Numerical Simulation of Aircraft Engine Related Two-Phase Flows
Introduction and Motivation
Airblast atomizers serve as the state of the art technology in modern high performance aircraft engines to assure fuel atomization. During airblast atomization a thin liquid film is passed by coflowing air streams, leading to the disintegration of the liquid sheet. The breakup process is complex and still not well understood. Therefore, a detailed insight into the phenomena of primary breakup is needed for understanding theses flow systems. Here, the relation between operating conditions and breakup behavior is of particular interest for facilitating the predictability of combustion chambers in aircraft engines.
For this research, highly resolved Direct Numerical Simulations (DNS) of airblasted liquid sheets are performed. In this context, a new approach, the embedded DNS (eDNS) concept, is used. The eDNS concept limits computational costs and makes DNS applicable to industrial applications.
Method and Theory
DNS provides the possibility to study the breakup behavior without any models as all relevant length scales are resolved. In addition the resolution of liquid structures and droplets (< 10 µm) requires an extremely refined mesh, the computational costs rises significantly. Here, the embedded DNS concept is set.
Central aspect is to keep the computational DNS domain as small as possible. Only the region of interest is simulated using DNS, while the DNS domain is embedded in a larger and wider domain. The wider domain is simulated using Large Eddy Simulations (LES), delivering high quality boundary conditions for the DNS simulation. With this methodology the highly refined DNS region can be reduced to the breakup region.
The phase interface of the two-phase flow is resolved applying the Volume of Fluid (VOF) methodology, an interface capturing approach. Here, an indicator function defines the volume fraction of each fluid in each gird cell. The region of the phase interface is marked with values between 0 and 1.
In order to understand the primary breakup of airblasted liquid sheets and to prove the applicability of the eDNS concept, computations of several operating conditions are necessary. A generic prefilming atomizer with a planar geometry is simulated, facilitating the analysis of multiple parameters. Concretely, the influence of the gas phase velocity, the density ratio between the two fluids and the surface tension of the liquid plays a significant role. In addition, the planar geometry of the prefilming airblast atomizer assures comparative studies together with experiments. Here, new analysis methods need to be developed to compare reliably numerical and experimental results.
The results enables a detailed view into the breakup phenomena. Besides the qualitatively appearance of the disintegrating liquid sheet also characteristic quantities are received: length and velocity scales of the accumulated liquid at the prefilmer trailing edge as well as of the resulting ligaments and droplets. Thus, droplet distributions for size and velocity can be derived. This valuable droplet information serve as input data for subsequent Lagrangian Particle Tracking simulations.