## 5 Conclusions  
In this study, the geometrical parameters and mass of coal fragments generated from WJD in coal seams were measured. According to the results, the particle cumulative probability distributions, size distribution density, shape features (major axis/minor axis) and fractal dimensions of coal fragments for various jet impact velocities and coal strengths were analyzed based on the three-parameter GEV function and fractal theory. The following conclusions were drawn.  
1. The NUM-based cumulative probability curves of coal fragments are more intensive in the sections with relatively small particle sizes, and then the curves become sparser with increasing particle size. With increasing jet velocity, there is an obvious shift in the distribution curves toward smaller sizes, implying that the fragments decrease as the jet impact velocity increases. Moreover, the higher the coal strength is, the larger the fragment sizes are when the jet impact energy is the same.
2. The size distributions of coal fragments are mainly determined by the dominant fragment size. The dominant fragment size increases logarithmically as the jet impact velocity decreases; the curves will become flat, and the particle size range is more scattered. With the increase in coal strength, the dominant fragment size increases, but there is no obvious change in the dispersion degree of the fragment size distribution.
3. The NUM-based cumulative probability curves for the shape (the ratio of the major axis to the minor axis) of coal fragments move toward the upper left with the increase in impact velocity. The curve for high impact velocity attains unity more quickly. Furthermore, there is a difference in the orders of magnitude for the cumulative probability of fragment quantity for different sizes. The larger the particle size is, the smaller its proportion is in all the fragments.
4. The coal fragmentation subjected to water jets in WJD has fractal characteristics. The fractal dimension value increases linearly with the increase in jet impact velocity. The fractal dimension of coal fragments from the M7 coal seam (f = 1.1) is smaller than that from the M8 coal seam (f = 0.5) under the same jet impact energy. In addition, the fractal dimensions obviously increase with the decrease in the dominant fragment size, which can be indirectly used to reflect the dynamic fragmentation of coal.
5. The size distribution, morphology and fractal characteristics of coal fragments are determined by the failure patterns of coal subjected to water jet impact. The shear fracture zone, circumferential fractures and radial fractures with a high density and conical fractures are conducive to generating powder particles, fine particles, and large fragments, respectively.  
[PERSON], [PERSON] and [PERSON] contributed to conceiving and designing the experiments, analyzing the data, and writing the paper. [PERSON], [PERSON], [PERSON] and [PERSON] performed the experiments.  
This research was funded by the National Science Fund for Distinguished Young Scholars grant number [51625401]; the National Natural Science Foundation of China grant number [51504046,51774055]; the National Science and Technology Major Projects of China grant number [2016 ZX05045] and the Science and Technology Project of Chongqing grant number [cstc2016 jcyjA0557].  
**Acknowledgments:** The authors acknowledge the Fengchun coal mine for providing access to the field experimental sites. The authors would like to thank Senior Engineer [PERSON] for technical guidance, and the American Journal Experts (AJE) Team (www.aje.com) for editing the English test of a draft of this manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.