Abstract

Atomization of liquids (pure liquids and suspensions) plays an important role in numerous industrial fields and applications. One of the main important applications is in thermal spraying processes which is the primary motivation for this study. The main trend in thermal spray processes is to coat with sub-micron and nano sized particles due to the superior performance of fine microstructured coatings. Recently, thermal spraying processes are using the suspension spraying technique. The breakup of a suspension in the atomization process differs from that of a pure liquid by the influence of the suspended particles on the fragmentation kinetics. In suspension spraying process, different types of atomizers are used but clogging problems can occur due to the suspension properties. Effervescent atomizers have shown to be a good alternative to the conventional atomizers to solve clogging issue when liquids with large variety of viscosity and density such as suspensions are atomized.
In this study, effervescent atomization of suspensions in a crossflow of air is investigated experimentally. The tests have been performed at different liquid-to-gas momentum flux ratios (q) and different gas to liquid ratios (GLR). Hydrophilic and hydrophobic particles are used in the experiments. Shadowgraphy and image processing have been used in order to capture the penetration height of the spray. New correlations have been developed to predict the spray penetration height of suspensions in case the non-aerated liquid jet (GLR= 0) and for the aerated liquid jet (GLR ≠ 0).
Moreover, suspensions properties such as viscosity and surface tension have a crucial effect on the atomization process. Because the atomization process and droplet formation occur in a very short timescale of the order of milliseconds, it is necessary to analyze the rapid change of the affecting suspension properties related to this timescale especially surface tension. Therefore, the time changing (dynamic) surface tension is more appropriate to be analyzed than static surface tension. In this work, the dynamic surface tension of suspensions is investigated using a combined analytical and experimental approach based on the physics governing the oscillation of elliptical jets. The dynamic surface tension of suspensions liquids in the timescale of milliseconds is calculated. The effect of the dynamic surface tension of suspension on its atomization process has been analyzed.