Xiaolong Gou

Studies on the Outwardly and Inwardly Propagating Spherical Flames with Radiative Loss

Outwardly and inwardly propagating spherical flames (OPF and IPF) with radiative loss are studied analytically and numerically. Emphasis is placed on investigating the effects of radiation on flame propagating speed, Markstein length, and flame extinction, as well as on examining whether the reactant can be completely consumed via an IPF. A general correlation between flame propagating speed and flame radius for OPF and IPF of large flame radii is derived and utilized to study the effects of radiative loss and Lewis number on flame propagation and extinction. A correlation for Markstein length at different Lewis numbers and radiative loss is also presented. It is shown that the Markstein length is strongly affected by radiative loss as well as Lewis number, and that only for mixtures not close to their flammability limits and without CO2 dilution is the effect of radiation on the Markstein length measured from expanding spherical flames negligible. Furthermore, the theoretical results are validated by numerical simulations. It is found that when radiative loss is considered, there exists unconsumed reactant after the extinction of IPF for mixture with Lewis number less than unity.

Parallel On-the-fly Adaptive Kinetics for Non-equilibrium Plasma Discharges of C2H4/O2/Ar Mixture

To enhance the computational efficiency for the simulation of plasma assisted combustion (PAC) models, three new techniques, on-the-fly adaptive kinetics (OAK), point-implicit stiff ODE solver (ODEPIM), and correlated transport (CoTran), are combined together to generate a new simulation framework. This framework is applied to non-equilibrium plasma assisted oxidation of C2H4/O2/Ar mixtures in a low-temperature flow reactor. The new framework has been extensively verified by both temporal evolution and spatial distribution of several key species and gas temperature. Simulation results show that it accelerates the total CPU time by 3.16 times, accelerates the calculation of kinetics by 80 times, and accelerates the calculation of transport properties by 836 times. The high accuracy and performance of the new framework indicates that it has great application potentials to many different areas in the modeling and simulation of plasma assisted combustion.

Direct Modeling of Auto-Ignition and Flame Propagation of N-heptane-Air Mixtures at HCCI Conditions by Using Dynamic Multi-Timescale Method

The ignition, flame propagation, and transition to detonation of n-heptane-air mixtures in a one-dimensional, cylindrical chamber are numerically modeled at homogeneously charged compression ignition (HCCI) conditions by using a multi-time scale (MTS) method with a comprehensively reduced kinetic mechanism. It is found that depending on the initial temperature and temperature gradient, there exist many new combustion regimes. At low temperatures, it is shown that there is a coupled low temperature flame (LTF) and high temperature flame (HTF) propagation regime. At intermediate temperatures, the results demonstrated that there are six different combustion regimes, an initial single flame front propagation regime, a coupled LTF and HTF double flame regime, a decoupled LTF and HTF double flame regime, a low temperature ignition regime, a single HTF regime, and a hot ignition regime. At high temperatures, only HTF and hot ignition are observed. Furthermore, it is found that the existence negative temperature coefficient (NTC) region dramatically changes the critical temperature for flame acoustic coupling. The rapid increase of the magnitude of critical temperature in the NTC region enhances the occurrence of supersonic ignition regime and suppresses detonation transition. The results show that the low temperature flame chemistry affects dramatically the flame regimes, flame transitions to ignition and detonation, and the temporal histories of pressure and heat releases.

A Mechanism Reduction Method Integrating Path Flux Analysis with Multi Generations and Sensitivity Analysis

ABSTRACT A novel mechanism reduction method that integrates path flux analysis with multi generations and sensitivity analysis (MPFASA) is proposed to reduce the complex detailed chemistry and to generate skeletal mechanisms. At first, the path flux analysis with multi generations (MPFA) method is used to efficiently reduce detailed mechanisms; then the sensitivity analysis (SA) method is applied to further eliminate the redundant species and their related reactions on the basis of the skeletal chemistry obtained by the MPFA method. Detailed mechanisms of methane and n-heptane are reduced by the MPFASA method, which are validated in the context of autoignition and perfectly stirred reactor for methane/air and n-heptane/air mixtures, over a wide range of operating conditions. The comparison shows that the skeletal mechanisms generated by the MPFASA method can well reproduce the results of detailed mechanisms and contain a much smaller number of species and reactions than those obtained by using the MFPA method only. It is illustrated that the MPFASA approach can overcome the shortcomings of the MPFA and SA methods and can be applied to obtain further reduced skeletal mechanisms in reactive flow modeling.

An Efficient Multi Time Scale Method for Solving Stiff ODEs with Detailed Kinetic Mechanisms and Multi Scale Physical Chemical Processes

In direct numerical simulations of reactive flow and other multi scale physical problems, a broad time scale distribution from seconds to a fraction of nano-seconds makes explicit schemes in-efficient. On the other hand, although implicit methods may improve computational efficiency, they lose the time histories and statistical values of the fast modes, which may be required for modeling of large scale slow modes in a turbulent reactive flow and multi physical process. In the present study, a dynamic multi time scale (MTS) method and a dynamic hybrid multi time scale (HMTS) model are developed to achieve efficient and time accurate integration of stiff ODEs with detailed kinetic mechanisms. The methods are applied to ignition of hydrogen, methane and n-decane/air mixtures and compared, respectively, with standard Euler and implicit ODE solvers by using detailed chemical kinetic mechanisms. The results showed that both methods can accurately reproduce the species time histories and ignition delay times. In addition, compared to the explicit Euler method, MTS is not only computationally efficient but also robust at larger time steps. Compared to the implicit ODE solver, MTS is about one-order more computationally efficient. In addition, unlike the implicit ODE solver, whose computation time is proportional to the square of the species number, the computation time required for MTS is only proportional linearly to the species number. As such, MTS has advantages particularly for large equation systems such as large chemical kinetic mechanisms. To further accommodate the specification of a limiting time scale of the equation system and to improve the computation efficiency and robustness at large time scales, HMTS is developed by integrating MTS with a fully implicit algorithm. Therefore, the present HMTS is a generalized scheme which includes the Euler scheme, MTS, and implicit scheme, and compatible to both incompressible and compressible flow solvers. The results showed HMTS is rigorous and efficient. This scheme can be used for direct numerical simulations and large eddy simulation with detailed chemical mechanisms to improve the computation efficiency, accuracy, and robustness.