| name | critical-heat-flux |
| description | Critical heat flux (CHF) — boiling crisis, DNB, Zuber correlation, Lienhard correction, DNBR for nuclear fuel, subcooled boiling CHF, flow boiling CHF, dryout, reactor safety, ASME correlations. |
| metadata | {"priority":7,"promptSignals":{"phrases":["critical heat flux","CHF","departure from nucleate boiling","DNB","burnout heat flux","boiling crisis"],"minScore":3}} |
Critical Heat Flux (CHF) — Complete Skill
CHF Fundamentals
CHF (boiling crisis): transition from nucleate boiling (high h) to film boiling (low h) at surface
Effect: surface temperature spikes dramatically (ΔT = 200–1000°C) → surface burnout or damage
Two regimes:
- DNB (Departure from Nucleate Boiling): subcooled or low-quality flow boiling; bubble coalescence blocks liquid access; sudden vapor blanket formation
- Dryout: high-quality annular flow; liquid film on wall evaporates; critical quality reached
Pool boiling CHF threshold:
On boiling curve: transition from nucleate (B) to film boiling at Leidenfrost point
q''_max (CHF) ≈ 10× q''_Leidenfrost
Pool Boiling CHF — Zuber Correlation (1958)
q''_max = C × h_fg × ρ_V × [σ × g × (ρ_L - ρ_V) / ρ_V²]^(1/4)
C = π/24 = 0.131 (Zuber theoretical)
C = 0.149 (Kutateladze-Rohsenow, empirical fit to data — most accurate)
Variables:
h_fg = latent heat of vaporization [J/kg]
ρ_L, ρ_V = liquid and vapor density [kg/m³]
σ = surface tension [N/m]
g = 9.81 m/s²
Example: water at 1 atm:
q''_max ≈ 1.1 MW/m² (pool boiling)
At 100 bar (PWR coolant): q''_max increases significantly (σ and (ρL-ρV) change)
Finite Heater Size Correction (Lienhard)
For flat surfaces:
L' = L × [g(ρ_L - ρ_V)/σ]^(1/2) [dimensionless heater dimension]
Correction factor C from L':
L' < 0.15: short heater; C = 0.149 × (L'/0.15)^0.25
L' > 0.15: C = 0.149 (Zuber asymptote)
For horizontal cylinder diameter D:
L' = D/2 × [g(ρ_L - ρ_V)/σ]^(1/2)
C = 0.12 × L'^(-0.25) for L' = 0.15–1.2
Subcooled Flow Boiling CHF
Bergles-Rohsenow (subcooled, low quality):
q''_CHF increases with subcooling (ΔT_sub = T_sat - T_bulk):
q''_CHF,sub = q''_CHF,sat × (1 + K × ΔT_sub) [K = subcooling enhancement factor, 0.002–0.01 depending on fluid and conditions]
Tong W-3 Correlation (nuclear reactor — most widely used):
q''_DNB = [(2.022 - 0.0004302P) + (0.1722 - 0.0000984P) × exp((18.177 - 0.004129P) × x)] × [(0.1484 - 1.596x + 0.1729x|x|) × G/10⁶ + 1.037] × (1.157 - 0.869x) × [0.2664 + 0.8357 × exp(-3.151D_h)] × [0.8258 + 0.000794(H_f - H_in)]
P = pressure [psia]; x = quality; G = mass flux [lb/(hr·ft²)]; D_h = hydraulic diameter [in]
Valid: P = 1000–2300 psia; G = 1×10⁶–5×10⁶ lb/(hr·ft²); x = -0.15 to 0.15
Biasi correlation (European, SI):
q''_CHF = 0.7249/(G^0.6) × (100P/1000)^n × h(P)/√(D_h) × (1-x) [MW/m²]
Valid: D_h = 3–20 mm; G = 100–6000 kg/(m²s); P = 0.27–14 MPa
DNBR (Departure from Nucleate Boiling Ratio)
Nuclear reactor safety criterion:
DNBR = q''_CHF / q''_actual ≥ MDNBR (Minimum DNBR)
PWR safety limit:
MDNBR ≥ 1.30 (thermal design limit); operating MDNBR ≥ 1.50 (with uncertainty)
DNBR calculation:
- Calculate actual heat flux q'' at each axial node (cosine flux shape for pin)
- Calculate q''_CHF from correlation at same location (local conditions)
- DNBR = min(q''_CHF/q'') across all locations and conditions (including hot channel factors)
Hot channel factors:
F_q = q''_max / q''_avg [total peaking factor; typical ≤ 2.8 for PWR]
Includes: nuclear peaking (F_N), engineering factors (F_E), power tilt
Flow Boiling CHF — Annular Flow / Dryout
Critical quality (dryout quality x_crit):
x_crit decreases with increasing G and L; increases with D_h
Shah correlation (flow boiling, wider range):
Bo = q'' / (G × h_fg) [boiling number]
Co = [(1-x)/x]^0.8 × (ρ_V/ρ_L)^0.5 [convection number]
CHF at high quality: x_crit from Katto-Ohno (1984) or Groeneveld lookup tables
Groeneveld lookup table (NRC, 1996):
Standard reference for rod bundle CHF in light water reactors
Input: D_e [m], G [kg/(m²s)], x [-], P [kPa] → output: q''_CHF [kW/m²]
Correction factors K₁ through K₉ for geometry, non-uniform flux, subcooling
Pressure Effect on CHF
CHF peaks near intermediate reduced pressure:
For water: q''_max peaks at P_r ≈ 0.2–0.3 (∼40–60 bar)
At critical pressure (P_c = 220.6 bar for water): q''_max → 0 (ρ_L = ρ_V)
Enhancement and Suppression Factors
CHF Enhancement (relative to smooth tube):
Surface enhancement (micro-fins, nano-coatings): +50–200%
High subcooling: +20–50%
Swirl flow (twisted tape): +30–80% (at same G)
CHF Reduction:
Dissolved gas: -10–20%
Surface contamination/fouling: significant reduction (film on surface)
Vibration: complex effect (low amplitude may enhance; high may reduce)
Standards
| Standard | Scope |
|---|
| ASME TDP-1 | Thermal design procedures for LWR fuel |
| NRC RG 1.105 | Instrument setpoints (safety margin calculation) |
| ASTM E2349 | Experimental CHF testing |
| IAEA-TECDOC-938 | CHF data and correlations compilation |
| ASME Section III | Nuclear component design |
Output
Provide: boiling configuration (pool/subcooled flow/annular), fluid and operating conditions (P [bar], T_sat, T_bulk, G [kg/(m²s)], x [-]), q''_CHF by applicable correlation [MW/m²] (Zuber, W-3, Biasi, Groeneveld), actual q'' [MW/m²], DNBR = q''_CHF/q'' vs. minimum DNBR limit (≥ 1.30), critical quality x_crit (if dryout regime), hot channel factor F_q, heater surface dimension correction (Lienhard L'), CHF enhancement/reduction factors, and applicable standard (ASME TDP-1, Groeneveld lookup table, ASTM E2349).
