Question

Name the different boiling regimes in the order they occur in a vertical tube during flow boiling.

Short answer

Answer: The four boiling regimes during flow boiling in a vertical tube are: 1. Partial nucleate boiling (PNB): Bubbles form at nucleation sites on the heated surface, while the bulk of the liquid remains subcooled. 2. Fully developed nucleate boiling (FDNB): A high-density of bubbles is present in the flow, leading to increased heat transfer rates. 3. Partial dry-out (PD): Local dry-out regions in the tube form, causing a sharp decline in heat transfer rates, while nucleate boiling continues in remaining liquid regions. 4. Film boiling (FB): A stable vapor film separates the heated surface and the liquid, resulting in a significant reduction in heat transfer rate due to its insulating properties.

Step-by-step solution

Identify the boiling regimes

There are four boiling regimes that occur during flow boiling in a vertical tube. These regimes are as follows: 1. Partial nucleate boiling (PNB) 2. Fully developed nucleate boiling (FDNB) 3. Partial dry-out (PD) 4. Film boiling (FB)

Describe each boiling regime

1. Partial nucleate boiling (PNB): PNB occurs at the initial stage of boiling when the heat flux is relatively low. In this regime, bubbles begin to form at nucleation sites on the heated surface, but the bulk of the liquid remains subcooled. 2. Fully developed nucleate boiling (FDNB): As the heat flux increases, FDNB occurs. In this regime, the boiling process is more advanced, and a high density of bubbles are present in the flow. The bubbles coalesce and grow, resulting in bubble columns and slugs. The heat transfer rate is much higher in this regime compared to PNB. 3. Partial dry-out (PD): In the PD regime, the heat flux is high enough to cause local dry-out regions in the tube. However, some liquid is still present, and nucleate boiling continues in those regions. This regime is characterized by a sharp decline in the heat transfer rate due to the dry-out regions. 4. Film boiling (FB): When the heat flux continues to increase, and the entire tube is in a dry-out condition, film boiling occurs. A stable vapor film forms, separating the heated surface and the liquid, which results in a significant reduction in heat transfer rate. This is due to the insulating properties of the vapor film that are much lower than the liquid's heat transfer properties. These boiling regimes occur in the mentioned order as the heat flux increases during flow boiling in a vertical tube.