Thermal conductivity of a material may be defined as the
* quantity of heat flowing in one second through a slab of the material of area 1 cm² and thickness 1 cm when its faces differ in temperature by 1 Degree Centigrade
* quantity of heat flowing in one second through one cm of material when opposite faces are maintained at a temperature difference of 1 Degree Centigrade
* heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

Gas liquefaction is physical conversion of a gas into a liquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures. Air refrigeration cycle is used in gas liquefaction process.

The important dimensionless numbers in fluid mechanics are
1. Reynold’s number
2. Froude’s number
3. Weber’s number
4. Euler’s number
5. Mach’s number or Cauchy’s number

Reynold’s number: Reynold’s number is the ratio of inertia force to the viscous force.

Froude’s number: Froude’s number is the ratio of inertia force to the gravity force.

Weber’s number: Weber’s number is the ratio of inertia force to the surface tension force.

Euler’s number: Euler’s number is the ratio of inertia force to the pressure force.

Mach’s number or Cauchy’s number: Mach’s number or Cauchy’s number is the ratio of inertia force to the elastic force.