High Fidelity Simulation of the Spray Generated by a Realistic Swirling Flow Injector

Abstract

Practical aero-engine fuel injection systems are highly complicated, combining complex fuel atomizer and air swirling elements to achieve good fuel-air mixing as well as long residence time in order to enhance both combustion efficiency and stability. While detailed understanding of the multiphase flow processes occurring in a realistic injector has been limited due to the complex geometries and the challenges in near-field measurements, high fidelity, first principles simulation offers, for the first time, the potential for a comprehensive physics-based understanding. In this work, such simulations have been performed to investigate the spray atomization and subsequent droplet transport in a swirling air stream generated by a complex multi-nozzle/swirler combination. A Coupled Level Set and Volume Of Fluid (CLSVOF) approach is used to directly capture the liquid-gas interface and an embedded boundary (EB) method is applied to flexibly handle the complex injector geometry. The ghost fluid (GF) method is also used to facilitate simulations at realistic fuel-air density ratio. Adaptive mesh refinement (AMR) and Lagrangian droplet models are used to efficiently resolve the multi-scale processes. To alleviate the global constraint on the time-step imposed by locally activated AMR near liquid jets, a separate AMR simulation focusing on jet atomization was performed for relatively short physical time and the resulting Lagrangian droplets are coupled into another simulation on a uniform grid at larger time-steps. The high cost simulations were performed at the U.S. Department of Defense high performance computing facilities using over 5000 processors. Experiments at the same flow conditions were conducted at UTRC. The simulation details of flow velocity and vorticity due to the interaction of the fuel jet and swirling air are presented. The velocity magnitude is compared with experimental measurement at two downstream planes. The two-phase spray spreading is compared with experimental images and the flow details are further analyzed to enhance understanding of the complex physics.

Detailed Numerical Simulations of Atomization of a Liquid Jet in a Swirling Gas Crossflow

“…An estimate of the Kolmogorov scale in the crossflow is given by η ∼ DRe − 3 4 g = 1.06µm. Though the smallest grid we use is ≈ 23η and doesn't capture the detailed flow, we believe it is good enough to capture the breakup dynamics of the droplets produced during breakup and is more resolved than most of the previous studies (for example, Li et al (2014) used a grid ≈ 130 times larger than η). Figure 2 shows the adaptively refined grid used in the simulations on a plane containing the annular axis and the injection point.…”

“…Thus a numerical investigation can complement the experimental observations to attain a better understanding of the involved physics. Due to the constraints on experimental conditions and choice of the working liquid, previous experimental studies on LJICF were mostly limited to high density ratio flows (Vich, 1997;Mazallon et al, 1999;Wu et al, 1997), whereas computational studies have been performed for both low and high density ratios (Aalburg et al, 2005;Elshamy and Jeng, 2005;Elshamy, 2007;Hermann, 2010Hermann, , 2011Behzad et al, 2015Behzad et al, , 2016Xiao et al, 2013;Li et al, 2014). A comparative study for density ratios of 10 and 100 by (Hermann, 2011) showed that with an increase in the density ratio: (a) penetration of the liquid jet increases, (b) bending of the liquid jet reduces, (c) spread of the spray in the transverse direction increases and (d) wavelength of the traveling wave on the jet and the surface wave seen on the jet decreases, thus producing smaller droplets.…”

“…Xiao et al (2013) performed high density ratio simulations of breakup of high speed turbulent jets and used the experimental results from Sallam et al (2004) and Elshamy (2007) for comparison of laminar and turbulent jets. Li et al (2014) performed simulations of breakup of swirling liquid jets. Although simulations of direct injection jets and swirling jets and that of breakup of low speed liquid jet by a crossflow have been performed, breakup of a liquid jet in a swirling crossflow has not been studied in detail.…”