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Micro Turbines : Turbo-expanders
New Solutions for Distributed Green & Waste Resources…..
P M V SubbaraoProfessor
Mechanical Engineering Department
Selection of An Expander
In search of A Suitable Principle of Momentum Exchange/Direction of Fluid Flow
• Primary characteristics of a source or need.• The cause/effect: p or h• The Capacity: Flow rate, Q (m3/s ).• Density of fluid: (kg/m3).
Time Scale of a Machine to Resource
Speed: N (rpm) or n (rps) of a turbo machine:
Scale Time Machine
Scale Time Resourcescale timeessDimensionl
n
T f1
scale timeDimension -Non
This is named as Specific Speed, Ns
4
3
p
QT f
4
3
p
QnN s
4 31000
76
h
QNN s
Selection of An Expander
Why Radial Flow Turbines
• Better ability to guide flow in an optimal direction into the expansion turbine wheel,
• Variable inlet guide vanes present the most important advantage of a radial turbine over an axial turbine.
• Suitable for highly variable natural sources of energy/waste energy recovery.
Turbo-Expanders
Compressible Flow Francis Turbine
• Through minor modifications standard radial inflow turbines can be optimized for different renewable thermal resources.
• They enable to smooth the seasonal variations by maintaining high efficiency levels at off-design conditions through the use of variable inlet guide vanes.
• Radial inflow turbines are less sensitive to blade profile in accuracies than axial turbines, which enable high efficiencies to be maintained as size decreases.
• Radial-inflow turbines are more robust under increased blade road caused by using high-density fluids as either subcritical or supercritical conditions.
Compressible Flow Francis Turbine
• Radial inflow turbines are easier to manufacture relative to axial turbines as the blades are attached to the hub.
• The rotor dynamic stability of the system is also improved due to a higher stiffness.
Parts of A Turbo-expander
Design of Spiral Casing
Rcasing
Risv
dpipe
Q
Select a suitable value of mass flow rate.
2
4 pipepipemain dVm
At any angle , the radius of casing is:
pipeisv dRR
2casing
A full spiral is generally recommended for high head 300m, semi-spiral is recommended for low head < 50m.
2mm
In general =1.0, however corrected using CFD.
Flow Distribution Analysis of Casing
Stay vanes or Guide vanes
Parts of A Turbo-expander
Geometrical Description of A Turbo-expander
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanes
R a d i a l v i e wrunner guide vanes and stay vanesR a d i a l v i e wrunner guide vanes and stay vanes
Water from spiral casing
Water particle
Design of the Details of Stay Vanes
StayVaneinletStayVaneinletwGuideVaneinletGuideVaneinletw rVrV
rexit stay Vane
rinlet Stay Vane
Besv
Theory of Relatively Whirling flow:
Bisv
isvn
wisvisvesvn
wesvesv rVrV
isvSratyVaneinletStayVaneinletf BD
QV
pipemainStayVaneainlet VV
Guide vanesGuide vanesGuide vanesGuide vanesGuide vanesGuide vanes
Runner inlet (Φ 0.870m)
Guide vane outlet for designα) (Φ 0.913m)
ClosedPosition
Max. Opening Position
Operation of Guide Vanes
Guide vane at DesignPosition = 12.21°
Guide vane at closed position
Guide vane at Max. openPosition = 18°
.
Design of the Guide Vanes
• The outlet angle can be calculated by assuming a vortex from the flow in the gap between the runner and the guide vanes
nwigvigv
nwegvegv igvegv
rVrV
egvegvfegv BD
QV
regv
rigv
Begv
Select appropriate value of n
The correlation between the turbinedischarge and the guide vane opening angle.
Pressure drop versus guide vane angle
Velocity trianglesrri
rre
UriVwi
Vri
Vfi
Vai
UreVwe
Vre
VfeVae
i
i
ee
rU
UU
r
rire
Inlet Velocity Triangles Vs Ns
Low Specific Speed : Slow Francis Runner
Vwi
Vai
Vfi
Inlet Velocity Triangles Vs Ns
Low Specific Speed : Normal Francis Runner
Vwi
Vai
Vfi
Inlet Velocity Triangles Vs Ns
High Specific Speed : Fast Francis Runner
Vwi
Vai
Vfi
Specfic Speed Vs Runner Shape
3D Reconstruction of Runner with Blades
Study of Velocity distribution on runner for improvement
Design Rule 190
-i
Design Rule 2
Design Rule 3