Question 1

What is the difference between nucleate and film boiling?

Accepted Answer

In nucleate boiling, individual bubbles form at discrete nucleation sites on the heated surface, maintaining good liquid contact and very high heat transfer coefficients (thousands to tens of thousands W/m²K). In film boiling, the entire surface is covered by a continuous vapor film that insulates the surface from the liquid, dramatically reducing the heat transfer coefficient (typically hundreds of W/m²K). The transition from nucleate to film boiling occurs when the surface heat flux exceeds the critical heat flux (CHF).

Question 2

What is critical heat flux (CHF) and why does it matter?

Accepted Answer

The critical heat flux (CHF) is the peak of the Nukiyama boiling curve — the maximum heat flux achievable in the nucleate boiling regime. For water at 1 atm it is approximately 1.2–1.4 MW/m². Exceeding the CHF causes a vapor film to envelop the surface, causing a dramatic drop in heat transfer coefficient and a rapid rise in surface temperature (burnout). CHF is a critical design parameter in nuclear reactor cooling, electronics liquid cooling, and boiler design.

Question 3

How does the Rohsenow correlation work?

Accepted Answer

The Rohsenow (1952) correlation for nucleate pool boiling is: q'' = μ_l h_fg [g(ρ_l−ρ_v)/σ]^0.5 [c_pl ΔTe / (C_sf h_fg Pr^n)]^3. Here C_sf is a surface–fluid combination coefficient (≈0.013 for copper–water) and n is a fluid-specific exponent (1.0 for water). Given a surface excess temperature ΔTe, the correlation directly yields the nucleate boiling heat flux q''.

Question 4

What is the Leidenfrost temperature?

Accepted Answer

The Leidenfrost temperature (Leidenfrost point) is the surface temperature at which the minimum heat flux in the transition boiling regime occurs and stable film boiling begins. For water at 1 atm it is approximately 150–180°C (ΔTe ≈ 50–80 K). Above this temperature a stable vapor cushion prevents liquid–surface contact. It is important in spray cooling, quenching (heat treatment), and droplet impact dynamics.

Boiling Heat Transfer Calculator

Theory — Rohsenow Correlation (Nucleate Boiling)