An experimental study of laminar combustion of LPG-hydrogen-air mixture at room temperature

Abnormal laminar combustion of LPG-hydrogen-air mixture, including flame instability and uncompleted combustion phenomena, was experimentally studied with high speed photography in a constant volume chamber for different excess air coefficient (0.6∼1.4), initial pressure (0.09∼0.124MPa) and hydrogen fraction (0∼60%) at room temperature. The experiments and analysis showed that, with the increase of excess air coefficient, the Markstein length increases and therefore the stability of laminar flame increases, but the flame propagation speed is lowered, then buoyancy plays an important role in the combustion process and flame propagation. When LPG-hydrogen-air mixture is either too lean or too rich, the flame propagates very slow and the profile of flame front is no longer spherical and so-called "flame brush" phenomenon occurs; moreover, the flame doesn't spread all over the chamber, but is limited to only part of the chamber volume, leads to uncompleted combustion. Based on the above analysis, the lean flammability limit (lower flammability limit) and rich flammability limit (upper flammability limit) of LPG-hydrogen-air mixture, in volume fraction of fuels and excess air ratio separately, were plotted in a 3D coordinate system for different hydrogen fraction and initial pressure. The plots indicated that the flammability limit is remarkably expanded with the increase of hydrogen fraction.