Tsao-Jen Lin

Predictions of flow transitions in a bubble column by chaotic time series analysis of pressure fluctuation signals

Abstract In this study, predictions of flow transitions of a bubble column are carried out by chaotic time series analysis of pressure fluctuation signals. Pressure signals are measured for four different distributor plates with the superficial gas velocity varying from 0 to approximately 0.22 m / s . Methodology of the chaotic time series analysis is presented, including procedures of the time-delayed phase-space reconstruction and calculation of the chaotic invariants. Four chaotic invariants, which include the largest Lyapunov exponent, metric entropy, correlation dimension and mutual information, of all pressure signals are computed. The critical gas velocities are determined by the sharp increase or decrease of the obtained chaotic invariants at the flow regime transitions. This simple criterion is consistent with the experimental observations. The obtained critical gas velocities by various chaotic measures for the four different distributor plates are quantitatively consistent and are in good agreement with the results by the gas holdup data.

Characterizations of flow regime transitions in a high-pressure bubble column by chaotic time series analysis of pressure fluctuation signals

Abstract In this study, experiments are carried out for a high-pressure bubble column with a perforated plate as the distributor under the operating pressure up to 15 MPa . Gas holdups and pressure fluctuation signals are measured. Analyses of standard deviation and chaotic time series on pressure fluctuation signals are conducted to determine the transition gas velocities. The results of chaotic measures, such as the largest Lyapunov exponent, metric entropy and the correlation dimension show that there are bands of transition velocities for each operating pressure. The general trend is that the transition velocities increase as the operating pressure rises. Similar trends are also found for the transition velocities obtained by the standard deviation. All the transition velocities resulting from the chaotic measures and the standard deviation show good agreements.

High temperature and high pressure three-phase fluidization-Bed expansion phenomena

Abstract Bed contraction and expansion are investigated in a high pressure and high temperature gas–liquid–solid fluidized bed. The bed is operated at pressures from 0.1 to 17.4 MPa and temperatures from 20 to 94°C. The bubble dynamic behavior in the bed is visualized through transparent windows. The study indicates that the pressure and temperature affect the bed expansion and contraction, mainly through the variations in bubble behavior and the changes of liquid properties. The extent of bed contraction decreases with an increase in pressure and/or temperature for 2.1 mm glass beads. This decrease is attributed to the reduction in size and number density of large bubbles. However, for 1 mm glass beads, bed contraction is observed to be more pronounced under higher pressures. The generalized wake model is modified to account for bed expansion or contraction phenomena by taking into account the bubble size distribution effects.

Heat transfer and bubble characteristics from a nozzle in high-pressure bubble columns

Heat transfer and bubble characteristics (including bubble size, bubble rise velocity, and bubble formation frequency) from a nozzle in high-pressure bubble columns are studied. The experiments are conducted at pressures up to 15.2 MPa and temperature at 27°C. As pressure increases, the bubble size and the bubble rise velocity decrease, but the bubble formation frequency increases in the bubbling regime. The effects of pressure on bubble characteristics are more significant at high nozzle gas velocities than at low nozzle gas velocities. The transition from the bubbling to the jetting is identified based on two methods: visualization and power spectra of the signals from a pressure transducer. The transition velocity from bubbling regime to jetting regime decrease with increasing pressure or temperature. In the bubbling regime, the heat transfer coefficient has a significant increase with increasing pressure and nozzle gas velocity. The higher the pressure, the larger the increasing rate of the heat transfer coefficient and the nozzle gas velocity. In the jetting regime, however, the influence of pressure on the heat transfer coefficient is small.

Effects of macroscopic hydrodynamics on heat transfer in a three-phase fluidized bed

The macroscopic hydrodynamics and heat transfer of a two-dimensional (2D) three-phase fluidized bed have been quantitatively studied using particle image analyzer (PIA) and heat transfer probe, respectively. Three flow regimes transiting with increasing gas velocity are investigated, including dispersed and coalesced bubbling regimes characterized by two flow conditions (4- and 3-region flow). These flow regimes are comprised of different macroscopic structures, which are profoundly dominate the local heat transfer distribution. Generally, the bubble stream can enhance the heat transfer due to inducing the turbulent intensity, while the vortex acts like a barrier to inhibit the heat transfer. Different solid holdups are introduced into the flow to examine the effect on the heat transfer. It is found that the solid particles can initially increase the collision frequency between the solid particles and heating object, which leads to the increase of heat transfer. However, increasing the solid holdup over 8% becomes a negative effect on the heat transfer. The average heat transfer coefficient increases with the liquid and gas velocities in the beginning, and then levels off with further increasing the liquid and gas velocities.

Effects of macroscopic hydrodynamics on heat transfer in bubble columns

Abstract The hydrodynamics and heat transfer of two-dimensional bubble columns operated in various flow regimes are simultaneously studied using particle image velocimetry and heat transfer probe, respectively. With increasing gas velocity, the flow structures change from dispersed bubble regime to coalesced bubble regime divided into the 4- and 3-region flows. At low gas velocity ( cm / s ) , the main flow structure is the dispersed bubble regime with the uniform distributions of gas holdup, average bubble size, liquid velocity, and heat transfer in the radial direction except near sidewalls. For gas velocity between 1 and 3 cm / s , the 4-region flow regime begins to take over with the bubble coalescence and interaction in the flow. Macroscopic structures operated in the 4-region flow regime comprise descending, vortical, fast bubble, and central plume regions. In this regime, the fast bubble region has the maximum gas holdup, average bubble size, liquid velocity as well as heat transfer rate due to bubble inducing the strongest turbulent intensity. The vortex in the vortical flow region acts like a close cell to prevent interacting with its surrounding, and leads to unfavorable heat transfer. The heat transfer in the central plume region is better than that in the dispersed bubble regime because it has higher gas velocity to generate larger bubbles from the injector. When gas velocity is over 3 cm / s , the regime becomes the 3-region flow with a large fast bubble stream, no central plume. The path of the center of descending vortex has the minimum heat transfer rate. On the whole, the 3-region flow regime has the highest heat transfer rate in all three regimes. It is found that the flow and heat transfer are profoundly dominated by the macroscopic hydrodynamics structures.