A system for measuring bubble voidage and frequency around tubes immersed in a fluidized bed of particles.
Gas-solid fluidized beds are common in chemical processing and energy production industries. These types of reactors frequently have banks of tubes immersed within the bed to provide heating or cooling, and it is important that the fluid dynamics within these bundles is efficient and uniform. This paper presents a simple, low-cost method for quantitatively analyzing the behavior of gas bubbles within banks of tubes in a fluidized bed cold flow model. Two probes, one containing an infrared emitter and one containing an infrared (IR) detector, are placed into adjacent glass tubes such that the emitter and detector face each other. As bubbles pass through the IR beam, the detector signal increases due to less solid material blocking the path between the emitter and detector. By calibrating the signal response to known voidage of the material, one can measure the bubble voidage at various locations within the tube bundle. The rate and size of bubbles passing through the beam can also be determined by high frequency data collection and subsequent analysis. This technique allows one to develop a map of bubble voidage within a fluidized bed, which can be useful for model validation and system optimization.
Observations of a fluidized bed using capacitance imaging
Abstract The Morgantown Energy Technology Center has developed a unique method for imaging voidage distributions within fluidized beds. This system allow high-speed three-dimensional imaging of the voidage distribution in a bed to be recorded. From these imaging data a variety of visualizations can be constructed and quantitative information extracted. Five materials with differing particles shapes and sizes were fluidized and imaged in a 15.24 cm diameter bed over a range of superficial velocities. The imaging revealed a variety of bubble sizes and shapes depending on the material and velocity. Bubble properties including a frontal diameter, length, velocity and spacing were determined. A correlation of bubble rise velocity developed from an earlier study was modified to include the bubble length as the primary dimension defining the bubble. The imaging also revealed detailed voidage distributions around bubbles and slugs. These images show round-nose bubbles and slugs for the fine materials examined. With the coarser materials, a very blunt slug was common. The technique has the potential to substantially improve design and scale-up of fluidized beds and other gas-solid systems by providing a detailed understanding of voidage distributions in these systems.
The bubbling behaviour of fine powders when fluidised
Narrow size cuts of particles in the range 40–260 μm have been examined by X-rays when fluidised by air. No discontinuity in behaviour was observed with decreasing particle size. The bubbles behave the same in all materials except that their velocity increases with decreasing particle size. The visible bubble flow is generally less than the excess over minimum fluidisation flow. Almost all flow occurs interstitially near the bottom of the bed but the proportion decreases with height to approach Umf near the top of the bed. Appreciable changes in dense phase voidage occur with fine particles and this varies with bed height.