Investigation On The Influence Of Combustion On Turbulent Flow And The Corresponding Turbulent Model Considering Combustion Effects

In the turbulent combustion process of aero-engine combustors,there exists a strong interaction between turbulent flow and chemical reactions.It is a key scientific problem in the fundamental research of turbulent combustion to accurately quantify this complex bidirectional coupling.This has significant implications for understanding the mechanism of turbulent combustion within the engine combustors and constructing corresponding numerical models.Over the decades of turbulent combustion and its modeling research,the relevant studies on the influence of turbulence on combustion have been very complete.However,most of the studies on the reverse influence of combustion on turbulence are limited to simple and ideal flow structures.In the swirl flame with high temperature,high pressure and high turbulence intensity,which has strong mean shear characteristics and average pressure gradient,the influence mechanism of combustion on turbulence remains unclear.In addition,the subgrid turbulence models commonly used in large eddy simulation(LES)are mostly developed based on non-reacting flows with constant density,without considering the local anisotropy and complex thermodynamic characteristics caused by combustion in reacting flows.Therefore,in LES of turbulent flames,these traditional models are insufficient for predicting subgrid stresses.Based on the aforementioned background,this paper designs premixed,diffusion,and partially premixed miniature combustors with aero-engine combustion chamber operating parameters,and conducts direct numerical simulations to comprehensively investigate the influence mechanism of combustion on turbulence in high-temperature,high-pressure,and highturbulence swirling flames.The paper also proposes for the first time a large eddy simulation(LES)model considering the effect of combustion,and finally verifies the model using DNS premixed flames and gas-liquid two-phase swirling flames at laboratory scale.This paper first designs premixed,diffusion,and partially premixed miniature combustors based on the high-temperature,high-pressure condition and swirling flame structure of the engine combustion chamber.The combustor uses aviation kerosene as fuel,operates at a pressure of 2 MPa,and an inlet air temperature of 760 K.Statistical analysis of the non-dimensional characteristic parameters of turbulent combustion at the flame front shows that the turbulent combustion state of the high-temperature and high-pressure swirling flames simulated in this paper are similar to those in the practical engine combustors.The study finds that combustion significantly reduces the vorticity intensity in the swirl,mainly due to the weakening of vortex stretching in the reactive flows.Secondly,combustion causes an increase in the average velocity in the swirl,while the fluctuating velocity changes are related to the flow area of the swirl.Analysis based on the turbulent kinetic energy transport equation shows that combustion has a inhibitory effect on mean shear production,gradient transport,and viscous dissipation.The coordinating mechanism of competition between multiple factors leads to local turbulence fluctuations in different flow areas.In addition,the gradient diffusion hypothesis of scalars in swirling flames is verified,and it is found that there is inverse gradient diffusion in some of the scalars in premixed,diffusion,and partially premixed flames.Finally,a multi-scale analysis of the flow field reveals that combustion decreases the small-scale kinetic energy by enhancing small-scale viscous dissipation and enhances large-scale kinetic energy by promoting large-scale pressure gradient work through thermal expansion effects.Near the flame front,the inverse scaled energy cascade phenomenon caused by combustion is observed,which means that kinetic energy tends to transfer from small scales to large scales.In addition,the direction of the strongest subgrid stress anisotropy near the flame front shows a strong alignment relationship with the normal direction of the flame front.Subsequently,based on the understanding of the mechanism,this paper develops a large eddy simulation(LES)turbulent model considering the influence of combustion.The model consists of a subgrid kinetic energy term that determines the size of the subgrid stress and a normalized velocity gradient modeling term that determines the direction of the subgrid stress.To solve the subgrid kinetic energy term,its transport equation needs to be modeled.This paper combines the Taylor series expansion and dynamic subgrid coefficient methods to model the pressure effect caused by combustion.Based on the principle of scale similarity,the correlation between the ratio of subgrid kinetic energy and the ratio of subgrid viscous dissipation at different scales has been assumed and tested,which enables the modeling of subgrid viscous dissipation effects.In the modeling of the velocity gradient term,the high-order velocity gradient in the subgrid stress Taylor expansion equation at the local grid is approximated by incorporating the velocity gradients at the neighboring spatial stencil locations.At the same time,the flame correlation factor is introduced near the flame front to describe the anisotropic characteristics of the subgrid stress tensor in the flame space.Finally,to verify the applicability of the new turbulence model,this paper conducts a priori and a posteriori works based on DNS premixed flames and gas-liquid two-phase swirl flames at laboratory scale,respectively.A priori comparison found that under different filter widths,the predictions of the subgrid stress tensor direction by the new model are significantly better than those of traditional eddy viscosity models and structure models,and the predictions of various modeling terms in the subgrid kinetic energy transport equation are accurate,as well.The new model is then used for large eddy simulations of these two flames,and the results are compared with the experimental results.The simulation results using the new model coincide with the experimental measurements,verifying its applicability in actual engine combustors.Based on the large eddy simulation results,the turbulence flame characteristics in the lean direct injection combustor are analyzed to provide a numerical simulation basis for understanding the mechanisms of turbulent combustion in practical combustors.