The Effect of Fluid Properties on the Physical Behavior of Adiabatic Annular Two-Phase Flow

Abstract

Three separate experimental facilities were used to explore the mechanics of both vertical and horizontal annular flow. The first facility described was a low-pressure two- phase refrigerant facility with a horizontal, adiabatic test section. The second apparatus described was an air/water facility with an identical horizontal, adiabatic test section to the refrigerant facility. The final test set-up described was an air/oil facility with a vertical, adiabatic test section. Air/water mixtures have been studied extensively in the present study as a vehicle for developing an understanding of two-phase flow behavior. In order to make use of air/water data to better understand the two-phase flow of refrigerants, it has been hypothesized that the best method to relate the pressure drop and behavior of the air/water and refrigerant data was to equate the kinetic energy of the refrigerant vapor to the kinetic energy of the air. The research effort summarized here directly compared the behavior of adiabatic air/water and R-123 vapor/liquid flows in identical test sections. The air/water and refrigerant flows were compared using pressure drop measurements, film thickness measurements, and qualitative visual observations including wave behavior. The pressure drop measurements were well correlated using vapor kinetic energy; however, the R-123 pressure drop was higher overall. The different fluids exhibited similar trends in film thickness, but the air/water film thickness was generally more circumferentially uniform. At similar vapor kinetic energies, each fluid pair appeared to be in similar flow regimes. However, the wave structure and behavior of R-123 was quite different than that of the air/water mixture. The R-123 liquid appeared to wet the inside tube surface better than the water. An important design problem in large refrigeration and air-conditioning systems is sizing of large diameter vertical vapor lines optimized to carry liquid up the pipe walls while attempting to minimize overall pressure loss. Of particular concern is the ability of refrigerant vapor flow to drive a liquid oil film through a refrigerant circuit so that oil does not accumulate outside the compressor under normal operation. Thus, this work also described a study aimed at characterizing the dynamic behavior of an annular oil film layer driven upward by air through a 50.8 mm I.D. pipe and a 25.4 mm I.D. pipe. The film thickness and gas mass flow at which flow reversal occurred were presented. Flow reversal in the oil film layer was identified both qualitatively (visually) and quantitatively by particle streak tracking. These results were compared and discussed with previously published experimental data and modeling work for air/water experiments. The significance of this work can be seen by applying the results of each study to the practical design of vertical risers. The refrigerant vapor kinetic energy required for flow reversal to occur was found using the method of equating the vapor kinetic energies.