What marks the end of nucleate boiling and the beginning of decreased heat transfer?

What marks the end of nucleate boiling and the beginning of decreased heat transfer?

• A. Departure from Nucleate Boiling
• B. Critical Heat Flux
• C. Nucleate Boiling
• D. Film Boiling

Answer: (A)

The key idea is the transition from efficient two-phase heat transfer in nucleate boiling to a regime where a vapor layer forms on the surface, reducing heat transfer. In nucleate boiling, bubbles form at sites on the heater and depart, which helps pull heat away. As heat input increases, this boiling becomes unstable and a vapor film starts to cover parts of the surface. Once this departure from nucleate boiling occurs, stable nucleate boiling can no longer be sustained, and the heat transfer begins to decrease as the surface becomes insulated by vapor. This point marks the end of nucleate boiling and the start of decreased heat transfer, with further transitions leading toward film boiling. While the maximum heat transfer occurs at the critical heat flux, the specific transition described is the departure from nucleate boiling.

The key idea is the transition from efficient two-phase heat transfer in nucleate boiling to a regime where a vapor layer forms on the surface, reducing heat transfer. In nucleate boiling, bubbles form at sites on the heater and depart, which helps pull heat away. As heat input increases, this boiling becomes unstable and a vapor film starts to cover parts of the surface. Once this departure from nucleate boiling occurs, stable nucleate boiling can no longer be sustained, and the heat transfer begins to decrease as the surface becomes insulated by vapor. This point marks the end of nucleate boiling and the start of decreased heat transfer, with further transitions leading toward film boiling. While the maximum heat transfer occurs at the critical heat flux, the specific transition described is the departure from nucleate boiling.