What is an afterburner in a jet engine? What does it do?

 An afterburner is a component present on some jet engines, mostly those used on military supersonic aircraft. Simply put, an afterburner works by injecting jet fuel into the exhaust air and igniting it. The ignition of the jet fuel increases pressure within the exhaust of the engine, thus increasing thrust. When you see flames coming out the back of a jet engine like on military aircraft, this is because they have engaged their afterburner.

The F-15 has two engines which under normal power produce 14,590 pounds of thrust. The F-15 can not reach its maximum speed of Mach 2.5 using normal thrust. When the afterburners are in use the F-15's engines each produce 23,770 pounds of thrust. The afterburners can be used as long as there is fuel. The objective of an afterburner is to increase the thrust by about a factor of 1.5 to 2.0 for short periods.

Jet engine thrust is governed by the general principle of mass flow rate. Thrust depends on two things: the velocity of the exhaust gas and the mass of that gas. The afterburner increases thrust primarily by accelerating the exhaust gas to a higher velocity. While the mass of the fuel added to the exhaust does contribute to an increase in thrust, this effect is small compared to the increase in exhaust velocity. The purpose is to provide an increase in thrust, usually for supersonic flight, takeoff, and combat situations.

Afterburning significantly increases thrust without the weight of an additional engine, but at the cost of very high fuel consumption and decreased fuel efficiency, limiting its practical use to short bursts. An afterburner uses the unburned portion of the air passing through the engine it is attached to, to add an enormous amount of extra thrust. Most afterburners will burn substantially more fuel than the jet engines they serve, with fuel consumption tripling compared to normal operation.

Are nuclear power plants just a sort of steam engine?

 Essentially, yes.

The nuclear fission produces enormous amounts of heat and you use the heat to produce steam to drive a steam turbine which in turn drives the alternator.

It’s basically a conventional coal powered plant on steroids. A conventional coal powered plant uses coal to heat the water instead of nuclear fission.

Apart from photovoltaics, electricity is generated by driving the alternator ( electric generator) using a mechanical device such as a steam turbine, hydraulic turbine, gas turbines, diesel engines or IC engines or just about any device that will output mechanical energy.

A steam turbine converts thermal energy from high pressure and high temperature steam into mechanical energy which is outputted at the shaft of the turbine which in turn drives an alternator ( short for alternating current generator ) that produces electricity.

Similarly a hydraulic turbine converts hydraulic energy into mechanical energy that drives the alternator.

The difference between a coal powered plant and nuclear plant is how the water is heated and made into steam to drive the steam turbine. A nuclear plant uses nuclear fission which can generate enormous heat and that is used to generate the steam that drives the steam turbine. The only difference is the source of the heat for the steam.

And for people commenting that Beta Voltaics, Magneto Hydrodynamics, Fuel cells, batteries etc can produce electricity without a mechanical prime mover with the exception of photovoltaics, can any of them power an electric grid or produce enough power to do so? This specific answer pertains to power generation for an electric grid and other than photovoltaics, most others are incompabale of generating power to supply a grid and batteries are for low power devices ( cars with Li-Ion or NiMH batteries have to recharge through the electric grid mostly powered by conventional electric power generation and some from photovoltaics.