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3.2

## University of Liverpool

Skip to 0 minutes and 2 seconds [HAIR DRYER RUNNING] This is good fun. If you haven’t done it before, then it’s something you can easily do at home. And it’s a simple demonstration of jet power. Inside the hair dryer, there’s a fan and a source of heat. It’s similar to a jet engine that sucks, squeezes, bangs, and blows air into a jet engine. We’re rather more interest in the force at which the air leaves the engine. And so, we provide a large amount of heat energy via a bang that’s an explosive combustion of paraffin. [JET FLIES BY]

Skip to 0 minutes and 40 seconds We can use mechanics to analyse the jet engine. Newton’s Law tells us that, for every action, there is an equal and opposite reaction. So the hot, high-velocity air leaving the back of the jet exerts a force on the surroundings, which react and push the airplane forward. However, since this is a course in thermodynamics, I would like to look at the process in terms of energy transfers and flows. Let’s go back to the hair dryer. A fan sucks air into the device. This requires work to be done, which is supplied through the electric motor in the hair dryer. Then we transfer heat to the air from the electric heating element. And finally, we blow the hot air out.

Skip to 1 minute and 27 seconds As the air is blown out, we squeeze it through a nozzle to increase its velocity at the expense of pressure. If we look at the energy transfers occurring during the operation of the hair dryer, then we have work being done by the electric motor driving the fan. We have thermal energy being supplied by the heating element. And finally, the air flows out with a higher energy than it flowed in.

Skip to 1 minute and 57 seconds Last week, we looked at the relationship between work and energy. This week, we’ll explore heat transfer mechanisms and the energy associated with flows. To analyse such processes as in the hair dryer and the jet engine, we’ll use a special form of thermodynamic system known as a control volume. A control volume has permeable boundaries across which matter flows in and out. So we could define our system as the volume occupied inside the hair dryer. Air flows in across one boundary and at a low energy level, and then flows out the other end at a higher energy level. Work is supplied by the fan and heat by the element.

Skip to 2 minutes and 47 seconds Of course, the other common feature is that both hair dryers and jet engines make a lot of noise. This is energy being dissipated and lost, or wasted. We talk about energy dissipation and losses next week. In the meantime, if you want to learn more about jet engines, then search YouTube for “How Does a Jet Engine Work?” More on that later.

# Heat and mass flows

What is the connection between balancing a table tennis ball in the hot air stream from a hair dryer and shouting into a hairy microphone while a jet aircraft roars overhead? Eann explains that the hair-dryer and jet engine are both control volumes involving heat and mass flows.