Boiling and Condensation

Why it matters

Boiling and condensation are crucial in many industrial applications, such as power generation, refrigeration, and chemical processing. Understanding these processes helps engineers design efficient thermal systems that manage heat transfer effectively.

Key ideas

• Boiling: The process where a liquid turns into vapor when it reaches its boiling point. It involves heat transfer from a surface to the liquid, causing phase change.
  • Nucleate Boiling: Occurs at the surface where bubbles form and detach.
  • Film Boiling: A vapor layer forms between the surface and the liquid, reducing heat transfer efficiency.
• Condensation: The process where vapor turns into liquid upon cooling.
  • Film Condensation: A liquid film forms on the surface, which can impede heat transfer.
  • Dropwise Condensation: Occurs when droplets form and coalesce, offering higher heat transfer rates.
• Heat Transfer Coefficient: A measure of the heat transfer rate per unit area and temperature difference, crucial for analyzing boiling and condensation.

Applicability

Saturation temperature depends on pressure. In surface boiling use wall superheat T_s - T_sat; for condensation use T_sat - T_wall when quoting positive heat transfer to the wall. An effective h depends strongly on regime, fluid and surface; do not keep it constant across the entire boiling curve. Nucleate boiling reaches a critical heat flux, after which transition toward a vapor blanket can cause a large wall-temperature rise. If latent heat dominates, vapor generation/condensation rate is mdot = q/h_fg.

Formulas

• q = h·A·ΔT
  • q: Heat transfer rate (W)
  • h: Heat transfer coefficient (W/m²·K)
  • A: Surface area (m²)
  • ΔT: Temperature difference (K)
• Nu = h·L/k
  • Nu: Nusselt number (dimensionless)
  • h: Heat transfer coefficient (W/m²·K)
  • L: Characteristic length (m)
  • k: Thermal conductivity (W/m·K)

Worked example

Given an effective coefficient for the specified operating state:

• Surface area, A = 0.5 m²
• Heat transfer coefficient, h = 1000 W/m²·K
• Temperature difference, ΔT = 20 K

Find: Heat transfer rate, q

1. Use the formula: q = h·A·ΔT
2. Substitute the values: q = 1000 W/m²·K · 0.5 m² · 20 K
3. Calculate: q = 10000 W

Final Answer: 10000 W

Common mistakes

• Confusing nucleate and film boiling, leading to incorrect assumptions about heat transfer efficiency.
• Ignoring the effects of surface conditions on condensation type (film vs. dropwise).
• Miscalculating the heat transfer coefficient due to incorrect units or assumptions.

For GATE ME

Questions often involve calculating heat transfer rates during boiling or condensation, understanding the differences between nucleate and film boiling, and analyzing the effects of surface conditions on condensation. Practice problems involving the application of heat transfer coefficients and Nusselt numbers.

Quick check

1. What is the main difference between nucleate and film boiling?
2. How does dropwise condensation differ from film condensation?
3. What is the formula for calculating the heat transfer rate?

Answers: 1. Nucleate boiling involves bubble formation at the surface, while film boiling involves a vapor layer. 2. Dropwise condensation involves droplets forming and coalescing, while film condensation involves a continuous liquid film. 3. q = h·A·ΔT