Numerical investigation on heat and mass transfer characteristics of LiCl solution falling film on finned tube of frost-free evaporator

Frost-free air source heat pumps (FFASHPs), which integrate the functions of closed-type heat-source towers, play a crucial role in promoting the development of low energy, ultra-low energy or even zero energy buildings. The frost-free evaporator is one of the key equipment in the system, and its performance is affected by the inlet gas-liquid parameters. In order to provide theoretical support for the efficient operation of the frost-free evaporator, a heat and mass transfer model is established in this study. The upper critical Re L and lower critical Re L of complete film flow are obtained by analyzing the influence of liquid film Re L on the plain finned tube surface with contact angle of 10 degrees . The corresponding prediction correlation of dimensionless average film thickness is given, and the deviation is less than 2.5 % compared with the previous correlation of water film thickness. Subsequently, the influence of inlet air and solution parameters on the heat and mass transfer characteristics of LiCl solution in falling film on finned tube is studied. The results show that the air side performance of the frostfree evaporator is affected by inlet air flow rate, solution flow rate, concentration and temperature. With the increase of inlet air flow rate, the average nusselt number at the surface rises slowly and then decreases rapidly, and reaches its peak at the inflection point of air flow rate of 0.6 m/s. The increase of the solution flow rate can effectively promote the heat and mass transfer process outside the frost-free evaporator, which is due to the increase of the contact area and the enhancement of the interface disturbance. The increase of solution concentration will also increase the partial pressure difference of steam near the gas-liquid interface, thus enhancing the mass transfer process. However, the solution concentration should not exceed 40 %, because too high a concentration will cause it to crystallize at low temperatures.