Introduction
High-velocity gas-particle flows in risers are accompanied by persistent fluctuations in pressure, velocities and particle concentration, and the presence of particle clusters. Risers are seldom used in isolation; instead, they are employed in conjunction with several other devices. For example, in circulating fluidized beds, they are used along with cyclones, standpipe, particle flow control device such as slide valve, etc. Analysis of the power spectral density of temporal fluctuations (say, in gas pressure at some location in the riser) observed experimentally in the various components of the circulating fluidized beds including the riser often reveals both low and high frequency components [ 11. Our recent experiments on fluctuations in circulating fluidized beds suggest that the low frequency fluctuations in circulating fluidized beds are likely to be associated with the interaction between the various components of the system and have their origin in the standpipe. A manuscript based on these experiments is attached to this report as appendix A. The high frequency component of the fluctuations in the riser does not appear to be correlated with those observed elsewhere in the circulating fluidized bed [ 11, suggesting that it is associated with local phenomena.