Mechanical energy conversion

Drag

The force exerted by any moving fluid past any object

Lift

Created by a pressure difference between the opposing faces of the wing/sail

Limitations of drag turbines

The wind will hit the returning blade

Tip speed cannot exceed the tip speed

Vertical axis wind turbines

Limited by tip speed ratio

Difficult to control the speed of direction

Must be aligned to the direction of the wind

Can be simplified

Can be ground mounted

Horizontal axis wind turbine

Simple

Preferred for small scale applications where wind is unpredictable

Definitions of wind frontal area

Upstream

Where the wind hits the turbine

Downstream

Assumptions about wind energy

Conserved mass flow rate

Density in the air remains constant throughout

Why can the density be assumed constant?

The change in pressure upstream compared to downstream is minimal

What happens when an aerofoil is symmetric?

No lift, the path for the wind to follow above and below are the same, so we say the air speed is the same too (therefore same pressure)

What is the angle of attack?

The angle between the direction of the wind velocity and the aerofoils chord

How do aerofoils “work”?

Non identical pathways around the upper and lower edge

Pathway above is longer, so airspeed is greater

Pressure drops as air speed increases, so there is a lower pressure above and a lower pressure below

Assumptions about aerofoil dynamics

Unequal pathways is an oversimplification

There is no reason for the air to travel the upper edge rather than the lower edge.

Constrictive- one promotes the latter

What is the reality of aerofoil dynamics?

Air speed across the upper edge is faster than expected, therefore greater lift

What happens when we increase alpha?

Increases lift until seperation

What is separation?

Occurs when streamlines over the upper edge can no longer maintain the smooth pathway across the edge, creating vortices

FIrst occurs at the trailing edge 

What are vortices?

Create a region of high pressure, counteracting the pressure difference, reducing lift

What is the stall zone?

The region across which the aerofoil increasingly works in a manner that is not intended by its design

Affects the rate at which separation moves forwards

Tip speed ratio

Defines the relationship between the linear speed of the tip of a turbine blade and the wind speed

Assumptions for the Brayton cycle

Closed loop

Steady flow

What does a high work ratio mean?

The turbine is outputting high proportion of the turbines work

Methods to harvest the excess energy

Regeneration- Utilises a heat exchanger to use this energy to heat the air, prior to combustion

Combined power plant- Heat water and drive steam turbines, to generate more power

How do we optimise the gas turbine?

Generator at the front

Inlet not a diffuser

Large turbine- takes more energy- less KE out

Ideal brayton to real

Pressure drop

Work in is greater

Work out is less

Why is there a difference from ideal to real?

Irreversibilities