ICMS ' 2000 The Physics of Boiling at Burnout
The basic elements of a new experimental approach for the investigation of burnout in pool boiling are presented. The approach consists of the combined use of ultrathin (nano-scale) heaters and high speed infrared imag|ng of the heater temperature pattern as a whole, in conjunction with highly detailed control and characterization of heater morphology at the nano and micron scales. It is shown that the burnout phenomenon can be resolved in both space and time. Ultrathin heaters capable of dissipating power levels, at steady-state, of over 1 MW/m 2 are demonstrated. A separation of scales is identified and it is used to transfer the focus of attention from the complexity of the two-phase mixing layer in the vicinity of the heater to a micron-scaled microlayer and nucleation and associated film-disruption processes within it.
Framework for a Unified Model for Nucleate and Transition Pool Boiling
An area and time-averaged model for saturated pool boiling heat fluxes has been developed. In the model, which is valid in the upper end of nucleate boiling and in transition boiling, the existence of stationary vapor stems at the wall is assumed. The energy from the wall is conducted into the liquid macro/micro thermal layer surrounding the stems and is utilized in evaporation at the stationary liquid-vapor interface. The heat transfer rate into the thermal layer and the temperature distribution in it are determined by solving a two-dimensional steady-state conduction equation. The evaporation rate is given by the kinetic theory. The heater surface area over which the vapor stems exist is taken to be dry. Employing experimentally observed void fractions, not only the nucleate and transition boiling heat fluxes but also the maximum and minimum heat fluxes are predicted from the model. The maximum heat fluxes obtained from the model are valid only for surfaces that are not well wetted and includes the contact angle as one of the parameters.
The dynamics of vapour bubbles in nucleate boiling
Abstract A photographic study of the growth and translational characteristics of vapour bubbles issuing continuously from a single nucleation site was made for two fluids: water and Arcton (Freon) 113. The liquids were heated uniformly by external radiation to various levels of superheat, where high-speed photographs were taken of the vapour bubbles as they grew on the horizontal surface of a solid body immersed in the liquid, and after detachment while rising a distance of several diameters from the site. The film was analysed to determine the bubble radius and distance from the nucleation site as a function of time. The bubble radius was found to vary as (time) 2 3 during the entire recorded history, for all runs. This dependence is reflected in the dimensionless correlation: which accurately represents all data for both liquids, to within 10 per cent, when the velocity parameter is taken to be: The bubble trajectories within the range of observation could be accurately predicted by a balance of buoyancy with inertial and viscous forces.