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Abstract
A digital flow visualization system was developed to investigate the emulsion residence times on bare horizontal tubes submerged in an air fluidized bed. This system utilized two Digital Equipment Corporation LSI-11/02 microcomputers; one hosting a digital image processor, the other an analog to digital (A/D) signal processing system.
The A/D system was used to determine the fluidization state of the bed and to monitor bed start-up operations. The system consisted of a network of thermocouple probes and pressure transducers. A procedure for calibration and operation was also developed.
The digital image processing system was used to measure the emulsion residence time. This system utilized a Recognition Concepts Inc. Trapix real-time image processor to digitize camera video signals. Emulsion/bubble activity on the tube surface within the bed cavity was observed using an Olympus borescope and input to the RCI digitizer in real-time via the video output of a Sony AVC 3250 television camera. Emulsion residence time was determined through software analysis of the digital images. A Fast Fourier Transform (FFT) subroutine was used to determine the extent of emulsion movement using harmonic average threshold cut-off criteria.
An investigation into the emulsion residence time as a function of position on the tube surface was made for various mass flow rates. Variation in radial and axial positioning produced very pronounced effects in the measured residence times. Effects due to geometric position with respect to the bed wall was also evident. Varying the mass flow rate thus changing the fluidization level likewise had a major effect on the measured residence times. Increasing the fluidization level either by increasing the flow rate or by moving away from the bed wall lowered the residence time. In general, the lower surface had the lowest residence time and the top of the tube had the highest residence time due to loss of fluidization or settling.
