Leidenfrost phenomenon in boiling heat transfer: the “Leidenfrost point” refers to which process condition?

Introduction:
The Leidenfrost point is a hallmark of boiling heat transfer behavior at very high surface temperatures. Above this temperature, liquid droplets levitate on a stable vapor layer, dramatically altering heat transfer mechanisms and visual appearance.

Given Data / Assumptions:

• A hot solid surface is in contact with a cooler liquid.
• Surface temperature can exceed the liquid’s saturation temperature significantly.
• Boiling regimes transition from nucleate to transition and then to film boiling.

Concept / Approach:
When a surface is sufficiently hotter than the liquid, vigorous nucleate boiling gives way to unstable transition boiling and ultimately to film boiling. The Leidenfrost point marks the onset of a stable vapor film that insulates the liquid from the surface, leading to droplet “skating” and greatly reduced heat transfer coefficients compared with nucleate boiling.

Step-by-Step Solution:
Recognize the regime: very hot surface, droplet levitation observed.Identify the definition: the temperature at which a continuous vapor film forms beneath the liquid.Associate this strictly with boiling at a hot surface, not condensation or mixing.

Verification / Alternative check:
Classical boiling curves show a minimum in heat flux near the Leidenfrost region; beyond this point, heat transfer occurs through a vapor layer (film boiling).

Why Other Options Are Wrong:

• Condensation or evaporation of corrosives: Not related specifically to Leidenfrost behavior.
• Viscous liquid heat transfer and natural convection: Different phenomena.

Common Pitfalls:
Assuming higher surface temperature always increases heat transfer; in film boiling, heat transfer can actually decrease due to insulation by vapor.

Final Answer:
boiling of a liquid on a hot surface.