ENERGY CONVERSION

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• ENERGY CONVERSION
• MME 9617A
• Eric Savory
• www.eng.uwo.ca/people/esavory/mme9617a.htm
• Lecture 9 Prime movers and turbomachinery
• Department of Mechanical and Material Engineering
• University of Western Ontario

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Definition

• Turbomachinery describes machines that transfer
  energy between a rotor and a fluid, including
  both turbines and compressors.
• A turbine transfers energy from a fluid to a
  rotor, a compressor transfers energy from a rotor
  to a fluid.
• The two types of machines are governed by the
  same basic relationships including Newton's
  second law of motion and Euler's energy equation.
  
• Centrifugal pumps are also turbomachines that
  transfer energy from a rotor to a fluid, usually
  a liquid.
• Energy is converted from kinetic to potential and
  vice versa with the aid of mechanical energy.

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Pump classes and types
Class Type
Centrifugal (rotating impeller increases the pressure energy of a fluid) Volute Diffuser Regenerative turbine Mixed flow Axial flow
Rotary (positive displacement pump produces the same volume output regardless of pressure) Gear Vane Cam and piston Screw Lobe
Reciprocating (pistons or plungers displace the fluid) Direct acting Diaphragm Rotary piston
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Positive displacement pumps
External gear pump
Reciprocating piston
Double screw pump
Sliding vane
Three-lobe pump (left) Double circumferential
piston (centre)
Flexible tube squeegee (peristaltic)
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Pump types
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Centrifugal pump cutaway schematic
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Formulation of the concept

• We will focus on the centrifugal pump. However,
  the principles are the same for compressors and
  turbines with a geometry change and appropriate
  boundary conditions.
• The dominant direction of the flow during the
  energy transfer process is radial.
• Rotor (impeller) rotating element where the
  energy transfer process occurs.
• Diffuser stationary element which is
  responsible for the transformation of the
  velocity head into static pressure.
• Velocity head - V2/2g

Centrifugal pump with volute and diffuser
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Energy transfer mechanism

• The energy transfer mechanism results from the
  change in angular momentum of the fluid
• The torque on the shaft is
• Where Vu denotes the component of the vector V in
  the direction of U, the tangential wheel speed
  (at a given Urw), assuming steady-state
  frictionless flow.
• Further assumptions of uniform flow at the inlet
  and outlet and an effective mean radius, give

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• Power becomes
• The increase in Head is
• (Euler pump equation)
• U2Vu2 gt U1Vu1 the device functions as a
  compressor
• U2Vu2 lt U1Vu1 energy is extracted from the flow
  and the device function as a turbine

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The velocity triangle

• V - absolute velocity
• U - tangential velocity
• Vr - relative velocity

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Centrifugal-compressorschematicandvelocitytri
angles
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• From figure (a) in previous slide, fluid enters
  the rotor with an absolute velocity that is
  completely radial (zero pre-swirl), therefore,
  Vu1 is zero. The increase in Head is
• Denoting the radial component of the exit
  velocity as Vm, then
• And from the exit velocity
• triangle fig. (c)
• For an impeller of width w, the volume flow rate
  is

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Head (H) versus Volume flow rate (Q) relationships

• The increase in Head is a function of the
  volumetric flow rate, Q
• Defining
• We obtain
• The sign on K2 (which depends on the exit angle
  ?2) establishes the characteristics of the machine

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H - Q characteristics

• Three separate cases can be considered
• (1) Radial exit blades (b2 90o)
• (2) Backward-curved blades (b2 lt 90o)
• (3) Forward-curved blades (b2 gt 90o)

Ideal H versus Q curves
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Actual H - Q relationships
Losses inside pump (e.g. friction and turning
losses)
Head H
Volume flow rate Q
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Manufacturers pump characteristics
Index of pumps from Goulds Pumps Inc
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TheGoulds 3196family of pumps
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Composite ratingcharts for theGoulds
3196family of pumps
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Performance characteristics
Symbol Parameter Imperial Units
H Head (m) ft-lbf/lbm
Q Flow rate (m3/s) ft3/s
N Speed (rpm or rad/s) rpm
? Mechanical efficiency none
D Geometry (m) ft
? Density (kg/m3) lbm/ft3
? Viscosity (kg/ms) lbm/ft-s
P Power (W) ft-lbf/s
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Buckingham P theoryA dimensional analysis of
all the variables involved yields a number of
non-dimensional groups called ? parameters
Note that although the viscosity ? is an
appropriate parameter to include and it yields
the Reynolds number (?4), in practice this is not
a dominant parameter for turbomachine scaling
analysis
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Scaling relationships for turbomachines of the
same geometry (geometrical similarity)
For a change in diameter only For a rotational speed change only

