Phase change heat transfer .Pool boiling

Boiling is the process of heat transfer from a heated wall to a liquid, associated with a phase change resulting in the formation of vapor bubbles in the liquid. Free boiling refers to boiling in a stationary liquid in the absence of thermal stress, as opposed to convective boiling, in which the fluid is set in motion by an external device such as a pump. The phenomenon of boiling plays an important role in many fields of industry, as it enables high flow densities to be dissipated with relatively small temperature differences between the wall and the fluid. It is used to cool dissipative systems (electronic components, motors, etc.), heat exchangers (evaporators in refrigeration systems, thermo-mechanical energy conversion systems, etc.), chemical engineering and process engineering.

Whether the technical devices mentioned above use free boiling or convective boiling, understanding the fundamental phenomena (thermodynamics of phase equilibria, bubble growth dynamics) governing boiling is essential to good control of heat transfer and its coupling with the hydraulic behavior of the liquid volume (boiling regimes). This understanding is the key to considering improved designs for these devices by taking advantage of local phenomena, after sizing constituting a sketch based on macroscopic heat exchange laws.

As a result of the quest for greater compactness to limit fluid quantities, or to meet the constraints imposed by the miniaturization of a large number of systems that dissipate ever-increasing flow densities (notably electronic components), two-phase exchangers have evolved towards designs that take advantage of machining microtechnologies. Macroscopic laws established for conventional geometries are no longer applicable, as other phenomena come into play on smaller scales.

Boiling fluid mixtures is a common practice in process engineering, for example in distillation operations. Similarly, the changing nature of the fluids used in refrigeration evaporators, linked to environmental concerns, means that refrigerants are increasingly non-azeotropic mixtures. The boiling of fluid mixtures differs from the boiling of pure bodies due to the difference in volatility of each species, as well as the diffusion of one species within the other. Specific tools have therefore been developed for these situations.

Last but not least, boiling under very low pressure (i.e. close to the fluid's triple point) has become a major topic in recent years, thanks to its application to sorption refrigeration evaporators, which are enjoying renewed interest thanks to their reduced environmental footprint. These special thermodynamic conditions give rise to phenomena that have a major impact on the thermal-hydraulics of boiling. Over and...