Effect of liquid viscosity on flow reversal of two-phase mixture in crank-type vertical tubes

Abstract

The present work is to examine the flow reversal phenomenon occurring in crank-type vertical tubes. The crank-type test sections have 10.7 mm I.D. with the overall height of 2 m. The length of the horizontal part has been changed from zero (straight vertical tube) to 0.81 m. Air and the aqueous glycerin solution were used as the test fluids. Ranges of the gas and liquid flow rate were $(2~30)×10^{-4} kg/s$ and $(8~750)×10^{-5} kg/s$, respectively, and the absolute viscosity has been changed from $1.1×10^{-3}$ to $130×10^{-3} N s/㎡$. In general, the gas flow rate preventing the flow reversal increased with increasing of the liquid viscosity. Except for the low-viscosity/low-flow-rate conditions of the liquid phase, the gas flow rates for the flow reversal remained almost the same when other parameters were fixed. However, with low viscous flows (i.e., air-water flows), marginal increase of the gas flow rate (for the flow reversal) was observed with decreasing of the liquid flow rate. Variation of the horizontal part length also affected the flow reversal criterion. The gas flow rate for the flow reversal in the crank-type tubes appeared larger than in the straight vertical tube. However, the increasing rate was getting smaller as the horizontal part became longer. The effect of the horizontal part length appeared prominent at the low-viscosity and high-flow-rate condition of the liquid, but the effect diminished as the liquid flow rate was decreased and/or the liquid viscosity became larger. Finally, a tentative correlation representing the flow reversal condition was proposed within an accuracy of ±5 %.