Q1: In spite of large heat transfer co-efficients in boiling liquids, fins are used advantageously, where the entire surface is exposed to __________ boiling.

ANS:D - all modes of

"Boiling" in the context of heat transfer refers to the phase change from liquid to vapor that occurs when a liquid is heated to its boiling point. There are several modes of boiling:

1. Nucleate Boiling: This is the most common mode of boiling and occurs when small bubbles of vapor form at discrete nucleation sites on the heated surface. These bubbles detach from the surface and rise through the liquid. Nucleate boiling provides high heat transfer rates because the bubbles carry away heat as they rise.
2. Film Boiling: In film boiling, a vapor film forms between the heated surface and the liquid, inhibiting direct contact between the liquid and the surface. Heat transfer in film boiling is much lower than in nucleate boiling because of the insulating effect of the vapor film.
3. Transition Boiling: Transition boiling occurs during the transition between nucleate and film boiling. It involves a combination of nucleate boiling and film boiling phenomena. The heat transfer characteristics during transition boiling vary depending on the specific conditions.

When fins are used for heat transfer, they are typically most effective in nucleate boiling conditions, where the bubbles formed at the fin surface help to enhance heat transfer. In transition and film boiling, the presence of the vapor film can reduce the effectiveness of the fins. Therefore, while fins can still provide some heat transfer enhancement in all modes of boiling, their effectiveness is typically greatest in nucleate boiling.