Drop and spray impact onto a hot substrate: Dynamics and heat transfer

• Main Author: Breitenbach, Jan
• Format: Others
• Language: en
• Published: 2018
• Online Access: https://tuprints.ulb.tu-darmstadt.de/8097/1/070119_Dissertation_Breitenbach.pdf; Breitenbach, Jan <http://tuprints.ulb.tu-darmstadt.de/view/person/Breitenbach=3AJan=3A=3A.html> (2018): Drop and spray impact onto a hot substrate: Dynamics and heat transfer.Darmstadt, Technische Universität, [Ph.D. Thesis]

Non-isothermal spray/wall interaction is an important process encountered in a large number of existing and emerging technologies, such as fuel injection in aircraft gas engines and internal combustion engines, and is the underlying phenomenon associated with spray cooling technology. Spray cooling is a very promising technique for the cooling of devices with very high heat flux densities (as encountered in the fields of metalworking, cooling of electronic components or light-water nuclear reactors), surpassing all other conventional cooling methods. The effectiveness of spray cooling is influenced by a large number of parameters, including spray characteristics like drop size, velocity and number density, the surface morphology, but also on the temperature range and thermal properties of the materials involved. Indeed, the temperature of the substrate can have significant influence on the hydrodynamics of drop and spray impact, an aspect which is seldom considered in model formulation. This process is extremely complex and current approaches are highly empirical in nature. In the present thesis the single drop impact as a central element of spray impact is experimentally investigated for various thermodynamic and hydrodynamic conditions. Understanding single drop impact is an important and necessary preliminary work in the description and modeling of non-isothermal spray impact. The observed outcomes of single drop impact are classified for various impact conditions according to the well-known heat transfer regimes: single phase cooling, nucleate boiling, transition boiling and film boiling. Observations from the present work also introduce the thermal atomization regime. The phenomenon is characterized by the dewetting of the substrate, caused not by rim dynamics but induced by thermal effects, and an intensive evaporation leading to a fine secondary spray. Various theoretical considerations for the heat transfer regimes single phase cooling, nucleate boiling, thermal atomization and film boiling are obtained to describe the quantities involved in the non-isothermal drop impact. The theories allow predictions of the heat transferred from the hot substrate to the impinging drop, the typical time of drop contact, and the secondary spray. These quantities are of paramount importance for spray cooling application, since they can be used to determine the optimum spray. The theoretical predictions account for the underlying physical phenomena and are validated with existing data. Finally, the consideration for the single drop impact is used for the development of a theoretical model for an average heat transfer coefficient for spray cooling in the film boiling regime. The model captures the influence of spray characteristics and accounts for the probability of drop interactions on the wall, when the droplet number density in the spray is high. The theory agrees well with existing experimental data.