Heat transfer and evaporation during single drop impingement onto a superheated wall

by Stefan Batzdorf

Institution: Technische Universit├Ąt Darmstadt
Department: Technische Thermodynamik
Degree: PhD
Year: 2015
Record ID: 1097993
Full text PDF: http://tuprints.ulb.tu-darmstadt.de/4542/


This thesis is aiming for the numerical simulation of the impingement process of a single droplet onto a wall which is superheated against the fluid's saturation temperature corresponding to the bulk pressure. The heat transfer during drop impingement is of particular importance in spray cooling which is a promising technology for the removal of high heat fluxes at a small temperature difference. While the hydrodynamics of an impinging droplet have been studied extensively in the past, the heat transfer to the droplet during the impingement process in the non-isothermal case is not yet fully understood, in particular if evaporation comes into play. Moreover, many studies on pool boiling heat transfer have demonstrated that the evaporation at the 3-phase contact line, where the solid, liquid, and gas phase meet, might contribute significantly to the overall heat transfer. Hence, it can be expected that a proper knowledge of the physical processes at the contact line might be crucial for the understanding of the entire process. However, up to now no attempt has been made to model the heat transfer of an impinging droplet just above the boiling point taking into account the microscale thermodynamic effects at the contact line. To shed light on the individual heat transfer processes involved in the overall process and to quantify their individual importance, a numerical simulation of the drop impingement is conducted within this thesis. Numerical simulations provide data on small length and time scales which cannot be resolved with available measurement techniques. The numerical model is based on the Volume of Fluid method to track the evolution of the droplet shape. Evaporation is accounted for at the surface of the droplet. Special attention is payed to the modeling of the evaporative heat transfer in the vicinity of the moving 3-phase contact line. The developed numerical model is validated with the help of highly resolved experimental data on single drop impingement. A good agreement of the model predictions to the measurements is achieved. At the same time the detailed information provided by the simulation are employed to identify the dominant phenomena governing the heat transfer during the entire impingement process. Moreover, the model is utilized to quantify the impact of the governing influence parameters. Thereby it is made use of the great advantage of numerical simulations that any parameter can be controlled individually without any additional effort. Even though the focus of this thesis is on single droplets, also the interaction of individual droplets during their impingement is addressed briefly.