Flow Visualization of R-134a in a Capillary Tube

This paper reports on a novel flow visualization study in which the two-phase flow behavior of R-134a in a small-bore capillary tube is viewed from inside of the tube using a fiberscope. All previous capillary tube flow visualization studies, some dating back nearly fifty years, have used glass tubes through which two-phase flow behavior was observed and photographed from the outside. Because glass tubes have extremely smooth inner walls in comparison to the relative roughness of a drawn copper capillary tube, the two-phase flow behavior may be different. The proper characterization of the two-phase flow field downstream of the flash point is necessary for an accurate prediction of pressure drop and flow rate in the capillary tube. The fiberscope used was a 0.020 in. (0.51 mm) diameter glass fiber bundle roughly 39 in. (1 m) in length with at teflon coating. The fiberscope was inserted into the upstream end of the capillary tube through a specially designed fitting. The fiberscope lens at the end of the fiber bundle was positioned approximately 32 in. (80 cm) downstream of the capillary tube inlet. By carefully controlling the upstream pressure and temperature, the location of the onset of vaporization, or flash point, could be positioned near the fiberscope lens. In this way, the twophase flow detail could be viewed in the region of the flash point. The flow visualization results presented herein clearly indicate that once the vaporization is initiated the two-phase flow appears to be plug/slug-like, in contrast to developing bubbly flow behavior reported for the glass tubes in previous studies.