Effects of boiling hysteresis on metastable two-phase flow refrigerant in a straight adiabatic capillary tube.

Experiments on adiabatic two-phase flow phenomena of R134a in a straight capillary tube of 0.977mm I.D. and 1.13m in length were carried out at inlet pressures from 0.8 to 1.0MPa and subcooled temperatures from 5 to 1K. The results show that the experimental procedures, such as increasing or decreasing the subcooled temperature at a given specified inlet pressure will significantly affect the mass flow rates. By measuring the surface temperature along the capillary tube with an infrared thermography camera, we found that the delay of flashing (i.e. the metastable flow, due to the actual flashing point occurred at the downstream of the theoretical one) is significantly affected by the experimental procedures. The maximum difference in the under pressure is about 120 kPa, and that in the mass flow rate is about 12% at the same inlet conditions. These results indicate that boiling hysteresis phenomenon affects the delay of flashing, and so affects the mass flow rate in the capillary tube. By taking into account the actual delay of flashing, the mass flow rate can be estimated within ±5% using the homogeneous model, which is combined with the correlation equation proposed by Cicchitti et al. for two-phase flow viscosity.