an exclusive conservation equation for ideal turbo-machines p m v subbarao professor mechanical...
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An Exclusive Conservation Equation for Ideal Turbo-machines
P M V SubbaraoProfessor
Mechanical Engineering Department
Invention of New Property for CVs with Work Transfer….
Conservation of Rothalpy
• A cornerstone of the analysis of steady, relative flows in rotating systems has, for many years, been the immutable nature of the fluid mechanical property rothalpy.
• "In a moving passage the rothalpy is therefore constant provided:– the flow is steady in the rotating frame;– no friction from the casing;– there is no heat flow to or from the flow.
gzUVV
hIRothalpy blade 2:
2
or
gzUVhIRothalpy blade 0:
gzUVVVV
hIRothalpy bladerx
2:
222
gzUVVVVhIRothalpy bladerx 22
1: 222
gzUUVVVhIRothalpy bladebladerx 2222
2
1:
Ideas for creation of a variety in turbo-machine.
Novel Idea for Creation of Variety
Blade Velocity Vs Tangential Component of Fluid Velocity
Ub
Ub
Vwi
Vai
Vfi
gzUUVVVhI bladebladerx 2222
2
1
Vri
In maridional plane at mean radius of rotor
Ub
Vwi
Vai
VfiVri
Ub
Vwi
Vai Vfi
Vri
VwiUb
Vai
Vfi
Vri
gzUVVVhIRothalpy bladerelrx 22,
22
2
1:
gzUVhIRothalpy bladerel 22
2
1:
Relative Angular Velocity
gzUVhgzU
hIRothalpy bladeblade
rel 0
2
,0 2:
Constant in an ideal turbo-machine
For stator Ublade =0
constant: 0,0 gzhgzhEnthalpyStagnation rel
For rotors :
constant2
: 0
2
,0 gzUVhgzU
hIRothalpy bladeblade
rel
For a true axial flow machines: Ublade constant
constant: ,0 gzhIRothalpy rel
Complex Geometrical Features of A Turbo-Machinne
A turbomachine working with incompressible fluid will be isothermal and hence U(T) is constant throughout the machine.
constant22
12
22 gzUVVp
gzUVp
bladebladerel
constant2
2
, gzUVpgzU
p bladestagnationblade
relstagnation
For an Ideal Hydro Power Plant :
constant2
2
, bladestagnationblade
relstagnation UVgzpU
gzp
A Two-Way Welfare for the Globe
Hydro Electric Plant with High Headspatm
H
gHpV
p atmpenstock
static
2
2
gHV jet
2
2
Option for High Head Hydro Station
In an ideal Penstock constant2
2
gHpV
p atmpenstock
static
In an ideal Nozzle constant2
2
jetstatic
Vp
In an ideal turbo-machine constant2
2
relstatic
Vp
constant2
1 22 gzUVp
bladerel
U
Vri
Vre
Vai
UVri
Vai
Inlet Velocity Triangle
U
VreVae
Exit Velocity Triangle
Vri
More Ideas
For an Ideal Hydro Power Plant :
constant2
1 22 gzUVp
bladerel
Turbo-machines working with Vapors/Gas
constant2
: 0
2
,0 bladeblade
rel UVhU
hIRothalpy
constant,2
,0,0
0
2
0
blade
T
pblade
T
p UVdTTpcU
dTTpcIrel
For an ideal gas:
constant2
0,0
0
2
0
blade
T
pblade
T
p UVdTTcU
dTTcIrel
constantgz
For simple compressible fluid: Like Inert Gas
constant2
: 0
2
,0 bladepblade
relp UVTcU
TcIRothalpy
constant2
0,0
0
2
0
blade
T
pblade
T
p UVdTcU
dTcIrel
The Fourth Generation Nuclear Power Plants
An Advanced Nuclear Power Plant
Geometrical Details along the Third Direction
• True flow through a turbo-machinery is three-dimensional.
• Flow and tangential flow velocities are very important for better operation of a turbo-machine.
• The third component, which is normal to flow and tangential direction is in general of no use.
• This direction can better represented as blade height direction.
Third Direction of an Axial Flow Turbo-Machines
• The third direction in an axial flow machine is the radial direction.
