Evaluation of turbulent co-flow effects on liquid fuel atomization including spray evolution from a pressure swirl atomizer

Optimizing combustion systems is imperative due to current environmental and energy demands. To achieve optimal performance once liquid fuel is used for firing such systems, the liquid fuel atomization process needs to be well controlled as it determines all the subsequent multiphase flow evolution in the system. In pressure swirl atomizers, the atomization process typically relies on the combined effects of turbulent kinetic energy and non-axial kinetic energy of the fuel as it exits the nozzle. Notably, the incorporation of co-flow in the spray burner provides additional energy due to the turbulent co-flow level. In this study, numerical techniques are employed for the first time to assess the impact of varying mass flow rates of turbulent co-flow on in-nozzle flow dynamics, liquid atomization, and subsequent processes of an N-heptane spray jet from a swirl simplex atomizer. Appropriate droplet size and velocity measurements, achieved utilizing Phase Doppler Anemometry (PDA) alongside microscopic shadowgraphy to visualize spray atomization phenomena for a single co-flow mass flow rate value, are used as reference validation data. Numerically, a seamless coupling of the Volume of Fluid method (VOF) and the Lagrangian Particle Tracking (LPT) approach within a Large Eddy Simulation (LES) framework is applied. Prior to any analysis, the consistent agreement observed between simulation results and available experimental findings underscored the effectiveness of the employed approach inaccurately predicting and thoroughly exploring the whole phenomena under study. Then, the impact of varying the co- flow mass rate is quantified on the in-nozzle flow-dynamics and the flow field in proximity to the gas-liquid interface. In particular, changes in the primary and secondary breakup, initial and outer spray cone angle are evaluated in terms of liquid fuel sheet thickness, breakup length and Weber number as a function of mass co-flow rates. In the dilute spray region, the effects of different co-flow turbulent conditions on the dispersion of the spray are quantitatively evidenced by means of various spray droplet statistics.