Fluid Film Boiling

Fluid film boiling is a phenomenon that occurs when a liquid comes into direct contact with a surface that is significantly hotter than the liquid's boiling point at the relevant pressure conditions. The fluid film boiling model assumes that boiling takes place either on hot film walls or inside film cells.

Boiling at hot walls influences the heat flux between the film and the wall. The mechanisms causing these effect are:

  • Evaporation (latent heat) transport—here heat is supplied near the wall to bubbles which then move away into the liquid film. In subcooled boiling, simultaneous evaporation at the base of a bubble and condensation at its tip can occur.
  • Micro-convection—includes growth and collapse of bubbles cause random liquid motion.
  • Vapor-liquid exchange—a Reynolds analogy model in which bubble growth causes an exchange of liquid between the wall and bulk regions.
  • Surface quenching—a variation of the vapor-liquid exchange, assuming transient conduction to the cold liquid in contact with the wall after bubble departure.
  • Wake flow—where liquid motion behind a departing bubble causes convection from the wall.
  • Thermo-capillary flow—small variations in surface tension due to temperature differences between the base and tip of a bubble cause a jet of hot liquid to flow away from the wall.

Fluid film boiling includes several regimes, each characterized by unique heat transfer dynamics and behaviors. These regimes include:

  • Liquid film evaporation—occurs when the wall temperature remains below the saturation temperature. Here, liquid sprayed onto the surface forms a delicate film and gradually evaporates.

  • Nucleate boiling—occurs when the wall temperature exceeds the saturation temperature but remains lower than the Nukiyama temperature. In this phase, overheated liquid near the wall forms vapor cavities, leading to bubble formation. The heat flux from the wall consumed by the phase change prevents the liquid temperature to exceed saturation.

  • Transition boiling—occurs for wall temperatures between the Nukiyama and Leidenfrost temperatures, which shows characteristics of both nucleate and film boiling, marked by intermittent nucleation and film formation. Heat transfer in this regime is more complex.

  • Film boiling—occurs when the wall temperature exceeds the Leidenfrost temperature and a stable vapor film forms between the solid surface and the liquid.

Understanding and modeling these regimes is important for modeling heat transfer during boiling in various engineering applications. Simcenter STAR-CCM+ provides the Rohsenow boiling and Habchi boiling models for modeling film boiling.