Lecture_32_notes - ChE 374Lecture 32Turbines Pumps add...

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Unformatted text preview: ChE 374Lecture 32Turbines Pumps add mechanical Energy to increase the fluid head. Turbines EXTRACT mechanical energy at the expense of fluid head (decreased pressure). Turbine types: Hydraulic, wind, steam, gas. Typically more efficient than pumps (larger, less separation, lower speed, narrower operating range). 2 Types: Positive displacement (for flow measurement), and DYNAMIC (our focus, for power production). Dynamic: pumps have impellers, turbines have runners. 2 types: Impulse and Reaction. Impules turbines (Pelton Wheel) High Head, Low Flow. Fluid through nozzle, deflected in a wheel of buckets. Pressure is converted to kinetic energy, that impinges on and rotates the bucket wheel. Analyse with a momentum balance. Optimal redirection angle is 180o in theory, and 160o as a practical compromise. Optimal bucket speed is u = Vj /3, where Vj is the jet velocity. (Book says its u = Vj /2). Efficiencies up to 90% Reaction Turbines. As for pumps, have Centrifugal, Mixed Flow, and Axial. 2 Types: Fracis and Kaplan Francis: Radial and Mixed Flow: Med. Head and Med. Flow. Kaplan: Axial Flow (propeller with variable pitch): Low Head, High Flow. = bhp/gH V . Wind Turbines: Wavail = K.E. x flow rate = 1 v 2 m = 1 v 3 A. 2 2 3 Power A, Power v . = Cp = W /Wavail . Show with momentum balance and B.E. that Cp,max = 16/27 = 0.5926. This is the best possible, and the best in practice is around 0.45. Scaling laws are similiar as for pumps: CH , CQ , Cp , , but use Cp , not CQ as the independent parameter. Choose turbine type based on the Turbine specific speed Nst . Impulse for Nst < 0.3 Francis for Nst = 0.3 - 2 Kaplan for Nst > 2 CAREFUL OF THE UNITS ...
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Lecture_32_notes - ChE 374Lecture 32Turbines Pumps add...

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