A theoretical model with experimental verification for heat and mass transfer of saline water droplets

摘要

Heat transfer to and mass transfer from NaCl-water droplets are investigated both numerically and experimentally. A new model is presented and used to simulate saline water droplet evaporation. The model is robust enough to be applied for various initial concentrations and conditions of the droplet, ambient conditions, and dissolved media properties. The model is validated using experimental data obtained in this study on top of those already available in the literature. The experimental apparatus as well as the processing routines to optically measure droplet evaporation at a range of ambient conditions are presented. The droplet was suspended using a glass filament. Data were collected for droplets with an initial radius of 500 μm at three temperatures 25 ℃, 35 ℃, and 45 ℃ and three air velocities 0.5 m/s, 1.5 m/s, and 2.5 m/s to provide a comprehensive validation dataset. Based on experimental and simulation data, a correlation is presented that captures the start time of solid formation. This time plays an important role in cooling tower design as it shows the time that the outer surface of the droplet dries. Using the validated model, it is shown that for 500 μm radius droplets with 3% initial mass concentration the start time of reaching the final size is 17% less than evaporation time of a pure water droplet. Also, the net energy required to evaporate the droplet falls by 7.3% compared to a pure water droplet. For 5% initial concentration these values are 24.9% and 12.2%, respectively. Using saline water in spray-cooling has two major effects: the energy extracted from the air per unit droplet volume is reduced (which can be compensated for by increasing the liquid flow rate). Moreover, compared to the time taken for the evaporation of a pure water droplet, the period with wet surface is shorter as a result of crust formation around the saline water droplet. This allows a shorter distance between spray nozzles and heat exchangers.