ANS:B - pool

In pool boiling, the heat transfer process occurs when a heated surface is submerged in a pool of liquid. Here's how it works:

1. Heat Input: Heat is applied to the submerged surface, raising its temperature above the saturation temperature of the liquid. This causes the liquid in immediate contact with the surface to reach its boiling point and form vapor bubbles.
2. Bubble Formation: As heat continues to be transferred to the liquid, vapor bubbles nucleate and grow on the heated surface due to the local temperature exceeding the saturation temperature. These bubbles are composed of the vaporized liquid.
3. Bubble Departure: Once the vapor bubbles reach a critical size, they detach from the heated surface and rise through the liquid. This process is known as bubble departure. As the bubbles rise, they carry latent heat away from the surface.
4. Liquid Replacement: As the vapor bubbles depart, fresh liquid flows in to replace the void left behind. This replenishes the liquid in contact with the heated surface and ensures continued heat transfer.
5. Cycle Continuation: The process of bubble formation, departure, and liquid replacement continues as long as heat is supplied to the surface and a temperature gradient exists between the surface and the bulk liquid.

Pool boiling is commonly observed in various applications such as boiling water in a pot, industrial boilers, and nuclear reactors. It is characterized by the presence of multiple nucleation sites on the heated surface, leading to the simultaneous formation of multiple vapor bubbles. This process can significantly enhance the overall heat transfer rate compared to single-phase convection.