Question

How does film boiling differ from nucleate boiling? Is the boiling heat flux necessarily higher in the stable film boiling regime than it is in the nucleate boiling regime?

Short answer

Answer: The main difference between film boiling and nucleate boiling lies in the mechanism of heat transfer. In film boiling, a stable vapor film forms between the heating surface and the liquid, leading to a lower overall heat transfer rate. In nucleate boiling, bubbles form at the heating surface and enhance the heat transfer rate due to better mixing and direct contact between the heating surface and the liquid. Generally, the boiling heat flux is higher in the nucleate boiling regime due to the more efficient heat transfer mechanisms.

Step-by-step solution

Define Film Boiling

Film boiling is a type of boiling that occurs when the surface temperature of a heating element is significantly higher than the saturation temperature of a liquid. In this situation, a stable vapor film forms between the heating surface and the liquid, preventing direct contact between the two. As a result, the heat transfer occurs mainly through the vapor film, which has a much lower thermal conductivity compared to the liquid, leading to a lower overall heat transfer rate.

Define Nucleate Boiling

Nucleate boiling, on the other hand, occurs when the surface temperature of a heating element is only slightly higher than the saturation temperature of a liquid. In this case, bubbles form at the heating surface and then rise through the liquid, causing turbulent mixing and enhancing the heat transfer rate. The formation and collapse of these bubbles help facilitate direct contact between the heating surface and the liquid, leading to a higher overall heat transfer rate.

Describe Differences between Film Boiling and Nucleate Boiling

The main difference between film boiling and nucleate boiling lies in the mechanism of heat transfer. In film boiling, a stable vapor film forms between the heating surface and the liquid, which means that the heat transfer takes place mainly through this vapor film. This results in a lower overall heat transfer rate due to the poor thermal conductivity of the vapor film compared to the liquid. In contrast, nucleate boiling involves bubble formation at the heating surface, which leads to better mixing and more direct contact between the heating surface and the liquid. This enhances the heat transfer rate and results in a higher overall heat transfer rate compared to film boiling.

Compare Boiling Heat Fluxes

To determine if the boiling heat flux is necessarily higher in the stable film boiling regime than in the nucleate boiling regime, we need to analyze the heat transfer mechanisms for both regimes. In nucleate boiling, heat transfer is enhanced due to the formation of bubbles and direct contact between the heating surface and the liquid. This results in a high boiling heat flux. On the other hand, in film boiling, the heat transfer takes place mainly through the vapor film, which has lower thermal conductivity than the liquid. This results in a lower boiling heat flux compared to nucleate boiling. Thus, the boiling heat flux is not necessarily higher in the stable film boiling regime than in the nucleate boiling regime. In reality, the boiling heat flux is generally higher in the nucleate boiling regime due to the more efficient heat transfer mechanisms.