In local or sub-cooled nucleate boiling, where do steam bubbles form?

In local or sub-cooled nucleate boiling, where do steam bubbles form?

• A. Surface irregularities
• B. Within the bulk liquid away from the surface
• C. At cracks in walls
• D. At the liquid-vapor interface away from the surface

Answer: (A)

Bubbles form at tiny features on the heated surface. Local heating creates superheating right at the wall, and microscopic crevices, pits, and other irregularities trap vapor nuclei. These surface sites lower the energy barrier for phase change, so vapor embryos can form there and grow into bubbles that detach into the subcooled liquid. That’s why surface irregularities are the correct description. If you look away from the surface into the bulk liquid, there isn’t the heat source needed to nucleate bubbles, and bubbles don’t typically originate there. Likewise, cracks in walls are still surface imperfections, but the most general and representative nucleation sites are the ordinary surface roughness and cavities. And bubbles forming away from the liquid–vapor interface near the surface wouldn’t be sustained without the heat input from the wall, so nucleation isn’t described as occurring away from the boundary.

Bubbles form at tiny features on the heated surface. Local heating creates superheating right at the wall, and microscopic crevices, pits, and other irregularities trap vapor nuclei. These surface sites lower the energy barrier for phase change, so vapor embryos can form there and grow into bubbles that detach into the subcooled liquid.

That’s why surface irregularities are the correct description. If you look away from the surface into the bulk liquid, there isn’t the heat source needed to nucleate bubbles, and bubbles don’t typically originate there. Likewise, cracks in walls are still surface imperfections, but the most general and representative nucleation sites are the ordinary surface roughness and cavities. And bubbles forming away from the liquid–vapor interface near the surface wouldn’t be sustained without the heat input from the wall, so nucleation isn’t described as occurring away from the boundary.