Role of surface conditions in the enhancement of heat transfer rates in convective boiling

The primary purpose of this study was to investigate the improvement of the low heat transfer rates associated with film boiling process. The general method to improve heat transfer rates investigated here was to initiate greater liquid-solid contact in convective film boiling by means of macroroughnened surfaces and the application of coatings. Three types of tubes were experimentally tested in a vertical upward annular flow system , namely (1) a Union Carbide high flux stainless steel tube, (2) a Gewa-Y copper-Nickel tube, and (3) a smooth tube for base line comparisons. The experimental results obtained indicated that surface macroroughnesses and surface projections affect (improve) the film boiling characteristics of the enhanced surfaces significantly. The tubes had improvements in heat transfer coeficients of up to 80%, but more importantly resulted in a significant shift of the minimum film boiling temperature to higher values. The shift in this temperature to higher values is very important in that the range of temperatures associated with the nucleate and transition regimes is expanded and extended to higher values. In other words, the high heat transfer rates associated with the nucleate and transition regimes are present in what would normally be a film boiling regime; Hence better cooling and quenching rates are achieved at higher temperatures. In an effort to explain the observed shift in the minimum film boiling temperature to higher values, an analytical study, consisting of modeling the highly transient two-dimensional conduction phenomena occuring at the tip of the surface projections or fins, was undertaken. The conduction model employed showed that local surface temperature depressions occurred very near to the surface of a fin tip. Once this temperature drop occurs, the surface's ability to repel any additional liquid decreases, and thus breakdown of the film boiling vapor film occurs in the immediate vicinity of these locally quenched areas. Consequently, the surface then becomes exposed to the much higher heat transfer rates associated with nucleate and transition boiling regimes. Coating the tips of the fins with low thermal conductivity material was also investigated analytically. It was found that as the thermal conductivity of the coating is decreased, the magnitude of the local surface temperature drop increased, which also would have the effect of forcing a transition to nucleate boiling sooner than for an uncoated surface, thereby resulting in higher heat transfer rate from the surface.