Orientation effects on pool boiling critical heat flux (CHF) and modeling of CHF for near-vertical surfaces

Photographic studies of near-saturated pool boiling at various surface orientations were conducted in order to determine the critical heat flux (CHF) trigger mechanism associated with each orientation. Based on the vapor behavior observed just prior to CHF, it is shown that surface orientations can be divided into three regions: upward-facing (0- 60 degrees), near-vertical (60-165 degrees), and downward-facing (>165 degrees), each region is associated with a unique CHF trigger mechanism. In the upward-facing region, the buoyancy forces remove the vapor vertically off the heater surface. The near-vertical region is characterized by a wavy liquid-vapor interface which sweeps along the heater surface. In the downward-facing region, the vapor repeatedly stratifies on the heater surface, greatly decreasing CHF. The vast differences between the observed Vapor behavior within the three regions indicate that a single overall pool boiling CHF model cannot possibly account for all the observed orientation effects: but instead three different models should be developed for the three regions. Upward-facing surfaces have been examined and modeled extensively by many investigators and a few investigators have addressed downward-facing surfaces, so this paper focuses on modeling the near-vertical region. The near-vertical CHF model incorporates classical two-dimensional interfacial instability theory, a separated flow model, an energy balance, and a criterion for separation of the wavy interface from the surface at CHF. The model was tested for different fluids and shows good agreement with CHF data. Additionally, the instability theory incorporated into this model accurately predicts the angle of transition between the near-vertical and downward-facing regions. (C) 1998 Elsevier Science Ltd. All rights reserved.