Experimental investigation on spark ignition of multi-swirl spray flames under sub-atmospheric pressures and low temperatures

Altitude relight is an increasingly important issue limiting the development of low-emission combustors due to the current trend towards lean combustion. This paper experimentally investigates the flow field features and spark ignition processes inside a multi-swirl staged combustor to further reveal the internal mechanism leading to the deterioration of ignition performances under altitude conditions including sub-atmospheric pressures and low temperatures. Laser diagnostic technologies and high-speed imaging are applied to acquire the flow struc-tures, droplet distributions, and detailed kernel motions under various inlet conditions. A flame-tracking algo-rithm is developed to capture the flame features (flame intensity-centroids and flame object edges) and provide the flame propagation trajectories during 10 ms following the spark. A typical "dipper-type propagation" pattern is summarized from numerous spark events and its formation mechanism can be clarified by statistical char-acteristics of flow and spray inside such staged combustors. The variation of ignition limits quantitatively characterizes the degree of ignition deterioration under sub-atmospheric pressures (10 -70 kPa) and low temperatures (253 -301 K). From the analysis of air to liquid ratio and Weber number, it is demonstrated that the increased droplet size and shrunken spray cone under altitude conditions are induced by three aspects, including the insufficient aerodynamic force, increased liquid viscosity force, and increased surface tension. The poor atomization quality is detrimental to the kernel generation and flame propagation and finally leads to the deterioration of ignition performances.