Numerical modeling of gas-liquid flows in a swirl nozzle

The present dissertation belongs to the research area of fluid dynamics. In particular, scale-resolving computational fluid dynamics (CFD) methods are utilized to investigate liquid injection and atomization processes in industrial context. The two-phase flows are solved using the volume-of-fluid (VOF) method utilizing a sharp interface capturing framework. The work is motivated by high-viscosity, biomass-based renewable fuel injection where the atomization process and the size of the liquid droplets may relate to the overall quality of the combustion process. In this work, liquid injection from a large-scale asymmetric pressure-swirl atomizer is studied in different conditions. In contrast, previous research has mainly concentrated on small-scale nozzles with symmetric designs. Detailed simulations of both the inner-nozzle flow characteristics and the onset of liquid sheet breakup are carried out. Publications I and II investigate the low-Reynolds-number regime (420 ≤ Re ≤ 5300) in detail. Full understanding of the atomizer flow characteristics and key spray parameters has been so far lacking for the studied parameter range. In addition, comparison with experimental measurements is carried out in Publication II. In Publication III, bubbly flow in the atomizer is studied with relevance to injection scenarios where significant volume of gas is present inside the nozzle. Thus far, such conditions have remained relatively unexplored by detailed simulations in the context of swirl nozzles. The novelty of the present work relates to the application of highly resolved numerical simulations to study a large-scale swirl atomizer in conditions relevant for industrial applications. The main outcomes of the dissertation are as follows: (1) Detailed description of the inner-nozzle flow characteristics is provided. (2) Near-nozzle liquid sheet characteristics are analyzed and the existence of different spray patterns at different Reynolds numbers is demonstrated. (3) The effects of bubbly flow on the atomizer performance and liquid film characteristics are assessed. In addition, practically relevant nozzle key parameters, such as the spray opening angle, discharge coefficient, and film velocity, are reported. Also, the applicability of the used computational approach in the present application field is demonstrated via sensitivity tests, numerical benchmarks and experimental validation.