Secondary breakup of water and surrogate fuels: Breakup modes and resultant droplet sizes

Abstract

Secondary breakup of water and single-component surrogates of diesel such as n-dodecane and n-hexadecane, has been experimentally investigated in the present study. The experimental arrangement involves injection of mono-disperse droplets of the test liquid into an air jet issuing from a contoured nozzle. The mono-disperse droplet generator (MDG) is used to produce liquid droplets of diameter around 200 ± 4-µm, which is close to those found in practical sprays. The aerodynamic Weber number (We) is varied by changing the air-jet velocity. In the present study, apart from the most-reported regimes namely bag breakup, bag and stamen breakup, shear breakup and catastrophic breakup - two more regimes, viz. dual-bag breakup and multi-bag breakup regimes, have also been observed. The dual bag regime occurs in the Weber number range of 30 to 46 followed by the multi-bag regime in the Weber number of 46 to 80. These two regimes have been very rarely reported in the technical literature. The temporal droplet deformation observed in the present study during bag regime and bag and stamen regime is observed to match well with data reported in the literature. The size of core droplet in the bag and stamen mode is observed to increase with Weber number and after a Weber number of 30 it is of sufficient size for its Weber number to be above the critical Weber number. This core droplet undergoes a subsequent breakup resulting in dual-bag breakup mode. Variation of SMD of resultant droplets is measured and reported for all the breakup modes ranging from We = 10 up to 500 and data at some salient points is observed to match well with data from literature. Detailed droplet probability distributions for bag, bag and stamen, dual bag and multi-bag regimes has been reported. A tri-modal droplet size distribution is observed in bag and bag and stamen breakup regimes, where the first peak corresponding to film, second peak corresponding to rim and the third corresponding to nodes on the rim and stamen. The rim, nodes and stamen of the bag is observed to contribute up to around 90% of mass from the parent droplet. With increase in Weber number, the droplet distribution transforms in to a bi-modal distribution. The droplet size distribution resulting from water breakup is different from those of the other two liquids in the larger droplet size range due to the higher surface tension of water. The experimental data presented is useful to validate computational models in terms of their capability to predict the droplet deformation dynamics, resultant droplet sizes and distribution with water and diesel fuel surrogates such as n-dodecane and n-hexadecane.