Theoretical analysis of flamelet model for supersonic turbulent combustion

Abstract

Being physically intuitionistic and computationally efficient, the flamelet model has obtained increasing attention and becomes popular in numerical simulations of supersonic combustion. However, the flamelet model was firstly built for simulations of the low-speed flows. Therefore it is still unclear whether the assumption of the flamelet model is reasonable in supersonic combustion. This paper tries to identify the existence of the flamelet mode in supersonic combustion. Firstly, the interaction of the turbulent fluctuation and the flame is discussed, based on which the combustion regime is distinguished. Then the characteristic length scale of the turbulent flows and the combustion inside the scramjet combustor are calculated and compared, which are used to identify whether the assumption of the flamelet model is reasonable in supersonic flows. The results show that for premixed combustion, due to the low fluctuation velocity in the recirculation zone and the shear layer, the assumption of the flamelet model is established for all the flight Mach numbers, while for non-premixed combustion the assumption is also established for most of the flight Mach numbers except for very high Mach number under which the slow reaction mode dominates the combustion. In order to quantitatively examine the combustion mode, numerical calculations are performed to simulate the strut-injection supersonic combustion which has been experimentally investigated in German Aerospace Center. The results show that the supersonic combustion occurs in the fully-developed turbulent regions and the assumption of the flamelet model is established in the whole flow field. flamelet model, characteristic scale, supersonic turbulent combustion, thin-reaction-zone mode Citation: Fan Z Q, Liu W D, Sun M B, et al. Theoretical analysis of flamelet model for supersonic turbulent combustion.

Full-scale Detached Eddy Simulation of kerosene fueled scramjet combustor based on skeletal mechanism

“…The difference in the time and spatial scales of turbulent flow and chemical reactions determines the micro mixing and reaction process of the fuel, leading to different flame regimes. The turbulent chemistry interaction (TCI) in supersonic combustors depends not only on the flow condition, but also is affected by the fuel type [71,105]. In the Da (Damkohler number) -Re (Reynolds number) diagram shown in Fig.…”

“…Thus in scramjet combustors, the flame thickness may become smaller than the smallest turbulent eddies, which hence can only wrinkle the flame front to form corrugated flame rather than thicken the flame thickness through entering the reaction zone. For hydrogen fuels with small ignition delay time scale, experimental data and theoretical calculation show that most of the supersonic combustion region is in the regime of wrinkled laminar flames (Ki<1) or the upper regime of flamelets in eddies (1<Ka<100), where flamelet based turbulent combustion models are correct or approximately applicable [71,105]. Whereas for complex hydrocarbon fuels (e.g.…”

Application and theoretical analysis of the flamelet model for supersonic turbulent combustion flows in the scramjet engine

“…Firstly, the oblique-shock compression caused by the inlet side wall and the strut makes the flow conditions of the air entering the combustor very different from free stream conditions, which produces uncertainty in the determination of the oxidizer boundary conditions in Equation (12). Secondly, the pressure in the combustion region will definitely differ from free stream pressure because of the inlet compression and the jet/cross-flow interaction.…”

“…All of the numerical and theoretical studies by Bray et al [9], Balakrishnan and Williams [10], Gao and Lee [11] and Fan et al [12] indicated that the combustion processes inside the scramjet engine satisfy the flamelet assumption. Moreover, Waidmann et al [13] performed an analysis on the experimental data of a hydrogen-fuelled scramjet engine at the German Aerospace Centre (DLR) [9] and it was concluded that the combustion is in a flamelet mode.…”

“…Oevermann [17] was the first to apply the flamelet model in a 2D simulation of a practical scramjet combustor model, and he proposed a strategy such that only the species mass fractions are calculated through the PDF integration of the flamelet library, while the mean temperature is solved implicitly from the solution of the energy equation. By this strategy, the flamelet model developed originally for low-Mach-number flows can be extended to more complicated supersonic flows, so this strategy became in practice a standard method for subsequent supersonic combustion simulations based on the flamelet model, including both RANS [18] and large eddy simulation (LES) [12,19] research.…”