نشریه سوخت و احتراق
سال هشتم شماره 2 (پیاپی 16، پاییز و زمستان 1394)

In this study, the combustion of a fuel jet with supersonic air flow has been studied numerically.The injected gas was a preburned mixture of hydrogen and air with equivalence ratio of 4.5. The flow field was investigated with and without incident oblique shock wave. The simulations have been performed using openFoam software in a three-dimensional field. A two-equation turbulence model, SST-kω, and the PaSR combustion model were used. The aim of this study was to examine the impact of the shock wave on flame stabilization. According to the results, the reaction occurs at low rates without the oblique shock. However, when the incident shock hits the bottom surface downstream of the injection slot, reaction rate increases, indicating the flame stabilization in this case. These findings are in good agreement with experimental results.

In the present study, the effects of inlet velocity on characteristics of Repetitive Extinction-Ignition Dynamics are numerically investigated. Hydrogen-air mixture (with equivalence ratio = 0.5) enters into a heated micro channel with a prescribed wall temperature. Low Mach number approach is considered for governing equations in numerical simulation and also detailed chemistry, and different mass diffusivity of species is utilized. The dynamic behavior is studied by two parameter, amplitude and frequency. The results show that the amplitude of repetitive extinction-ignition dynamics increases with increasing the inlet velocity, while the frequency has a descending-ascending behavior. For detailed study of this phenomenon the chemical reaction approach is used by considering the reaction rate parameter. The results illustrate that for high inlet velocitis the reactions tend to produce light species such as O, H and OH. The effects of inlet velocity on flame propagation velocity are also studied. For lower inlet velocities, flame stays longer in an extinction-ignition period in the channel, while increasing the inlet velocity causes the flame to extinguish faster.

Reduction of environmental pollutants caused by combustion in power plant systems is one of the main challenges for the researchers. To pinpoint the mechanisms of formation and transport of combustion pollutants, it is necessary to have an accurate prediction of temperature field and combustion products. For this reason, simulation of turbulent combustion flows has attracted much attention in recent years. An appropriate combustion model is required for simulation of these flows. Flamelet model is the most favorite combustion model, due to inherent separation of the turbulent flow field and the chemical reactions. Moreover, the consideration of unsteady flamelet in modeling complex physical phenomena such as radiation heat transfer and slow chemical processes (of pollutants) leads to better results than the steady flamelet assumption. The purpose of this study is to investigate the application of steady and unsteady flamelet models in the simulation of turbulent diffusion bluff body flame. Predictions of temprature and mean mixture fraction using steady flamelet model have shown very good agreement with experiment data. NO mass fraction in steady-state simulations using two different chemical mechanisms GRI3.0 and GRI2.11 is over predicted. While NO mass fraction in the unsteady flamelet modeling using mechanism GRI2.11 have shown good agreement with the experimental data. Thus, unsteady effects are important in slow processes such as the formation of NO.