) and Tshaft is the torque supplied to the shaft.

15. Department of Civil Engineering Efficiency of Hydraulic Machines • ηturbineis defined as the ratio of brake horsepower output (actual turbine output shaft power) to water horsepower (power extracted from the water flowing through the turbine) • Note that turbine efficiencyηturbineis the reciprocal of pump efficiencyηpump, since bhp is the actual output instead of the required input

16. Department of Civil Engineering Reference Fluid Mechanics by Yunus A. Cengel. 2th Edition. (McGraw Hill)(Available in SCET Lib.) Fluid Mechanics by John F. Douglas. 5th Edition. (Pearson) (Available in SCET Lib.) Fluid Mechanics by E. John Franzini. 10th Edition. (McGraw Hill) (Available in SCET Lib.) A text book of hydraulics, fluid mechanics and hydraulic machines by R. S. Khurmi