8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P6.21CFD simulation of condensation and growth of liquiddroplets on surfacesChristian Jungreuthmayer, 1 Christoph Körber, 1 and Helmut Kühnelt 11 AIT Austrian Institute of Technology GmbH, Giefinggasse 2, 1210, Vienna, AustriaCondensation and growth of droplets play a critical role in many fields of science and engineering.The involved processes are non-trivial and the quantitative prediction of condensation phenomenais still a major challenge - despite the huge effort put into this branch of physics for more thana century. In order to numerically calculate the condensation and growth of liquid droplets onsurfaces, an extension of the CFD program Fluent was developed in form of a UDF (User DefinedFunction). The basis of the implemented UDF was a transient Eulerian-Eulerian multi-phase caseusing two phases: a) a non-condensable carrier gas and b) vapor of a condensable liquid. Each timestep the multi-phase solver calculates a set of physical properties, such as temperature, pressure,volume concentration, etc. Based on these properties the UDF calculates the number of dropletscreated by nucleation on each face of the investigated walls utilizing an energy balance approach,which considers the energy released by the phase transition from vapor to liquid and the energyrequired to overcome the surface tension. The number and size of droplets per face are trackedby a histogram approach using a limited number of discrete droplet diameters per face. Everysingle droplet created is assigned to a specific diameter range increasing the droplet count of theassociated droplet container. Starting from the initial diameter, droplets grow by “consuming”vapor molecules of the surrounding atmosphere. This grow process is determined by consideringthe involved heat and mass balances and causes a shift of droplets from containers for smallerdroplets to containers for larger ones. The results of the simulations are compared to measurementsperformed by our group using a con-focal microscope, a Peltier cooled polycarbonate substrate,and a sophisticated Matlab program to automatically scan, detect, and statistically evaluate waterdroplets.21