Category Archives: Engineering

Sizzling sausages

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130-3071_IMGIn my post on 19th June 2013 [Closed system on the BBQ], I discussed the thermodynamics of sausages cooking on a barbeque in the context of the first law of thermodynamics.  This is an everyday example of engineering principles [see my post entitled ‘Bridging cultures’ on June 12th, 2013].  I mentioned that the energy gained by a sausage causes it to be cooked and for the water-content to boil as the temperature is raised.  The rise in temperature causes the pressure inside the sausage to increase, which is Gay-Lussac’s law in action.  When the water-content of the sausage starts to boil, the steam produced raises the pressure even further providing the sausage skin remains impervious to the transfer of matter, i.e. the steam.  The sausage as a closed system that becomes a miniature pressure vessel.

Pressure vessels fail as a result of the stresses in their wall.  In engineering, stress is defined as force divided by the area of  material carrying the force.  My sausages always fail longitudinally, i.e. they burst open with splits running along their length.  This is because the stress across the split, known as the circumferential or hoop stress, is the largest stress in the skin.

It is relatively simple to use Newton’s Third Law, about there being an equal and opposite reaction for every action force, to show that the circumferential stress is larger than the longitudinal stress; but it is a level of detail beyond what I feel is appropriate here.  Bursting sausages are a good illustration of Everyday  Examples of Engineering, which became the ‘poster-child’ of the NSF-funded project that developed them in the USA .  The pedagogy underpinning the use of Everyday Examples is explained in detail in a paper in the European Journal of Engineering Education (vol 36, pages 211-224, 2011) and a 5Es lesson plan is available here [for more on 5Es lesson plans see my post entitled ‘Disease of the modern age’ on June 26th, 2013].

You can see a video of me talking about these sausages at http://www.youtube.com/watch?v=nsSxKuRo4H0

EJEE paper: http://www.tandfonline.com/doi/abs/10.1080/03043797.2011.575218#.UbG9TZyPMx4

Impossible perfection

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Carnot's equation for ideal efficiency of a cyclic device converting heat to work and operating between two temperatures specified on the Kelvin scale

Carnot’s equation for ideal efficiency of a cyclic device converting heat to work and operating between two temperatures specified on the Kelvin scale

In my last post [National efficiency on 29th May, 2013] I calculated the efficiency of the nationwide process of electricity generation in the UK [35.8%] and made no comment on the relatively low value.  It will be similarly in all industrialised countries as a consequence of the second law of thermodynamics and the requirement for all real processes to increase entropy.  A French engineer / scientist, Sadi Carnot [1796-1832] demonstrated from the second law, that the maximum efficiency achievable in ideal conditions by a process operating in a cycle to convert heat into work is a ratio of the temperatures of the heat source and cold sink to which excess heat is dumped.  In a power station the heat source might be a fossil-fuelled furnace, a nuclear reactor or a solar concentrator.  The cold sink is usually the environment, perhaps in the form of river or sea water.  So both source and sink temperatures are limited.  The sink by the local climate and the source by the temperatures that modern materials can withstand.

The very best efficiency based on Carnot’s expression for a maximum material temperature of 350 degrees Centigrade [=623 Kelvin] and environment temperature of 5 degrees Centigrade [278 Kelvin] is 55%.  Of course a real power station will never operate at this level because ideal conditions are not achievable – perfection is impossible.

The ideal efficiency improves as the operating temperatures of the heat source and sink are moved further apart and this quest to raise this temperature difference drives a substantial proportion of materials research.  However, even operating with a heat source at 800 degrees Centigrade, using expensive, high temperature alloys, such as Hastelloy N  [a nickel-chromium alloy], on a winter day in the Canadian capital, Ottawa where the average January daytime temperature is -7 degrees Centigrade, the Carnot efficiency of a power station would be only 75%  [=1-(266/1073)].

Energy diversity

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Probably most people never give a thought to where the power comes from to switch on the light or their TV.  Engineers are primarily responsible for ensuring that the right number of power stations are available to supply exactly the right amount of electricity to match demand.  If supply exceeds demand then energy needs to stored, for instance at the Dinorwig storage scheme [ http://www.fhc.co.uk/dinorwig.htm ]; however if demand exceeds supply then someone’s lights will dim or go out until an additional power station can be switched on or the output increased from one that is running.  The latter is a relatively quick process but switching on a power station takes longer than half time in a televised football match when everyone switches on the kettle or makes some toast.

You can see how national demand in the UK varies in real-time at the National Grid website [ http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Demand/demand24.htm ].  There is a similar “national electricity meter”  for Spain  [ https://demanda.ree.es/demandaEng.html ], which also shows the blend of energy sources being used.

Blending sources such as fossil fuels, hydro, nuclear, solar, tidal and wind is the key to a cost-effective sustainable energy supply with the diversity to adapt to unexpected circumstances.

Noise transfer

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This is not the author's house!

This is not the author’s house!

We are privileged to have magnificent views of the river and mountains beyond from our city centre house.  However, the house was built before the motor car was invented when the loudest event outside might have been rowdy party-goers heading for home.  We still have some party-goers walking home under our bedroom window at night but most of them travel by noisy taxis.  I look forward to when the price of fossil fuels, or legislation will force taxis to become electric-powered.  In the meantime, we have been designing secondary glazing that will offer a high resistance to noise transmission and be in keeping with the early 19th century windows.  Noise is a form of energy transfer by vibrations, acoustic energy would be an alternative term for it, and so the combined resistance of the outside wall of my bedroom can be calculated using Kirchhoff’s law, as discussed for heat transfer in my last post [Born in a barn, 20th March, 2013].  In this case, the thin and badly-fitting but antique glass is the dominant component of both the heat and noise resistance.  We were happy to deal with the poor resistance to heat transfer by using plenty of bedclothes, i.e. adding a large resistance in series, but the same approach does not work with noise because earplugs are uncomfortable, fall out in your sleep and have a low resistance at the frequency of taxi-generated noise.  So, the solution is secondary glazing and the best performance is achieved using an acoustic laminate consisting of a polymer sandwiched between two sheets of glass which should be different thickness to avoid resonant effects.  Of course this will also improve the resistance to heat transfer which will be advantageous in winter, but perhaps not in summer…