Use of Synchronized, Infrared Thermometry and High-Speed Video for Generation of Space- and Time- Resolved High-Quality Data on Boiling Heat Transfer

Nucleate boiling is an effective mode of heat transfer; one of the most studied physical phenomena in science and engineering, and a key thermal limit in nuclear systems. However, for decades, modeling of nucleate boiling heat transfer has been relying on speculative hypotheses and a good dose of empiricism. For example, the widely popular Rosenhow's correlation for nucleate boiling is based on the assumption that single-phase convection and nucleate boiling are analogous physical processes, and can be both correlated in terms of the Reynolds and Prandtl number of the liquid phase; for nucleate boiling the characteristic velocity and length are assumed to be the downward liquid velocity and most unstable Taylor wavelength, respectively; then, an empirical constant, Csf, is determined to fit the experimental data for any fluid/surface combination. As researchers are now finally moving away from the rough empiricism of the past, and start to develop more mechanistic models of nucleate boiling heat transfer, the need for high-quality high-resolution data on the bubble nucleation and growth cycle is becoming increasingly big. Specifically, nucleation site density, bubble departure diameter and frequency data are necessary input for the source terms in interfacial area transport models and CFD 'multi-fluid' models as well as semi-empirical models for boiling heat transfer, such as the RPI's heat flux partitioning model and Kolev's bubble interaction model. Furthermore, time-resolved temperature distribution data for the boiling surface and direct visualization of the bubble cycle are needed for validation of 'first principle' models of bubble nucleation and growth, based on interface tracking methods, in which the geometry of the vapor/liquid interface is not assumed, but rather calculated from a marker function advected according to the Navier-Stokes equations. However, gathering the detailed data needed for validation of advanced simulation models is not straightforward. The traditional approaches based on thermocouples and high-speed visualization of the boiling process suffer from several shortcomings; for example, the thermocouples can only measure temperature at discreet locations on the boiling surface, thus no information on the temperature distribution about a nucleation site can be obtained. Further, thermocouples (including micro-thermocouples) have relatively long response time, thus are unsuitable for studying the bubble nucleation and growth phenomena, which have time scales of the order of milliseconds. The usefulness of high-speed video is typically limited by poor optical access to the nucleation site and interference from adjacent bubbles. Second-generation two phase flow diagnostics, such as multi-sensor conductivity and optical probes and wire-mesh probes, can measure bubble diameter and velocity near the boiling surface. However, these approaches are intrusive, and also produce data only at discreet locations within the boiling fluid. It was not until the early 2000's that new possibilities for generating time-resolved multidimensional data on the bubble nucleation and growth cycle have opened up with the introduction of infrared-based visualization of thermal patterns on the boiling surface by Theofanous et al. In this paper we will present an approach based on synchronized infrared thermometry and high speed video 'through' the heater that enables simultaneous measurement of the nucleation site density, bubble growth rate (including bubble departure diameter), bubble departure frequency (including wait time), time-resolved 2D temperature distribution and phase distribution on the boiling surface, all in a relatively effortless manner. (authors)