Influence of the velocity gradient of the carrier medium on the crushing characteristics of non-Newtonian liquid droplets on the example of a highly concentrated coal-water suspension

The article presents the results of the experimental studies of the processes of fragmentation of coal-water slurry fuel (CWS) drops in an air flow. The directions of movement of the CWS drops and the air flow coincided. The effect of the velocity gradient ($\nabla V$) of the carrier medium (air) on the characteristics and conditions of CWS drop fragmentation was analyzed based on the experimental results. It was found that the $\nabla V$ values have a significant effect on the characteristics and conditions of CWS drop fragmentation. An increase in the velocity gradient of the carrier gas medium leads to a significantly nonlinear and nonmonotonic change in the critical Weber numbers for CWS drops of typical sizes in the case of using highly concentrated suspensions (with a coal component concentration in the fuel of φ_coal = 50–55 %). The analysis of the effect of the coal type and its concentration in the water-coal suspension has shown that the characteristics and conditions of CWS drop fragmentation are significantly affected by the rheological properties of the water-coal fuel. It is shown that during fragmentation of drops of highly concentrated suspensions (at φ_coal = 50–55 %) the dependence of the critical Weber number on the velocity gradient of the carrier medium flow demonstrates significant non-monotonicity and non-linearity. With an increase in the velocity gradient from $\mathrm{gradV}=80$ s⁻¹ to $\mathrm{gradV}=160$ s⁻¹ the values of the critical Weber number decrease (by 20 %), while a further increase in the velocity gradient leads to an increase in the We_cry values by 40 %. The latter is due to a significantly non-linear relationship between the rheological characteristics of the CWS and the concentration of coal, as well as the degree of its metamorphism. The hypothesis describing this non-trivial result has been developed. The hypothesis has been substantiated that the nature of the process of fragmentation of typical CWS drops is greatly influenced by a complex of hydrodynamic processes occurring inside the drop in the time period immediately preceding the fragmentation.