General concept for autoignitive reaction wave covering from subsonic to supersonic regimes

We consider a one-dimensional (1D) autoignitive reaction wave in reactive flow system comprising unburned premixed gas entering from the inlet boundary and burned gas exiting from the outlet boundary. In such a 1D system at given initial temperature, it is generally accepted that steady-state solutions can only exist if the inlet velocity matches either the velocity of deflagration wave, as determined by the burning rate eigenvalue in the subsonic regime or the velocity of detonation wave as dictated by the Chapman-Jouguet (CJ) condition in the supersonic regime. In this study, we developed the general concept of "autoignitive reaction wave" and theoretically demonstrate that two distinct regimes that can maintain steady-state solutions both in subsonic and supersonic conditions. Based on this theory, we selected inlet velocities that are predicted to yield either steady-state or flashback solutions, and conducted numerical simulations. This novel approach revealed that steady-state solutions are possible not only at the velocity of the deflagration wave in the subsonic regime and the velocity of the detonation wave in the supersonic regime, but also across a broad range of inlet velocities. Furthermore, we identify a highly stable "autoignitive reaction wave" that emerges when the inlet velocity surpasses the velocity of detonation wave, devoid of the typical shock wave commonly seen in detonation waves. This "supersonic autoignitive reaction wave" lacks the instability-inducing detonation cell structure, suggesting the potential for the development of novel combustor concepts.Comment: Prior to publication please use: "The following article has been submitted to Physics of Fluids. After it is published, it will be found at Link.

Dense core response to forced acoustic fields in oxygen-hydrogen rocket flames

Oscillatory combustion representative of thermo-acoustic instability in liquid rockets is simulated by experiment and LES calculation to investigate the flame behavior in detail. In particular, we focus on how the velocity and pressure fluctuations affect the behavior of the dense oxygen jet, or ‘LOx core’. The test case investigated is a high pressure, multi-injector, oxygen-hydrogen combustor with a siren for acoustic excitation. First, the LES calculation is validated by the resonant frequencies and average flame topology. A precise frequency correction is conducted to compare experiment with LES. Then an unforced case, a pressure fluctuation case, and a velocity fluctuation case are investigated. LES can quantitatively reproduce the LOx core shortening and flattening that occurs under transverse velocity excitation as it is observed in the experiments. On the other hand, the core behavior under pressure excitation is almost equal to the unforced case, and little shortening of the core occurs. The LOx core flattening is explained by the pressure drop around an elliptical cylinder using the unsteady Bernoulli equation. Finally, it is shown that the shortening of the LOx core occurs because the flattening enhances combustion by mixing and increase of the flame surface area

Carleman linearization approach for chemical kinetics integration toward quantum computation

The Harrow, Hassidim, Lloyd (HHL) algorithm is a quantum algorithm expected to accelerate solving large-scale linear ordinary differential equations (ODEs). To apply the HHL to non-linear problems such as chemical reactions, the system must be linearized. In this study, Carleman linearization was utilized to transform nonlinear first-order ODEs of chemical reactions into linear ODEs. Although this linearization theoretically requires the generation of an infinite matrix, the original nonlinear equations can be reconstructed. For the practical use, the linearized system should be truncated with finite size and analysis precision can be determined by the extent of the truncation. Matrix should be sufficiently large so that the precision is satisfied because quantum computers can treat. Our method was applied to a one-variable nonlinear dy/dt = -y^2 system to investigate the effect of truncation orders in Carleman linearization and time step size on the absolute error. Subsequently, two zero-dimensional homogeneous ignition problems for H2/air and CH4/air gas mixtures were solved. The results revealed that the proposed method could accurately reproduce reference data. Furthermore, an increase in the truncation order in Carleman linearization improved accuracy even with a large time-step size. Thus, our approach can provide accurate numerical simulations rapidly for complex combustion systems

Interpretation of the response of cryogenic rocket flames to forced acoustics using large eddy simulation

Experimental measurement of flame response to acoustics under conditions relevant to industrial engines is challenging and so the scope of such measurements is often limited. High fidelity CFD can be used to model the interaction of acoustic waves with cryogenic flames, and modelling an experimental test case can not only serve as a code validation exercise but also be useful in better characterising the experimental results. This work explores this potential by extending the interpretation of high-speed imaging of representative rocket flames based on comparison with a large eddy simulation of the experiment

Obtaining pseudo-OH∗ radiation images from CFD solutions of transcritical flames

The quantitative comparison of experimental data and results from CFD simulations is still an ongoing challenge in the investigation of high pressure combustion in rocket combustion chambers. This is due to the extreme environment which develops in liquid propellant rocket engines, which represent a challenge for experimental data collection. OH∗ radiation emitted from the flame has often been designated as an indicator of the combustion zone, because of its relative ease of detection with appropriate cameras. A method was developed to compare OH∗ radiation originating from cryogenic oxygen-hydrogen flames in an experimental combustor with the CFD simulation results. Pseudo-OH∗ images were obtained from CFD results of two combustors. The method consists in obtaining the path of a ray of light by a reverse ray tracing algorithm and sampling the thermodynamic properties along the path of the ray, simulating the emission and absorption spectra in the wavelength range of interest, in this case of OH∗ emission during combustion. The spectral radiance is then determined by solving the differential radiative transfer equation. Finally, the total radiance is calculated integrating the spectral radiance. The results obtained applying this method are then compared with former results of two test cases, a laminar and a turbulent flame, and with the related experimental data. An improvement of the comparison with the experimental data was achieved in terms of the prediction of self-absorption, which was underestimated in previous works by a factor of 15, and in terms of radiance near the injection plane, where difference is estimated to be about 40% when including refraction. The method allows for more direct comparison between 3D CFD results and 2D experimental images collected by the optical setup and probes