Droplet burning flame diameter

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

This demonstrates a quadratic dependence of burnout on the diameter of the initial liquid droplet. For burning droplets, the droplet flame front acts as a nearby source of heat for droplet vaporization, otherwise the mechanism is the same as pure vaporization. For a singlecomponent, uniform droplet burning under quasi-steady gas-phase conditions... 

The model assumes that liquid evaporation is always the rate controlling step. At some point the model must fail, since as droplet size approaches zero the predicted MIE approaches zero rather than the MIE of the vapor in air. In practice, droplets having diameters less than 10-40 /rm completely evaporate ahead of the flame and burn as vapor (5-1.3). The model also predicts that the MIE continuously decreases as equivalence ratio is increased, although as discussed above, combustion around droplets is not restrained by the overall stoichiometry and naturally predominates at the stoichiometric concentration. It is recommended that the model be applied only to droplet diameters above about 20/rm and equivalence ratios less than about one. 

A two-color pyrometer has been used along with the phase-Doppler anemometer to simultaneously measure the local velocity and size of kerosene droplets and the temperature of burning soot mantle in a swirl burner.[648] The measurements were conducted within the flame brush that develops in the shear layer of a swirl-stabilized, gas-supported kerosene flame with a swirl number of about 0.19 and potential heat releases of 10.6 and 15.5 kW, respectively. The results showed that the maximum burning fraction of the droplets occurs adjacent to the region denoted as gas flame but the value ranges from 20 5 to 40 5% depending on the axial station, and decreases sharply across the shear layer. The flame mantle temperature was found to be independent of droplet diameter, which agrees with previous results in the literature. 

The behavior of a single droplet during burning is the foundation for understanding and analyzing the process. Barnard el al. 11361 proposed that, if a liquid droplet exceeds some critical size but less than about one millimeter in diameter, the combustion takes the form of a spherical diffusion flame round the droplet and the burning rate is determined by the vaporization from the surface of the droplet. The fact... 

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