New Evidence Uncovered of Anomalies During Asymmetric Radiant Droplet Vaporization

Asymmetric fuel droplet vaporization may arise in liquid-fueled microcombustors, whose construction and operating conditions can result in high combustor wall temperatures, leading to an increased effect of asymmetric thermal radiation on the vaporizing fuel droplets. Radiant heating can also play an important role within non-premixed combusting sprays at conventional scales.  Our recent experimental results have revealed a prominent asymmetry in the vapor distribution around a monodisperse stream of single and bi-component droplets exposed to asymmetric radiant heating.

Planar laser-induced fluorescence (PLIF) was used as a diagnostic tool, with acetone as the fluorescent tracer, to image the vaporizing droplets. Contour plots of the acetone vapor mole fraction around the droplets revealed asymmetries, as shown in the figure below.

Figure 1.  Acetone vapor concentration around a 145 µm acetone droplet exposed to asymmetric radiant heating. The contour plot reveals an excess of acetone vapor on the irradiated right side of the droplet.

It is believed that the peak acetone concentration located between the droplet and the radiant heater is due to vapor outgassing from the droplet surface, the result of nucleate boiling.  Plan view images (Figure 2) of a liquid hydrocarbon-acetone mixture undergoing pool boiling show highly non-uniform acetone vapor concentrations just above the liquid surface exposed to radiant heating.  Such behavior is not accounted for in current numerical models of droplet vaporization, and these results will provide insight that will lead toward improvements in such models.

Figure 2.  Acetone vapor concentration directly above a hydrocarbon-acetone mixture undergoing pool boiling due to radiant heating.