Numerical Simulation of a Cross Flow Marine Hydrokinetic Turbine

Taylor Hall

Alberto Aliseda

University of Washington Mechanical Engineering

Northwest National Marine Renewable Energy Center (NNMREC)

Introduction

• Tidal energy resource realized

• Renewable

• Clean

• Predictable

• Many similarities to wind energy

Source: windows2universe

Axial Flow Turbines Cross Flow Turbines

Turbine Classifications

Source: Antheaume - Hydraulic Darrieus turbines efficiency for free fluid flow conditions versus power farm conditions

Turbine Design Concepts

Source: Department of Energy

Cross Flow Turbine Advantages

• High energy density typically found in narrow constricted channels

• Packing critical to efficiency and economic feasibility

• Cross flow turbines can be stacked, optimally utilizing limited space

• Work in any direction of flow

Source: Zanette - A design methodology for cross flow water turbines

Helical Cross-Flow Turbine

• Micropower Generation Project as benchmark study

• 4 helical blades, each with 90 degree sweep: Gorlov turbine

• Initial CFD simulation: flow over single helical blade in static position

Experiment/Simulation Parameters

• ReC =𝑉∞𝐶

𝜈= 28,000

• Aspect Ratio =𝐻

𝐷= 1.4

• Blockage ratio= 2𝑅𝐻

𝐶ℎ𝑎𝑛𝑛𝑒𝑙 𝐴𝑟𝑒𝑎 = 0.12

• Solidity Ratio =NC

2π𝑅= 0.075

• Tip Speed Ratio =ωR

V∞= 0

• Fluent v12.0

• Reynolds-Average Navier- Stokes (RANS) equations

• SST-kω turbulence closure model

• Transient simulation: Δt=0.5 sec

Turbine and Channel Flow Numerical Modeling

Discretized Domain • Gambit 2.4.6 • Domain sized to represent

experiment flume • 185,000 Elements • Same mesh for all blade

positions

Discretized Domain

o Structured in most volumes o Wall Functions

• 30<y+<300

•𝐹𝑖𝑟𝑠𝑡 𝐿𝑒𝑛𝑔𝑡ℎ

𝐶ℎ𝑜𝑟𝑑 𝐿𝑒𝑛𝑔𝑡ℎ= 0.15

First Length

Source: Antheaume - Hydraulic Darrieus turbines efficiency for free fluid flow conditions versus power farm conditions

Static Torque for a Single Blade

𝐶𝑇 =𝑇

12 𝜌𝑉∞

2𝑆𝑟𝑒𝑓

Flow fields for angles of attack (α) α=5⁰ α=60⁰

α=205⁰ α=25⁰

Static Torque for a Single Blade

𝐶𝑇 =𝑇

12 𝜌𝑉∞

2𝑆𝑟𝑒𝑓

Modeling in Near Wall Region Wall Functions

• 30< y+ < 300

•𝐹𝑖𝑟𝑠𝑡 𝐿𝑒𝑛𝑔𝑡ℎ

𝐶ℎ𝑜𝑟𝑑 𝐿𝑒𝑛𝑔𝑡ℎ= 0.15

• Total Elements = 185,000

Near Wall Approach • y+ < 1

•𝐹𝑖𝑟𝑠𝑡 𝐿𝑒𝑛𝑔𝑡ℎ

𝐶ℎ𝑜𝑟𝑑 𝐿𝑒𝑛𝑔𝑡ℎ= 0.0001

• Total Elements = 4.0 million

Near Wall: 20x computation time, 20-25% reduction in error

Near Wall Simulations

𝐶𝑇 =𝑇

12 𝜌𝑉∞

2𝑆𝑟𝑒𝑓

Sliding Mesh Stationary Domain Rotating Domain:

Tip Speed Ratio=3.6

Summary • Research into helical cross flow hydrokinetic turbines

• Validation of numerical simulations with laboratory experiments

• Simulation scaled to match laboratory experiments

• Investigated start up torque characteristics

– Many simulations in good agreement with experiments

– Positions with discrepancies to be improved

• Working toward simulation of steady state operation for rotating turbine blade