• The direction of Centrifugal forces!
• Strong centrifugal forces are exerted on blades & fluid in radial direction.
• The centrifugal field distorts the flow velocity profiles considerably.
• Fluid particles tend to move outwards rather than passing along cylindrical stream surfaces as classically assumed.
• Particularly in tall blade (low hub: tip) ratio designs.
• An approach known as the radial equilibrium method, widely used for three-dimensional design calculations in a an axial flow machine.
Radial Equilibrium Theory of Turbo-machines
P M V SubbaraoProfessor
Mechanical Engineering Department
A Model for Stable Operation of A Machine
A guiding equation for distribution of load along blade length ….
Radial Variation Blade Geometry
Radial Equilibrium Theory
• Assumes that flow is in radial equilibrium before and after a blade row.
• Radial adjustment takes place through the row.
• More important for Axial Flow Machines.
Radial Equilibrium Analysis
The centrifugal force = (rdrd)2r V = r
The centrifugal force is
The pressure force on the element
drdVF lcentrifuga2
rdpdFpressure
If the two forces are the only ones acting (viscous and other effects neglected), the particle will move at constant radius if:
lcentrifugapressure FF
r
V
dr
dp 2
r
drV
dp 2
Equilibrium Condition for A Rotating Fluid
An equivalent equation for compressible flow can be developed by using the following thermodynamic relation:
0dp
dhvdpdhTds
dp
dh r
drVdh 2
The radial variation of whirl velocity should be according to above equation.
How to implement on a machine?
2222
2222
0VVV
hV
hh rf
0222
222
0
VVV
hddh rf
Total Energy Equation for A Rotating Fluid
Stagnation enthalpy should conserve, as there are not interactions with rotor at inlet or exit.
r
drVdh 2
0222
2222
0
VVV
dr
drVdh rf
0222
22220
VVV
dr
d
r
V
dr
dh rf
02
0 dr
dVV
dr
dVV
dr
dVV
r
V
dr
dh rr
ff
Radial component of velocity should be constant (zero) along radial direction for radial equilibrium of flow.
02
0 dr
dVV
dr
dVV
r
V
dr
dh ff
gzUVhU
hIRothalpy bladeblade
rel 0
2
,0 2:
Constant in a turbo-machine along meridonial Plane
0
2
12
dr
rVd
r
V
dr
dV f
Stagnation enthalpy is Constant in a turbo-machine along radial direction at intake and discharge.
Twisted Blades for Large Turbines
Lessons from Nature
• In the case of a vortex, the flow field is purely tangential.
ziiW ln2
The complex potential function:
THE VORTEX
•Free Vortex Whirl:
•Forced Vortex Whirl :
General Rules for Selection of Whirl Component
r
CV
constantfV
rCV
221C rCV f
0
2
12
dr
rVd
r
V
dr
dV f
More complex Models
• Weighted mean of free and forced vortices
• General Whirl Distribution
Inlet Exit
Radial Variation of Flow Velocity in Real Machine
Intake
Discharge
Radial Variation of Whirl Velocity
Intake
Discharge
Radial Variation of Mass flow rate
Intake
Discharge
Design of Compact Machine
Kaplan Turbine
DESIGN OF THE BLADE
Two different views of a blade
90% or better in efficiency
Basic Rules for Design of An Ideal Turbo-machine
Basic Rules for Design of An Ideal Turbo-machine
• Enumerate the details of source or demand.
• Calculate Specific speed and identify the fundamental concept of operation.
• X1 (Impulse)+X2(Reaction)+(1-X1-X2)(centrifugal)
• Y1 (Radial)+(1-Y1 )(Axial)
• Design of Flow Path using Conservation of rothalpy.
• Design blade cascade using conservation of mass and momentum.
• Design of Radial Geometry using Radial Equilibrium Theory.
• A design of an Ideal Machine…..
• Real Performance will be lower ……
Basic Rules for Design of A Real Turbo-machine
• More customized rules along with the general rules.
• Customized rules are specific to application:• Power consumption Vs Power Generation.• Radial Vs Axial.• Incompressible flow Vs Compressible.• In Reality:• Design analysis of A Real Machine is an
Exclusive Scientific Art.