Experimental Study On Heat Transfer And Wall Nucleation Of Subcooled Flow Boiling In Narrow Channel

The phenomena in narrow channels of subcooled flow boiling is widespread in many engineering fields, such as nuclear reactor fuel components, high performance plate heat exchangers, the cooling system of micro electromechanical system and so on. The influence of the limited space narrow channel to heat transfer characteristics is more complex and related research is relatively few, heat transfer and surface nucleation characteristics are unclear. This text used water as the working fluid, under different heat fluxes, different degrees of subcooling and different flow fluxes in different duct sizes, heat transfer and visual experiments had been taken.The study found that the heat transfer characteristics of the subcooled flow in the narrow channel were closely associated with the nucleation feature on the wall, the heat transfer coefficient along narrow channel increased with the increase of heat flux and inlet water temperature, but the influence of channel size and flow flux were complex Under the same conditions the instantaneous total number of bubbles in the local area presented cyclical change, the evolution process of the typical surface bubbles mainly included bubble growth, wall sliding, lifting, in addition merger, shrinkage, deformation. Periodic pressure drop was associated with the cycle of bubble oscillation evolution.The statistic results of active nucleation sites in different areas verified that the distribution of active nucleation sites on the copper surface was uniform.The experiential result showed that the density of active nucleation sites would increase with heat flux, and the increase was based on the foundation of pre-existing sites, only a few of pre-existing active nucleation sites were disappeared caused of the change of flow and heat transfer condition. It was obvious that the influences of wall superheats and channel size on the density of active nucleation sites.The experiment found that the bubble departure frequency of one certain active nucleation site showed a normal distribution, bubble departure frequency increased with wall superheat, and decreased with channel size.The cycle of instantaneous total bubble number in macro view had relationship with bubble departure frequency and the bubble sliding time on the copper surface. Bubble departure diameter and bubble lift-off diameter showed a normal distribution, there was a proportional relation between departure diameter and lift-off diameter in one certain active nucleation site. The lift-off diameter increased with wall superheat and channel size, decreased with mainstream subcooled degree.According to the pre-existing literature and based on experiment data, new correlations of wall nucleation features were correlated, including density of active nucleation sites、bubble departure frequency、bubble lift-off diameter,the new correlations were coincided well with experiment data for its considering of the effect of channel size.