1.5 More on the applications of thermal energy transfer science

Focus on the humble 'thermos' flask, known as Dewar flask in science

See also More on methods of reducing heat transfer eg in a house and investigating insulating properties of materials

Thermal conductivity of good insulators OR good conductors

The thermal conductivity of a material is a measure of efficiently heat is transferred through a material by conduction.

Materials like metals are very good heat conductors and transfer thermal energy very quickly.

Materials like stone, brick, wood and concrete etc. are poor heat conductors and have low thermal conductivities.

Thermal conductivity data is important when considering the material required to fulfil a particular application e.g. in heating systems when in one situation you might want good insulation (e.g. in loft) and in another rapid heat transfer (copper piping inside a hot water tank).

The design of a vacuum flask and other examples of a 'thermos flask'

The 'thermos' vacuum flask is a container designed to keep hot liquids hot and cold liquids cold. Diagram on the right.

It is designed to minimise thermal energy (heat) transfer by conduction, convection or radiation, both in an out of the container.

The flask is double walled with a vacuum (of nothing!) between the walls.

The vacuum ensures there is no thermal energy transfer by conduction - no material to vibrate.

If the double-walled flask is made of glass, the inner surfaces exposed to the vacuum are silvered to reflect back any thermal radiation (infrared).

If the flask is steel, the surfaces are shiny and reflect infrared in the same way.

The top cap should be a poor conductor of heat energy (thermal insulator), and is often made of plastic or incorporates a plastic seal.

Both double walls and the cap help minimise losses by convection. The design ensures no air can pass over any surface that is in contact with the fluid.

Insulated cups, flasks and jugs come in all sizes - illustrated by the pictures below.

 

Very low temperature storage flask use

Note that in science 'thermos flasks' (Dewar vacuum flasks) are used to keep liquid mixtures cold, based on the same principles of minimising thermal energy transfer by conduction, convection and infrared radiation e.g. liquid nitrogen at -196^oC !!!

These flasks are used to store delicate biological material including embryos.

This technology is referred to as 'cryogenic science'

 

Use of 'thermos' type flask technology in the kitchen

Left picture:

A double walled plastic jug and several steel vacuum flasks

Right picture:

As above plus on the left a plastic insulated coffee cup, sometimes with an added cardboard hand holder for extra insulation.

Your knowledge of examples of heat transfer situations should include the ...

The design of a vacuum flask

How to reduce the energy transfer from a building

How humans and other animals cope with low temperatures by ways of increasing thermal insulation - adaptations.

See Homeostasis - thermoregulation, control of temperature