Flow boiling of ammonia and flow instabilities in mini-channels

A simultaneous visualization and measurement study has been carried out to investigate the flow boiling and two-phase flow instabilities of ammonia in a mini-channel evaporator at various heat and mass fluxes. The evaporator consisted of 4 parallel 1 × 1.1 mm channels with a uniformly heated length of 250 mm. The visualization study showed that periodic reverse flow occurred in the mini-channel evaporator with liquid rewetting stage and annular film evaporating stage repeated alternately. Both damped and stable oscillations of pressure drop, wall temperature and system pressure were observed. The liquid rewetting stage approximately coincided with the decreasing stage of system pressure. Three typical two-phase flow patterns: bubbly flow, elongated bubble/slug flow and annular flow were identified. Low-frequency high-amplitude oscillation and high-frequency low-amplitude oscillation were identified through wavelet analysis. The high-frequency oscillation mainly occurred at the wave crest of the low-frequency oscillation. In addition, high-order harmonics were found in almost all the low-frequency oscillation waves. The effects of mass flux and saturation temperature on flow boiling instabilities were also investigated.