Boiling & Critical Heat Flux Simulator

What the model is actually calculating

1. Nucleate pool boiling — Rohsenow

The Rohsenow correlation relates wall superheat to nucleate-boiling heat flux using liquid properties, latent heat, surface tension, gravity, Prandtl number and an empirical surface–fluid coefficient. It is useful for engineering estimates within nucleate boiling, but the empirical coefficient matters.

2. Saturated pool CHF — Zuber

Zuber's hydrodynamic limit is used as the peak saturated pool-boiling heat flux. It captures the instability that limits vapor removal from a large horizontal heated surface.

3. Margin

The tool reports CHF ratio as q″_CHF/q″_applied. That has the same mathematical form as a critical-to-actual heat-flux ratio, but it is not a reactor DNBR unless the CHF itself comes from the applicable validated DNB method.

Engineering limitations that matter

• Pool versus flow boiling: forced-flow CHF depends on pressure, mass flux, quality, geometry, heated length, power shape and flow regime. A pool-boiling CHF formula should not be substituted for a validated tube or fuel-bundle correlation.
• Surface condition: nucleation-site density, roughness, oxidation, coatings, fouling and wettability can materially shift boiling behavior.
• Subcooling: the Zuber implementation here is for saturated pool boiling. Subcooled CHF enhancement is not included.
• Transition and film boiling: the post-CHF branch in the first visualization is deliberately schematic. It is not used for numerical heater sizing.
• Properties: use saturated properties at the operating pressure. Near the critical point, simple correlations become especially sensitive and may lose validity.

Boiling regimes and two-phase flow patterns

Pool boiling: the regime explorer distinguishes natural convection / incipient boiling, developed nucleate boiling, the CHF or boiling-crisis region, transition boiling, and stable film boiling. Only the nucleate branch and saturated-pool CHF point are calculated quantitatively in this page; transition and film-boiling shapes remain educational.

Vertical flow boiling: the regime explorer demonstrates single-phase/subcooled boiling, bubbly, slug, churn, annular and mist/post-dryout morphology. The selected pattern is deliberately a visual study choice, not a prediction from vapor quality alone.

Real flow-pattern boundaries depend on superficial phase velocities, pressure, fluid properties, channel geometry, orientation and upstream history. The tool therefore reports homogeneous-equilibrium void fraction and superficial velocities without pretending those quantities define a universal regime map.

How to use the regime explorer

Choose the morphology you want to inspect, then enter equilibrium quality and mass flux. The engineering cards calculate void fraction under the homogeneous-equilibrium assumption, liquid and vapor superficial velocities, homogeneous mixture density and mass flow through the selected tube diameter.

Use the separate Flow-boiling energy tab to estimate outlet equilibrium quality from heat input. Then use the regime explorer to understand what different two-phase structures look like at a conceptual level.

Important: bubbly → slug → churn → annular → mist is a common vertical-upflow progression as vapor content increases, but the actual transitions are not universal and can shift substantially with operating conditions and geometry.

Useful engineering quantities

Prandtl number: Pr = μc_p/k. It appears explicitly in Rohsenow.

Jakob number: Ja = c_pΔT/h_fg. It compares sensible superheat energy with latent heat.

Capillary length: ℓ_c = √[σ/(gΔρ)]. It gives the gravity/surface-tension length scale associated with bubble and interface behavior.

Boiling number: Bo = q″/(G h_fg) for flow boiling. It compares imposed wall heat flux with the latent-energy carrying capacity of the mass flux.

Equilibrium quality: the flow-energy tab computes an enthalpy-based quality. A negative value means the bulk is still subcooled; 0–1 denotes a two-phase equilibrium enthalpy range.

HEM void fraction: the regime explorer assumes thermodynamic equilibrium and equal phase velocities (slip ratio = 1). Real void fraction can differ substantially when vapor and liquid velocities are unequal.