Kiyarash Rahbar, Saad Mahmoud , Raya K. Al-Dadah, Ahmed Elsayed

School of Mechanical Engineering

University of Birmingham

MODELING AND CFD ANALYSIS OF A MINIATURE RADIAL TURBINE FOR DISTRIBUTED POWER

GENERATION SYSTEMS

SusTEM Special Sessions

on

Thermal Energy Management

• Introduction Steam/water Rankine cycle Vs organic Rankine cycle Importance of of expander

• Aims and objective • Design methodology of radial turbo-expander Preliminary design Detailed design

• Results Preliminary design Detailed design

• Proposed specifications • Conclusion

Introduction • Accelerated world’s energy consumption has led to scarcity of fuel

resources and severe environmental pollutions

• New solutions and alternatives are required

• Distributed (on site) Power Generation (DPG) is a promising solution for supplying energy demands and reducing environmental problems

• DPG is an electric power source connected directly to the distribution network or the customer site of the meter

Category Power Rating

Distributed Micro Power Generation 1Watt to 5kW

Distributed Small Power Generation 5kW to 5MW

Distributed Medium Power Generation 5MW to 50MW

Distributed Large Power Generation 50MW to 300MW

Ackermann T et al .Distributed Generation: a definition. J Electric Power Systems Research 2001;57:195–204

Steam/Water vs. Organic Rankine Cycle

Water/steam Rankine cycle

1. Has uneconomically low thermal efficiency when exhaust steam temperature drops below 370ᵒC

2. Bulky equipments due to high specific volume of steam

3. High capital cost, safety concerns and complex system due to requirements of high temperature and pressure

4. High maintenance cost due to erosion and corrosion of blades caused by steam droplets

5. Unavailability of high temperature heat sources in DMPG

Organic Rankine cycle

1. Suitable to be powered by low grade heat sources in temperature range of 60-200ᵒC

2. Small size due to high fluid density (Steam=2.4kg/m3 ,R245fa=17.6 kg/m3 at 5bar,200ᵒC)

3. Simplicity and alleviation of safety concerns due to low pressure and temperature

4. Low capital and maintenance cost due to use of non-eroding and non-corrosive working fluids

5. Availability of low grade heat sources when supplied by renewable energies

Importance Of Expander

• Key component of the DPG

• Plays a major role in determining the overall cycle efficiency

1. Velocity type: turbo expanders (Radial and Axial)

2. Displacement type: scroll, screw and reciprocal piston expanders

Radial Turbo Expander

• Radial turbo expanders offer many advantages over axial turbo expander and displacement type expanders

Simple structure and easier manufacturing (one-piece casting) compare to axial turbo expander (blades and disk)

Compact size due to greater specific power than equivalent axial stage (Euler turbomachinery equation)

High efficiency

Light weight

Aims And Objectives • Design and CFD analysis of a small size radial

turbine

• Applicable for distributed micro power generation systems with power capacity of 5kW

• Operating in organic Rankine cycle

• Suitable to be powered by low grade heat sources such as solar or geothermal energies in temperature range of 60-200ᵒC

Design Methodology Of The Radial Turbo-Expander

• Main goal is to minimize the losses and maximize the efficiency of turbine with following constraints

Geometric Physical Economic • This goal is accomplished by a systematic approach

consisting of two main phases:

Preliminary phase Detailed phase

Preliminary Design • Determines the overall characteristics and the performance levels • Highly iterative since it requires comprehensive trade studies of many

different designs by variation of large group of input parameters • 1-D code based on conservation of mass, momentum and energy and

Euler turbo-machinery equation and appropriate loss models • Mean streamline through the stage represents an average of the

passage conditions at each key calculating station

Detailed Design • Concentrates on 1 or small number of design candidates that offer the

optimum combination of features based on preliminary design results

• Investigates the aerodynamics of the flow field with much greater accuracy

• CFD analysis employed using ANSYS CFX (full three-dimensional Reynolds-Average Navier-Stokes equations with appropriate turbulence modeling)

Results- Preliminary Design Algorithm for systematic variation of input parameters

