Droplet flames in reactive environments

ARO and AFOSR Contractors Meeting in Chemical Propulsion.

“…The traditional secondary breakup regime map for drops due to Hinze (1975) was not effective for these results because regime boundaries were significantly affected by liquid gas viscosity ratios when effects of liquid viscosity were large. A better approach was to account for liquid viscous effects directly by plotting the ratio of drag forces to liquid viscous forces, We 17 /Oh, as a function of the ratio of surface tension to liquid viscous forces, Oh" , where We and Oh are the Weber and Ohnesorge numbers of the flow. These results are illustrated in Fig.…”

“…The extinction velocity was obtained by selecting an initial freestream velocity that resulted in an envelope flame and then incrementing the freestream velocity by 1 cm/s between successive quasi-steady solutions until the evaporation constant exhibited a sharp decrease. Experimental data under normal-gravity for various fuels in air at "room" temperature and atmospheric pressure; (o) -kerosene (Spalding [12]), (0) -n-butyl alcohol (Agoston et al [13]), (*) -n-heptane (Gollahalli and Brzustowski [15]), (D) -previous point corrected for natural convection, (V) -gasoline (Agafonova et al [16], lower: aiding natural convection, upper: opposing natural convection), (A) -n-heptane (Gore et al [17]), (shaded box) -n-heptane (Chauveau et al [18]). …”

Numerical simulation of fuel droplet extinction due to forced convection

“…The literature indicates that the extinction velocity increases monotonically with the droplet diameter (linearly [6] or like d 0.5 [8]). Regardless of the dependence, as a result of the monotonic increase in extinction velocity with droplet diameter, it has been argued that, under extinction conditions, natural convection becomes negligible (forced convection dominates and Ri is small) at large "droplet" (porous sphere) diameters (e.g., [12]). However, the effect of natural convection on extinction velocity depends on the orientation of the gravitational vector relative to the forced flow.…”

“…Experimental studies on convective extinction of fuel droplets (mostly simulated with porous spheres) under normal gravity are also available in the literature [6][7][8][9][10][11][12][13]. The Richardson number (the inverse of the Froude number) provides a ratio of the strength of buoyancy-induced to forced convection flows in the experiments.…”

“…However, the effect of natural convection on extinction velocity depends on the orientation of the gravitational vector relative to the forced flow. In general, three different orientations have been used in experimental studies: the buoyancy-induced flow either (1) aids [7,[9][10][11][12], (2) opposes [8], or (3) is normal to [6] the forced convection. The characteristic lengths in the Richardson number would differ greatly between these orientations.…”