Characteristics of gas–liquid two-phase flows through a sudden contraction in rectangular microchannels

• Single-phase and two-phase flows across sudden contraction in microchannels were studied.
• Experimental data on pressure drops across the sudden contraction were obtained.
• Empirical correlation of the contraction coefficient for single-phase flow was developed.
• The two-phase pressure change experimental data were correlated with slip flow model.
• The numerical simulation data for two-phase flow agreed with the experimental data.

A series of experiments were conducted to study the effects of contraction ratio and different liquid properties on gas–liquid two-phase flow through sudden contraction in micro-channel. Two rectangular microchannels with different contraction ratio were used. The widths of the larger channels upstream of the contraction were 0.53 or 0.78 mm (0.230 mm in height), while those of the smaller ones were fixed at 0.270 mm (0.230 mm in height). Distilled water, ethanol 49 wt% aqueous solution, pure ethanol and hydrofluoroether (HFE)-7200 were used as the test liquids, and nitrogen as the test gas. Flow pattern, bubble length, liquid slug length and bubble velocity were determined by analyzing video images of the flows. In addition, pressure profile upstream and downstream from the contraction was also measured for single-phase and two-phase flows. The pressure loss due to the contraction was determined from the pressure profiles. In the analysis, bubble velocity data and the pressure loss data were compared with calculations by some correlations in literatures. The bubble velocity data both upstream and downstream from the contraction were correlated with Kawahara et al.’s correlation (2010) where the correlation was accounted for aspect ratio of the channel. The correlation of the contraction coefficient for single-phase liquid flows was newly developed with Reynolds number and contraction ratio. The two-phase pressure drop data reasonably agreed with a slip flow model combined with the present contraction coefficient correlation and drift flux model based void fraction correlation. Moreover, numerical simulations were performed using the computational fluid dynamics software FLUENT (Fluent Inc.) and the volume of fluid (VOF) model. The present numerical simulation data agreed well with the experimental data for flow pattern, bubble velocity, bubble length, bubble pitch and pressure changes through the contraction.