Pumps and Turbines *Pumps and turbines: Fluid machines. Pumps: Add energy to the fluid-they do work on the fluid. ** Turbines: Extract energy from the fluid - the fluid does work on them. 3
Fluid Machines *Positive displacement machines (denoted as the static type) ** Turbomachines (denoted as the dynamic type)
Positive Displacement Machines S*Force fluid into or out of a To body Pulmonary artery Aorta To lungs Right atrium Piston motion chamber by changing the volume of the chamber. The pressure developed and the work done are a result of essentially static forces rather than dynamic effects Oxygenated blood from ungs Force of piston on fluid Left atrium Venous blood Right ventricle Left valve Suction Discharge Typical positive displacement pumps: (a) tire pump, (b) human heart, (c) gear pump.
Turbomachines Turbomachines involve a collection of blades, buckets. flow channels, or passages arranged around an axis of rotation to form a rotor. Turbomachines are mechanical devices that either extract energy from a fluid (turbine) or add energy to a fluid (pump) as a result of dynamic interactions between the device and the fluid. The fluid used can be either a gas or a liquid.
Operating Principles of Turbomachines The basic operating principles are the same whether the fluid is a liquid or a gas. *Cavitation may be an important design consideration when liquids are involved if the pressure at any point within the flow is reduced to vapor pressure. *Compressibility effects may be important when gases are involved if the Mach number becomes large enough.
Structure of Turbomachines **Many turbomachines contain some type of housing or casting that surrounds the rotating blades or rotor, thus forming a n internal flow passageway through which the fluid flows. * Some turbomachines include stationary blades or vanes in addition to rotor blades. These stationary vanes can be arranged to accelerate the flow and thus serve as an nozzles. * These vanes can be set to diffuse the flow and act as diffusers.
Classification of Turbomachines Axial-flow machines: The fluid maintains a significant axial-flow direction component from the inlet to outlet of the rotor. Mixed-flow machines: There may be significant radial- and axial-flow velocity components for the flow through the rotor row Radial-flow mahcines: The flow across the blads involves a substantial radial-flow component at the rotor inlet, exit, or both.
Basic Energy Considerations By considering the basic operation of Household fan (pump) Windmill (turbine) 10
Household Fan 1/2 * Consider a fan blade driven at constant angular velocity by the motor. * Absolute velocity is the vector sums of relative and blade velocities. The blade velocitv U-or The absolute velocity V seen by a persorn sitting stationary at the table on which the fan rests. The relative velocity seen by a person riding on the fan blade W
Household Fan 2/2 2) Inlet surface Blade cross section Axial e motion angential Exit surface 0 Idealized flow through a fan: (a) fan blade geometry: (b) absolute velocity, V; relative velocity, W, and blade velocity, U at the inlet and exit of the fan blade section. 12
Windmill Consider the windmill. Rather than the rotor being driven by a motor, it is rotated in the opposite direction by the wind blowing through the rotor, 2 2 (0 Blade motion Idealized flow through a windmill: (a) windmill blade geometry; (b) absolute velocity, V; relative velocity, W, and blade velocity, U at the inlet and exit of the windmill blade section, 13
Example 12.1 Drag from Pressure and Shear Stress Distributions * The rotor shown in Fig. E12.1a rotates at a constant angular velocity of 100 rad/s. Although the fluid initially approaches the rotor in an axial direction, the flow across the blades is primarily radial. Measurements indicate that the absolute velocity at the inlet and outlet are Vi-12 m/s and V2- 15 m/s, respectively. Is this device a pump or a turbine? 2 60 (2) r2 = 0.2m r0.1mm o100 rad/s Blade
Example 12.1 Solution22 The inlet velocity triangle The outlet velocity triangle Radial U2 20 m/s Known quantities shown in color Outlet 2-600 V2 15 m/s Circumferential U1 10 m/s Radial Inlet VI 12 m/s Circumferential At the inlet there is no component of absolute velocity in the direction of rotation: at the outlet this component is not zero, That is the blade pushes and turns the fluid in the direction of the blade motion, thereby doing work on the fluid. - This device is a pump. 16
Angular Momentum Considerations 26 All of the turbomachines involve the rotation of an impeller or a rotor about a central axis, it is appropriate to discuss their performance in terms of torque and angular momentum. 19
Angular Momentum Considerations 56 shaftshaf (3) shaftT (3) (4) (5):The basic governing equations for pumps or turbines whether the machines are radial-, mixed, or axial-flow devices and for compressible and incompressible flows.
Angular Momentum Considerations 66 Another useful but more laborious form. Based on the velocity triangles at the entrance or exit. v2 +U2-w2 Wshaft Turbomachine work is related to changes in absolute, relative, and blade velocities, 23
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