What do bubbles do in local or sub-cooled nucleate boiling?

What do bubbles do in local or sub-cooled nucleate boiling?

• A. Collapse
• B. Move away from surface
• C. Coalesce into larger bubbles
• D. Remain stable

Answer: (A)

Subcooling reduces the tendency for bubbles to grow; it promotes condensation. In local nucleate boiling, vapor bubbles form at the heated surface, but the surrounding liquid is cooler than the saturation temperature. That cooler liquid extracts latent heat from the vapor, causing the vapor inside the bubble to condense and the bubble to shrink and collapse. Bubbles may detach and rise briefly, but their size is limited by this condensation, so they don’t persist or grow into larger bubbles. Coalescing into larger bubbles or remaining stable don’t fit the subcooled condition, where condensation dominates.

Subcooling reduces the tendency for bubbles to grow; it promotes condensation. In local nucleate boiling, vapor bubbles form at the heated surface, but the surrounding liquid is cooler than the saturation temperature. That cooler liquid extracts latent heat from the vapor, causing the vapor inside the bubble to condense and the bubble to shrink and collapse. Bubbles may detach and rise briefly, but their size is limited by this condensation, so they don’t persist or grow into larger bubbles. Coalescing into larger bubbles or remaining stable don’t fit the subcooled condition, where condensation dominates.