A Numerical Study of Aero Engine Sub-idle Operation: From a Realistic Representation of Spray Injection to Detailed Chemistry LES-CMC

Abstract

High altitude relight is a matter of increasing importance for aero engine manufacturers, in which combustion plays literally a vital role. In this paper we want to evaluate the predictive capability of a combined Smoothed Particle Hydrodynamics (SPH) and Large Eddy Simulation with Conditional Moment Closure (LES-CMC) approach for a spray combustion process at these extreme conditions. The focus is on the SPH modelling of the kerosene primary atomization, the extraction of realistic spray boundary conditions for LES-CMC and the effect of the spray on combustion. Interestingly, it will be demonstrated that the fragment size distributions resulting from the airblast atomization are characterized by bimodal behaviour during the relight process and that small and large fragments differ significantly in their dynamical behavior. This is shown to affect the combustion in the Central Recirculation Zone (CRZ). Very large fragments are even able to supersede the flame from the CRZ, such that endothermic pyrolysis becomes dominant, but simultaneously essential to sustain and stabilize the remaining flame with reactive pyrolysis species. The study proves the ability of our methodology for extreme operating conditions, in which experimental insights are hardly possible.

Modeling Multivariate Spray Characteristics with Gaussian Mixture Models

“…To perform these simulations, we utilized the smoothed particle hydrodynamics (SPH) method, which was extensively employed in this context. The specific setup for these simulations follows the methodology outlined by Okraschevski et al [22]. In this setup, the primary atomization process is reduced to two dimensions in a necessary compromise between physical fidelity and computational cost.…”

“…In this setup, the primary atomization process is reduced to two dimensions in a necessary compromise between physical fidelity and computational cost. Despite its simplified two-dimensional approach, the SPH method has demonstrated its capability to accurately capture the spray characteristics [10,11,21,22,25,26]. Figure 1 displays two snapshots from the primary atomization process, one at OP1 and the other at OP4, clearly illustrating how the emerging ligament structures and, subsequently, the downstream droplet characteristics are impacted by the change in operating conditions with the decreasing air-fuel ratio.…”

“…The parametric ansatz functions used in some models, while suitable for specific geometries and operating ranges, may fail to accurately represent spray behavior under varying conditions, limiting their applicability in critical situations for combustion behavior and the design of atomizers. Of special interest are situations in which the spray characteristics deviate from the optimum, such as extreme operating conditions [22], or through temporal fluctuations induced by thermoacoustic instabilities [23,24], both of which have a significant impact on flame stability and emission characteristics. Another aspect that has to be considered is that a sufficiently sophisticated spray model might feasibly be employed to aid in the design process of atomizers.…”