An Investigation of Two-Phase Flow Regimes for Microchannels Based on Void Fraction

Estimation of void fraction is of prime importance for the two-phase flow boiling in microchannels for understanding the heat transfer and the pressure drop across the channel. In this paper, experimental investigation is carried out using five different mass fluxes of deionized water on a silicon microchannel having three parallel channels, by applying gradually increasing power input in the range of 4-29 W. The flow boiling patterns are observed by employing a high-speed camera, and the vapor void fractions are calculated using image processing technique. In order to facilitate the clear identification of different flow regimes, the threshold and cutoff void fraction values, respectively, are defined from repeated experimental trials and are validated with the previously published results. Flow regime maps are generated by using the thermodynamic quality, void fractions, input power, and the mass flow rates. The lengths occupied by the bubbles in various regimes are measured, and qualities at different locations are found for calculating the chordal void fractions. By using the void fractions, the liquid film thickness ratios are calculated over the whole range of annular regime, with inclusion of a factor associated with water droplet entrainment. The heat transfer coefficient for the annular regime is also evaluated using the liquid film thicknesses.