ILASS - Europe, Institute for Liquid Atomization and Spray Systems
Abstract
Heat and mass transfer mechanisms related to multiphase flows occur in several applications such as spray cooling, quenching, internal combustion engines and plasma spraying. These mechanisms have become increasingly important due to the need of achieving higher heat rate coefficients associated with phase-change processes, such as evaporation and condensation. Specifically, the phenomenon of droplet impact onto non-heated liquid films has been extensively researched, both experimentally and numerically. However, the influence of temperature on droplet impact and liquid film stability has been overlooked in the literature, which is a focal point in understanding interfacial phenomena. The main objective of this work is to experimentally study droplet impact onto heated liquid films. Therefore, an experimental facility was designed for this purpose. A borosilicate glass surface is used to contain the liquid film. This surface is placed above an aluminium block with four embedded cartridge heaters of 250W each, heating the liquid film by conduction. Immersion type-k thermocouples are employed for liquid film temperature measurements. Liquid film evaporation rates are calculated in order to ensure the liquid film thickness prior to the droplet impact.
Water and n-decane are the fluids adopted due to their differences in thermophysical properties and saturation temperature. The impact conditions are 100 < W e < 300, 0.5 < h* < 1.5, and a dimensionless temperature of θ < 0.6. Qualitative analysis is performed regarding crater and central jet evolution, and quantitative data regarding evaporation rate and central jet height are measured. The dimensionless temperature affects the droplet impact phenomena, creating recirculation zones near the crater and the impact surface, and affecting the crater formation. The central jet height increases with increasing values of θ for h* = 1.0 and h* = 1.5, whereas for h* = 0.5, the measurements do not follow a similar tendency. The emerging time of the central jet is delayed for higher values of θ, meaning that future studies regarding crater evolution should be considered. The dimensionless temperature also promotes central jet breakup, as well increasing the number of secondary droplets originated from the breakup.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
