In spite of large heat transfer co-efficients in boiling liquids, fins are used advantageously, where the entire surface is exposed to __________ boiling.

Solution (By Examveda Team)

Option A: Film Boiling
In film boiling, a stable layer of vapor forms between the heating surface and the boiling liquid.
This vapor layer acts as an insulator, creating significant thermal resistance and reducing heat transfer efficiency.
Fins are helpful in film boiling because they increase the surface area available for heat transfer, partially overcoming the insulating effect of the vapor layer.
Therefore, fins can improve the overall heat transfer rate in this mode of boiling.

Option B: Nucleate Boiling
In nucleate boiling, bubbles form and release directly at the heating surface.
This direct liquid-to-surface contact results in very high heat transfer rates.
Since nucleate boiling already has an efficient heat transfer mechanism, adding fins typically offers minimal additional benefit.
Fins are generally not necessary in nucleate boiling, as the high localized heat transfer reduces the need for increased surface area.

Option C: Transition Boiling
Transition boiling is an unstable phase between nucleate and film boiling, where the heat transfer rate fluctuates due to intermittent formation and collapse of the vapor layer.
Due to these fluctuations, the use of fins does not consistently improve heat transfer, as the conditions vary between efficient and inefficient phases.
Fins are less effective in this mode, as the intermittent nature of heat transfer in transition boiling limits their impact.

Option D: All of the Above
This option suggests that fins are beneficial across all boiling regimes.
However, fins are primarily effective in film boiling, where the insulating vapor layer creates a thermal resistance that fins can help mitigate.
In nucleate and transition boiling, the benefits of fins are limited or negligible due to already efficient heat transfer mechanisms or unstable conditions.

Conclusion:
The correct answer is Option A: Film boiling.
Fins are used advantageously in film boiling to increase the surface area and improve heat transfer efficiency, overcoming the thermal resistance created by the vapor layer.