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Tom-ClarkCTO

Green-Tide Turbines Ltd

Analyses And Explorations of Ducted Marine Turbines

ANSYS User Conference, November 2011 © Green-Tide Turbines Ltd 2011

Tuesday, 1 November 2011


Abstract• At G-TT we had a problem: For early stage design there are multiple

mechanical performance indices for ducted turbines. moreover, efficiency values can be misleading and poorly bounded (depending on the denominator of the performance metric). In order to create a concrete performance index, G-TT integrated cost and geometric models of a ducted turbine in order to improve the economic performance of the system.

• This presentation looks at the system used to achieve the optimisation, from creation of a fully parameterised geometry (with SolidWorks TM), analysis in ANSYS CFX to investigation and optimisation using ANSYS Design Explorer. the aim of this talk is to highlight the potential for effective use of ANSYS tools for the early-stage design process. in this case, that usage allowed optimisation of a product beyond just the mechanical performance - minimising the cost per unit energy (£/kWh).



Introduction• What is a ducted turbine?

– An overview of G-TT’s Mk.8 ‘Manta-Ray’ turbine– Early stage design: Ducted and Open turbines.– Duct and rotor design

• Writing a new rule book– Early stage parameter searches using DesignExplorer– An economic problem: Putting eggs in the correct basket


• Power from 12.8m diameter turbine = 360kW @ 2.4ms-1

– Equiv. power to Marine Current Turbines design at same size & speed


Mk. 8 ‘Manta-Ray’


Key Features (Mk. 8 Manta Ray)


Ducted Turbinefor improved fluid dynamic efficiency, robustness and acoustic signature

Fixed Pitch Rotorfor improved reliability, longer maintenance periods and reduced mechanical and electrical complexity

Statorfor improved fluid dynamic efficiency, debris resistance and reduced downstream wake deficit (closer spacing of turbines in a farm)

Low Speed, High Solidity Rotorfor improved robustness, marine life survivability and high fluid inertia (damping unsteady flow)



• Efficiency of a well-optimised ducted turbine can be 1.3x that of an equivalently optimised open bladed turbine.*

• Acoustic emissions are shielded, and can be altered through design (to meet legal limits) via system mass and internal structure

• Blade rows are more efficient (loaded out to the tip) - can afford to dispense with controllable pitch systems (high failure rates)

• Incredibly robust• Less sensitive to unsteady inflow than open-bladed designs• Cost effective

– Shorter blades -> parabolic relationship reducing blade stress requirements

– Designed for extremely flexible and easy deployment– No specialist equipment required for assembly or deployment

Benefits - Ducted Design

*C.J. Lawn [2003] ‘On the performance of ducted turbines’. Note that some items of literature give values much larger than this, but do not compare like-for-like...


• Much easier initial design!• There are many ways of

determining performance:– Momentum balance (Betz)– Streamline Curvature analyses

(GGS)– Lifting line– Lifting Surface / Panel methods...

• Viscous limits well understood (2D aerofoil theory)

• Lots of experimental verification/validation

• Well-defined performance metrics


Open Bladed Turbines









• Well-defined performance metricsSource: Gorban A.N., Gorlov A.M. and Silantyev V.M. (2001) Limits of the turbine efficiency for free fluid flow ASME Journal of Energy Resources Technology Vol. 123, p311

practice. For example, the unit cost of the Kaplan turbine jumpsby a factor of 4 when the water head falls from 5–2 m.The principal difference between exploiting high-head and free-

flow turbines is that the latter need large flow openings to captureas much water masses as possible with low velocities and pres-sure. Conventional turbines, in contrast, are designed for highpressure and relatively small water ducts where all water has nochance to escape the turbine installed in the dam body. Accordingto the Bernoulli theorem, the density of potential energy of flow isproportional to the pressure, while the density of the kinetic en-ergy is proportional to the square of velocity. Conventional waterturbines utilize mostly the potential component at the expense ofthe kinetic one. In order to do so, they need so-called ‘‘high so-lidity’’ where turbine blades cover most of the inside flow pas-

sage, resisting water flow and building up the water head. Thiscauses the fluid velocity to fall and the kinetic component of Ber-noulli equation to become negligibly small compared to the po-tential component. That is the reason why the higher water headscorrespond to higher efficiency of hydraulic turbines, an effi-ciency that comes close to 90 percent in some cases. However, thesituation is completely reversed for free water flows. In this case,the kinetic part dominates, and conventional turbines performpoorly, becoming very expensive.Unlike the commonly used wheel-type turbines, the Darrieus

reaction turbine for free flow, patented in 1931, has a barrelledshape with a number of straight or curved-in plane airfoil bladesand a shaft that is perpendicular to the fluid flow. This turbineallows high torque to develop in slow flows, maintaining a large

