Heat transfer characteristics and mechanisms along entire boiling curves under steady-state and transient conditions

Abstract

The paper presents a survey of results found by the authors and their groups during recent years. An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wetting fluids and fluids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean. However, even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run. The situation is different under transient conditions: heating and cooling transients yield different curves even on clean surfaces. Measurements with micro-optical probes give an insight in the two-phase dynamics above the heating surface in the different boiling regimes. Temperature signals from microthermocouples at the surface and the optical probe data lead to some conclusions on the physics of boiling in different regimes such as the macrolayer configuration, the dry-spot size and dynamics, the nucleation site density etc. The high density of active nucleation sites in the region between fully developed nucleate boiling and critical heat flux (CHF) and the resulting highly turbulent two-phase boundary layer let us conclude that the strong increase of heat flux in heating transients and vice versa in a cooling process is mainly due to the intensive two-phase convection heat transfer from the wall to the bulk and not due to inertia effects in the two-phase structure dynamics. The latter is not significantly affected by temperature transients. An interfacial-area-density model is proposed. It needs data from the optical probes and enables the prediction of entire boiling curves by employing only one fitting parameter. Furthermore the concept of a reaction–diffusion model is presented to predict CHF. Here the triggering of CHF is due to an instability of dry spots on the heating surface. Data from microthermocouples and optical probes are needed to evaluate this model. Many aspects of the extremely complex mechanisms of boiling are still not sufficiently understood. The problems should be tackled from both the experimental and the theoretical end and both approaches should be closely linked.