--- title: "Pool boiling curve and its regimes" subject: "Heat Transfer" chapter: 5 key: "semester_5/heat_transfer:pool-boiling-curve-regimes" source: https://www.bctengineer.com/q/semester_5/heat_transfer/pool-boiling-curve-regimes site: BCT Engineering --- # Pool boiling curve and its regimes Heat Transfer · Chapter 5: Phase Change Heat Transfer - Asked in: year not given, 5 marks - Video: [HT EP 80 POOL BOILING CURVE](https://www.youtube.com/watch?v=NMER0LrbgZ0) — Professor kartik - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/heat_transfer/pool-boiling-curve-regimes - Whole chapter: https://www.bctengineer.com/md/semester_5/heat_transfer/chapter-5 ## Question 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. ## 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}. ## Answer **Pool boiling** is boiling on a heated surface submerged in a stagnant liquid. The behaviour depends on the excess temperature $\Delta T_e = T_w - T_{sat}$. #### Regimes ```text q" ^ . Critical (burnout) log | / \ | / \ _.-- film | / \ _.-' boiling | / nucleate '. Leidenfrost | / A B C D E +-------------------------> log dT_e ``` | Region | $\Delta T_e$ (water) | Description | |---|---|---| | A: natural convection | below about 5 °C | Liquid superheated slightly; evaporation at free surface | | B: nucleate boiling | 5 to 30 °C | Bubbles form at nucleation sites, rise and break; $h$ very high; heat flux rises steeply | | C: critical point | about 30 °C | Maximum heat flux $q_{max}$ (about 1.1 MW/m$^2$) | | D: transition boiling | 30 to 120 °C | Unstable vapour film patches; $q$ falls with $\Delta T_e$ | | E: film boiling | above about 120 °C | Stable vapour blanket; radiation becomes important; $q$ 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 $q_{max}$ 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 $q_{max}$. #### Correlations - **Rohsenow (nucleate boiling):** $q'' = \mu_lh_{fg}\left[\dfrac{g(\rho_l-\rho_v)}{\sigma}\right]^{1/2}\left[\dfrac{c_{pl}\Delta T_e}{C_{sf}h_{fg}Pr_l^n}\right]^3$, $C_{sf}$ depends on the surface-liquid pair. - **Zuber (critical flux):** $q''_{max} = 0.131\,\rho_v^{1/2}h_{fg}\left[\sigma g(\rho_l-\rho_v)\right]^{1/4}$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the water pool boiling curve, the correct order of regimes with increasing excess temperature is A. nucleate, natural convection, transition, film B. natural convection, nucleate, transition, film C. film, transition, nucleate, natural convection D. natural convection, film, transition, nucleate Answer: B. The curve runs natural convection, nucleate boiling, the critical point, transition boiling and then stable film boiling. (Key point: Boiling regimes) 2. Why must the design heat flux of an electric heater be kept below q_max? A. Because beyond q_max the surface temperature jumps to the film boiling value and may exceed the melting point (burnout) B. Because h becomes infinite C. Because bubbles stop forming at low flux D. Because natural convection takes over Answer: A. In flux-controlled equipment, exceeding q_max moves the surface to film boiling at a very high temperature. (Key point: Critical heat flux importance) 3. Which describes nucleate boiling? A. Stable vapour blanket with radiation B. No bubbles, evaporation only at the free surface C. Unstable vapour film patches with falling q D. Bubbles form at nucleation sites; h is very high and heat flux rises steeply Answer: D. Region B (about 5 to 30 C) has bubble formation and very high heat transfer coefficients. (Key point: Nucleate boiling features) 4. Which correlations are used for nucleate boiling and critical heat flux? A. Rohsenow for nucleate boiling and Zuber for critical flux B. Zuber for nucleate boiling and Rohsenow for critical flux C. Nusselt for both D. Dittus-Boelter for nucleate boiling and Zuber for film boiling Answer: A. Rohsenow gives nucleate boiling flux and Zuber gives q_max. 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