Fig。 4。 Numerically obtained gas hold-up distribution of (a) SRT and (b) DRT under different operating conditions。

1750 F。 Yang et al。 / Chinese Journal of Chemical Engineering 23 (2015) 1746–1754

Fig。 5。 Axial profiles of gas hold-up at different radial positions: (a) 20, (b) 40, (c) 60 and (d) 80 mm from the shaft in the left and right half of the vertical plane。 Operating condition: N = 600 r·min−1, Q = 0。6 m·h−1。

about 6%–18% was achieved。 This finding further validates the merit of DRT in the mixing of gas–liquid in stirred vessels。

4。2。3。 Measurement of dissolved oxygen

Experimental set-up is shown in Fig。 7。 The dissolved oxygen exper- iments were carried out at 23 °C in the same stirred vessel as that employed in the numerical study。 At this temperature, the saturated dissolved oxygen in water is 8730 μg·L−1。 The activated-sludge effluent was used as the working fluid since it contained aerobic bacteria。 Before论文网

experiment, no air was aerated and the dissolved oxygen level is zero because the dissolved oxygen was completely consumed by the bacte- ria。 Air was aerated via an OLF-3540 air compressor (Shengyuan    Air

Compressor Manufacturing Co。, Wenling, Zhejiang, China), and the flow rate was controlled by a LZB-10 rotor flowmeter (Qiquan Flowme- ter Co。 Ltd, Yuyao, Zhejiang, China)。 When aeration, the aerobic bacteria began to consume the dissolved oxygen, and after some time there reached a balance between the aerated and consumed oxygen。 For different operating conditions, the time needed for reaching the equilib- rium was different and generally 20 min were required。 Then the dissolved oxygen under the steady state was measured with the InPro 6050 O2 sensor (Mettler Toledo, Switzerland)。 The dissolved oxygen at two representative points located in the vertical plane containing the baffles was measured。 Point 1 located 100 mm below the liquid surface and point 2 was at the middle place between the upper and lower

F。 Yang et al。 / Chinese Journal of Chemical Engineering 23 (2015) 1746–1754 1751

Fig。 6。 Axial profiles of gas hold-up at different radial positions: (a) 20, (b) 40, (c) 60 and (d) 80 mm from the shaft in the left and right half of the vertical plane。 Operating condition: N = 300 r·min−1, Q = 0。4 m·h−1。

impeller。 Both points were positioned at a distance of 30 mm from the stirred vessel wall。 For each operating condition, the measurements were repeated four times and the average results were given in Fig。  8。

From Fig。 8 we can see that DRT outperforms SRT under all the con- ditions studied here。 An increase up to 16% was obtained。 The most pro- nounced increases exist at monitoring point 2 and the average increase

Table 2

Average gas volume fraction

Operating  condition Impeller Radial position/mm

r = 20 r = −20 r = 40 r = −40 r = 60 r = −60 r = 80 r = −80

N = 600 r·min−1 Q = 0。6 m · h−1 N = 300 r·min−1 Q = 0。4 m · h−1

SRT 0。124 0。131 0。135 0。141