Table of variation range of input parameters

Parameter Range

Inlet Total Temperature(˚C) 60 – 200

Inlet Total Pressure (kPa) 150 – 400

Pressure Ratio 1.5 – 3

Mass flow rate (kg/sec) 0.03 - 0.1

Rotational speed (rpm) 40000 – 60000

Velocity Ratio 0.65 - 0.85

Inlet relative flow angle (degree)

-60 - -15

Exit absolute flow angle (degree)

-10 – 10

Results- Design Space

70

72

74

76

78

80

82

84

0.4

0.6

0.8

1.0

1.2

0.660.68

0.700.72

0.740.76

0.780.80

0.82

Ro

tor

tota

l to

to

tal

eff

icie

ncy(%

)

Spe

cific

Spe

ed(N

s)

Velocity Ratio(U/C)

Design Space

Each point represent an individual turbine design with different operating conditions and geometry

Results- Preliminary Phase

Variation of inlet total temperature and pressure

Variation of mass flow rate and pressure ratio

Results- Preliminary Phase

Variation of velocity ratio and rotational speed

Variation of rotor relative inlet and absolute exit flow angles

0

1

2

3

4

5

-65 -55 -45 -35 -25 -15

Po

wer

(kW

)

Rotor relative inlet flow angle(deg)

Alpha 2= 10 deg

Alpha 2= 0 deg

Alpha 2= -10 deg

0

20

40

60

80

100

-65 -55 -45 -35 -25 -15

Ro

tor

inle

t d

iam

eter

(mm

)

Rotor relative inlet flow angle(deg)

Alpha 2= 10 deg

Alpha 2= 0 deg

Alpha 2= -10 deg

72

74

76

78

80

82

-65 -55 -45 -35 -25 -15

Ro

tor

tota

l to

sta

tic

effi

cien

cy(%

)

Rotor relative inlet flow angle(deg)

Alpha 2= 10 deg

Alpha 2= 0 deg

Alpha 2= -10 deg

Results- Detailed Phase

• 3 different blade profiles were investigated with the aim of achieving appropriate blade loading and uniform flow

• The case with best blade profile was investigated for the appropriate number of rotor blades using CFD analysis

Parameter Selected Value

Inlet Total Temperature(˚C)

60

Inlet Total Pressure (kPa)

200

Pressure Ratio 2

Mass flow rate (kg/sec) 0.09

Rotational speed (rpm) 55000

Velocity Ratio 0.685

Inlet relative flow angle (degree)

-35

Exit absolute flow angle (degree)

0

Results- Detailed Phase-Variation Of Blade Profile

Case “a”

Total Power output= 3.871kW

Total Power output=3.834kW

Total Power output=3.791kW

Case “b”

Case “b”

Results- Detailed Phase-Variation Of Rotor Blade Counts

Z=15

Z=8 Z=12

Specifications Of The Proposed Radial Turbo-Expander

Parameter Unit Value

Power kW 4 Total to total isentropic efficiency % 85.3 Nozzle diameter at TE mm 94.8

Nozzle vane height mm 14.5

Nozzle throat area mm2 253

Nozzle blade inlet angle to radial degree 0

Nozzle blade exit angle to radial degree 75

Nozzle blade number - 27

Rotor inlet diameter mm 82.5 Rotor exit diameter at tip mm 53.6

Rotor exit diameter at hub mm 24.7

Rotor blade inlet angle to radial degree 0

Rotor blade inlet angle to axial at RMS

degree -70

Rotor blade number - 12

Conclusion • There is a need for designing a small scale radial turbo expander for

distributed micro power generation systems based on organic Rankine cycle

• Two techniques were employed as preliminary design phase and detailed design phase

• An algorithm was developed for the preliminary phase in order to explore

a large number of designs based on a parametric study to determine the best initial design for the system of interest

• Preliminary design tool does not provide adequate information regarding to the complex 3-D behavior of the fluid inside the expander

• CFD analysis tool was also employed as the detailed design tool to investigate in greater details the characteristics of design candidate that was recommended by the preliminary phase

• Turbine with efficiency of 85.3% , power of 4kW and rotor diameter of 8cm is suitable to be used for supplying energy demands in DMPG systems

Closing

• Thanks for listening and patience

• Any questions?