A supplementary study of ignition phenomena of sprays using a shock tube

When a cetane droplets column freely falling from an ultrasonic atomizer is ignited behind a reflected shock, no ignition occurs at a temperature below ca, 1100K even if the pressure is sufficiently high (∼1 MPa). Although a higher temperature ensures ignition, no flame so luminous as to be recorded by high-speed photography appears, and even if a luminous flame lump appears, it disappears without spreading throughout the droplets column. If, on the other hand, fuel is injected into a diesel cylinder or an electric furnace, ignition occurs even at a temperature as low as 650K with a luminous flame spreading over the entire spray. Supposing that the above difference resulted from 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 down to ca. 840K, and the combustion made shifted from a blue-flame mode or locally confined ignition mode into an ordinary spray combustion mode.