ANS:B - vapour density.

The burnout heat flux in the nucleate boiling regime is not a function of the vapor density. In the nucleate boiling regime, heat transfer occurs primarily through the formation and collapse of vapor bubbles at the heated surface immersed in the liquid. The burnout heat flux refers to the maximum heat flux that can be sustained before a critical heat flux (CHF) condition is reached, leading to a rapid reduction in heat transfer efficiency. The burnout heat flux is influenced by several factors, including the liquid properties and the characteristics of the boiling process. However, it is not directly dependent on the vapor density. Instead, it is primarily influenced by factors such as:

1. Liquid Density: The density of the liquid affects the mass flow rate and convective heat transfer in the liquid phase. However, the burnout heat flux is not directly dependent on liquid density.
2. Temperature Difference: The temperature difference between the heated surface and the bulk liquid affects the driving force for heat transfer. A higher temperature difference can lead to higher heat fluxes, but it is not the sole determinant of the burnout heat flux.
3. Heat of Evaporation: The heat of evaporation (latent heat) is the amount of heat required to change a substance from liquid to vapor phase. While it influences the energy required for vaporization, it does not directly determine the burnout heat flux.