Condensation Heat Transfer and Pressure Drop of Acoustically Actuated Horizontal Two-Phase Flow

Abstract

An acoustic actuation technique to enhance condensation inside horizontal tubes is developed and experimentally demonstrated. Heat exchanger headers are used as Helmholtz resonators, allowing low-frequency (∼10 Hz) acoustic waves to propagate and be reflected within the condenser, improving mixing saturated vapor and condensed liquid, thereby enhancing heat transfer. Heat transfer coefficients and pressure drops are measured to evaluate the heat transfer enhancement and pressure drop penalty as a function of vapor quality, mass flux, temperature difference, and acoustic wave amplitude and frequency. Experiments are conducted using a tube-in-tube heat exchanger with an inner diameter of 14.45 mm that partially condenses refrigerant R134a at 950 kPa across vapor qualities ranging from 5% to 80% as mass fluxes of 120 and 180 kg m−2 s−1 and temperature differences of 6 and 12 K. The actuation amplitude and frequency are varied from 0.5 to 6 mm and 2 – 20 Hz, respectively to characterize the acoustic response. Heat transfer enhancement mechanisms are hypothesized and evaluated based on the experimental results, concluding that changes in two-phase flow patterns and increased convective effects lead to significant enhancement at low qualities but negligible enhancement at high qualities when the flow is annular. Existing correlations and modifications to existing models are proposed to explain the observed enhancement, and a simple correlation based on a heat-and-momentum analogy is proposed, which predicts both baseline and enhanced heat transfer data within 2.7%.