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Pool boiling curve and its regimes

Short answer

Pool boiling curve: natural convection (<5°C), nucleate boiling (5–30°C) with high flux, critical heat flux (~1.1 MW/m²) at ~30°C, transition boiling where flux drops, and film boiling (>120°C). Critical heat flux is crucial because exceeding it may cause burnout in heat-flux-controlled equipment. Rohsenow correlation governs nucleate boiling; Zuber: q''_ max =0.131 ρ_v^{1/2} h_fg [σ g (ρ_l−ρ_v)]^{1/4}.

Explain the pool boiling curve for water at atmospheric pressure, showing the different regimes of boiling. Why is the critical heat flux important in the design of heating equipment? State the correlations used for nucleate boiling and critical heat flux.

Pool boiling is boiling on a heated surface submerged in a stagnant liquid. The behaviour depends on the excess temperature ΔTe=Tw−Tsat\Delta T_e = T_w - T_{sat}.

Regimes

 q"  ^           . Critical (burnout)
 log |          /  \
     |        /      \       _.-- film
     |      /          \  _.-'    boiling
     |    /  nucleate    '.  Leidenfrost
     |  / A    B     C   D   E
     +-------------------------> log dT_e
RegionΔTe\Delta T_e (water)Description
A: natural convectionbelow about 5 °CLiquid superheated slightly; evaporation at free surface
B: nucleate boiling5 to 30 °CBubbles form at nucleation sites, rise and break; hh very high; heat flux rises steeply
C: critical pointabout 30 °CMaximum heat flux qmaxq_{max} (about 1.1 MW/m2^2)
D: transition boiling30 to 120 °CUnstable vapour film patches; qq falls with ΔTe\Delta T_e
E: film boilingabove about 120 °CStable vapour blanket; radiation becomes important; qq rises again

The minimum of the curve is the Leidenfrost point.

Critical heat flux

In heat flux-controlled equipment (electric heaters, nuclear fuel rods), exceeding qmaxq_{max} makes the surface temperature jump to the film boiling value, often above the melting point of the material (burnout). So the design heat flux is kept well below qmaxq_{max}.

Correlations

  • Rohsenow (nucleate boiling): q′′=μlhfg[g(ρl−ρv)σ]1/2[cplΔTeCsfhfgPrln]3q'' = \mu_lh_{fg}\left[\dfrac{g(\rho_l-\rho_v)}{\sigma}\right]^{1/2}\left[\dfrac{c_{pl}\Delta T_e}{C_{sf}h_{fg}Pr_l^n}\right]^3, CsfC_{sf} depends on the surface-liquid pair.
  • Zuber (critical flux): qmax′′=0.131 ρv1/2hfg[σg(ρl−ρv)]1/4q''_{max} = 0.131\,\rho_v^{1/2}h_{fg}\left[\sigma g(\rho_l-\rho_v)\right]^{1/4}.

✓ Answer checked by Claude Sonnet 5.5

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