Retention and overlapping in long-term multi-nozzle water spraying

In the present paper, water spraying is designed for an embedded air conditioning application in order to enhance heat transfer performances. The system was studied dynamically on a long-term evolution during spraying and after spraying. A focus has been done on the water retention to evaluate its effect on the efficiency. This work has characterized separately, then compared the thermal heat flux and the retention rate, obtained with the ratio of the efficient cooling surface on the total spraying surface. The total spraying surface (TSS) corresponds to all the impacted surface and the efficient cooling surface (ECS) corresponds only to the parts with no water retention. These surfaces were obtained by using a thermal image analysis. This analysis was based on several image analysis techniques, as image subtraction, grey-scaling, smoothing or binarizing. The present work has shown that retention encounters the heat transfer enhancement. Thus, after spraying, the evaporation of a water film on the heat exchanger surface increases the heat-exchanger efficiency. It means that retention induces a phenomenon of thermal energy storage in the water film. However, beyond a 25 min of spraying time, this thermal energy storage is no more interesting because the heat loss due to retention is too important. Besides, it has been noticed that the retention effect was independent from air flow rate, water flow rate and wall temperature: it depended only on spraying time. •Retention is not immediate: it appears after 5 min.•300 cm2 of retained water can decrease the heat-exchanger heat flux of 14%.•Retention induces a phenomenon of thermal energy storage in the water film.•Retention effect depends only on the spraying time.•For multi nozzles, the heat transfer enhancement is cumulative without overlapping.