When boiling a saturated liquid, one must be careful while increasing the heat flux to avoid burnout. Burnout occurs when the boiling transitions from boiling. (a) convection to nucleate (b) convection to film (c) film to nucleate (d) nucleate to film (e) none of them

Boiling is a heat transfer process that involves phase change from liquid to vapor, often due to the application of heat. There are two dominant types of boiling, nucleate boiling and film boiling. Nucleate boiling occurs when vapor bubbles form at the solid-liquid interface due to localized heating. This is considered the most efficient way of boiling as the heat transfer coefficient is high during this process. Film boiling, on the other hand, occurs when a stable vapor film covers the solid-liquid interface, which causes a sharp reduction in heat transfer efficiency. The heat is transferred mainly by radiation, and it is less efficient than nucleate boiling.

Let's go through each option and analyze if it corresponds to the burnout phenomenon: (a) Convection to nucleate boiling: This transition would represent an increase in heat transfer efficiency, which is not associated with burnout. (b) Convection to film boiling: This transition would represent a decrease in heat transfer efficiency. When the heat flux is increased beyond a certain point during nucleate boiling, the bubbles formed can merge into a continuous vapor film, leading to a decrease in heat transfer efficiency and potential burnout. (c) Film to nucleate boiling: This transition would represent an increase in heat transfer efficiency, which is not associated with burnout. (d) Nucleate to film boiling: Similar to option (b), this transition represents a decrease in heat transfer efficiency, leading to the possible occurrence of burnout. (e) None of them: Considering options (b) and (d), this option is incorrect because burnout can occur during certain transitions.

Since burnout occurs when the boiling transitions from nucleate boiling to film boiling due to a significant decrease in heat transfer efficiency, the correct answer is: (d) nucleate to film