Void Fraction Distributions in Cold-gassed and Hot-sparged Three Phase Stirred Tanks with Multi-impeller

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration （a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT＋2WHu） recommended in previous work has been used in this work. The operating temperatures were 24℃ and 81℃, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are .investigated and discussed. Results show thatth-e increasing of agitator speed or gas flow rate leads to an increase in local-void fraction at the majority of measurement points in both cold and hot systems. However, the unifo,rmity of gas dispersion does not always in crease as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems.have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase＇stirred tanks.

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT + 2CBY, was used in this study. The effect of the impeller diameter D, ranging from0.30 T to 0.40T(T as the tank diameter), on gas dispersion in a stirred tank of 0.48 m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT + 2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand(RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity VSof 0.0078 m·s-1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the system with D/T = 0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model(PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.

Indoor Environment Control Effect Based on Multi-Impeller Air Conditioner

Air conditioner is the key air supply device to improve the indoor air heat transfer cycle and thermal environment. In this paper, according to the similarity theory, six kinds of the multi-impeller air conditioners(MIAC) with different numbers of axial-flow impeller are first proposed based on air conditioners with cross-flow fan(CFAC). The effects of different multi-impeller air supply modes on the indoor airflow field and temperature field are investigated by using the numerical simulation method. The validity of the computational model and method is verified by comparing the experimental measurements and numerical results. Compared with other air supply modes, the air supply distance is relatively far and the indoor environment is also improved obviously when the six-impeller air conditioner(SIAC) is used. The independent multiple impellers and outlets of SIAC make the mutual interference between the outlet airflow less, which effectively reduces the friction between the airflow and the degree of loss along the way and makes the cold and heat exchange of the whole indoor space more sufficient. The indoor thermal environment is also analyzed by using the PMV-PPD method and prototype experiment. Under the working condition of no wind, the indoor thermal comfort of human body with SIAC is better than that of CFAC. Especially in the horizontal plane of 0.6 m above the human foot, the indoor thermal environment improvement effect is the most obvious. Simultaneously, the temperature difference along the indoor height direction with the SIAC is only 1.7℃, which is 70% lower than that of the CFAC. The blowing angle between the left and right side of the SIAC is 106°, which is 31% higher than that of the CFAC. Therefore, the overall thermal comfort of the six-impeller mode is better, and the control effect of indoor environment is satisfactory.