Flow patterns and bubble departure fundamental characteristics during flow boiling in microscale channels

This paper presents an experimental investigation into the fundamental characteristics of flow boiling in microscale channels based on diabatic high-speed flow visualizations. Experiments were conducted with R134a and R245fa refrigerants flowing in a 0.40 mm circular horizontal channel for mass velocities ranging from 100 to 900 kg/m2 s and heat fluxes of up to 226 kW/m2. Flow images were captured at recording speeds of up to 100,000 frames/s. Results for bubble departure diameter and frequency, bubble growth ratio, slug frequency and velocity, flow pattern transitions, characteristics of the liquid film and liquid-vapor interface are provided. The experimental data obtained are carefully analyzed, discussed and compared against previous results for tubes of 1.00 and 2.00 mm internal diameter. Predictive methods available in the literature for bubble departure diameter and frequency are evaluated by comparing their predictions against the data obtained in the present study. New bubble departure diameter and frequency correlations are proposed for small channels. The following conclusions can be drawn from the present study: (i) bubbles can detach from the wall with diameters much smaller than the tube diameter; (ii) the bubble growth process has a square root time-dependence; (iii) two different methods for estimating the average surface heat flux based on flow boiling videos have been developed; (iv) bubble active nucleation sites are observed for all flow patterns; (v) buoyancy effects are still present for a 0.40 mm tube, and (vi) six different sources of vapor-liquid interface oscillations have been identified.