Extensive research on horizontal tube falling film evaporation in desalination has been conducted on flow modes, flow and heat transfer characteristics and enhanced heat transfer structures. Studies on the flow modes have found that the heat transfer rate of the droplet mode is was markedly greater than that of the column mode and sheet mode due to the effects of thinning liquid film and increasing interface area in the droplet falling film evaporation.
The current measures to attain droplet falling film evaporation on smooth horizontal tubes involve reducing its flow density to the extent that the reduced flow density is less than the critical flow density (defined as the minimum flow density for the fully wetted horizontal tube). Therefore, the occurrence of dry patches on the tube surface [27,28] deteriorates the heat transfer rate in droplet falling film evaporation, resulting in the impossibility of its practical application in the desalination industry. The micro-nano structures on the surfaces of horizontal tubes manufactured by the micro-electromechanical system, play an important role in addressing the issue of dry patches in the droplet falling film evaporation by decreasing the contact angle of the horizontal tubes and increasing their wettability.
In contrast with the smooth horizontal tube, the critical flow density of the micro-nano structured horizontal tube is decreased by 90 %, and its falling film heat transfer coefficient is increased by more than two times.