Characterization of oscillations during flow boiling of water in parallel microchannels

Several challenges coexist in the field of flow boiling in microchannels, ranging from high superheat required for boiling incipience to boiling instabilities and early dryouts. The aim of this study is to hinder or overcome some of the challenges and develop an image processing algorithm for analysis of the boiling oscillations in multiple parallel channels. The experimental results were measured in an array of 64 parallel 25 × 25 μm microchannels with a synchronized high-speed visualization and measuring system. The small cross section of the microchannels allowed only the formation of annular two-phase flow and a computer algorithm was developed for tracking the meniscus oscillations during boiling. The applied image analysis focuses on reliability with the simultaneous use of brightness variation and brightness derivative along with an image subtraction. Moreover, the images were preprocessed to determine the number of microchannels and their orientation with applying different filtering and Radon transformation. The data extracted from the visualization was used to determine the peak to peak amplitudes and fundamental frequencies of the oscillating meniscus. The results exhibit lower amplitudes and higher fundamental frequencies with increasing heat flux. The mass flux was kept constant at 83 kg/m2s, whereas the heat flux varied from 150 kW/m2 to 250 kW/m2. The amplitudes and the fundamental frequencies of the meniscus oscillations are determining the length and duration of the microchannel with periodically alternating liquid and vapor phase.