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.
Use of optical probes to characterize bubble behavior in gas‐solid fluidized beds
Optical probes are used to study gas-solid fluidized-bed hydrodynamics. The probes each consisting of a light source and photodetector separated by a gap are suitable for use at combustion-level temperatures. The methodology to process the signal for calculation of bubble properties such as bubble frequency, local bubble residence time, bubble velocity, pierced length, bubble size, and visible bubble flow is presented. The signal processing technique is independent of bed operating conditions. The probe signal processing methodology is validated by comparing calculated bubble properties based on the probe signal with properties observed on videotapes of a 2-D bed.
Influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution in a fluidized bed
Abstract Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles were made in a pressurized fluidized bed, using capacitance and Pitot-static pressure probes, respectively. The influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution was studied. The bed, of 0.2 × 0.3 m cross-section, was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities between 0.1 and 0.6 m/s. The bed material was silica sand with a mean particle diameter of 0.7 mm. The mean bubble frequency, the mean bubble rise velocity, the mean bubble volume fraction and the visible bubble flow rate were found to increase with both increasing pressure and excess gas velocity. The mean pierced length of bubbles increases with increasing excess gas velocity but decreases, after an initial increase, wtih increasing pressure. With both increasing pressure and excess gas velocity, the bubbles tend to redistribute themselves towards the center of the bed cross-section. The pressure probe measurements show that the through-flow velocity of gas inside the bubbles decreases with increasing pressure. Furthermore, gas flow balance calculations show that the ratio of dense phase superficial gas velocity to minimum fluidization velocity increases with both increasing pressure and excess gas velocity. Pressure drop measurements show a decrease in minimum fluidization velocity with increasing pressure, in accordance with predictons from the Ergun equation.