A visual study of vapor bubble growth and departure in vertical upflow and downflow forced convection boiling is presented. A vertical flow boiling facility was constructed with a transparent, electrically-heated test section in which the ebullition process could be observed. High-speed digital images of flow boiling phenomena were obtained, which were used to measure bubble growth, departure diameters, and lift-off diameters. Experiments were conducted for flow of FC-87 over a commercially-finished nichrome heating surface, with mass flux ranging from 190 to 666 kg m(-2) s(-1) and heat flux ranging from 1.3 to 14.6 kW/m(2). The flow was slightly subcooled (Delta T-sub = 1.0-5.0 degrees C), and boiling occurred at isolated nucleation sites. A major conclusion of this work is that the observed vapor bubble dynamics between upflow and downflow are significantly different. In the upflow configuration, bubbles departing the nucleation site slide along the heater wall, and typically do not lift off. In the downflow configuration, bubbles either lift off directly from the nucleation site or slide and then lift off, depending on flow and thermal conditions. The process of vapor bubble sliding appears to be responsible for enhanced energy transfer from the heating surface, as evidenced by larger heat transfer coefficients for upflow than for downflow under otherwise identical operating conditions. (C) 1998 Elsevier Science Ltd. All rights reserved.
