An experimental study on flow boiling in a narrow channel: From convection to nucleate boiling

Heat transfer experiments were performed in a narrow rectangular channel to investigate the transition from forced convection to nucleate boiling. The flow configuration was a narrow (2 x 20 x 375 mm), vertically oriented channel with one wall heated at uniform heat flux and the others approximately adiabatic. Liquid crystal thermography was used to acquire the surface temperature data. The experiments covered a range of mass velocities from 60 to 5257 kg/m2s ; the liquid inlet subcoolings, from 4.7 to 15.3°C; and the inlet pressures, from 0.14 to 0.18 MPa.;The transition process was found to be influenced by pressure, inlet subcooling, and mass velocity. An incipience transition number was introduced to describe this transition. Various analytical models and correlations for boiling incipience and developed nucleate boiling were examined. Semitheoretical equations were developed using classical nucleation theory for prediction of boiling incipience superheat and heat flux. An empirical correlation for the calculation of wall superheat from heat flux in developed nucleate boiling was also presented. In addition, the influence of asymmetrical heating on single-phase convection was discussed.;Observations of the boiling incipience superheat excursion and hysteresis were made. Thermographic images revealed two distinct types of incipience transitions from boiling incipience to sustained nucleate boiling. In Type One, boiling incipience was initiated near the channel exit, and shifted gradually upstream as the heat flux increased. A sudden and sharp wall temperature drop occurred at the incipience point. In Type Two, the transition region was wider and less distinct. Boiling incipience occurred at approximately the same heat flux level over most of the channel.