Granular temperature in a gas fluidised bed

Contradictory evidence over the role of bubbles in inducing particle velocity fluctuations has been presented in the literature - are bubbles required to induce particle velocity fluctuations in gas FBs or not? Applying the non-intrusive method of diffusing-wave spectroscopy (DWS) to a gas fluidised bed of particles whose minimum bubbling velocity is ~5 times that of the minimum fluidisation velocity, it is shown that bubbles are not necessary. Fluctuations are first observed at or just above fluidisation and increase in magnitude with superficial velocity. Following Batchelor (1988), assuming the square of the fluctuations are proportional to the square of the superficial velocity, (δv)² =f(θ) U_s² the function f(θ) was found to rise from zero at a voidage of θ≈ 32% to a maximum around θ≈ 40% from where it tails off to zero at θ≈52%. The distribution of velocity fluctuations in the bed is also presented. They are symmetrical about the centreline where they are maximal and increase with height above the distributor.