Session G25: Reacting Flows: Sprays Emissions and Soot

Numerical Simulations of Soot Formation in Turbulent Premixed Flames of Jet Fuels

Numerical simulations of soot formation in a fuel-rich turbulent premixed flame of a gasified jet fuel are studied in a canonical setup. The fuel kinetics is based on 62 species mechanism developed by T. Lu & co-workers at University of Connecticut and includes reactions for polycyclic aromatic hydrocarbon (PAH) formation and growth. Two different modeling approaches, Method of Moments with Interpolative Closure (MOMIC) [Frenklach, M. Chemical Engineering Science 57.12 (2002): 2229-2239.] and a three-equation model [Franzelli, B. et. al.  Proceedings of the Combustion Institute 37.4 (2019): 5411-5419.], are assessed under identical flow conditions to determine and compare their predictions of global soot quantities such as number density, surface area and volume fractions. The effects of different equivalence ratios of the premixed mixture as well as different turbulence intensities on soot formation and growth are studied. Results are analyzed to determine stages of soot nucleation in a turbulent premixed flame based on different PAH species and their sensitivity to predictions will be addressed using both models.

    

Evolution of the soot particle size distribution in turbulent nonpremixed bluff body flames using the Bivariate Multi-Moment Sectional Method

To address current and emerging soot particle size regulations, understanding the evolution of the soot particle size distribution (PSD) is a critical need. Traditionally, sectional methods have been utilized to simulate the evolution of the PSD in turbulent reacting flows utilizing Large Eddy Simulation (LES) but suffer from simultaneous shortcomings of excessive computational cost and physical inaccuracy due to their limitation to a single size coordinate incapable of accounting for the aggregate structure of soot. The Multi-Moment Sectional Method (MMSM) tackles the former shortcoming by considering two moments per section and a locally linear reconstruction of the size distribution within a section but fewer sections so fewer overall unknowns. In this work, the Bivariate MMSM (BMMSM) extends MMSM to consider a joint description of soot aggregate morphology in volume and surface area and overcome the latter shortcoming. BMMSM is first validated in one-dimensional laminar flames and then implemented in LES and applied to simulation of a series of turbulent nonpremixed bluff body flames. The evolution of the PSD is analyzed in the different regions of the flame.   

Flame Dynamics of a reactive spray in vitiated crossflow

This work investigates experimentally and numerically the effect of the air-to-liquid massflow ratio (ALR) on the topology of a Jet A-1 airblast spray flame. The spray is injected transversely into a turbulent vitiated crossflow composed of products of a lean natural gas flame. The spray flame thermal power is varied between 2.5 and 5 kW and the ALR between 2 and 6. The reaction zone is characterized using OH* chemiluminescence and OH and fuel planar laser induced fluorescence. The Large Eddy Simulations of the multiphase reactive flow agree closely with the experiments. Our study shows a substantial effect of the ALR on the mean flame topology and dynamics due to its influence on flow field and spray characteristics. A low ALR results in a relatively small jet to crossflow momentum ratio (J) and a large spray Sauter mean diameter (SMD). A thick windward reaction region is formed due to poor mixing between the fuel spray and the crossflow. Meanwhile, the correspondingly high spray SMD leads to isolated penetration and combustion of bigger droplets. At high ALR, the reaction fronts are more homogeneous and distributed as a result of the efficient entrainment-induced mixing on the leeward side caused by the high J and the faster droplets evaporation due to the lower spray SMD.   

Topology transition of the multiphase reacting flow in a Lean Azimuthal Flame (LEAF) combustor

The pressing need to reduce the aviation carbon footprint has propelled several research endeavors to develop new combustor concepts aiming at clean combustion of sustainable aviation fuels (SAF). The current work presents the flame characteristics in a Lean Azimuthal Flame (LEAF) combustor, which has shown ultra-low NOx and soot-free liquid fuel combustion. In the presented work, three Jet-A1 sprays are injected axially into a whirling main airflow, where the interaction between them leads to a spray in a vitiated crossflow configuration. The continuous entrainment and mixing of hot products result in the sequential combustion of the sprays by each other. The combustor is investigated for the Jet-A1 thermal power range from 15 kW to 25 kW, with varying atomization-air to liquid mass flow ratio (ALR) using OH-chemiluminescence, Mie scattering, and OH-PLIF. The flame shows the LEAF configuration at high ALRs, whereas, at low ALR, the flame exhibits a tubular topology. It is shown that the convective and spray evaporation timescales are the two prominent parameters that govern the observed flame behavior. A simple model based on empirical correlation is derived, allowing us to predict the change of flame topology associated with the ALR variation in this complex LEAF configuration.   

Transcritical Phase Change in High-Pressure Spray Combustion

The requirement of high power outputs and high efficiencies of combustion engines such as rocket engines, diesel engines and gas turbines has resulted in the increment of the system pressure close to or beyond the critical point and hence often leads to the fluids becoming supercritical in state. This has led to increased interest in both the high-pressure multicomponent phase change phenomena as well as their chemical reactions. Most existing multiphase research is concentrated on either a single- or two-component system, hence less relevant to the multicomponent phase change and reactions occurring in real engines. This work will employ our previously developed thermodynamic model based on vapor-liquid equilibrium (VLE) theory, which can predict the high-pressure phase separation near mixture critical points. This model is coupled with a reacting flow solver developed in OpenFOAM to simulate and study the effects of high-pressure phase change on transcritical combustion. An ECN Spray A type configuration is chosen to study the high-pressure phase change and combustion of fuel/oxidizer mixtures as well as differences due to different thermodynamic conditions (i.e., subcritical and supercritical).