In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.

ANS:B - local

1. Nucleate boiling:
   • Nucleate boiling occurs when the liquid temperature is below its saturation temperature, but there are localized regions on the heated surface where boiling occurs.
   • At these nucleation sites, small bubbles of vapor form due to the intense heat flux from the surface to the liquid.
   • These bubbles then detach from the surface and rise through the liquid, carrying heat away from the surface.
   • Nucleate boiling is characterized by the presence of many small bubbles, which provide efficient heat transfer due to the increased surface area available for vaporization.
   • It is commonly observed in various industrial and everyday applications, such as in boilers, heat exchangers, and cooking processes.
2. Local boiling:
   • Local boiling refers to a situation where boiling occurs only in specific regions or spots on the heated surface.
   • This can happen due to localized heating, surface irregularities, or impurities on the heated surface.
   • Local boiling can result in uneven heat transfer and potential hot spots on the surface.
   • It is often observed in situations where the heating is non-uniform or where there are surface imperfections.
3. Pool boiling:
   • Pool boiling occurs when a liquid is heated in an open container or pool, and boiling takes place over a large area of the liquid surface.
   • It is commonly encountered in everyday situations such as heating water in a pot on a stove.
   • In pool boiling, bubbles form at various locations on the heated surface and rise through the liquid, carrying heat away from the surface.
   • Unlike nucleate boiling, where boiling occurs at specific nucleation sites, pool boiling involves boiling over a broader area.
   • Pool boiling can exhibit different regimes, including nucleate boiling, transition boiling, and film boiling, depending on the heat flux, liquid properties, and surface characteristics.
4. Saturated boiling:
   • Saturated boiling occurs when the liquid temperature equals its saturation temperature at a given pressure.
   • At saturation temperature, further addition of heat results in the phase change of the liquid to vapor without any increase in temperature.
   • Saturated boiling is often used in thermodynamic and heat transfer analyses as a reference condition.
   • It is important in the design and operation of various systems such as steam generators, refrigeration systems, and heat exchangers.