Frictional pressure drop and void fraction analysis in air-water two-phase flow in a microchannel

Adiabatic air-water two-phase upward flows were analyzed in a circular pipe with 1.2 mm inner diameter. Superficial velocities were varied from 0.1 to 3.5 m/s for water and 0.1 to 34.8 m/s for air. Gas quality of up to 0.38 was obtained. Void fractions were experimentally determined using a 15 kHz resistive impedance sensor. Pressure drop and void fraction results were obtained for a wide category of patterns. The churn/annular transition was found to occur at superficial gas velocities of up to 6 m/s. Pressure drop correlations based exclusively on fluid refrigerant data appeared not to be appropriate for air-water flows. Small tube correlations created with air-water data showed the best performance in the prediction of the pressure drop for annular flows with superficial gas velocities of over 18.6 m/s. Traditional correlations, generally used for large pipes, presented the best performance for bubbly, slug, churn and annular patterns with superficial gas velocities below 14.5 m/s. The flow pattern and the liquid flow regime (given by the bulk Reynolds number of the liquid phase) appeared to affect the pressure drop predictions. The Lockhart-Martinelli void fraction correlation was used successfully to predict the experimental results. The mean slip factor increased from approximately 1 for bubbly flows to around 16 for annular flows. Instantaneous void fraction analysis was found to be useful for monitoring purposes or for improving pressure drop predictions.