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