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F。 Yang et al。 / Chinese Journal of Chemical Engineering 23 (2015) 1746–1754 1747

provides better gas–liquid mixing performance than the traditional Rushton impeller。 Since then, other impellers with non-flat blades were continuously being developed。 For example, the hollow half el- liptical blade dispersing turbine (HEDT) was applied to the gas–liq- uid and gas–liquid–solid mixing in stirred tanks by Chen et al。 [13]。 The void fraction distributions under different operating conditions, including impeller rotational speed, power input, gas flow rate and temperature, were investigated。 Vasconcelos et al。 [14] concluded that the best performance may be expected from retrofitted Rushton impeller with streamlined blades, which lowers the power number, results in less aerated power drop, retards impeller flooding and accordingly, are confirmed more efficient and capable of handling gas。 This may attribute to the fact that impellers with streamlined blades could increase the blade curvature and alter the trailing vorti- ces characteristics [15]。

Another promising improvement of the standard Rushton impeller is the self-inducing Rushton type impeller。 The mixing mechanisms were explored, and the advantages of this impeller in applications of gas–liquid–solid and gas–liquid dispersion were experimentally and numerically validated by several researchers [16–19]。 Bakker et al。 [20–22] invented the Scaba (also named as BT-6) impeller which has vertically asymmetric blades。 It was shown that this impeller has a flatter aerated power curve and can disperse more gas before flooding than impellers with symmetric blades。 In addition, there are also flexible blade impellers [23–25]。 Compared with the rigid blade impeller, the flexible blade impeller is allowed to flex into the desired curved configurations under the action of the fluid。 Unfortunately, there is no available information so far for the application of flexible blade impeller in the processes involving  microorganisms。

In this work, we investigate the dislocated-blade Rushton impeller in gas–liquid mixing in a baffled stirred vessel using the experimental and CFD methods。 This impeller has the same component dimensions as the standard Rushton impeller, except that the blades are mounted above and below the impeller disc alternatively。 To be exact, three blades are above the impeller disc, and the bottom edge of each blade is aligned with the bottom surface of the disc。 For the other three blades, they are below the disc, and their top edges are parallel to the top surface of the disc。 For more details about this impeller, the readers may refer to the literature [26]。

2。 Stirring System

Fig。 1 depicts the configuration of the stirring system studied in this paper。 The impellers are the standard Rushton impeller and the dislocated-blade Rushton impeller (hereafter referred to as SRT and DRT, respectively)。 The stirred vessel (dia。 T = 0。21 m) is an elliptical- bottomed cylindrical vessel with four standard baffles。 The offset from baffle   to   the   vessel   wall   is   T/60。   Tap   water   (density:   ρl          =

998。2 kg·m− 3, dynamic viscosity: μl = 0。001 Pa·s) was used as the

working liquid。 For the experimental measurements of dissolved oxygen, the activated sludge effluent was employed, which has the same density and viscosity as the tap water。 Air (ρg = 1。225 kg·m−3,

Fig。 1。 Schematic of the stirring system: (a) stirred tank; (b) DRT impeller。