MECH0024: Advanced Thermodynamics and Fluid Mechanics
Advanced Thermodynamics and Fluid Mechanics
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MECH0024: Advanced Thermodynamics and Fluid Mechanics
Thermodynamics
Attempt ALL questions. Where stated do not exceed maximum word count.
1.
The urgent need to decarbonise power networks and reach net-zero has seen the
decommissioning of coal-fired thermal power stations in the UK. There is therefore a
need to provide alternative generative capacity to ensure continuity in the UK power
supply.
a) One proposal is for the construction and operation of a 1000 MW nuclear power
plant. What specific issues should be considered in the undertaking of an
Environmental Impact Assessment of this proposal (max. word count 300)?
[10 marks]
b) An alternative proposal is for the construction of a large array of solar collectors.
Determine the total area of collectors required to receive 3 MW of solar incident
radiation at a latitude of 50.7° in the northern hemisphere at 5 pm on a day when the
angle of the sun’s declination is -2°. Assume that the collectors are orientated to be
normal to the sun at noon on a day when the sun’s angle of declination is 6°. Clearly
state any other assumptions made in determining the total area required. The
expression for the intensity of radiation normal to the Earth’s surface is given at the
end of this question and the Solar flux, , is found by the following equation:
= 1.353(0.057 + 0.83 exp(−0.5)) kWm−2
where is the air mass ratio.
[10 marks]
Useful Information: The expression for the intensity of solar radiation normal to the
Earth’s surface is
= cos sin sin + sin cos
where the symbols take their usual meanings.
c) On the basis of your answers to parts a) and b) of this question, and considering
end-use demand, would you recommend that either proposal proceed (max. word
count 150)?
[5 marks]
2.
Thermal power plants are used throughout the world for electricity generation,
representing significant consumers of primary energy and emitters of greenhouse
gases.
a) A thermal power plant in operation provides steam to a high-pressure turbine from
a boiler at a steady flow of 20 kg/s, pressure of 60 bar and a temperature of 450 °C,
while the surrounding air is at 25 °C and 1.0 bar. Determine the steady flow exergy
of the steam entering the turbine.
[4 marks]
b) Following expansion, the steam leaves the turbine at a pressure of 1.1 bar, 111.4
°C and at a dryness fraction of 0.95, while the surrounding air is at 25 °C and 1.0
bar. Calculate the exergetic efficiency of this process assuming a loss of heat from
the turbine of 12 kW.
State with brief justification whether you would advise attempting further energy
recovery from the steam following expansion through the high-pressure turbine.
[8 marks]
c) Following the turbine, the external surface of the pipe conveying steam at 1.1 bar
and 111.4 °C is not insulated and is exposed to the surrounding air. Given that the
pipe has an external diameter of 0.4 m, determine the sum of the radiative and
convective heat losses for the first 1 metre length of the exposed pipe.
Assume that the emissivity of the pipe surface is 0.2, the Stefan-Boltzmann constant
is 5.67E-11 kW/m2K4, and that the convective heat transfer coefficient between the
pipe surface and surrounding air is 0.00272 kW/m2K. State any further assumptions
made.
[7 marks]
d) Excluding nuclear energy, identify two renewable primary energy sources suitable
for thermal power generation and briefly discuss the relative advantages and
disadvantages of each (max. word count 200).
[6 marks]
3.
Increased awareness of the effect of combustion emissions on air quality and public
health has seen the introduction of low emissions zones in cities globally, including
London. To reduce exhaust emissions of pollutants two options are the use of after-
treatment devices and the utilisation of oxygenated fuels.
a) A three-way catalyst is an exhaust after-treatment device used to reduce emissions
of carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons from spark ignition
internal combustion engines. Describe how the simultaneous reduction of all three
pollutants is achieved when the engine is operated with a stoichiometric air fuel ratio.
[6 marks]
b) Find the gravimetric air fuel ratio of a direct injection spark ignition engine which is
combusting bio-ethanol, assuming that the combustion is stoichiometric, and that the
intake air consists of only nitrogen and oxygen. By what percentage would the fuel
mass flow rate supplied have changed if the same engine is now combusting
renewably produced methanol? Clearly state any further assumptions made and
comment on the practical implication of your result.
[11 marks]
c) The same spark ignition engine continues to be fuelled with methanol but at a
gravimetric air to fuel ratio of 11:1. How would you expect the levels of pollutants in
the engine exhaust gases to change in abundance relative to the operation of the
engine with a stoichiometric air fuel ratio (max. word count 250)?
[8 marks]
Useful information
Molecular composition of ethanol = C2H6O
Molecular composition of methanol = CH4O
Volumetric composition of air = 21 % O2 and 79 % N2
Atomic mass values:
Carbon [C] = 12
Oxygen [O] = 16
Hydrogen [H] = 1
Nitrogen [N] = 14
4.
Solid polymer H2O2 fuel cells (also known as proton exchange membrane fuel cells)
are seeing increasing utilisation in road transport applications, especially in densely
populated urban environments.
a) With the use of a sketch, state the reactions occurring at the fuel cell electrodes,
and apply the steady flow energy equation to show how ideal power out from a fuel
cell can be related to the change in Gibbs function of the reactants.
[6 marks]
b) An inner-city bus with a peak power requirement of 340 kW is powered by a
hydrogen oxygen fuel cell. Given that reactions within the fuel cell are occurring at 450
K and the following data, calculate the actual output voltage of the cell:
Δ (change in Gibbs Function) = -176,200 kJ/kmol of H2
Δ (change in reaction) = -244,800 kJ/kmol of H2
Faraday’s constant = 96.5 x 106 C/kmol
[3 marks]
c) Given that the maximum current loading on the anode of the fuel cell is 2.7
mA/mm2 find the size of the anode necessary to meet the peak power requirements.
[4 marks]
d) Given that the molecular weight of H2 is 2.016 g/mol, determine the mass flow rate
of H2 necessary for meeting of the peak power requirement.
[2 marks]
e) Discuss current options for the provision of H2 for the fuel cell powered bus
described in part b) of this question, considering both on-board storage and the
wider infrastructure necessary (max. word count 300).