Thermodynamics

What is the Clausius statement of the second law?

It's impossible to construct a heat engine which operates in reverse without a network input from the surroundings

what is the Kelvin-Plank statement of the second law?

It is impossible for any heat engine to receive heat from a single thermal reservoir and to produce an equivalent quantity of work

What is the first law of thermodynamics?

Q12-W12= E2-E1

What is a system?

a fixed quantity of material

What is the difference between control volume and system?

For control volume material crosses the boundary

what is rp for a typical open-cycle gas turbine?

10 to 30

what is the efficiency for a typical open-cycle gas turbine?

20-40%

1-2 compression stroke in a real otto cycle

compression of air and petrol vapour mixture

2-3 in a real otto cycle

spark ignites mixture, increasing pressure

3-4 power stroke in a real otto cycle

expansion of combustion products

4-1 in a real otto cycle

exhaust stroke, induction stroke

why might engine power be less than expected?

air breathed by the engine is at a lower density than the ambient air

1-2 compression in a real open cycle gas turbine engine

compression of air in compressor

2-3 in a real open cycle gas turbine engine

combustion of fuel in chambers, small pressure drop

3-4 expansion in a real open cycle gas turbine engine

expansion of combustion products in turbine

stage 1-2 of the joule cycle (compressor)

isentropic compression

stage 2-3 of the joule cycle (combustor)

isobaric heat addition

stage 3-4 of the joule cycle (turbine)

isentropic expansion

stage 4-1 of the joule cycle

isobaric heat rejection

why is stage 2-3 modelled as isobaric in a diesel cycle?

Combustion is slow compared to the movement of the piston

what happens to joule cycle efficiency when pressure ratio increases?

increases

When is specific net work in the joule cycle a maximum?

at medium pressure ratio

what is the difference between a jet engine and joule cycle?

in a jet engine the turbine provides only enough work to drive the compressor

what is the dew point temperature of a gaseous air/water vapour mixture?

The temperature which corresponds to the saturation temperature at the partial pressure of the water vapour

gibbs-dalton law

Each species in a gas mixture occupies the same volume and has the same temperature as the mixture

What's the worst COP for a refrigerator?

0

What's the worst COP for a heat pump?

1

Steps 1-2 of VCR (compressor)

Isentropic compression

steps 2-3 of VCR (condenser)

isobaric condensation

Steps 3-4 of VCR (valve)

Valve

steps 4-1 of VCR (evaporator)

Isobaric evaporation

how does Q being negative affect entropy?

Decreases entropy

how does Q being positive affect entropy?

Increases entropy

why are two stage cascade cycles used?

to improve COP

heat pump pros

Low CO2 emissions, low running costs

heat pump cons

High capital costs, efficiency drop in cold temperature

how do air source heat pumps work?

Use heat from the outside to heat an indoor space

how do ground source heat pumps work?

Absorb heat from the ground

Pros of ground source heat pumps

high COP

cons of ground source heat pumps

expensive

Two conditions to liquify a gas

Reach temp below Tcrit and a state under the saturation line

two methods to liquify a gas

High pressure, three stage cascade refrigeration plant

what is the linde process?

gas is compressed, cooled and throttled

why is gas behaviour real in the linde process?

Throttling has to cause a temperature drop

Turbomachine definition

A device that transfers energy between a rotor and a fluid due to dynamic action

What two ways can turbomachines be classified?

Fluid type, flow direction

What does a turbomachine stage consist of?

Rotor row, stator row

When is stagnation enthalpy constant?

Any process that doesn't involve heat or work transfer

Condition needed to define a stagnation state

Isentropic process

How do internal irreversibilities affect stagnation pressure?

Reduce stagnation pressure

What is the requirement for a real gas to be liquefied by throttling in a Linde process?

the Joule-Thomson coefficient must be greater than 0

what is the absolute reference frame in turbomachinery?

fixed in relation to stator

what is the relative reference frame in turbomachinery?

fixed in relation to the rotor

Reaction definition

the ratio of static enthalpy change between in rotor to stagnation enthalpy change in stage

pros of impulse turbine

smaller tip leakage

pros of reaction turbine

smaller friction loss

cons of impulse turbine

higher friction loss

cons of reaction turbine

Higher tip leakage

why are many stages for a reaction turbine in a compressor needed for an effective pressure ratio?

If the reaction is too high it can cause boundary layer separation