Figure 9 . Sample images of spherical LDPE particles fluidization and 100 cylindrical wood pellets with a superficial gas velocity of (a) 1.5Umf, (b) 2.0 Umf,(c) 3.0 Umf, (d) 3.7 Umf.
Figure 10 and 11 shows the temporal evolution of the LDPE particle velocity field at 2.0 Umf and 3.0 Umfrespectively. The general slugging behavior resembles that observed for the LDPE only case. The process generally involves (1) the slug formation and rising at an almost constant speed (Figure 10: t=0 - 0.313 s, and Figure 11: t=0 - 0.213s) and (2) disintegration of the slug by raining down the particles (Figure 10: t=0.313 -0.763s, and Figure 11: t= 0.213 - 0.606s). However, the characteristic size and velocity of these processes differ significantly between each case. For example, at 2Umf , both LDPE only case and binary mixture case form a slug of around 5 cm. At 3Umf the slug length of the binary mixture case increases to 14 cm. As for the slug rising speed, at 2Umf, the rising speed is 35 cm/s and 44 cm/s for LDPE only case and the binary fluidization case respectively. At 3Umf, the rising speed of the slug increases to 101 cm/s. Similarly, at 2Umf, the maximum falling speed is 56 cm/s for the LDPE only case, and increases to 119 cm/s for the binary fluidization case. At 3Umf, it reaches as high as 224 cm/s.
To compare the general trends of fluidization with and without non-spherical wood pellets, the spatially averaged vertical velocity is calculated using all available velocity vectors in the field of view, denoted as <v >FOV. The comparison of the temporal evolution of <v >FOV for LDPE only and binary fluidization case at both 2Umf and 3Umf is shown in Figure 12. As expected, the LDPE only case at 3Umf remains almost constant due to the transition from the slugging to the spouting regime. Except for the LDPE only case at 3Umf, the <v >FOV varies quasi periodically with varying amplitude and frequency. To quantify the amplitude and frequency, the local troughs and peaks are detected for the whole duration of the available dataset. The corresponding amplitude and period are measured and the mean values are calculated across all detected local troughs and peaks. At 2Umf, the mean amplitude of the <v >FOVsignal increases significantly with the addition of wood pellets from 29.4 cm/s to 58.7 cm/s. At 3Umf the mean amplitude for the binary fluidization case increases to 123.0 cm/s. The mean frequency of the signal is 1.72 Hz for LDPE only case at 2Umf and increases to 2.37 Hz with the addition of wood pellets at the same superficial gas velocity. At 3Umf, the mean frequency decreases significantly to 1.54 Hz, presumably due to the longer traveling range of the slugs in the bed.