Fig. 1 Betz and GGS models for plane propeller in incompressible fluid flow—„a… Betz rectilinear flow model; „b… suggestedcurvilinear flow model „‘‘GGS’’ model…

312 Õ Vol. 123, DECEMBER 2001 Transactions of the ASME







• Well-defined performance metrics



ηBETZ =

Wx12 ρArU∞

3 ≤ 0.593



Performance Metrics• How can we assess mechanical performance?

ηBETZ =

Wx12 ρAROTORU∞

3 ηDUCT =

Wx12 ρADUCTU∞

3

ηCOMBINED =

Wx12 ρ(αAROTOR + βADUCT )U∞

3

(α + β =1)

?Tuesday, 1 November 2011


Viscous Performance• Stall curve of a duct difficult to model

– Like a 2D aerofoil, but...– Axisymmetric geometry– Pressure drop at rotor position (variable)– 3D effects (e.g. tip gap effects and tip vortices alter boundary layer

properties inside duct)– Hub interaction effects



Viscous Performance

X (mm)

Y (mm)

Calibration: Cal001C, Test #0007, Carriage Velocity = 2m/s

10005000500300

200

100

0

100

200

300

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0Normalised Axial Velocity (u/carriageVelocity), positive downstream

X (mm)

Y (mm)

Calibration: Cal001C, Test #0009, Carriage Velocity = 1m/s

10005000500300

200

100

0

100

200

300

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0Normalised Axial Velocity (u/carriageVelocity), positive downstream



So What?

What have all these problems got to do with ANSYS?

... and why am I at an ANSYS User Conference?



Key Point

We don’t know where in the design space to put our operating point, but...

Then we can:- Characterise design space without using general rules of

thumb or idealised models

- Investigate design space / optimise product using Cost Of Energy as a performance metric, instead of an arbitrary index of mechanical efficiency

Given that:– Simple CFD simulations are now cheap and quick (even on

desktop workstations)– We can build a cost model (lots of work, but relatively easy)



Ingredients...• DDS SolidWorks

– Definition of geometry

• ANSYS SolidWorks Geometry Interface– Handles geometry update / transfer

• ANSYS Design Explorer– Definition of parametric ranges– Response and characterisation of design space

• ANSYS Workbench– Overarching management of Geometries, CFD Simulations, Data, etc

• MATLAB– Rotor / stator design and analysis tools– [To Do] Cost model (object-oriented code allows bucketed design space)

• MS Excel– Cost model framework (Carbon Trust MEC for offshore installations)


Scope of ANSYS Workbench

Setup


Define and parameterise geometry (Named Selections)

Import / Update Geometry Meshing

CFD (with momentum drop zones etc)

Blade design & analysis

Integrate performance characteristic curve over tidal cycle (changing inflow speed)

Per-turbine energy output, peak capacity requirements and geometric constraints used to determine cost of energy (50MW tidal farm)

PARAMETERS Geometric Dimensions

Axial Loading (Rotor thrust distribution) CFD Results

RESPONSE LAYER (DesignExplorer): Performance anywhere in design space



In WorkBench



• Betz’s analysis gives an ideal distribution of axial load for an open turbine (over the blade length)...

• No such luck for ducted turbines: The optimal distribution is a function of the duct performance (governed by viscous behaviour and the chordwise lift distribution of the duct).

Load Distribution: Replacing Betz

• Here, Betz’s analysis is replaced with design exploration of the 3 parameters used to define the axial load distribution.



Understanding The System• The effect of changing flow regime



Summary• In applications where idealised models are limited (e.g. by invalid

assumptions such as inviscid flow), DesignExplorer can be used with simplistic, representative CFD / FEA models to understand and characterise the fundamental design space.

• Create your own preliminary design tools!

• We can use design exploration results as input to a cost (or other external performance) model – Especially where performance of a design cannot be described by a

single metric– Response surface from DesignExplorer prevents re-run of CFD

(thousands of evaluations when optimising a cost model, hundreds of thousands where Monte-Carlo or similar models are deployed!!)



Thank you - Questions?


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