Understanding Nucleate Boiling

Nucleate boiling is a regime of boiling heat transfer that occurs when the surface temperature is higher than the saturation temperature of the liquid, but not so high as to cause film boiling.

Let's look at the conditions and characteristics of nucleate boiling mentioned in the options:

Required Temperature Condition

For boiling to occur, the surface from which heat is transferred to the liquid must be at a temperature higher than the saturation temperature of the liquid at the given pressure. This temperature difference, known as superheat, is essential for vapor bubble formation. Nucleate boiling specifically happens when this superheat is relatively low to moderate, leading to the formation of discrete vapor bubbles at specific points on the surface.

Therefore, the statement "The temperature of the surface is greater than the saturation temperature of the liquid" is a fundamental condition for nucleate boiling to take place.

Bubble Formation Mechanism

In nucleate boiling, vapor bubbles form at specific sites on the heating surface. These sites are often microscopic cavities or scratches where gas or vapor is trapped. These trapped pockets serve as nucleation points where the phase change from liquid to vapor can easily begin. As the surface temperature is above the saturation temperature, heat transfers to the liquid at these sites, causing it to vaporize and form bubbles. The bubbles then grow and detach from the surface, allowing fresh liquid to come into contact with the hot surface.

So, the statement "Bubbles are created by expansion of entrapped gas or vapour at small cavities in the surface" accurately describes the typical mechanism of bubble formation in nucleate boiling.

Comparing Nucleate and Film Boiling Temperatures

Nucleate boiling occurs at relatively lower surface temperatures compared to film boiling. As the surface temperature increases beyond the critical heat flux (CHF), a vapor film starts to cover the surface, leading to film boiling. Film boiling requires a much higher surface superheat because heat must be conducted or radiated through the insulating vapor film to reach the liquid interface. Thus, the temperature of the surface in film boiling is significantly greater than that in nucleate boiling.

The statement "The temperature is greater than that of film boiling" is incorrect; nucleate boiling generally happens at lower surface temperatures than film boiling.

Conclusion

Based on the analysis of the characteristics, nucleate boiling requires the surface temperature to be greater than the saturation temperature of the liquid. This superheat drives the boiling process and the formation of vapor bubbles.