Chemical Engineering Science 54 (1999) 2047}2054 Gamma radiation densitometry for studying the dynamics of #uidized beds R.F. Mudde*, W.K. Harteveld, H.E.A. van den Akker, T.H.J.J. van der Hagen, H. van Dam Kramers Laboratorium voor Fysische Technologie, J.M. Burgers Centre for Fluid Mechanics, Delft University of Technology, Pr. Bernhardlaan 6, 2628 BW Delft, Netherlands Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, Netherlands Abstract In this paper -radiation techniques are used to study the behaviour of a 40 cm #uidized bed, consisting of 1.9 mm spherical polystyrene particles. The particles are #uidized at high velocity, i.e. 50% of their terminal velocity, the bed being in the turbulent #uidization regime. Two parallel source}detector pairs are used to study the time-averaged void fraction pro"le. Instead of a direct inversion using the Abel-transform, a least-squares based algorithm with basis functions is proposed. The dynamics are investigated by inspection of the probability density functions of the recorded time series. It is shown, that close to the static bed height the bed can be almost packed to almost completely transparent. The bed is seen to move violently: large voids and particle clusters travel through the column. Using generalized cross-correlation techniques both upward and downward velocities are found.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluidized bed; Gamma-radiation densitometry; Void fraction pro"le; Velocity distribution; Tomography 1. Introduction In many industrial processes, contacting of gases and solids can be found. One way of doing so is in a #uidized bed, which is appreciated for its high interfacial area, good mixing, uniform temperature and high heat and mass transfer rates. The #uidized bed has been a research topic for several decades, nevertheless scale-up remains di$cult and there is still a need of reliable techniques to extract information from experiments. These are even more important in view of the increasing interest in Computational Fluid Dynamics applied to #uidized beds. To validate these simulations detailed experimental work concerning the #ow characteristics is needed. While intrusive probes are used frequently (see e.g., Wherther 1972; Van Lare et al., 1997; Groen et al., 1997), more recently electrical capacitance tomography (ECT) is em- ployed (Hallow et al., 1993; Xie et al., 1990; Ku K hn et al., *Corresponding author. Tel.: 0031 15 278 1400; fax: 0031 15 278 2838; e-mail: r.f.mudde@klft.tn.tudelft.nl. 1996). Though tomography is a promising technique, it faces serious problem with its spatial resolution, espe- cially in larger equipment (see e.g. Ku K hn et al., 1997). Gamma radiation and X-ray techniques are non-intru- sive as well and are applied to study e.g. bubble columns (Yang et al., 1993; Kumar et al., 1997), gas-#uidized beds (Lin et al., 1985), or three-phase #uidized beds (Larachi et al., 1996). The radiation techniques can provide information about the distribution of the solids (Weimar et al., 1994; Yates and Simons, 1994; and Van Santen et al., 1997; Shollenbeger et al., 1997). A classical example is found in the work of Rowe and Partridge (1965), showing the shape of a bubble inside a #uidized bed. Furthermore, single radioactive particle tracking techniques have been developed that allow studying the solids #ow and #ow patterns inside engineering equipment like #uidized beds (Parker et al., 1993; Stellema et al., 1998; Yang et al., 1993). An advantage of radiation techniques over the electri- cal capacitance measurements is the absence of the so- called soft "eld e!ect in the former. However, the data rate of ECT systems is easily 100 Hz or higher, whereas radiation techniques are limited in this respect due to the 0009-2509/99/$ } see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 8 ) 0 0 3 7 2 - 8