Unconfined Pool Boiling

Unconfined Pool Boiling, ANSYS Fluent Tutorial

Description

This project simulates unconfined pool boiling of saturated water inside a cylindrical chamber using the Eulerian multiphase model in ANSYS Fluent. Pool boiling is a fundamental heat transfer process across many industrial and engineering applications, and understanding its behavior — including key concepts like the Onset of Nucleate Boiling (ONB), Departure from Nucleate Boiling (DNB), and Critical Heat Flux (CHF) — is essential for designing efficient heat exchange systems, preventing equipment burnout, and ensuring safety in high-heat applications such as nuclear reactors.

The cylindrical chamber geometry was designed in Design Modeler and meshed in ANSYS Meshing using a structured grid, with mesh quality specifically optimized to capture the complex interphase behavior characteristic of multiphase boiling simulations.

Methodology

Liquid water and vapor interaction was captured using the Eulerian multiphase model, with the RPI (Rensselaer Polytechnic Institute) boiling sub-model enabled to represent the underlying boiling physics. Several interphase forces were configured, including drag, lift, and wall lubrication, alongside wall adhesion to realistically capture bubble detachment behavior from the heated surface.

Boiling mass transfer mechanisms were activated to capture the liquid-to-vapor phase change directly, with the energy equation enabled to resolve the resulting temperature distribution and the Realizable k-epsilon model applied for turbulence. Bubble dynamics were modeled in detail, including bubble departure diameter and frequency, nucleation site density, an area influence coefficient, and a quenching model correction incorporating bubble waiting time — together capturing the full cycle of bubble formation, growth, and departure from the heated wall.

Conclusion

Results include vapor volume fraction contours that reveal the bubbles' changing shape and the mass exchange occurring through condensation as they ascend through the liquid, along with the convective motion this boiling process induces throughout the surrounding fluid.

These results illustrate the complete physical picture of nucleate pool boiling — from bubble nucleation at the heated wall, through growth and departure, to condensation and convective mixing as bubbles rise — providing insight directly applicable to heat exchanger design optimization and to predicting the onset of burnout conditions in high-heat industrial and safety-critical applications.