EXPERIMENTAL INVESTIGATION ON THE BOILING HEAT TRANSFER OF WATER IN VERTICAL UPWARD SMOOTH TUBES AT SUB-CRITICAL AND NEAR CRITICAL PRESSURES

摘要

Under sub-critical and near critical pressure, the heat transfer characteristics of water in a vertical upward smooth tube with the diameter of φ 32 ?? 3mm were researched experimentally. The experiments were performed under conditions of the pressure p=13~21.5Mpa, the mass velocity G=600~1200kg/(m<'2>s) and the internal wall heat flux q=200~600kW/m2. Based on the experiments, it was found that the effect of the pressure on the heat transfer deterioration in sub-critical pressure region was different from that in near critical pressure region. The departure from nucleate boiling (DNB) mainly occurred when the critical heat flux (CHF) was high and the dryness was low. The increasing of the mass velocity was favorable to eliminate DNB, and the design mass velocity must surpass the critical mass velocity under the corresponding work condition in the engineering design. Correlations of the critical mass velocity, CHF and the minimum heat transfer coefficient have been provided and can be used as the reference for the design of the engineering practice.