Dependence of particle fluctuation velocity on gas flow, and particle diameter in gas fluidized beds for monodispersed spheres in the Geldart B and A fluidization regimes

In this paper we present new experimental data on the steady-state, mean squared, fluctuation velocity, or granular temperature, of Geldart B polymer, glass, nickel, and stainless steel monodispersed spheres averaged over the wall of a gas fluidized bed, as a function of gas flow and sphere diameter. The granular temperature is obtained by Acoustic Shot Noise technology—namely power spectral analysis of the steady state vibrational energy of the wall excited by random sphere impact, and calibrated by hammer excitation over the wall. The new data extends to polymer and metallic spheres the experimental discovery of a 1996 paper of Cody et al. that the fluctuation velocity of Geldart B glass spheres when scaled to the gas superficial velocity, Us, is inversely proportional to sphere diameter, directly proportional to a fundamental length scale, DoB, and is a universal function of U = (Us / Umf). We also demonstrate that the new data is consistent with the diameter dependence of the fluctuation velocity that can be derived from both the 1997 paper of Menon and Durian, who measured random sphere motion near the wall through the spectroscopy of scattered laser light, and the 1992 paper of Rahman and Campbell, who measured the average granular pressure of random sphere impact on a porous steel membrane. While the inverse scaling of the fluctuation velocity with sphere diameter, and the existence of a fundamental length scale for gas fluidization, DoB, had not been a feature of any published fundamental model, or computer simulation, of the steady state granular temperature of spheres in gas fluidized beds, we show that it is a feature of two recent dense kinetic fluidization models published in 1999, by Buyevich and Kapbasov, and Koch and Sangani. Both theories implicitly define a fundamental length scale for the fluctuation velocity, D⁎ = (μf2 / ρp2g)1 / 3, where ρp is the sphere density, μf is the gas viscosity, and g is the laboratory gravitational field. The new data for polymer, glass, nickel and stainless steel spheres presented in this paper, defines DoB = (56 ± 2)D⁎. We use the Anderson–Jackson stability model to show that the length scale DoB, also defines a stability length scale, such that for D < DoB(D > DoB), the uniform dense phase of the fluidized bed is stable (unstable), against one dimensional, first order fluctuations in sphere concentration. The length scale, DoB is thus the theoretical equivalent to the empirical scaling length introduced by Geldart, DB/A, to distinguish spheres (D > DB/A) that bubble at fluidization, from spheres (D < DB/A) that fluidize before bubbling. Finally, we present new experimental data, on the remarkable changes in the granular temperature, bed expansion, and bed collapse time, between Geldart B and Geldart A monodispersed glass spheres, and compare that data to granular temperature, and bed expansion, for Geldart A rough, non-spherical, log-normal dispersed diameter catalytic particles.

New data on the fluctuation velocity, Vn, of monodispersed spheres as a function of superficial gas velocity, Us, show that (Vn / Us), as a function of U = (Us / Umf), scales inversely with sphere diameter, defining a fundamental length scale for gas fluidization, D⁎ = (μf2 / ρp2g)1 / 3 which is a feature of recent dense kinetic theories of fluidization. For glass spheres, experiment and theory define the transition to Geldart A fluidization by DG = 60D⁎.Figure optionsDownload as PowerPoint slide