Chapter II Fluid Machinery

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MEC 442 FLUID MECHANICS II

FLUIDMACHINERY


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INTRODUCTIONA fluid machine is a device which converts the energy storedby a fluid into mechanical energy or vice versa . The energystored by a fluid mass appears in the form of potential, kinetic

and intermolecular energy. The mechanical energy, on theother hand, is usually transmitted by a rotating shaft.Machines using liquid (mainly water, for almost all practicalpurposes) are termed as hydraulic machines. In this chapterwe shall discuss, in general, the basic fluid mechanicalprinciple governing the energy transfer in a fluid machine andalso a brief description of different kinds of hydraulicmachines along with their performances. Discussion onmachines using air or other gases is beyond the scope of thechapter.


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MEC 442 FLUID MECHANICS IIClassification Based General Types

Positive Displacement Machines [PD]:

R Hydraulic Pumps & Motors; Gas Compressors & Expanders:

X Piston, Sliding Vane, Scroll, Screw, Lobed (Roots)

Rotodynamic (Turbo) Machines [RD]:

R Hydraulic Pumps & Turbines; Gas Compressors; Steam and Gas

Turbines; Hydraulic Couplings; Windmills; Propellers and Fans:

X Radial, Mixed-flow, Axial; Impulse & Reaction; Single and Multi-stage


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MEC 442 FLUID MECHANICS IIPositive Displacement Machines [PD]:Piston


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MEC 442 FLUID MECHANICS IIPositive Displacement Machines [PD]:Sliding Vane


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MEC 442 FLUID MECHANICS IIPositive Displacement Machines [PD]:Scroll


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MEC 442 FLUID MECHANICS IIPositive Displacement Machines [PD]:Twin Screw


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MEC 442 FLUID MECHANICS IIPositive Displacement Machines [PD]:Roots


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Rotordynamics Machines [RD]:Hero turbine


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Rotordynamics Machines [RD]:Pelton Impulse Turbine


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Rotordynamics Machines [RD]:Francis Turbine


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Rotordynamics Machines [RD]:Kaplan Turbine


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Rotordynamics Machines [RD]:Hydraulic coupling


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Rotordynamics Machines [RD]:Centrifugal compressor


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Rotordynamics Machines [RD]:Turbocharger (Centrifugal compressor + turbine)


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Rotordynamics Machines [RD]:Steam turbine


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Rotordynamics Machines [RD]:Rocket engine turbopumps


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Rotordynamics Machines [RD]:Ship screws and aircraft propellers


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CLASSIFICATIONS OF FLUID MACHINESThe fluid machines may be classified under differentcategories as follows:

Classification Based on Direction of Energy Conversion.The device in which the kinetic, potential or intermolecularenergy held by the fluid is converted in the form ofmechanical energy of a rotating member is known asa turbine . The machines, on the other hand, where the

mechanical energy from moving parts is transferred to a fluidto increase its stored energy by increasing either its pressureor velocity are known as pumps, compressors, fans orblowers .


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CLASSIFICATIONS OF FLUID MACHINES

The fluid machines may be classified under different categories as follows:

Classification Based on Principle of Operation

The machines whose functioning depend essentially on the change of volumeof a certain amount of fluid within the machine are known as positivedisplacement machines . The word positive displacement comes from the factthat there is a physical displacement of the boundary of a certain fluid massas a closed system. This principle is utilized in practice by the reciprocating

motion of a piston within a cylinder while entrapping a certain amount offluid in it. Therefore, the word reciprocating is commonly used with the nameof the machines of this kind. The machine producing mechanical energy isknown as reciprocating engine while the machine developing energy of thefluid from the mechanical energy is known as reciprocating pump orreciprocating compressor.


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MEC 442 FLUID MECHANICS IIClassification Based on Principle of Operation (cont)

The machines, functioning of which depend basically on the principle of fluiddynamics, are known as rotodynamic machines . They are distinguished frompositive displacement machines in requiring relative motion between thefluid and the moving part of the machine. The rotating element of themachine usually consisting of a number of vanes or blades, is known as rotoror impeller while the fixed part is known as stator. Impeller is the heart ofrotodynamic machines, within which a change of angular momentum of fluidoccurs imparting torque to the rotating member.


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For turbines, the work is done by the fluid on the rotor, while, in case of pump,compressor, fan or blower, the work is done by the rotor on the fluid element.Depending upon the main direction of fluid path in the rotor, the machine istermed as radial flow or axial flow machine . In radial flow machine, the maindirection of flow in the rotor is radial while in axial flow machine, it is axial. Forradial flow turbines, the flow is towards the centre of the rotor, while, forpumps and compressors, the flow is away from the centre.


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Therefore, radial flow turbines are sometimes referred to as radially inward flow machines and radial flow pumps as radially outward flow machines.Examples of such machines are the Francis turbines and the centrifugal pumpsor compressors. The examples of axial flow machines are Kaplan turbines andaxial flow compressors. If the flow is party radial and partly axial, theterm mixed-flow machine is used. Figure 1.1 (a) (b) and (c) are the schematicdiagrams of various types of impellers based on the flow direction.


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TEST 1 17th

October 2012, 8 pm,Location to be announced (most likely at DKB)

Topic: Compressible Flow


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump - this pump consists of an impeller rotating within a

casing. Fluid enters axially through the eye of the casing, is caught up in theimpeller blades, and is whirled tangentially and radially outward until itleaves through all circumferential parts of the impeller into the diffuser partof the casing. The fluid gains both velocity and pressure while passingthrough the impeller.


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump - The doughnut-shaped diffuser,or scroll, section of

the casing decelerates the flow and further increases the pressure. Theimpeller blades are usually backward-curved but there are also radial andforward-curved blade designs, which slightly change the output pressure.The blades may be open, i.e., separated from the front casing only by anarrow clearance, or closed, i.e., shrouded from the casing on both sides byan impeller wall. The diffuser may be vaneless or fitted with fixed vanes tohelp guide the flow toward the exit.


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump - Basic Output Parameters


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump - Theory


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Effect of Blade Angle on Pump Head


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Pump performance Curves


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Measured Pump performance Curves


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Dimensionless Pump Performance


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Similarity Rules


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MEC 442 FLUID MECHANICS IIThe Centrifugal Pump Effect of Viscousity


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MEC 442 FLUID MECHANICS IIThe Mixed- and Axial-Flow Pump


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MEC 442 FLUID MECHANICS IIThe Mixed- and Axial-Flow Pump


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Documents

Chapter II Fluid Machinery