Vaporization (evaporation) is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. See also: Vaporizer.
The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.
If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapour increases. When the process of escape and return reaches an equilibrium, the vapour is said to be saturated, and no further change in either vapour pressure and density or liquid temperature will occur.
Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the entropy change for evaporation (ΔHevaporation) is always positive.
Forced evaporation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component (e.g. water), leaving behind the dry, less volatile, component.
It is a misconception that at 1 atm, water vapour only exists at 100°C. Water molecules are in a constant state of evaporation and condensation flux near the surface of liquid water. If a surface molecule receives enough energy, it will leave the liquid and turn into vapour pending an allowable vapor pressure. Under a pressure of 1 atm, water will boil at 100°C.
Factors influencing rate of evaporation
The fuel droplets vapourise as they receive heat by mixing with the hot gases in the combustion chamber. Heat(energy) can also be received by radiation from any hot refractory wall of the combustion chamber.
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