Tag Archives: Engineering

Thunderous applause

2 Replies

2d543f31-6f09-43ba-875c-c2d5d3bd0cebI have had cause to applause enthusiastically on two recent occasions. We went to see ‘Dead Dog in a Suitcase’ at the Everyman Theatre in Liverpool. It was fantastic and we joined in a standing ovation at the end. It’s a beggar’s opera that throughly deserved the rave reviews that it has received. It is full of energy, music, wit and spectacular performances.

The second occasion was my son’s graduation in Durham Cathedral. The programme asked us not to applaud as each graduand’s name was read out and they walked onto to the podium to shake the hand of the Chancellor, Sir Thomas Allen, but to hold our applause until the end. So, the last graduand walked off the podium to thunderous applause. Sir Thomas is an opera singer with a sonorous speaking voice and he gave a theatrical entertaining speech that we applauded enthusiastically and appreciatively. It is the tradition at Durham to applaud the new graduates as they walk down the aisle to leave the Cathedral. It’s a long aisle, there were a lot of graduates and we clapped energetically so that by the time the end of the line reached the door our hands were smarting.

You are right. There has been no mention of engineering, yet. However, here it comes. The heat and stinging sensation in the palms of my hands as the last graduate left the cathedral reminded me briefly of an example from my first year thermodynamics lectures in which I estimated the temperature rise in the skin of the hand from vigorous clapping ten times. This was more an exercise in estimating and problem definition than thermodynamics as you will see from the attached worksheet (clapping_example.pdf), but those skills are as important to an engineer as a knowledge of the laws of thermodynamics. Its also another Everyday Example and the experimental part can be performed at home.

Conflict Resolution

Leave a reply

conflict pyramidEngineers need to be experts in resolving conflicts…

Every man-made device that moves required energy to make it and uses energy when it moves. Heavier devices have greater inertia than small ones and hence more energy is needed to set them in motion – think about peddling an old-fashioned steel-framed bike compared to a modern alloy one. So, designing for sustainability requires engineers to minimise the quantity of raw materials and energy used to manufacture a device AND to minimize its weight if the device moves as part of its function.

Now, here comes the conflict.

Sustainability also implies that devices should have a long, maintenance-free service life so that resources used in maintenance and replacement are minimized. Service life is usually limited by fatigue and, or wear and the probability of these failure mechanisms occurring can be reduced by lowering stress levels. However, stress is inversely proportional to cross-section area and so can be reduced by adding material, i.e. increasing the mass of the device which will also increase its inertia, or resistance to motion. The probability of failure can be reduced by using stronger, more sophisticated materials that are lightweight and almost always more expensive, e.g. composites. Customers also want performance and additional expense might be acceptable if it is accompanied by additional performance – some people will pay for a carbon-fibre frame for their bicycle. Elegant engineering design requires resolution of the conflict between cost, safety and reliability, performance and sustainability.

This is why engineers are trained in conflict resolution or as it is more commonly known: problem-solving.

Energy efficiency

2 Replies

We were sent a summary of our annual gas and electricity consumption recently by our local utility company. The utility quantified our consumption of both gas and electricity in units of kilowatt-hours (kWh). It is usual to be sold electricity in kilowatt-hours but most people are confused by this unit. Perhaps because they learnt at school that the units of energy are Joules in the SI system and the power rating of appliances is usually given in Watts. They might know that a Watt is a Joule of energy per second, so what is a kilowatt-hour? Well it is about 3.6 x 106 Joules or 3.6 MJ, because it is 1000 Joules per second (= Watt) for one hour. So, I think the utility company should be telling me how many MegaJoules (MJ) we have consumed. After all we are used to seeing the energy content of our food quoted in kiloJoules (kJ), as well as calories.

The situation with our gas consumption is rather different because the utility does not supply energy but gas. The amount of energy that I get from it depends on what I do with it. If I burn it under conditions of constant volume, e.g. in a closed rigid container with exactly the correct concentration of oxygen then it will generate more energy in terms of heat than when it is burnt in constant pressure conditions, such as at atmospheric pressure in air. This is because in constant pressure conditions some of the energy released by combustion is used to expand the exhaust gases against the constant pressure, i.e. to do work, and only what is left is released as heat. So the utility should sell the gas by weight. If they sold it by volume then I would be paying more for the same amount of gas (i.e. number of hydrocarbon molecules) when the supply pressure was reduced.

Oil companies don’t sell gasoline or diesel in Joules for the same reason but they can sell by volume because it is always supplied to our cars at atmospheric pressure and the volume of a liquid is essentially constant.

We like to compare the efficiency of cars in terms of miles per gallon, or kilometres per litres. Efficiency can be loosely defined as what you want divided what you have to put in [See my post entitled ‘National Efficiency‘ on May 29th, 2013]. So for a car, what you want is kilometres travelled and what you put in is litres of fuel. However, when we are all driving plug-in electric cars then we will probably talk about how many kilometres per megajoule our car achieves [see my post entitled ‘Are electric car back?‘ on May 28th, 2014] . Unfortunately, while we are in transition with plug-in hybrids, car manufacturers like to quote very attractive kilometres per litre and ignore the electricity supplied via the plug – as if it were free!

Image courtesy KKN Liebstadt NPP from http://www.nucleartourist.com/systems/ct.htm

Watched kettle never boils

Leave a reply

boiling kettleThe phrase ‘a watched kettle never boils’, or a watched pot as Americans might prefer say, is a familiar phrase.  We have probably all stood waiting for water boil thinking it is taking a long time.  This might be in part because the rate of boiling does indeed slow down during the heating process and then speed up towards the end.

When an electric kettle is first switched on the element in the bottom of the kettle heats up causing heat to be transferred by conduction to the water.  The water adjacent to the element rises in temperature becomes less dense, moves towards the surface and transfers heat by natural convection to the contents of the kettle.  As the temperature of the water rises, tiny bubbles form on the element due to local boiling.  Bubbles are dislodged by new ones forming and float up to the surface giving the appearance that complete boiling is imminent.  However, as the temperature rises further the element becomes completely covered by a film of vapour that insulates the element from the water and slows down heat transfer to the water.  This delays boiling until the element has pumped enough energy (heat) into this film for heat transfer to occur across it from the element to the water. Sections of the film tend to break away and belch onto the surface of the water.  This process of large bubble formation and belching on the surface usually establishes itself fairly quickly once the first one has broken free and we see the familiar violent boiling of the kettle.

So the watched kettle has boiled but only after what might have seem like an interminable delay.  If you have a transparent electric kettle then you can watch this happen, otherwise you could watch a YouTube video – possible the most boring video on YouTube?

The process described above is known as the Liedenfrost effect and is illustrated graphically in the chart below, which is based on Figure 6.16 in ‘The Design and Simulation of Thermal Systems‘ by NV Suryanarayana and Oner Arici published by McGraw-Hill.  There are a number of more comprehensive explanations available, for example by Jearl Walker.  The Leidenfrost effect is also responsible for the way water disperses in liquid droplets across a very hot surface instead of evaporating as steam, see this Youtube clip for more explanation.

boiling graph