Investigation Of Premixed Flame Propagation Of Natural Gas-Air Mixtures In A Closed Rectangular Tube

In recent years, with the optimization of energy structure and development of green low carbon strategy in China, natural gas, a kind of clean and environmentally friendly energy with high quality, has played an increasingly important role in the industrial or our daily life. It is practically significant to make proper measures depending on the scientific research to prevent the fire and explosion during natural gas pipeline transport and reduce people’s lives and property damage. Therefore the investigation of early flame propagation characteristics and mechanism of natural gas-air premixed in closed rectangular tube will not only prevent the fire explosion accidents effectively but also benefit the wide promotion of natural gas application.Based on the survey of experimental and theoretical literature in China and abroad, we carefully study and investigate the characteristic of premixed natural gas-air flame structure, propagation and pressure rise at different equivalent ratios by using high-speed schlieren photography and pressure transducer. The premixed flame propagation and the underlying mechanisms of Tulip flame are explored through the experimental measurement and analysis. Meanwhile, the flame behavior is also compared to that of premixed methane-air flame and acetylene-air flame at the same conditions.The results show that the influence of equivalence ratio on the flame propagation lies directly in the changes in flame structure, flame propagation speed and pressure in the tube. Buoyancy effect is more obvious in lean or rich fuel conditions, leading to asymmetric flame structure. However, the classical Tulip flame structure is found in the stoichiometric premixed natural-air flame in the closed duct with the fastest flame tip velocity and shortest time of flame propagation. Moreover, the Tulip flame is always formed if the flame propagation speed fell sharply. Also flame-induced flow, pressure wave and thermal diffusion instability have important impact on the Tulip flame formation.By contrast, with the methane-air premixed propagation in the same conditions, it illustrates that the mixture of a highly reactive gas to a binary gas cannot only affect the flame structure, propagation speed and pressure in the duct. Meanwhile, we found that the properties of gas have significant effect on flame propagation characteristics in comparison to the acetylene-air premixed flame propagation with higher laminar flame speed. What’s more, the results show that the combination effect of wall and boundary effect, hydrodynamic instability and flame-induced flow is probably the key reason for Tulip distortion. The prediction of flame skirt position by Bychkov does agree well with the experiment under the stoichiometric premixed gas, especially for acetylene-gas, which has large expansion ratio and high laminar flame speed.