The liquid-phase processes occurring during fuel droplet combustion are important in deciding the behaviour of the overall combustion process, especially, for the multicomponent fuel droplets. Hence, understanding these processes is essential for explaining the combustion of the multicomponent fuel droplet. However, the very fast combustion of the too small fuel droplet makes experimental investigation of these processes uneasily affordable.
In the present work, a high speed backlighting, and shadowgraph imaging and subsequent image processing leading to quantitative analysis of the multicomponent fuel droplet combustion including liquid-phase dynamics are performed. Two categories of multicomponent fuels – in which diesel is the base fuel – are prepared and utilized. The first category is biodiesel/diesel and bioethanol/diesel blends, while the second category is the water-in-diesel and diesel-in-water emulsions. The portion of the added components is set to 10%, 20%, and 30% of the total mixture volume for all the multicomponent fuel mixtures (blends and emulsions).
Specific optical setups are developed in-lab and used for tracking droplet combustion. The first setup is associated with the backlighting imaging with the resulting magnification of the droplet images being 30 times the real size. The second optical setup is used for Schlieren and shadowgraph imaging, with the resulting magnification being 10 times the real size for both techniques. Those magnifications made it possible to visualize the droplet interior at high imaging rates (250, 1000, 10000, and 40000 fps) so that tracking of the droplet liquid-phase processes is easily performed.
Using the aforementioned optical setups, spatial and temporal tracking of nucleation, bubble generation, internal circulation, puffing, microexplosion, and secondary atomization during the combustion of the isolated multicomponent fuel droplets are performed. This offered the privilege of full sequential tracking of droplet secondary atomization from initiation to sub-droplet generation.
Emulsion fuel droplet fragmentation has also been tracked and visualized using Schlieren imaging. The effect of water content of the emulsion on the intensity of the resulting droplet explosion wave has also been evaluated.
Spatial and temporal tracking of the sub-droplets generated by secondary atomization, and their subsequent combustion, in addition to their overall lifetimes have also been performed. Accordingly, a comparison of the burning rate constant between the parent droplet and the resulting sub-droplets is carried out.
Specifically written and developed algorithms are used for image processing and feature extraction purposes. These algorithms are executed using Matlab. Using these algorithms, droplet projected area, perimeter, equivalent diameter, flame height and width, and sub-droplet generation rate have been temporally evaluated.
The high speed magnified imaging and subsequent image processing revealed that the rate of droplet secondary atomization is higher than those obtained by relatively low imaging rate. Additionally, it is shown that during a large portion of its entire lifetime, the droplet geometry has been affected by combustion significantly.
The combustion of two-interacting multicomponent fuel droplets at different spacing distances has also been investigated. The liquid-phase processes inside both droplets have been conceived. The effect of secondary atomization from one droplet on the other neighbouring droplet has also been studied.
The burning rate constants evaluated for the interacting fuel droplets are found to have the same trends as the isolated droplet combustion. However, the ratio of the droplet burning rate constant of the interactive droplet combustion to that of the isolated droplet combustion is higher than unity. The nucleation rate within the interacting fuel droplets is also found to be higher than that within the corresponding isolated fuel droplets.
