Application of Large Scale Schlieren in Characterization of Swirling Plume Shape And Characteristics

Abstract: Schlieren imaging is one of the most promising flow visualization techniques, but its application to large-scale systems has been limited due to physical constraints. In this study, a large-scale background-focusing Schlieren imaging system was developed to investigate the effects of swirling parameters on the thermal plume generated by a natural draft dry cooling tower model. A comparison between the Schlieren images and known swirl conditions revealed that background-focusing Schlieren can effectively capture swirl characteristics and respond to ambient conditions, such as crosswind. Introducing swirling flow into the model increased the plume's diverging angle and altered its shape, which was directly correlated with the estimated swirl number. The swirling characteristics were further quantified through field measurements of flow velocity. Turbulent features visible in the entrainment area of the plume were used to approximate the axial velocity of the plume series. The results showed that increasing the swirl velocity enhanced the axial velocity, leading to improved heat rejection from the cooling tower. The development of large-scale focusing Schlieren imaging is a crucial advancement for diagnosing fluid flow in natural convection heat transfer projects, particularly in the context of meeting global carbon emission targets. Keywords: Swirling Thermal plume, background focusing schlieren, Flow visualisation