Investigation On Large Eddy Simulation Of Turbulent Two-Phase Dilute Spray Flames And The Flamelet Model

Dilute Spray Flame is widely applied in industry and is concerned by many researchers. With the enhancement of computing power and the optimization of numerical models, numerical simulation plays an increasingly important role in the study of dilute spray combustion and burner design and so on. Large eddy simulation (LES) is able to resolve the energetic large scale turbulence; the flamelet model can be coupled with the detailed reaction mechanism and is able to predict the unsteady process; thus the application of these two kinds of methods in dilute spray combustion simulation is inevitable. However, the flamelet model needs for improvement when extended to the two-phase combustion to consider more physical processes. Study on the model application scope, the theoretical foundation and the gas-liquid interaction in the field is a hot topic. Based on this, this paper focuses on the research of the application of flamelet model in dilute spray combustion. The main work is as follows:Large eddy simulations of a turbulent CH4/H2/N2jet diffusion flame with the steady flamelet (SFM) model or the flamelet/progress-variable (FPV) approach are conducted to valuate these two models. Results show that both models predict the low-order moments well while the latter one captures the second-order moment more accurately. Much larger deviations can be found when evaluating the minor species (NO, OH), which can be avoided by solving the specie’s transport equation. When analyzing the data using Proper orthogonal decomposition (POD), it is found that the first five modals occupies about60%of the total turbulent energy and the central jet region contains various turbulent coherent structures.To study the characteristics of droplet dispersion and evaporation in gas-liquid turbulent flows, numerical simulations of the cold evaporating dilute sprays are conducted in the Eulerian-Lagrangian framework. The gas turbulent flow in Eulerian framework are simulated using large-eddy simulation while the discrete liquid phase is described using the Lagrangian particle trajectory model, considering interphase mass, momentum, and energy exchange. And the equilibrium evaporation model is adopted to describe the evaporation effect. The resulting droplet velocity field, particle size distribution and mass flow rate are in good agreement with experimental data. In the confined spray with recirculation zone, droplets with big stokes number are affected by the initial inertia obviously while small droplets with stokes number around’one’are more controlled by the gas structures, and the evaporation rate in unit volume is closely related to the droplet dispersion.Direct numerical simulation (DNS) of the auto-ignition of the liquid n-heptane droplets in hot flows is conducted to make a contribution to the evaluation of the heat release indicators and analyze the combustion mechanism. A detailed reaction mechanism is adopted and all the species transport equations are solved directly. Time evolution, instantaneous contour plots and scatter plots are applied to evaluate the indicators’performance. Results show that [OH]x[CH20] is a proper heat release rate indicator when auto-ignition prevails, and the proportional relationship between the indicator and the actual local heat release rate is also verified both in premixed and non-premixed regions with small difference in the coefficients.The piloted ethanol-air spray flames are simulated by LES in Euler-Lagrangian framework, and to consider the evaporation heat loss effect in the FPV approach, two new methods are proposed. One is to correct the gaseous temperature in the flamelet calculation, and the other is to couple this heat loss into the computational fluid dynamics (CFD) process. Comparisons of the results obtained from the modified models show that the N-FPV approach which reduces the flamelet temperature and the T-FPV approach which considers the local energy balance in the CFD can both effectively simulate the evaporation heat loss effect. The modified models can simulate the gaseous and liquid statistics well. Instantaneous plots show that the FPV approach is able to capture part of the ignition and extinction upstream of x/D=20, and the strongest evaporation occurs in the high-temperature area and the shear layer near x/D=20. Results also show that the evaporation rate is affected by both the droplet density and the gaseous temperature.The auto-ignition process in the methanol and n-heptane jet with high temperature co-flow is simulated using LES and the FPV approach, to verify the flamelet model in capturing the ignition process in the dilute spray flames. Considering the fact that there exist three thermo-chemical states in the jet inlet (the liquid fuel, the carrier and the hot co-flow), the FPV approach is extended via introducing a new conserved scalar and forms the’three-inlet states flamelet model’. Results show that the instantaneous flame image from the new model can reasonably predict the temperature and mass fraction distribution, especially near the nozzle exit. And the new model shows better performance than the conventional model. However, the over-estimation of the lifted-off height should be solved to effectively predict the ignition process in the dilute spray combustion. So this paper adopts the auto-ignition model coupled with the proposed ’three-inlet states model’ to simulate the ethanol dilute spray flames. Results show that a series of lifted-off heights are all in good agreement with the experiment, which proves the validity of the auto-ignition model in simulating the lifted-off flames. It is also easy to find that the lifted-off height decreases with the increase of the liquid fuel loading, and that ignition occurs first in the lean state with low dissipation rate. Radical OH often appears upstream of the’flame generation point’, and there are a few small pockets (kernels) in the shear layer indicating the occurrence of the ignition.An automated method to optimize the definition of progress variable in flamelet-based dimension reduction is proposed to deal with the pyrolysis in heavy oil and such cases. In this method, progress variables are defined according to the first two principal components from principal component analysis (PCA) or KErnel-Density-weighted PCA (KEDPCA) of a set of flamelets. A priori validation of these optimized progress variables and the new chemistry table is implemented in a CH4/N2/air lifted-off flame. Results show that, the new definition can guarantee the monotonicity between thermo-chemical parameters and reaction parameters. The reconstruction of the lifted-off flame shows that optimized progress variables perform better than conventional ones especially in the high temperature area. R2statistics show that bi-KEDPCA approaches perform slightly better than PCA approach except for some minor species. The main advantage of KEDPCA that this approach is less sensitive to the database is also verified. Meanwhile, the criteria for this optimization are proposed and discussed.