Characteristics of primary air condensation in indirect evaporative cooler: Theoretical analysis and visualized validation

In hot and humid regions, an indirect evaporative cooler (IEC) could be utilized to pre-cool and pre-dehumidify fresh air by capturing the cooling potential of exhaust air from indoor air-conditioned space. The condensate retained on the surfaces of primary air channels has a profound impact on the heat transfer performance of the IEC. Although the studies of IEC considering condensation have received extensive attention recently, the micro scale condensation mechanism has seldom been addressed. In this paper, the analytical model and experimental investigation on the condensation heat transfer of the IEC under different dehumidifying conditions are presented based on visualized observation and measured data. An image processing method is developed to determine the area ratios of dropwise condensation (DWC) and filmwise condensation (FWC) on the plate surface. Results show that, with the increase of inlet primary air temperature, the area ratios of the DWC and FWC remain relatively stable at 0.4 and 0.6 respectively, while the overall heat flux slightly increases. The relative humidity of primary air can significantly influence the overall heat flux on the plate surface, as the increased FWC under high relative humidity results in a decreased growth rate of heat transfer coefficient. Higher air velocity can greatly improve the condensation heat transfer performance by enlarging the area ratio of the DWC. The results of this study enlighten the condensation heat transfer characteristics of primary air on the plate surface of the IEC, providing references for heat transfer enhancements of air-conditioning systems by promoting dropwise condensation.