A course in Turbomachinery Lecturer Dr Naseer AlJanabi

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi 4. 3 29 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 (Fig. 4. 4 below). Fig. 4. 4 A reaction turbine differs significantly from an impulse turbine; instead of using water jets, a volute is filled with swirling water that drives the runner. For hydroturbine applications, the axis is typically vertical. Top and side views are shown, including the fixed stay vanes and adjustable wicket gates.

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi 30 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. Unlike the impulse turbine, the water completely fills the casing of a reaction turbine. For this reason, a reaction turbine generally produces more power than an impulse turbine of the same diameter, net head, and volume flow rate. There are two main types of reaction turbine—Francis and Kaplan. The Francis turbine is somewhat similar in geometry to a centrifugal or mixed flow pump, but with the flow in the opposite direction. Note, however, that a typical pump running backward would not be a very efficient turbine. The Francis turbine is named in honor of James B. Francis (1815– 1892), who developed the design in the 1840 s. Fig 4. 5 Interior view of the 1. 1 -million hp (820 -MW) turbine units on the Grand Coulee Dam of the Columbia River, showing the spiral case, the outer fixed vanes (“stay ring”), and the inner adjustable vanes (“wicket gates”).

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi 31 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). Fig 4. 6 The five-bladed propeller of a Kaplan turbine used at the Warwick hydroelectric power station in USA. There are five runner blades of outer diameter 12. 7 ft (3. 87 m). The turbine rotates at 100 rpm and produces 5. 37 MW of power at a volume flow rate of 63. 7 m 3/s from a net head of 9. 75 m.

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi 32 Fig. 4. 7 The distinguishing characteristics of the four subcategories of reaction turbines: (a) Francis radial flow, (b) Francis mixed flow, (c) Kaplan mixed flow, and (d) Kaplan axial flow. Power P extracted by the runner: P = w = pw. Q(r 2 Vt 2 -r 1 Vt 1 ) = p. Q(u 2 V 2 cosa 2 -u 1 V 1 cosa 1 ) 4. 4 where Vt 2 and Vt 1 are the absolute inlet and outlet circumferential velocity components of the flow.

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi 33 Fig. 4. 8 Inlet and outlet velocity diagrams for an idealized radial flow reaction turbine runner. Fig. 4. 9 Typical setup and terminology for a hydroelectric plant that utilizes a Francis turbine to generate electricity; drawing not to scale. The Pitot probes are shown for illustrative purposes only.

A course in Turbomachinery…………………. . …. …Lecturer: Dr. Naseer Al-Janabi The absolute inlet normal velocity Vn 2 = V 2 sin α 2 is proportional to the flow rate Q. If the flow rate changes and the runner speed u 2 is constant, the vanes must be adjusted to a new angle α 2 so that w 2 still follows the blade surface. Thus adjustable inlet vanes are very important to avoid shock loss. EX: An idealized radial turbine is shown in Fig. below. The absolute flow enters at 30° and leaves radially inward. The flow rate is 3. 5 m 3/s of water at 20°C. The blade thickness is constant at 10 cm. Compute theoretical power developed at 100% efficiency. 34