A study of departure of nucleate boiling (DNB) mechanisms on forced convective channel flow

We studied a departure of nucleate boiling (DNB) on upward forced convective channel flow on vertical large-area (10mm×120mm) ITO heater (Indium Tin Oxide), using synchronized high speed visualization which views bubble side and its bottom. The D.I. water flow has 250kg/m²s, subcooled 5-10℃ and atmospheric pressure (1 Bar) condition. As a heat flux increase to 70% of DNB, numerous bubble coalescence and being confined by channel (15mm × 15mm) form the elongated massive bubble. In addition, the massive bubble moves slidingly along the channel wall, giving rise to thin liquid film on heating surface. Under the thin liquid film condition, vigorous nucleation activity and dry patch behavior have been observed through total reflection method visualization. When a heat flux reach to near DNB condition, the elongated massive bubble length become much longer and the dry patch is formed. As the sliding time for the elongated massive bubble increase, the dry patch grows larger and has enough time for heating up the surface. After the elongated massive bubble sliding, a strong rewetting process with the liquid front has been observed. In this study, the formation and growth of the dry patch has been explained with the visualization results, and the heating process and re-wetting ability has been discussed. Through the above visualization, this study can give a deep insight for the mechanisms understanding of the DNB on relatively low mass flow condition.