La práctica docente en la sociedad del conocimiento 34

Local solids concentration measurements were performed using the same type of thermistor used for local gas velocity measurements to demonstrate the usefulness of the probe. A preliminary understanding of the solids concentration distribution in the separator region below the gas outlet was made possible by the measurements. This information was used to gain further insight into the gas / solids separation process.

3.4.9.1

Effect of Superficial Solids Loading

Local solids concentration profiles were measured in the separator region below the gas outlet using the same type of thermistor used to measure local gas velocities in 3.4.7. A temperature method similar to those described by McMillan et al. (2006) and by Fushimi et al. (2012) was used in this study. The downer air was pre-heated to ~ 50 °C before injection in the unit, and the solids were injected at room temperature. Local solids concentrations were determined by measuring the heat transfer between the thermistor and the gas/solids mixture passing over the probe.

Figure 3.31 shows the temperature-specific thermistor power (Q*) radial profiles at three values of the superficial solids loading ratio. The measurements were performed for LS/D

= 0, z = 8.5 cm, and at Ug = 0.79 m/s (Reg = 5600), which ensured fully turbulent superficial gas flow. Larger values of Q* indicated higher solids concentrations since heat transfer increased with increasing solids concentration. One could reasonably expect that the solids concentration would be highest at the walls and minimum at the centerline. However, as shown in Figure 3.31, Q* was maximum at r/RD = 1 and decreased across the pipe diameter to a minimum at r/RD = -1. The trends were similar at all tested superficial solids loading ratios.

The apparent minimum solids concentration at r/RD = -1 can be explained by the location of the radial measurement profile. As shown in Figure 3.31, the radial measurement profile was in line with the gas outlet as viewed from above. Therefore, some solids that would otherwise have reached the thermistor between -1 < r/RD < 0 were deflected away from the measurement profile line by the gas outlet. Furthermore, this effect was amplified going from r/RD = 0 to r/RD = -1 since the gas outlet pipe was oriented downward at a 45° angle with decreasing r/RD, meaning that the gas outlet was closer to the measurement point at r/RD = -1. A measurement profile unobstructed by the gas outlet would have been preferable; however, the available measurement profile was limited by the location of existing ports on the downer unit.

Figure 3.31 – Effect of solids loading ratio on gas/solids mixture heat transfer radial profiles (Ug = 0.79 m/s, Reg = 5600, z = 8.5 cm, LS/D = 0)

A relationship was developed between the measured thermistor power and the superficial solids loading ratio in order to estimate the local solids loading ratio based on thermistor power measurements. Figure 3.32 shows the superficial solids loading ratio plotted against the area-weighted measured thermistor power for the same conditions described above. As mentioned above, the gas/solids mixture heat transfer increased with the solids loading. The data in Figure 3.32 were fitted very well by a power law relationship. It should be noted that the curve fit applied only to the specific operating conditions and separator geometry of the measurements shown in the figure. In other words, unique curves of the type shown in Figure 3.32 would be generated for different vertical positions, separator geometry, and superficial gas velocities. The curve fit shown in the figure was then used to estimate the local solids concentration along a measurement profile for a given set of operating conditions.

Figure 3.32 – Relationship between superficial solids loading ratio and area-weighted specific thermistor power (Ug = 0.79 m/s, Reg = 5600, z = 8.5 cm, LS/D = 0)

Figure 3.33 shows normalized local solids loading radial profiles for various superficial solids loading ratios using the curve fit shown in Figure 3.32. The local solids loading measurements were normalized to the superficial solids loading. As shown, the data were overlaid for all tested superficial solids loading ratios at z = 8.5 cm. This indicated that the radial profile was independent of superficial solids loading ratio at z = 8.5 cm.

Figure 3.33 – Normalized local solids loading ratio radial profiles for various superficial solids loading ratios (Ug = 0.79 m/s, Reg = 5600, z = 8.5 cm, LS/D = 0)

3.4.9.2

Solids Concentration Distribution with Height

Figure 3.34 shows specific thermistor power radial profiles at three different heights below the gas outlet. The tests were performed with Ug = 0.79 m/s (Reg = 5600), with a solids loading ratio of 6.4, and at LS/D = 0. In general, the thermistor power at the downer centerline (r/RD = 0) increased relative to the walls with increasing distance below the gas outlet. This indicated that particles were migrating increasingly from the walls to the central core as the distance below the gas outlet increased. Increased solids concentration in the central core far below the gas outlet was also observed by Yu et al. (2014) in numerical simulations of the same setup. The work by Yu et al. is provided for reference in Appendix E. Yu et al. predicted that solids migrated back toward the centreline with increasing z for m&sol m&g > 10 at m&g = 4 g/s (Ug ~ 0.7 m/s, Reg ~ 4000). Although the

migration of solids toward the centerline was undesirable, the gas velocity was low far below the gas outlet, and thus the impact on gas/solids separation was thought to be small.

Figure 3.34 – Effect of thermistor height on gas / solids mixture heat transfer (Ug = 0.79 m/s, Reg = 5600, m&sol/m&g= 6.4, LS/D = 0)