Numerical Analysis of the Primary Breakup Applying the Embedded DNS Approach to a Generic Prefilming Airblast Atomizer

Abstract

An improved understanding of the breakup processes of two-phase flows is essential to effectively control the fuel atomization for future aircraft engines. A detailed insight into the phenomena of primary breakup is a major limitation in gaining this knowledge. Aircraft engines use airblast atomizers to provide the fuel atomization. The geometries of airblast atomizers are complex, the operating conditions are characterized by high Reynolds-and Weber numbers. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) concept aims to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A zonal Large Eddy Simulation (LES) of the turbulent channel flow is performed prior to the DNS. The parameters are stored transiently on the "virtual" DNS inlet planes. These variables are then mapped to the DNS. The Volume of fluid (VOF) method is used to solve for the two-phase flow. DNS are performed for a shear-driven liquid wall film and for a generic planar prefilming airblast atomizer. As the Reynolds and Weber number for the first operating point (OP) are low (Re air = 5,333/We film = 1.9), the liquid wall film as well as the liquid sheet show no surface waves. For the second case with Re air = 13,333 and We film = 11.9, the surface appears more wrinkled and streamwise waves are transported along the wall for the shear-driven wall film. Instantaneous snapshots in 2-D and 3-D illustrate the qualitative behavior of the liquid sheet in time. Leaving the prefilmer trailing edge, the liquid sheet starts to oscillate in a sinusoidal fashion. This oscillation appears crucial for the onset of primary breakup. The qualitative characterization of the breakup for OP 1 and OP 2, yield to the distinction of the stretched ligament breakup for the former, and the torn-sheet breakup for the latter OP. The breakup time for OP 1 is longer than for OP 2. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.

Euler–Lagrangian simulation of the fuel spray of a planar prefilming airblast atomizer

“…Using the Smoothed Particle Hydrodynamics (SPH) method, characteristic breakup phenomena known from experimental findings of Gepperth et al [6] like liquid accumulation, ligament flapping and bag breakup are captured well numerically [8,9]. In simulations based on the Volume of Fluid method [10,11] and the Level Set method [12] the same good agreement with experimental observations is achieved. Precise numerical investigations enable a comprehensive understanding of primary atomization and serve as a basis for the development of new primary breakup models.…”

A novel inlet boundary condition for DNS-VOF simulations of the primary breakup of prefilming airblast atomizers

“…Details on the fluid properties are summarized in Table 1, corresponding to a moderate Reynolds (Re = ρū g h c µ ≈ 13500) and Weber (W e l = ρū 2 g h l σ ≈ 8). This condition has been chosen since it has been extensively studied via experiments [9,10,12,21] and simulation through different methods [14,15,16,19,23]. Thus, a large amount of data is readily available for validation.…”

“…Given the simplicity of the planar configuration while still being representative of the breakup taking place in annular atomizers, most numerical studies have focused on the former geometry. DNS studies include Volume of Fluid (VOF) [14,15,16] frameworks; coupled VOF-Level Set [17,18,19]; or Smoothed Particle Hydrodynamics (SPH) in 2D [20,21] and 3D [22]. Studying the full atomizer geometry through DNS is unfeasible.…”

“…Inflow boundary conditions are thus key to the simulation development and the predicted atomization features. While some authors acknowledge using constant velocity profiles for the liquid and gas phases at the inlet [19,22], Sauer et al [14,15,16] introduced the embedded DNS concept in which they prescribe turbulent fluctuations at the inlet thanks to the use of precursor LES. Warncke et al [23] compared two DNS using a mean gaseous velocity profile or a turbulent time-varying profile mapped from the precursor LES.…”

Experimental investigations of atomization characteristics of an airblast atomizer at high ambient pressure

“…Harani and Sher [30] showed that the most uniform droplet distribution is obtained when angle of impact between the air jet and the liquid jet is 30°. Recently, the design and optimization of airblast atomizer geometry highly relies upon high-accuracy numerical simulations to shorten the research time and to reduce research expenses [31,32]. Additionally, physical properties of fuels also greatly affect the size of spray droplets in fuel spray.…”