DETERMINATION AND CHARACTERISTICS OF THE TRANSITION TO 2-PHASE SLUG FLOW IN SMALL HORIZONTAL CHANNELS

Two-phase pressure drop and fluctuating static pressuresf were measure d in a small horizontal rectangular channel (hydraulic diameter = 5.44 mm). The two-phase fluid was an air/water mixture at atmospheric pres sure tested over a mass flux range of 50 to 2000 kg/m2.s. Two-phase pa tterns were identified and an objective method was found for determini ng the flow pattern transition from bubble or plgu flow to slug flow. The method is based on an RMS static pressure measurement. In particul ar, it is shown that the transition is accompanied by a clear and abru pt increase in the RMS pressure when plotted as a function of mass qua lity. Use of the RMSf pressure as a two-phase flow pattern transition indicator is shown to have advantages over pressure-versus-time trace evaluations reported in the literature. The transition is substained b y a clear local change in slope in in the literature. The transition i s substained by a clear local change in slope in the curve of two-phas e pressure drop plotted as a function of either Martinelli parameter o r mass quality. For high mass fluxes, the change in slope in distingui shed by a local peak. Some degree of substantiation was found in previ ous work for both of the results (the RMS static pressure change and t he local pressure drop change) at the transition to slug flow.