Effects of swirl on nonreacting and reacting flows in a single-element lean direct injection combustors

Purpose - The swirl intensity imposed on the flow plays a vital role in aerodynamics, flame shape, flame stabilization and combustion intensity. In lean direct injection (LDI), the air and fuel are fed through separate channels, and the swirling air flows have a strong impact on fuel-air mixing and heat release. The literature indicates that the effects of swirling on helical axial LDI systems are limited to nonreacting flows studied through experimental methods, but not many studies have been reported on the reacting flows of a single swirler. The objectives of the paper are divided into two parts. The first part presents the role of swirl in nonreacting LDI systems and the second part describes spray combustion in LDI systems for low (swirl < 0.5) to high (swirl > 0.5) swirl numbers. Design/methodology/approach - The numerical model incorporates all the necessary features of the single helical axial swirler, starting from the hollow circular section to the outlet of a long mixing chamber. The commercial solver FLUENT is used to predict the flow field around the axial swirler. The first step is to establish a numerical procedure (based on computational fluid dynamics [CFD]) to predict the nonreacting flow behavior for different swirlers and the CFD results are validated against literature data. The spray atomization, droplet evaporation and the effects of interaction between the two phases are modeled by implementing various spray submodels in FLUENT. The large Eddy simulation (LES) reacting flow results for a vane tip angle of 60 degrees are compared with test data and presented at multiple cross planes. Findings - The numerical simulations were carried out on a nonreacting single helical axial swirler for various vane tip angles, such as 40 degrees, 50 degrees, 55, and 60 degrees, and the results were validated against test data. The centerline mean axial velocity and radial velocity profiles at several axial locations are in good agreement with the literature data. For reacting swirling flows, the experimental data is available only for a 60 degrees vane tip angle. The S60 reacting flow LES mean predictions are compared at different cross planes. The axial momentum increases due to the liquid spray combustion in the gas phase and the reacting flow central recirculation zone is substantially shorter than the nonreacting flow. The impact of spray atomization due to interaction with the gas phase is verified, and the droplet mean diameter trends are consistent across different cross planes. The LES predictions of reacting flows for low to high swirls are investigated, and the effects on combustion performance are summarized. Originality/value - The novelty of the paper is highlighted in two key conclusions. First, the paper presents numerical methods for studying the role of swirl in a nonreacting LDI system and validates the results against experimental data. Second, the effects of combustion on the gas phase, spray combustion modeling and droplet atomization are numerically established and compared with literature data for a 60 degrees vane tip angle. In addition, the role of swirl in the reacting flow field for vane tip angles of 40 degrees, 50 degrees and 60 degrees is numerically investigated, and its effect on flame stability, pressure drop and NOx emissions is presented. The paper describes LES grid guidelines for the LDI swirler and presents a numerical modeling approach that helps to develop a robust swirler design through a parametric investigation of swirler geometry. The methodology can be extended to study multi-element swirler configurations, to understand the effect of swirl on droplet breakup, momentum exchange with adjacent swirlers, flame propagationand emissions.