Large Eddy Simulation of Swirled Spray Flame Using Detailed and Tabulated Chemical Descriptions

Abstract

Accurate characterization of swirled flames is a key point in the development of more efficient and safer aeronautical engines. The task is even more challenging for spray injection systems. From one side, spray interacts with both turbulence and flame, eventually affecting the flame dynamics. On the other side, the structure of turbulent spray flame is highly complex due to equivalence ratio inhomogeneities caused by evaporation and mixing processes. The first objective of this work is to numerically characterize the structure and dynamics of a swirled spray flame. The target configuration is the experimental benchmark named MERCATO, representative of an actual turbojet injection system. Due to the complex nature of the flame, a detailed description of chemical kinetics is necessary and is here obtained by using a 24-species chemical scheme, which has been expressly developed for DNS of spray flames. The first LES of a swirled spray flame using such a detailed chemical description is performed here and results are analyzed to study the complex interactions between the spray, the turbulent flow and the flame. It is observed that this coupling has an effect on the flame structure and that flame dynamics are governed by the interactions between spray, precessing vortex core and flame front. Even if such a detailed kinetic description leads to an accurate characterization of the flame, it is still highly expensive in terms of CPU time. Tabulated techniques have been expressly developed to account for detailed chemistry at a reduced computational cost in purely gaseous configurations. The second objective is then to verify the capability of the FPI tabulated chemistry method to correctly reproduce the spray flame characteristics by performing LES. To do this, results with the FPI method are compared to the experimental database and to the results obtained with the 24-species description in terms of mean and fluctuating axial gas velocity and liquid phase characteristics (droplet diameter and liquid velocity). Moreover, the flame characterization obtained with the FPI approach is compared to the results of the 24-species scheme focusing on the flame structure, on major and minor species concentrations as well as on pollutant emissions. The potential and the limits of the tabulated approach for spray flame are finally assessed.

Investigations of Evaporative Cooling and Turbulence Flame Interaction Modeling in Ethanol Turbulent Spray Combustion Using Tabulated Chemistry

“…Although both interactions are strongly coupled, they are not always comprehensively addressed in numerical simulations. The cooling down of the gas mixture caused by the evaporation process, namely the evaporative cooling (EC), is often neglected [4][5][6][7][8] or simplified [9,10] in the context of tabulated chemistry. Herein, the consideration of heat losses is not trivial and typically involves high computational efforts [11][12][13].…”

“…Nevertheless, as analyzed in [16] for one-dimensional flames propagating in droplet mists, the inclusion of the evaporative cooling is quite relevant in the spray combustion modeling. Yet the importance of the proper modeling of the turbulence-flame interaction is a well known topic in the spray combustion community [1,4,[6][7][8][9][10]13,[17][18][19][20][21]. In view of the application of the artificially thickened flame (ATF) approach, many works have been using a similar value for the exponent of the power-law function used to model the flame surface wrinkling (FSW), namely 0.5.…”

“…An initial approach to account for evaporative cooling effects is to consider heat losses in the mixture without including modifications in the chemistry table [9,10]. According to this formalism, influences of heat losses concentrate on the computation of droplet evaporation rates which indirectly interfere with the combustion process.…”

“…The ATF model stands out among methods based on deterministic approaches to characterize the turbulence-flame interaction in turbulent spray flames [9,10,17,20,21]. This model showed a great capability to predict the behavior of turbulent spray flames in the Eulerian-Eulerian two-phase flow approach (e.g., [9,17]) as well as in the Euler-Lagrangian framework (e.g., [10,20,21]). Initially proposed to model laminar premixed flames [26], the ATF was gradually expanded to address turbulent premixed [27,28] and stratified flames [29].…”

“…In view of turbulent spray combustion, a further extension to include effects of the dispersed phase in the Euler-Lagrangian context are recently presented in [21]. A common characteristic to all of these preceding works involving turbulent flames (i.e., [9,10,20,21]) refers to the usage of a global and presumed definition of the exponent of the power-law function (β) proposed by Charlette et al [28] to model the FSW. According to the analysis conducted for premixed flames in [30], no universal value exists for this parameter, which may be determined by adjusting with experimental data or by some modeling approach (e.g., the dynamic method proposed by Charlette et al [30]).…”