HEAT TRANSFER CHARACTERISTICS AND BUBBLE BEHAVIORS DURING NUCLEATE FLOW BOILING FOR SODIUM CHLORIDE SOLUTION

Deep understanding of nucleate boiling heat transfer mechanism of saline solution is of great importance for the design and safe operation of steam generation equipment. In this paper, the nucleate flow boiling process of saline solution in a vertical heated pipe was experimentally studied within the concentration range of 0 % similar to 6 %. In order to realize the visualization, the vertical heated pipe was made of transparent silica glass and a transparent ITO heater was used to provide energy for boiling. The high-speed high-resolution camera was used to capture the vapor-liquid two-phase flow structure. The bubble behaviors such as bubble departure diameter, bubble departure frequency, bubble growth time and waiting time were investigated under different operating conditions. The experimental results showed that the heat transfer deterioration did not occur within the solution concentration of 6% in this work. Under some low heat flux conditions, the heat transfer coefficients of solution can be higher than those of pure water. The reason for this phenomenon can be explained by the different bubble behaviors. Comparing to pure water, the bubble departure diameter of saline solution is bigger and bubble departure frequency is lower. The influences of operating parameters, including concentration, mass flux (200 kg/m(2)s similar to 600 kg/m(2)s), heat flux (30 kW/m(2) similar to 180 kW/m(2)) and subcooling of fluid (5 K similar to 35 K), on the nucleate boiling heat transfer coefficients and bubble parameters were comprehensively studied.