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

Abstract

Adiabatic air–water two-phase upward flows were analyzed in a circular pipe with 1.2mm inner diameter. Superficial velocities were varied from 0.1 to 3.5m/s for water and 0.1 to 34.8m/s for air. Gas quality of up to 0.38 was obtained. Void fractions were experimentally determined using a 15kHz 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 6m/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 18.6m/s. Traditional correlations, generally used for large pipes, presented the best performance for bubbly, slug, churn and annular patterns with superficial gas velocities below 14.5m/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.