Effects of turbulent mixing on spray ignition

Abstract

When a column of cetane droplets freely falling from an ultrasonic atomizer was ignited behind a reflected shock, no ignition occurred at a temperature below ca. 1100 K, even if the pressure was as high as ca. 1 MPa. Although a higher temperature condition ensured ignition, no flame so luminous as to be recorded by high-speed photography appeared, and even if a luminous flame lump appeared at an extremely high temperature, it disappeared without spreading over the entire column of droplets. It is known however that, if a fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650 K with a luminous flame spreading over the entire spray. Supposing that the above difference was caused by the difference in intensity of turbulent mixing of droplets with hot air, turbulence-generating rods were placed on the upstream side of the spray column. As a result, the ignition limit was lowered to ca. 840 K, and the ignition delay was reduced as the intensity or turbulence was increased. In addition, the light emission of the flame was intensified, and normal spray combustion was maintained in the low-temperature atmosphere after the shock tube ceased its operation.