Baoqing Liu

The Influence of Impeller Combination on the Gas-Liquid Dispersion Performance of a Coaxial Mixer in Viscous Fluids

Abstract As a kind of mixer with wide adaptability, coaxial mixer has a wide-application in the process industry. With the help of experiment and numerical simulation, the gas-liquid dispersion performance of four impeller combinations in viscous system was studied in terms of the global gas holdup, local gas holdup, local bubble size and liquid phase flow field. Wall-scraping anchor is the common outer impeller of the four impeller combinations, and the four inner impellers are Rushton turbine, six-straight-blade turbine (SBT), 45° six-pitched-blade turbine pumping downwards (PBTD) and 45° six-pitched-blade turbine pumping upwards (PBTU). The experimental results indicate that the global gas holdup of the Rushton impeller combination is the highest among the four impeller combinations and that of PBTU is the lowest. The local gas holdup of Rushton and PBTU combinations are all high, but the gas distribution with the Rushton combinations is more uniform. Besides, the local bubble size of the four impeller combinations has the same tendency with the local gas holdup. Under the same conditions, the flow field and distribution of gas holdup of the four impeller combinations in stirred tank were simulated numerically. Compared with the experimental results, the simulation results show a good agreement on the value and distribution of local gas holdup. Among the four coaxial mixers, the Rushton impeller combination is more suitable for gas-liquid dispersion.

Experimental Research on Fluid-Elastic Instability in Tube Bundles Subjected to Air-Water Cross Flow

Abstract Fluid-elastic instability is the major factor in causing the vibration of tube bundles. Design guidelines on fluid-elastic instability in tube bundles is necessary to avoid damage due to excessive tube vibration. However, the design guidelines on fluid-elastic instability in tube bundles subjected to two-phase cross flow have no consistent conclusions. Accordingly, this technical note researches the vibration characteristics of three tube bundle distributions, namely, normal square tube bundles with pitch-to-diameter ratios of 1.28 and 1.32 and a normal triangular tube bundle with a pitch-to-diameter ratio of 1.32. Comparison of the present fluid-elastic threshold results with previously published data shows good agreement in single-phase flow. The effects of pitch-to-diameter ratio and tube bundle configurations on fluid-elastic instability induced by air-water cross flow were also compared and analyzed by measuring unstable behavior of tube bundles. It was found that fluid-elastic instability is more prone to occur with a decrease of pitch-to-diameter ratio and that the normal square tube bundle is more stable than the normal triangular tube bundle. From the perspective of the tube bundle configurations, it was recommended that the instability constant K in normal triangular and normal square tube bundles be 3.4 and 4.0, respectively.