Experimental study of heat transfer and bubble behaviors of NaCl solutions during nucleate flow boiling

Concentrated salt solutions are used as working fluids in heat exchangers for heavy oil drilling to meet the requirements of environmental protection. Deep understanding the nucleate flow boiling heat transfer mechanism of salt solutions is of great importance for the design and safe operation of modern steam power equipment. In this work, the nucleate flow boiling for NaCl solution and pure water was experimentally studied within the concentration (C) range of 0%–6% in a vertical heated tube. To simultaneously obtain the heat transfer coefficient (HTC) and bubble parameters, the test section was built to be transparent and the energy for boiling was provided by a transparent ITO heater. The bubble parameters, including bubble departure diameter (Dw), bubble departure frequency (f), bubble growth time (tg) and waiting time (tw), were measured in the experiments. A wide range of operating conditions is considered in this study, covering the inlet fluid temperature (Tinlet) between 337 and 365 K, the mass flux (G) between 200 and 600 kg/m2s and the heat flux between 10 and 180 kW/m2. Based on the experimental results, the difference of nucleate flow boiling heat transfer performance between pure water and solutions was compared. It was found that the HTC of pure water was higher than that of salt solutions under the operating conditions of Tinlet 351 K and q > 180 kW/m2, in some cases, it was lower than that of solutions. Dw in the solution is bigger than that in pure water while f in the solution is lower. The corresponding reasons for those heat transfer differences between pure water and solutions were revealed by considering the differences in bubble behaviors, physical properties and mass transfer rate. In addition, the influences of operating parameters on the heat transfer characteristics and bubble behaviors of salt solution during nucleate flow boiling were demonstrated.