# Limiting Conditions for Droplet Fragmentation of Stabilized Suspension Fuels
## 4 Results and Discussion  
### Droplet Behaviors  
We used the following terms for the analysis of the research findings: droplet swelling, drying, destruction, and intact state. By droplet swelling we mean an increase in the droplet size due to lower effective density of the fuel composition and higher vapor pressure. Drying is the release of volatile components from the porous structure of fuel blends. Destruction occurs when fragments break off from the parent droplet due to higher vapor pressure in the porous structure of fuel blends. Intact state means no significant changes in the droplet shape and no fragments breaking off upon heating. Figure 2 presents typical snapshots showing the patterns of liquid film evaporation, near-surface layer dehydration, and fragmentation of C50W49D1 (a), C50W49 LE1 (b), C50W49L11 (c), C50W49R1 (d), and C50W50 (e) blends at \(T_{\rm a}\approx 450\) K and \(R_{\rm d0}\approx 1.0\) mm. Droplets of fuel blends with different effective viscosities had different shapes on the same holder. When droplets of these fuels are injected using various atomizers, their shapes follow the same patterns: droplets take an ellipsoidal, spherical, disk shape, etc. At the same time, earlier experiments [46; 47; 48] have shown that the shape of a moving droplet transforms continuously, though more slowly with an increase in the liquid viscosity. Note that the shape of CWS and CWSP fuels is not always spherical even in practical applications because it contains a high concentration (more than 50%) of solid particles. Typical videframes of droplet breakup in puffing and microexplosion modes are shown in Supplementary Materials A. The experiments showed two breakup regimes occurring for different physical reasons. In particular, droplets of the compositions without flammable liquids broke up after they were dehydrated and their porous structure was filled with water vapor. Afterward, due to an increase in the water vapor pressure, some of the fuel broke off from the droplet surface. If a blend contained a flammable liquid, the vapor bubble nucleation was intensified at the interface between water and liquid combustible component. Droplets were heated more rapidly if they contained solid particles. The latter also served as vaporization centers upon intense heating. The videos obtained using high-speed recordings do not make it possible to observe bubble nucleation near the liquid/liquid interface or around solid particles. In the near-surface layers, however, these effects were observed quite consistently. The slurry droplet surface tended to dry somewhat, followed by local vapor bubble nucleation and implosion, which, in turn, was followed by puffing of liquid and solid particles. The larger the droplet volume was, the more the droplet surface deformed with consecutive solid particle surface drying in the near-surface layer, vapor bubble nucleation and growth, droplet swelling, bubble implosion, separation of liquid fragments, entrainment of small solid particles, and large agglomerates breaking off when the dehydrated remnants of the composite droplet broke up. The analysis of physical patterns in the processes under study has shown that solid particles accelerate droplet heating due to an increase in the thermal conductivity and diffusivity. Faster heat exchange, however, does not give enough time for cascade bubble nucleation and growth inside the droplet and at its surface. A liquid film rapidly evaporates from the droplet surface, and the particle surface dries. It is important to keep a certain balance between the thermal processes and mass exchange processes to intensify the vaporization and boiling in a heterogeneous droplet. Note that child droplets initially break up due to relatively high velocity splash toward the upstream part. Then, child droplets fall freely by gravity force. Typical video-frames of droplet breakup with different coal particle concentrations are shown in Supplementary Materials B.