Figure 7. Ratio of \(U_{\text{mb}}/U_{\text{mf}}\) at different
temperature.
To further understand the mechanism underlying the fluidization
transition for silica particles used in this work, we studied the
pressure overshoot at the incipient fluidization, which is also shown
closely related to the cohesive inter-particle
forces.6, 24-26 Figure 8 shows pressure overshoot,
which was obtained by subtracting the local minimum from the maximum of
pressure drop over the superficial gas velocity, as shown in Figure 5.
The pressure overshoot becomes higher with the increase in temperature,
suggesting that the cohesive inter-particle forces are enhanced with the
increase in temperature. It was reported that the existence of alkali
metal elements in silica particles would cause the change in particle
surface property at high temperature, and further increase the cohesive
inter-particle forces.30 From Table 1, it can be seen
that the silica particles is mainly composed of Si and O, together with
some minor compositions of Al and K. Therefore, the fluidization
transition of silica particles from Geldart B to A at elevated
temperatures is mainly due to the enhanced cohesive inter-particle
forces.
Castellanos et al.43 found that the pressure overshoot\(P_{t}\) can be directly related to the tensile yield strength\(\sigma_{t}\) of particles via
\(P_{t}=\left(\frac{\sigma_{t}}{\rho_{p}\left(1-\varepsilon_{\text{mf}}\right)gH_{\text{mf}}}+1\right)P_{c}\)(16)
where \(H_{\text{mf}}\) is the bed height at the minimum fluidization.
The tensile yield strength \(\sigma_{t}\) can be further used to
estimate the cohesive inter-particle forces following
Molerus44:
\(F_{c}=\frac{\varepsilon_{\text{mf}}}{1-\varepsilon_{\text{mf}}}d_{p}^{2}\sigma_{t}\)(17)