# Limiting Conditions for Droplet Fragmentation of Stabilized Suspension Fuels
## 4 Results and Discussion
### Droplet Behaviors
The comparative analysis of the footage from the CWS and CWSP experiments indicates significant differences between droplet breakup regimes with each of the added components. At the same time, the temperature range of consistent breakup differed significantly for droplets of different component composition. The greater the concentration of solid particles in a droplet, the faster it reached the breakup temperature. The required temperature was lower, but the breakup was quite weak. Large agglomerates were the most frequent outcome. Two-liquid droplets without solid particles typically broke up in the puffing and microexplosion regime to form a large number of small liquid fragments. The longer it took for a droplet to reach the breakup temperature, the more secondary  
Figure 2: Images showing the evaporation, drying, and breakup of C50W49D1 (**a**), C50W49 LE1 (**b**), C50W49L1 (**c**), C50W49R1 (**d**), and C50W50 (**e**) at \(T_{\mathrm{a}}\approx 450\) K and \(R_{\mathrm{d0}}\approx 1.0\) mm. fragments were formed in the process. A high proportion of water in such initial droplets led to a transition from microexplosion to puffing. Three-component droplets (for instance, based on water, coal, and oil) also broke up in two regimes: puffing and microexplosion. The component concentration played a fundamental role in the regime transition. In particular, the microexplosion could be intensified by increasing the concentration of the liquid combustible component and minimizing the proportion of solid particles and water in a fuel slurry. The breakup regimes of composite liquid fuel droplets containing solid particles were found to depend greatly on the surface properties of those solid particles. For instance, we compared the heating and breakup behaviors of composite droplets based on coal and filter cake. Under identical heating conditions, droplets with coal particles broke up more intensely than composite fuels based on filter cake. This effect was observed throughout the temperature range under study. It can be attributed to the significantly different surface properties of the filter cake and coal. Filter cake particles were more hydrophilic. Liquid penetrated the pores between particles and was retained in the heterogeneous structure. Coal particles became wet after being in contact with water for a longer period of time. The preparation of slurries based on water and coal particles took a longer time than that of filter-cake-based slurries. The coal washing technology that produced the filter cake played a fundamental part. Such technology often uses special flocculants, so the solid particles were covered with wetting agents. These substances reduce the contact angle and increase the agglomeration of liquid droplets and solid particles [49]. Under such conditions, the breakup of composite droplets requires more outside effort than for droplets containing coal particles. The CWS and CWSP droplet breakup regime was significantly affected by the temperature and velocity of the oncoming air flow (and, hence, the heat flux). For instance, the experimental series established that consistent droplet breakup requires certain average ambient gas temperatures (450 to 850 K). At relatively low temperatures (under 450 K), the thermal energy is not enough for water boiling and droplet breakup. Under such conditions, the near-surface layer of a droplet heats steadily, the liquid evaporates, and solid particles are dried. The latter, however, do not break off from the main droplet because there is no pressure from the inside, i.e., no vapor bubble nucleation. At high temperatures (more than 850 K), rapid liquid evaporation was observed from the near-surface layer. Owing to rapid vaporization in a closed space, the solid particles breaking off from the droplet surface are suppressed by the increasing pressure of the gas-vapor mixture. Another important factor is the high temperature and radiant heat flux from the flame, which intensifies the pyrolysis of solid particles.