Two-Phase Flow Characteristics During Convective Boiling of Halocarbon Refrigerants Inside Horizontal Small-Diameter Tubes

Two-phase flow observations were performed for R134a and R245fa in 1.1- and 2.32-mm ID horizontal tubes. The tests were run for mass velocities ranging from 100 to 600 kg/m2-s and saturation temperatures of 22, 31, and 41 degrees C. Additionally, an objective method to characterize two-phase flow patterns was developed. This method is based on simultaneous processing of signals from the following devices: a pair of diodes laser-sensors with a transparent tube between them within which the two-phase flow occurs, a micro-piezoelectric pressure transducer to determine the variation in the local pressure, and a microthermocouple within the fluid. The method was developed based on the k-means clustering algorithm, which consists of the gradual agglomeration of data of similar average characteristics. Simultaneous images of two-phase flow were obtained through a high-speed camera (10,000 frames/s) and used to identify the following flow patterns: bubbly, elongated bubbles, churn, and annular flows. The maps obtained by the objective method were compared against flow pattern results segregated based on flow visualization and a reasonable agreement was obtained between them. The vapor quality for the transition between churn and annular flow pattern decreases with decreasing the tube diameter, whereas the vapor quality for the transition between elongated bubbles and churn flow decreases with increasing tube diameter. Effects of saturation temperature and mass velocity were also verified. Additionally, elongated bubble velocities, frequencies, and lengths were determined based on the analysis of high-speed videos and the processing of signals of the diode/laser-sensor. The elongated bubble velocity was correlated as a linear function of the two-phase superficial velocity. A new image treatment method was developed to automatically identify the entrainment frequencies, which were segregated according to two groups: low and high frequency. The former group was characterized by frequencies lower than 20 Hz and the later by 50500 Hz frequency ranges.