Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

Large-eddy simulation (LES) of a lean-premixed hydrogen turbulent jet flame with combustion instability (CI) in a low-swirl combustor (LSC) is performed by employing a dynamically thickened flame model with a detailed chemical reaction model with 9 chemical species and 20 reactions, and the LES validity and the CI characteristics are investigated in detail. The results show that the present LES can accurately reproduce the experimentally observed characteristics of the CI such as intensity, frequency, sporadic decay of pressure oscillations, and a flame–flow interaction inducing the periodic transitions of an inverted conical flame structure and a flat flame structure in the LSC. The sporadic decay of pressure oscillations and the flame–flow interaction are caused by the temporal decoupling of pressure and heat release rate and the periodic outward and inward deflections of the inflow, which is associated with the flow behavior in the upstream injector channel, respectively.

Large-eddy simulation of a lean-premixed hydrogen flame in a low-swirl combustor under combustion instability

Nagao,

Pillai,

Shoji

et al. 2023

Experimental investigation of combustion instability in a centrally staged combustor under self-excited oscillation conditions

Ma,

Han,

Han

et al. 2023

This paper presents an experimental investigation of combustion instability and flame dynamics in a laboratory-scale lean premixed prevaporized centrally staged combustor under self-excited oscillations. The macrostructure of the flame is captured by CH* chemiluminescence images using a high-speed camera, and the two-dimensional flow field of the combustor center plane is obtained by using particle image velocimetry. The effects of the pilot stage swirl number and main stage equivalent ratio on the thermoacoustic oscillations and flame dynamics are analyzed. The results indicate that there are intermittent oscillations, limit cycle oscillations, and mode switching in the combustor. As the pilot stage swirl number increases, the system transitions from intermittent oscillation to limit cycle oscillation. Additionally, the case with a pilot stage swirl number of 0.5 also occurs mode switching. Based on dynamic mode decomposition, the heat release fluctuation is primarily concentrated in the shear layers under different operating conditions. Furthermore, the intensity of thermoacoustic oscillations in a system is determined by the coupling strength between pressure and heat release fluctuations. From the results of the flow field, it is observed that as the pilot stage swirl number increases, the time-averaged axial strain rates and vorticities increase, but the time-averaged axial velocity is generally lower for the swirl number of 0.7 compared to the other two cases. On the other hand, the time-averaged axial strain rates and vorticities for the case with a swirl number of 0.5 decrease with the increase in the main stage equivalence ratio before and after the mode switching.

Effects of bulk viscosity, heat capacity ratio, and Prandtl number on the dispersion relationship of compressible flows

Bhaumik,

Deepak

2023

Here, we present the variation of the dispersion characteristics of the three-dimensional (3D) linearized compressible Navier–Stokes equation (NSE) to bulk viscosity ratio, specific heat ratio (γ), and Prandtl number (Pr). The 3D compressible NSE supports five types of waves, two vortical, one entropic, and two acoustic modes. While the vortical and entropic modes are non-dispersive, the acoustic modes are dispersive only up to a specific bifurcation wavenumber. We illustrate the characteristics and variation of relative (with respect to the vortical mode) diffusion coefficient for entropic and acoustic modes and a specially designed dispersion function for acoustic modes with depressed wavenumber η=KM/Re, the bulk viscosity ratio, γ, and Prandtl number Pr of the flow. Here, K, M, and Re denote the absolute wavenumber of disturbances, Mach number, and Reynolds number of the flow, respectively. At lower wavenumber components, the deviation of the dispersion function from the inviscid and adiabatic case is proportional to η2 at the leading order, and the relative diffusion coefficients increase linearly with bulk viscosity ratio and γ while varying inversely with Pr. With the increase in the bulk viscosity ratio, the shape and extent of the dispersion function alter significantly, and the change is more substantial for higher wavenumber components. The relative diffusion coefficient for entropic and acoustic modes shows contrasting variation with wavenumber depending upon bulk viscosity ratio, γ, and Pr. We also show by solving linearized compressible NSE that relatively significant evolution and radiation of acoustic and entropic disturbances occur when the bulk viscosity ratio is close to the corresponding critical value of maximum bifurcation wavenumber. Based on this criterion, we have presented an empirical relation for estimating bulk viscosity ratio depending upon γ and Pr, giving the corresponding range for obtaining relatively significant disturbance evolution.