DRT 0。131 0。148 0。153 0。167 0。178 0。192 0。199 0。186

SRT 0。139 0。137 0。141 0。136 0。161 0。166 0。174 0。177

DRT 0。141 0。149 0。151 0。155 0。167 0。170 0。182 0。178

1752 F。 Yang et al。 / Chinese Journal of Chemical Engineering 23 (2015) 1746–1754

Usually, power consumption can be expressed as the dimensionless power number。 The power number before gassing is given as follows:

where P is power consumption determined from the following torque equation:

P ¼ 2πNM ð9Þ

where M is the net torque before gassing。

For aerated mixing, the power number after gassing was calculated in the same way:

Fig。 7。 Experimental set-up of the dissolved oxygen measurement。

is as high as 10。8%。 By comparison, an average increase of 4。5% was found at point 1。 The reason is that point 1 approaches the liquid surface, and air above the surface was entrained into the liquid by the effect of agitation。 This phenomenon is more obvious especially at larger impeller rotational  speed  and  the  evidences  can  be  found  from Fig。 8(a) and (b)。 In addition, the amount of dissolved oxygen increases with the increase of gas flow rate, and the increase is more evident at lower impeller rotational  speed。

4。3。 Power consumption

Power consumption was calculated based on the torque acted on the impeller blades and shaft。 The torques consumed by bearing box and mechanical friction were subtracted when computing the net   torque。

where Mg  is the net torque after gassing。

When operated in the gas–liquid mixing, the Rushton type disc impeller developed ventilated cavities behind the blades, which result in the aerated power drop [2,39]。 The ratio of power consumption after and before gassing is named as relative power demand PRD:文献综述

PRD ¼ Pg=Po ð11Þ

Based on this correlation, smaller value of RPD indicates larger decrease of power number after gassing, which is adverse for the gas– liquid  mixing efficiency。

Study shows that when Re ≥ 2 × 105, the power number is constant。 It is independent of Re but dependent on the impeller type [40]。 For the

conditions studied here, the minimum Re is 3。18 × 105 and accordingly, the power number is invariable。 The power numbers of SRT and DRT configurations (composite impellers) are listed in Table 3。 It can be seen that, before gassing, the simulated power number for SRT is 7。31。 By comparison, power number prediction of DRT is 6。96, which is about 5% lower than the counterpart of SRT configuration。 The measured power numbers of SRT and DRT are 9。02 and 8。57, respective- ly。 Overall, the simulated power numbers of SRT and DRT are under- predicted with the experimental data and the difference is within 20%。