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The relative positions of the thermal convection column and the smoke plume from a variety of smoldering cigarettes were measured using a combination of schlieren and visual optical systems. The schlieren technique is an optical method used to observe refractive index gradients in gases and other clear media. The refractive index gradients can be caused by variations in pressure, composition or temperature. The convection column of heated air and combustion gases rising from a cigarette coal was observed with a two mirror schlieren system. A video camera was used as the observing device rather than the usual photographic camera. A second video camera was arranged to view the smoke plume rising from the coal region. The two video images were combined with a video special effects generator and were viewed on a single monitor. The behaviour and relative positions of both columns were thus observed in real time with two non-invasive optical methods. The schlieren images of the convection column were compared to those of model systems such as a heated cylinder and a small flame. Results for experimental cigarettes with paper porosities of 12 cm/min to 48 cm/min (Coresta) during smolder showed that the thermal convection column was centered 2 mm to 3 mm in front of the paper char line directly over the hottest part of the coal as determined by infra-red imaging. The smoke plume was centered 2 mm behind the paper char line and the position did not change with paper porosity. Results for experimental cigarettes made with a commercially available low sidestream paper showed that the position of the convection column did not change. However, the position of the smoke plume changed considerably. In addition to being markedly decreased in visibility, the plume now appeared to be centered directly over the paper char line. This change in position provides a valuable insight into the mechanism of smoke reduction. The low sidestream papers seem to work by preventing the escape of smoke forming condensibles through the paper behind the paper char line. The condensibles are therefore forced to escape at the paper char line and undergo increased combustion and pyrolysis. As a result, some are converted to lower molecular weight materials and are unable to condense as readily to form the smoke. In addition, the remaining condensibles are released into a hotter and faster rising gas stream. This serves to reduce smoke formation by suppressing condensation and increasing dilution.

Relative Positions of the Thermal Convection Column and Smoke Plume Generated from a Smoldering Cigarette