Transient flow boiling in a semi-annular duct: From the Onset of Nucleate Boiling to the Fully Developed Nucleate Boiling

Abstract

This paper presents an investigation of the transient Onset of Nucleate Boiling (ONB) on a vertical heated surface cooled by an upward flow. The duct geometry used (semi-annulus with a heated inner wall) make this study very similar to a fuel rod in a Pressurised Water Reactor (PWR). This test section is then heated with different power steps to show the ONB and observe its propagation along the wall. The fluid used is the refrigerant HFE7000 flowing with flow rates corresponding to Reynolds numbers from 0 to 60,000 and subcoolings Jakob numbers from 8 to 33. Synchronised infra-red (IR) thermal measurements and high-speed camera visualizations provided new data which helped to develop models to characterise the Onset of Nucleate Boiling and the transition to the Fully Developed Nucleate Boiling regime (FDNB). This very finely characterized local behaviour provides an original insight for the onset of nucleate boiling in such sub-cooled convective configuration. It is shown that boiling is initiated by nucleation at a few specific sites and then propagates in the wake of a large vapor pocket flowing along the wall. Analysis of wall to fluid heat transfer evidences a short heat transfer degradation phase below the first vapor pocket, followed by a very effective cooling in its wake. The wall to fluid heat transfer during this transition toward fully developed nucleate boiling appears uncorrelated to the power dissipated within the heating element or to flow conditions.

Experimental study of steady and transient subcooled flow boiling

“…Thanks to an infrared camera and a high-speed camera, it is possible to evaluate the heat flux transferred to the liquid and study all the regimes from the convection (Baudin et al [9] ) to the film boiling while relating the time evolution of the heat transfer with the development of twophase flow structures along the wall. Onset of nucleate boiling has been already studied in details in a companion paper [10] .…”

“…The experimental set-up used for this study has been detailed in the previous works of Visentini et al [8] and Scheiff et al [10] . The main part of the cell consists of a subcooled flow of a refrigerant, HFE70 0 0, in a semi annular section composed of a thin metal foil of thickness e w = 50 µm and radius 4.1 mm rapidly heated by Joule effect.…”

“…The thickness of the paint has been chosen as a best compromise between getting the largest surface emissivity and sufficiently low thermal inertia. Solving a 1D conduction through the metal foil and the paint layer, showed that the thermal gradient between the paint and the metal foil is negligible (Scheiff et al [10] ). Since the surface is curved, its emissivity depends on the observation angle.…”

“…radiation is deduced from view factor of a concave geometry. More information can be found in Scheiff et al [10] and Scheiff [15] .…”

Enhancing Boiling Heat Transfer on a Superheated Surface by Surfactant-Laden Droplets

“…The initial temperature of vigorous nucleate boiling is defined as the Onset of Boiling Driven (OBD) temperature. 12,13 As the surface temperature continues to rise, the droplet will be lifted off by the vapor film from its fast vaporization, and the heattransfer efficiency of the liquid/solid interface decreases rapidly. 14−16 This is called the Leidenfrost phenomenon, or film boiling, 17−23 the corresponding surface temperature of which is called the Leidenfrost point (LFP) temperature.…”