Turbomachine_Performance

Turbomachine_Performance - ME 395 : Turbomachine...

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ME 395 : Turbomachine Performance TP-1 PERFORMANCE OF A CENTRIFUGAL TURBOMACHINE 1. INTRODUCTION Many pumps, turbines, and blowers are examples of devices, which are collectively known as turbomachines. Turbomachines can increase the energy of a flowing medium, as in the case of a pump, or they can extract energy, as in the case of turbines. Turbomachines change the energy (principally the kinetic energy) of the flow through a rotating element. (Other types of pumps exist which do not use a rotating element). Turbomachines are employed in many different tech- nologies, including power generation, automotive engines, rocket propulsion, and aircraft pro- pulsion. In general, if the flow is parallel to the axis of rotation, the device is considered to be an axial turbomachine (such as the engines on commercial airliners). Alternately, if there is a pro- nounced change in radius of the flow from the inlet to the discharge of the device, the turbom- achine is said to be centrifugal (such as is typical in industrial blowers). 2. TURBOMACHINE PERFORMANCE CHARACTERISTICS Turbomachines are usually characterized by "performance curves," and examples are given be- low for a centrifugal pump. In the first graph, the fluid energy rise per unit mass through the pump, Δ E , is plotted against the volume flow rate through the pump, , for a variety of pump impeller speeds. The fluid energy rise between in the inlet (subscript i ) and outlet (subscript o ) results from an input of mechanical energy into the fluid and is defined as Δ E = ( P o P i ) ρ + g ( z o z i ) + ( V o 2 V i 2 ) 2 (1) where ( P o P i ) = Δ P is the pressure rise across the pump, is the change in fluid eleva- tion, and are the fluid inlet and outlet velocities, is the fluid density, and is the gravi- tational constant. For many pumps, the change in elevation and flow velocity across the pump accounts for a negligible portion of the total fluid energy rise, and Δ E can then be approximated as , but the success of this approximation is limited to low speed flows. On the second graph, the pump efficiency,
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Turbomachine_Performance - ME 395 : Turbomachine...

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