User:Dream mind/Sandbox/Boiling

When evaporation occurs at solid-liquid interface,it is termed as boiling.

Boiling occurs when the surface temperature of say a boiling pan exceeds the saturaion temperature of say water.

According to Newton's law of cooling,the heat transferred from the pan to the liquid is given by

$$q''=h(T _s-T _{sat})=h\Delta\ T_e $$

$$\Delta\ T_e\equiv T_s-T_{sat}$$ is termed as the excess temperature.

This process results in formation of bubbles at the walls which grow and finally the bubbles detatch from the walls.The growth and dynamics of the vapour bubbles depend on a)excess temperature b)nature of the surface and c)thermophysical properties of the liquid

Pool Boiling
Pool boiling occurs due to free convection and due to mixing by bubble growth and detachment.

Free convection boiling
Free convection boiling occurs when $$\Delta\ T _e\le \Delta\ T _{e,A}$$ where $$\Delta\ T _{e,A}\approx\ 5^oC$$

Nucleate boiling
Nucleate boling occurs when $$\Delta\ T_{e,A}\le\Delta\ T_e\le\Delta\ T_{e,c}$$

where $$\Delta\ T_{e,c}=30^oC$$

In this case the detection of the number of surface for the nucleation sites is important.Yamagata et al provided an expression to show the influence of nucleation sites on the heat rate.

$$q''=C \Delta\ T_e ^a n^b$$

where n is active nucleation sites per unite area i.e site density

$$a\approx\ 1.2 $$

$$b\approx\ 1/3 $$

Transition boiling
Transition boiling occurs when $$\Delta\ T_{e,c}\le\Delta\ T_e\le\Delta\ T_{e,D}$$

where $$\Delta\ T_{e,D}\approx\ 120^oC$$

Film boiliing
Film boiling occurs when $$\Delta\ T_e\ge\Delta\ T_{e,D}$$