Dropwise condensation, which collects the condensate liquid in the form of droplets, has attracted a growing interest due to much higher heat transfer coefficient. One important and challenging issue in dropwise condensation is the presence of non-condensable gas (NCG) which drastically reduces its heat transfer performance. Concerning the mechanism understanding, this thesis is aiming to investigate dropwise condensation in case of NCG by combing different methods. Firstly, convective dropwise condensation out of moist air is experimentally investigated under controllable conditions. In modeling, some crucial aspects are reasonably captured: the coupled heat and mass transfer during droplet growth by a multi-scale droplet growth model; the inter-droplet interaction defined by a distributed point sink method; the enhancement of the convective mass transfer using the droplet Sherwood number. Furthermore, a multi-component multi-phase thermal pseudopotential-based LB model is developed to advance the directly numerical simulation of dropwise condensation.

In dieser Arbeit wird der Wärmeübergang bei der Tropfenkondensation aus feuchter, turbulent strömender Luft experimentell untersucht. Die Versuchsstrecke besteht aus einem horizontalen Kanal in den Abmessungen (12 × 32 × 750) mm . Die Kondensation findet auf einem mit Kunststoff beschichteten Wärmestromsensor statt, der in einer der vertikalen Kanalwände bündig eingelassen ist. Experimentell erfasst werden der Wärmedurchgangswiderstand der Kondensatschicht durch Anwendung eines theromografischen Verfahrens sowie der in den Sensor eindringende Wärmestrom. Messergebnisse werden für verschiedene Strömungsgeschwindigkeiten, Luftfeuchten und -temperaturen, sowie für variierende Temperaturen der Sensoroberfläche gewonnen. Die Messergebnisse werden mit den Vorhersagen eines aus dem Kontext der Filmkondensation entlehnten Modells verglichen. Systematische Abweichungen konnten mit der Größe der Tropfen auf der Oberfläche korreliert werden. Die ermittelten Korrelationsgleichungen werden angegeben. Die Übertragbarkeit des hier entwickelten Ansatzes auf andere Geometrien und Randbedingungen konnte durch seine Anwendung auf externe Messdaten gezeigt werden.

Dropwise condensation (DWC) has the potential to enhance heat transfer compared to filmwise condensation (FWC). The heat transfer rates achieved by DWC depend on the drop size distribution, which is influenced by nucleation processes of newly formed drops. In DWC modeling, the nucleation site density Ns is used as an input parameter to obtain the drop size distribution of small drops. However, due to the small scale of the condensate nuclei, direct observation is difficult, and experimental data on the nucleation site density are scarce. In the literature, values in the range of 109 m−2 to 1015 m−2 can be found for Ns. In this paper, we report DWC experiments on SiO2 and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) thin hydrophobic coatings that show significantly different nucleation site densities. Nucleation site densities are estimated from high-speed imaging of small drops during initial condensation and from model calibration using established DWC theory. We have found the values for Ns to be in the range from 1.1×1010 m−2 to 5.1×1011 m−2 for the SiO2 coating and 1011 m−2 to 1013 m−2 for the PFDTES coating. Our results show that there can be large differences in the nucleation site density under similar conditions depending on the surface properties. This underlines the importance of investigating nucleation site density specifically for each surface and under consideration of the specific process conditions used for DWC.