We delineate and examine the successive stages of ligament-mediated atomization of burning multi-component fuel droplets. Time-resolved high-speed imaging experiments are performed with fuel blends (butanol/Jet A-1 and ethanol/Jet A-1) comprising wide volatility differential, which undergo distinct modes of secondary atomization. Upon the breakup of vapor bubble, depending on the aspect ratio, ligaments grow and break into well-defined (size) droplets for each mode of atomization. The breakup modes either induce mild/intense oscillations on the droplet or completely disintegrate the droplet (micro-explosion). For the blends with a relatively low volatility difference between the components, only bubble expansion contributes to the micro-explosion. In contrast, for blends with high volatility differential, both bubble growth as well as the instability at the interface contribute towards droplet breakup. The wrinkling pattern at the vapor-liquid interface suggests that a Rayleigh-Taylor type of instability triggered at the interface further expedites the droplet breakup.
We experimentally investigate the dissolution of microscale sessile alcohol droplets in water under the influence of impermeable vertical confinement. The introduction of confinement suppresses the mass transport from the droplet to bulk medium in comparison with the non-confined counterpart. Along with a buoyant plume, flow visualization reveals that the dissolution of a confined droplet is hindered by a newly identified mechanism levitated toroidal vortex. The morphological changes in the flow due to the vortex-induced impediment alters the dissolution rate, resulting in enhancement of droplet lifetime. Further, we have proposed a modification in the key non-dimensional parameters (Rayleigh number Raꞌ (signifying buoyancy) and Sherwood number Shꞌ (signifying mass flux)) and droplet lifetime c , based on the hypothesis of linearly stratified droplet surroundings (with revised concentration difference C
The present investigation deals with the puffing and micro-explosion characteristics in the combustion of a single droplet comprising butanol/Jet A-1, acetone-butanol-ethanol (A-B-E)/Jet A-1 blends, and A-B-E. The onset of nucleation, growth of vapor bubble and subsequent breakup of droplet for various fuel blends have been analyzed from the high-speed images. Puffing was observed to be the dominant phenomenon in 30% butanol blend, while micro-explosion was found to be the dominant one in other fuel blends (blend with 50% butanol or 30% A-B-E or 50% A-B-E). It was observed that puffing always preceded the micro-explosion. The probability of micro-explosion in droplets with A-B-E blends was found to be higher than that of butanol blends. Although the rate of bubble growth was almost similar for all butanol and A-B-E blends, the final bubble diameter before the droplet breakup was found to be higher for 50/50 blends than that of 30/70 blends. The occurrence of microexplosion shortened the droplet lifetime, and this effect appeared to be stronger for droplets with 50/50 composition. Micro-explosion led to the ejection of both larger and smaller secondary droplets; however, puffing resulted in relatively smaller secondary droplets compared to micro-explosion.Puffing/micro-explosion were also observed in the secondary droplets.
We examine the complete sequence of events associated with the transition in the topology of a single droplet into multiple fragments of secondary droplets in the context of burning multi-component miscible mixtures. The multi-component blends consist of tetradecane as a lower volatile component, while butanol and acetone-butanol-ethanol (A-B-E) are used as the higher volatile constituents. In addition to the widely recognized theory of bubble growth via micro-bubble coalescence, we reveal that the vapor bubble growth also occurs through the merging of large bubbles during the combustion of droplets. The initial bubble growth (Regime I) and collapse cycles were found to increase the rate of bubble nucleation in the droplet, which in turn leads to the growth and merging of two or more vapor bubbles into a single larger bubble (Regime II). The final stage of bubble growth (Regime III) is associated with the Rayleigh-Taylor (RT) instability at the vapor-liquid interface. After the inception of the RT instability, capillary wave propagation is also witnessed on the droplet surface. The breakup of a vapor bubble results in the creation of a ligament that subsequently undergoes pinch-off into one or more secondary droplets. The ligament pinch-off mechanisms are categorized into two types, i.e., tip breakup and tip-base breakup, which govern the diameter and velocity of secondary droplets along with succeeding volumetric shape oscillations in the parent droplet. In particular, the ligament tip-base pinch-off mechanism results in a bimodal distribution of secondary droplets. After the initial breakup event, a vapor bubble may grow either in the secondary droplet or inside the developing ligament, leading to a sequential cascade of breakup events.
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