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5
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Pumps in series and parallel
Series
Equivalent pump
Parallel
Equivalent pump
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Pumps in Series
Add the heads (H) at each flow rate (Q) For
example, for two identical pumps the head will be
double that of a single pump.
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Pumps in Parallel
Add the flow rates (Q) at each head (H) For
example, for two identical pumps the flow rate
will be double that of a single pump.
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Pump-system operation
System resistance (losses) curves (typically H ?
Q2)
C operating point
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Jet propulsion
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History Before Turbojets
Thermojet Henri Coanda 1910
Aeolipile Hero of Alexandria 75 A.D.
Rocket Chinese Taoist Chemists 1st Century
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History The First Jets
Hans Von Ohain
Frank Whittle
Test engine - 1937
Test engine - 1935
W.1 Turbojet - 1939
He S-3 - 1938
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History More Modern Jets
Centrifugal Compressor Turbojet - Used by Whittle
Ohain - Short and fat - Must bend the airflow

• Axial Flow Compressor Turbojet
• Introduced by Anselm Franz
• (Junkers' Engine Div.) 1944
• Long and thin
• - Improved airflow

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Jet Types and Uses
Type Description Advantages Disadvantages
Thermojet A piston engine is used to run the compressor. Works like a regular turbojet minus the turbines.   Heavy, inefficient and underpowered
Turbojet Generic term for simple turbine engine Simplicity of design Very basic. Does not take advantage of improved efficiency of other designs.
Turbofan Uses an enlarged first stage compressor as a 'fan' to provide more thrust. Quieter, more efficient for subsonic airspeeds. More complex, large diameter, heavy, subject to foreign object damage.
Ramjet No moving parts. Intake air is compressed by the airspeed and duct shape. Lightweight, efficient above Mach 2.0. Needs high speed to operate, only efficient in a narrow speed range, used as accessory?
Turboprop Not really a jet. A gas turbine driving a propeller. High efficiency at low speed (300-450 knots) Limited top speed, noisy, complex propeller drive and gearbox.
Propfan Turboprop engine with one or more propellers. Like a turbofan without ducts. Very high fuel efficiency, higher speed. Very complex, more noisy than turbofans.
Scramjet Intake air is compressed but not slowed to subsonic. Intake, combustion and exhaust occur in a single constricted tube Operates at very high speed (Mach 8-15). Still in development. Need to be above Mach 6 to operate. Cooling problems.
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Principles - Physical

• Major Components of a Jet Engine
• Fan
• Compressor
• Combustor
• Turbine
• Mixer / Nozzle

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Principles - Physical

• Newtons 3rd Law of Motion
• For every action there is an equal and opposite
  reaction.
• Boyles Law
• there is a relationship between the pressure of a
  fixed amount of air and its volume.

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Principles - Physical

• Power is measured in pounds (lb) of thrust (or
  Newtons of thrust 4.45 N1 lb).
• 1 lb of thrust means that the engine will be able
  to accelerate one pound of material at 32 ft/s2.
• Approximate equation for net thrust of a jet
  engine

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Principles - Chemical

• Kerosene is usually used to power Jets in the
  form of Avtur, Jet-A, Jet-A1, Jet-B, JP-4, JP-5,
  JP-7, or JP-8.
• Kerosene is obtained from the fractional
  distillation of petroleum at 150C and 275C
• Kerosene consists of carbon chains from the C12
  to C15 range.

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Principles - Thermodynamic
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Efficiency

• Thermal Efficiency
• 45 - 50 for todays best engines.
• Propulsive Efficiency
• About 47 for low by-pass turbojets.
• About 80 for high by-pass turbofans.
• Overall Efficiency
• About 40 for modern jets at cruise speed.

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Future of Jets ?

• Small, personal jet aircraft using highly
  efficient jet engines.
• High speed, high altitude jet aircraft.
• Engines to be cooled by new coal derived jet fuel.

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Future of Jets ?

• MEMS Turbines (Power on a Chip)
• Turbine blades span an area smaller than a dime.
• Run for 10 hrs on a container of diesel fuel
  about as big as a D battery.
• Also could be used to power tiny planes for the
  military
• 15W to 20W output.
• Flying humans
• Tiny jet engines combined with a wing-suit.