ANS:A - film

In the context of boiling, "film boiling" refers to a regime of boiling where a thin film of vapor forms between the heated surface and the bulk liquid. This phenomenon occurs when the temperature of the heated surface exceeds the Leidenfrost temperature, causing the liquid to boil vigorously and form a layer of vapor. Here's an explanation of film boiling:

1. Initial Stage: Initially, as the surface temperature rises, small bubbles of vapor form directly on the heated surface through nucleation. These bubbles detach from the surface and rise through the liquid.
2. Transition to Film Boiling: As the temperature of the surface continues to increase, the rate of bubble formation and detachment also increases. Eventually, the heat flux from the surface becomes so intense that a continuous layer of vapor forms between the surface and the liquid. This layer of vapor effectively insulates the surface from direct contact with the liquid, leading to a sudden decrease in heat transfer.
3. Characteristics: In film boiling, the formation of the vapor layer results in a sharp increase in surface temperature due to the reduced heat transfer. The vapor film is typically thin and unstable, leading to large fluctuations in the heat transfer rate and surface temperature. The vapor film also obstructs the transfer of heat from the bulk liquid to the heated surface, causing a decrease in overall heat transfer efficiency.
4. Potential Hazards: Film boiling can pose challenges in various applications, including heat exchangers and nuclear reactors, where maintaining efficient heat transfer is essential. It can lead to overheating of surfaces, reduced heat transfer rates, and potentially dangerous operating conditions if not properly controlled.