Questions
Quick questions
You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!
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The Reynolds number is a measure of:
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The terminal velocity of a particle falling under gravity in a fluid is:
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The Stokes number is a measure of:
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The (specific) permeability of a porous medium is:
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The sintering process that leads to a relatively loose assembly of powder particles being transformed into a material with acceptable strength and stiffness:
Deeper questions
The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.
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It is proposed that a new type of Diesel Particulate Filter (DPF), with a very high efficiency for extracting fine carbon particles, could be made by creating a bonded assembly of fine alumina fibres (diameter ~0.1 μm). Use the Carman-Kozeny equation to estimate the expected permeability of such a material, assuming that it will be 80% porous. If the maximum gas flux through the DPF is 500 m3 hr-1, its surface area is ~1 m2 and its wall thickness is ~1 mm, estimate the pressure drop across it and comment on whether this value is likely to be acceptable for satisfactory operation of the engine. [The viscosity of the exhaust gas at the temperature concerned is about 3 10-5 Pa s.]
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It is proposed that thermal spraying will be used to create an alumina (MPt. ~2040˚C) coating on a metal artefact, using a combustion torch with a flame temperature of 2300˚C. The alumina powder to be used has an average particle size of about 20 µm and the heat capacity of alumina c, is ~3 106 J m-3 K-1. The stand-off distance to be used is 400 mm and the gas velocity is 200 m s-1. The interfacial heat transfer coefficient under these conditions is estimated to be 10 kW m-2 s-1. Assuming that injected particles reach the gas velocity very quickly, estimate their temperature at impact and hence decide whether the spraying process is likely to be successful.