Mechanism study of departure of nucleate boiling on forced convective channel flow boiling

We carried out a visualization study of Departure from Nucleate Boiling (DNB) of a vertical upward flow boiling condition. In order to evaluate the effects of the convective flow condition on the DNB mechanism, we synchronized three high-speed cameras (bottom view, side view and total internal reflection view) and captured the detailed dry patch dynamics and relevant bubble behavior. A convective flow of 250 kg/m² s and subcooled 5 °C (water) was controlled on a heating surface (10 mm × 120 mm), which is much larger than the Rayleigh-Taylor instability wavelength (approximately 25 mm). As a brief result, a high heat flux near DNB (1000 kW/m²) produced periodic massive elongated bubbles through numerous bubble coalescence, and a thin liquid film was developed on the heater surface. Local evaporation of the thin liquid film generated a dry patch, which was observed using a total internal reflection technique. Accounting for the dry patch size and lifetime, the local overheated condition of the dry patch was discussed as an important physics of the irreversible dry patch (DNB). This study may provide a deep insight for understanding the DNB mechanism on the relatively low mass flow boiling condition.