| Natural gas is a recognized high quality gas fuel,and it has been widely spread and applied at present.However,because of its low burning velocity,the usage of methane in the internal combustion engine will increase the combustion duration and reduce the thermal efficiency.Besides,the poor lean burn capacity makes the engine cyclic variation increase.Hydrogen is a renewable clean energy,and its burning velocity is about 5 times than that of natural gas.The ignition energy of hydrogen is low and the flammability limits is wide.The combustion performance of the international combustion engine will be greatly improved after the addition of hydrogen.The main component of natural gas is methane.Therefore,the experimental research on the turbulent premixed combustion characteristics of methane/hydrogen mixture is very important for establishing a sound turbulent premixed combustion model and realizing the optimal design of natural gas engine combustor.A turbulent premixed combustion bomb experimental system of constant volume was built.The experimental system was consisted of a constant volume combustion bomb,schlieren system,ignition system,intake and exhaust system,turbulence generation system,high-speed camera system and synchronous trigger system.The combustion experiments of methane/hydrogen/air laminar premixed flame were conducted based on the experimental system.Effect of hydrogen fraction and equivalence ratio on the laminar flame propagation speed and the effect of flame stretch on the flame propagation speed were studied.It is found that the growth rate of laminar burning velocity increases with the increase of the hydrogen fraction.Chemkin was used to simulate the experiments based the mechanism of GRI 3.0.The effect of hydrogen fraction and equivalence ratio on the flame structure was analyzed.The core element reactions influencing the chemical reaction process were cleared and the mechanism of the influence of H and OH on the laminar burning velocity was revealed.A high accuracy extraction of the flame front cellular structure is realized based on Matlab.Two parameters,namely mean area and unity degree cellular structure,were defined to quantitatively study the structure characteristics of the methane/hydrogen/air laminar premixed flame.The influence of hydrogen fractions and equivalent ratios on the evolution flame cellular structure characteristics was discussed.It was clarified that cellular structure in the flame front tended to stable when the flame development to a certain extent,and the mean area of cellular structure and unity degree of the cellular structure tend to be a constant value.The influence of flame inherent instabilities on the cellular structure of the flame front was analyzed,and it was clarified that the cellular structure was intensified as the flame inherent instabilities increased.The mechanism between the flame propagation speed and the evolution of the flame front cellular structure on the flame cellular structure was revealed.The propagation characteristics of methane/hydrogen/air turbulence premixed flame were studied.The influence of hydrogen fractio,equivalence ratio and turbulence intensity on the turbulent premixed flame evolution characteristics were studied.The effect of hydrogen fraction,equivalence ratio and turbulence intensity on the turbulent premixed flame propagation speed were obtained.The effect of flame stretch on the turbulent premixed flame propagation speed was discussed.The influence of hydrogen fraction,equivalence ratio and turbulence intensity on the turbulent burning velocity was revealed.The fitting formulas of fully developed turbulent flame burning velocity under different conditions were obtained.The accelerating mechanism of hydrogen fraction,equivalence ratio and turbulence intensity on burning velocity was analyzed.It is clear that the relative turbulent burning velocity of methane/hydrogen decreases non-linearly with the increase of hydrogen fraction.The influence of the turbulence on the methane/hydrogen burning velocity decreases with the increase of hydrogen fraction.The structural characteristics of methane/hydrogen/air turbulence premixed flame were studied.Effect of initial flame shape on the flame structure characteristics was discussed.It is clear that the influence of flame initial shape on the flame structure characteristics gradually decreases with the development of the flame.FFT was used to decompose the different scales disturbances in the flame front.The influence mechanism of different turbulence on flame structure is clarified.It found that the disturbances with smaller wavenumber were the mainly component of the flame macrostructure and the disturbances with larger wavenumber were the mainly component of the flame local structure.The mechanism of the turbulence on the local structure and local propagation characteristics of the flame front is revealed.The change of the flame structure caused by the increase of the turbulence scale is the core factor that causes the increase of local fluctuation of the flame front and the local flame propagation speed. |
