Visualization of dropwise condensation on vertical plate and horizontal tube geometry
Creators: name: Naccarato, Blake 
Publisher: name: University of Nevada, Las Vegas
Year: 2016

description: A droplet detection method has been developed to measure the distribution of droplet sizes on a flat plate under dropwise condensation. Dropwise condensation heat transfer may be modeled by combining an expression for the single droplet heat transfer rate with the droplet size distribution. The ability to measure this distribution is integral to the validation of such models. An example study is undertaken in which heat flux is obtained for a given surface treatment by implementing such a model and measuring the droplet size distribution. These results are compared with the heat flux measured by internal coolant temperature monitoring for external condensation on a tube featuring the same surface treatment.
The plate condensing heat exchanger is a modular design for condensate visualization. The core of the design is a four way pipe cross with open flanges on each end. Flange caps are designed to accomplish the goal of condensate visualization, and are easily exchangeable depending on design intent. The sample side flange features a conductive contact between an external cold plate and internally mounted sample. A viewing flange opposite the sample side flange allows for lighting and capture of video data of the condensation process. A third flange features an internal, concentric boiler for steam generation. A vacuum pump valve and ambient temperature and pressure sensors are fitted to the fourth flange cap. Dropwise condensation models are explored in this setup by detection of droplets in the captured video data.
Droplet detection is performed by a Circle Hough Transform that has been modified to handle the order of magnitude differences in droplet radii within the same image. The Circle Hough Transform is applied to detect a radius range corresponding to the largest droplets, then the next largest droplets, and so on until the smallest detectable droplets have been marked. Detections in any given stage of the modified Circle Hough Transform are used to mask the detection region for the next stage. This reduces detection noise emanating from larger droplets that would otherwise overwhelm detections of smaller droplets. Another technique used in reducing detection noise involves illumination leveling, morphological erosion, and morphological reconstruction of the video data. The combination of these methods yield measurements of the droplet size distribution suitable for heat transfer analysis.
The droplet size distribution is dependent upon a balance between droplet growth and sweeping of condensate, which is observable by analyzing the distribution in each frame of the video data. While the distribution is constant for very large condensing surfaces, the local droplet size distribution varies as droplets nucleate, grow, coalesce, and are swept away by departing droplets. This apparatus and detection method make it possible to observe time dependent growth and sweeping mechanisms as well as the droplet size distribution that emerges from these mechanisms. This study demonstrates the utility of the apparatus and detection method for the validation of dropwise condensation heat transfer models.