Boiling regimes — When vaporization initiates directly at the heating surface with discrete bubbles forming on nucleation sites, the phenomenon is called:

Introduction:
Boiling heat transfer exhibits distinct regimes as surface temperature rises: natural convection, onset of nucleate boiling, fully developed nucleate boiling, transition boiling, and film boiling. Recognizing the regime is critical for predicting heat-transfer coefficients and avoiding burnout.

Given Data / Assumptions:

• A heated surface in contact with a saturated liquid.
• Surface has micron-scale cavities acting as nucleation sites.
• Moderate temperature excess above saturation temperature.

Concept / Approach:
In nucleate boiling, vapor bubbles form at nucleation sites on the surface, grow, detach, and collapse in the bulk liquid. This regime provides very high heat-transfer coefficients due to vigorous mixing. At higher temperature excess, a continuous vapor film may form—film boiling—which drastically reduces heat transfer.

Step-by-Step Solution:
Identify “directly at the heating surface” with discrete bubble nucleation.Map to regimes: discrete bubbles at sites → nucleate boiling.Confirm alternatives: a stable vapor film indicates film boiling, not the stated condition.

Verification / Alternative check:
Pool-boiling curves show a sharp rise in heat flux with temperature excess once nucleate boiling starts, distinguishing it from natural convection and film boiling.

Why Other Options Are Wrong:

• Film boiling: vapor blanket separates liquid and surface, opposite of discrete nucleation.
• Vapour binding: term used in pumps/boilers for vapor lock, not a boiling regime.
• Transition boiling: unstable region between nucleate and film boiling; not the description given.

Common Pitfalls:
Confusing initial bubble formation (nucleate) with later regimes; using “film boiling” whenever vapor is present.

Final Answer:
Nucleate boiling