Bubble dynamic parameters during subcooled flow boiling under uniform and non-uniform transverse heat distribution

Abstract

Bubble behaviors and their interactions with each other affect the two-phase flow characteristics. In this study, experiments are conducted to investigate the bubble characteristics during subcooled flow boiling under uniform and non-uniform transverse heat flux distribution. The non-uniform heat flux distribution creates non-uniform bubble characteristics on the heated surface as opposed to under uniform heating. The working fluid is demineralized water that flows through a narrow rectangular channel heated from one side. The experimental loop used in the study operates at low pressure. A wide range of experimental operating conditions, such as inlet temperature (35–65 °C), thermal power (500–6250 W), and mass flow rates (0.03–0.13 kg/s), are applied to the upward flow channel. The bubble behaviors are visualized using a high-speed camera (2200 fps) at a resolution of 512 × 512 pixels. The results indicate that the bubbles exhibit different departure diameters, nucleation site density distributions, and bubble departure frequencies. In the uniform case, bubbles are generated uniformly across the whole transverse direction of the heated surface. In the non-uniform case, more bubbles are generated where the heat flux is concentrated, which disturbs the flow velocity profile in the transverse direction. The differences in bubble generation in the transverse direction result in differences in the two-phase flow instability through the heated channel. As a result, new empirical correlations are proposed based on the experimental results to estimate the bubble departure diameter, nucleation site density, and bubble departure frequency. The correlations are applicable for both heated surface conditions, uniform and non-uniform, under low-pressure conditions. CFD analysis using ANSYS FLUENT incorporates the RPI wall boiling model is conducted to validate the empirical correlations. Comparison of the CFD calculations with the experimental data for void fraction, wall temperature, and bulk temperature show good agreement. The simulation results show an accurate prediction of the ONB (Onset of Nucleate Boiling) and OFI (Onset of Flow Instability).