Question

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

Short answer

boiling.

Step-by-step solution

Understand convection, nucleate, and film boiling

Convection boiling is when heat is transferred from a hotter surface to a cooler liquid primarily through the bulk motion of the liquid caused by differences in temperature. Nucleate boiling occurs when a fluid reaches its boiling point and forms vapor bubbles at the solid-liquid interface. These bubbles then rise to the surface and release vapor, allowing the liquid to cool. Film boiling is when a thin layer of vapor forms between the hot surface and the liquid, insulating the liquid from the surface and preventing further boiling.

Analyze options provided

We need to determine which boiling transition corresponds to burnout. Let's look at the options: (a) Convection to nucleate boiling: This transition isn't related to burnout, as it only represents the transition from heat being transferred primarily by convection to heat being transferred by bubble formation. (b) Convection to film boiling: This is not the right answer as the transition from convection to film boiling would be from a less intense to an even less intense heat transfer process. (c) Film to nucleate boiling: This transition also doesn't correspond to burnout since it represents a transition from a less effective heat transfer process (film boiling) to a more effective one (nucleate boiling). (d) Nucleate to film boiling: This is the correct answer. When the boiling process moves from nucleate boiling to film boiling, it means that the heat flux has increased enough to cause a layer of vapor to form between the surface and the liquid. This layer of vapor prevents further boiling and can lead to burnout as the increased heat cannot be dissipated effectively. (e) None of them: We found the correct answer in option (d), so this option is not correct.

Choose the correct answer

We have analyzed all the given options and determined that the boiling transitions from nucleate boiling to film boiling when burnout occurs. So, the correct answer is: (d) nucleate to film