When vaporisation takes place directly at the heating surface, it is called

ANS:B - nucleate boiling

When vaporization takes place directly at the heating surface, it is called: Nucleate boiling. Explanation:

1. Film boiling: Film boiling occurs when a layer of vapor forms between the heating surface and the liquid, preventing direct contact between the heating surface and the liquid. This typically happens at very high heat fluxes or when the temperature of the heating surface is significantly higher than the boiling point of the liquid. It is characterized by a vapor film that insulates the surface, leading to reduced heat transfer rates compared to nucleate boiling.
2. Nucleate boiling: Nucleate boiling is characterized by the formation of small bubbles of vapor directly on the heating surface. These bubbles form due to localized heating at irregularities or nucleation sites on the surface. Nucleate boiling is the most common form of boiling and typically has higher heat transfer rates compared to film boiling due to direct contact between the heating surface and the liquid.
3. Vapor binding: This term does not describe a recognized phenomenon in the context of boiling or heat transfer. It is not related to the process of vaporization taking place directly at the heating surface.