Flow regimes and hydrodynamics of acoustically actuated saturated liquid–vapor flow

A new acoustic actuation technique to enhance in-tube condensation heat transfer is proposed and experimentally demonstrated. Heat exchanger headers are used as Helmholtz volumes, where low-frequency acoustic waves are propagated and reflected to cause pulsatile flow in the vapor phase, leading to dispersed and well-mixed liquid vapor flow. Pressure drop measurements and high-speed videos are recorded to characterize the effect of acoustic actuation and change in two-phase flow patterns, and a flow pattern classification algorithm is developed to track the change in flow pattern over several actuation cycles. Experiments are conducted on a 12.7-mm tube containing saturated refrigerant R134a at 950 kPa across vapor qualities from 5% to 85% flowing at mean mass fluxes of 150 and 240 kg m−2 s−1. The actuation frequency and amplitude are varied from 1.5 Hz to 25 Hz and 0.5 mm – 6 mm, respectively, and the volume of the Helmholtz resonator at the test section outlet varied from 150 to 1000 mL to characterize the acoustic response for various geometric and acoustic conditions. Correlations for the heat exchanger resonance frequency, frequency response, and relative change in flow regime are developed.