Phase change heat transfer Flow boiling

Convective boiling is widely used to cool environments, liquids and systems, thanks to more efficient heat transfer than free boiling. In the industrial field, the design of water-cooled nuclear reactors, energy recovery systems based on the organic Rankine cycle, refrigeration machines or heat pumps, boilers in the petrochemical industry and many process engineering installations is based on knowledge of the mechanisms controlling convective boiling. In convective boiling, heat exchange depends, on the one hand, on the phenomenon of forced convection and, on the other, on the process of nucleate boiling from a wall heated sufficiently to generate steam. These two mechanisms are closely interdependent, given the coexistence of the two phases. In addition to the viscous, inertial and pressure forces that characterize single-phase flows, two-phase flows are subject to interfacial tension forces and the exchange of momentum between the two phases. Heat transfer in nucleate boiling is mainly controlled by the temperature difference between the wall and the fluid, the properties of the liquid and the wettability of the wall. For convective boiling, the velocities of each phase and their distribution play a major role, requiring knowledge of flow patterns as a function of the system's position, which is usually horizontal or vertical. The associated boiling mechanisms lead to different boiling regimes, which need to be studied separately. In addition, the geometry of systems (intratubular, extratubular boiling) and their orientation significantly modify boiling heat transfer. Fluid mixing is of great importance in many chemical, petrochemical and industrial process applications. Because their behavior differs from that of the pure substances that make them up, they must be the subject of specific developments in convective boiling. Finally, in recent decades, more efficient two-phase cooling methods have emerged, such as microchannel flow.