a b s t r a c t   Towards the objective of improving the gas dispersion performance, the dislocated-blade Rushton impeller was applied to the gas–liquid mixing in a baffled stirred vessel。 The flow field, gas hold-up, dissolved oxygen, power consumption before and after gassing were studied using the computational fluid dynamics (CFD) technique。 Dispersion of gas in the liquid was modelled using the Eulerian–Eulerian approach along with the dispersed k–ε turbulent model。 Rotation of the impeller was simulated with the multiple reference frame method。 A modified drag coefficient which includes the effect of turbulence was used to account for the momentum exchange。 The predictions were compared with their counterparts of the standard Rushton impeller and were validated with the experimental results。 It is concluded that the dislocated-blade Rushton impeller is superior to the standard Rushton impeller in the gas–liquid mixing operation, and the findings obtained here lay the basis of its application in process industries。72603

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1。 Introduction

In the past several decades, there have been considerable  efforts to improve the bacteria and cell culture efficiency。 These aerobic bio-processes are mostly carried out in aqueous medium with ionic salts where the solubility of oxygen is limited。 Under such circum- stances, oxygen must be continuously supplied and the rate of oxygen mass transfer is a key task [1]。 Therefore, the desire to improve gas dispersion homogeneity has attracted many scientific researchers  and engineers。

The most widely used reactor in biochemical industries to carry out the bacteria and cell cultivation operations is the stirred vessel [2,3], within which the fluids are agitated by impellers。 For gas–liq- uid mixing, excellent gas dispersion performance is the essential re- quirement of the impeller。 It is responsible for bubble breakup and dispersion。 Many kinds of impeller exist and  are  continuously being invented to meet various needs。 Traditionally, for the opera- tion of gas–liquid dispersion, the standard Rushton impeller has been widely used since 1950s [4]。 This impeller has high volumetric gas–liquid mass transfer coefficient [5]。 So far, it has served as the

measuring yardstick to which other types of impellers are compared [2]。 However, in spite of its versatility, the standard Rushton impeller is not perfect and many weaknesses have been identified。 For exam- ple, the axial pumping capacity is low。 It is usually not sufficient to induce the necessary bulk flow to satisfy the oxygen absorption re- quirements。 It can only effectively disperse gas in regions adjacent to the impeller。 The uniform dispersion state of gas in the stirred vessel bulk is hard to achieve。 Large shear stress could be produced, which is adverse especially for the cultivation of animal cell having no protecting cell wall。 Due to the sweeping action of the impeller, there are low-pressure trailing vortices at the rear of the blades。 This result in great power drop after gas is introduced and the gas handling capacity is handicapped by flooding    [2,6]。

For the purpose of improving the gas dispersion uniformity, great efforts have been devoted to the modification of the standard Rushton impeller over the past several decades。 To date, many new types of modified Rushton impellers have been developed。 Through increasing the blade height and simultaneously adding perforations to the blades, Roman et al。 [7] designed the perforated Rushton impeller, which has a reduced power consumption and improved oxygen transfer efficiency。 Chen and Chen [8] studied the comb blade and the perforated blade disc impellers。 The latter was found to have higher volumetric gas–liquid mass transfer coefficient than the standard Rushton impeller at similar power input。 Warmoeskerken and Smith [9] proposed the CD-6 impeller which has concaved blade just like cutting from pipe sections。 This  and  subsequent  studies  [10–12]  confirmed  that  this  impeller