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but thepower number of two impellers is similar at high Re. Ingeneral, the bacterial cultivation in baffled stirred vesselsfollows this law that the power number became constantand maintained at five in turbulent conditions. But this lawis not applicable in the fungal cultivation for their specialphysicochemical characterization and metabolic behavior.Furthermore, the power number is lower than five andcontinues to decrease with increasing Re. Thus, the straightdiagonal-pitched blade stirrer is useful for the reduction ofthe mixing power at low speed. At the same time, thepower consumption of the new stirrer has been shown to belower than that of the Rushton turbine under conditions ofno ventilation. Cavitation occurs easily in the fermentation process with the Rushton turbine under ventilation condi-tions and the liquid mixing is therefore relatively poor. Theratios of mixing power consumption to Re with (Pg) andwithout (Po) ventilation as a function of speed and gas flowrate are shown in Fig. 3. If the ratio is less than 1, thestirring power declines after ventilation, the impellerreduces the flow of liquid, and this is detrimental to theliquid dispersion and mixing materials. The results showedthat the power ratio of this new impeller was maintained atone at the gas flow rate of 4 L/min, whereas the power ratioof the Rushton turbine increased with stirring speed, but itwas always less than 1 and therefore the liquid mixingcapabilities of the Rushton turbine are poorer than those ofthe new impeller.In the actual fermentation process, the mixing paddleneeds to transform mechanical energy into kinetic energy ofthe fluid, and this accounts for the majority of the totalenergy consumption of the entire fermentation process,which has a direct impact on the fermentation productioncosts. Stirring power consumption is therefore a criticalfactor for microbial fermentation and needs to be consid-ered. At the early stage of fermentation, where the mycelium is at the beginning of the growth stage, the viscosity of thefermentation broth is low, and the motor current gap issmall. As fermentation proceeded, the mycelium graduallythickened and intertwined, and the electric current showsunstable fluctuations. Furthermore, the motor current withthe straight diagonal-pitched blade stirrer in the entire fer-mentation process is less than the Rushton turbine, indicat-ing that the power consumption of the new stirrer is less thanthat of a conventional paddle. A quantitative estimate indi-cates that the power consumption of the new stirrer was2.8 % lower than the Rushton turbine.
Oxygen transfer performanceThe correct measurement and/or prediction of the volu-metric mass transfer coefficient, (KLa), are a crucial step inthe design, operation, and scale-up of bioreactors. In gen-eral, KLa was evaluated using the relationship for thespecific surface area, a, related to gas hold-up and Higbie’spenetration theory for KL, which is widely accepted forgas–liquid transfer description. In this study, the initialdissolved oxygen concentration (C0) was set to zero in theEq. (3). As shown in Fig. 4, the oxygen transfer coefficientincreased with power input. The difference between theoxygen transfer coefficients for the two impellers wassmall at low power inputs but large at high power inputs.The oxygen transfer coefficient of the new impeller islower than that of the Rushton turbine. Figure 4 also showsthat the oxygen transfer coefficient increased with gas flowrate at the stirring speed of 300 r/min. The oxygen transfercoefficient of the new impeller is lower than that of theRushton turbine at a given flow rate. The greater number ofblades of the Rushton turbine generates a higher shearstress than the new impeller. In addition, the disc on theRushton turbine hinders gas flow and increases the gasdispersion. Furthermore, Rushton turbine consisting ofhollow mixing elements fixed to a rotating tubular shaftsuitable for mixing, dispersion and partly for transport ofthe gas is also known. The hollow mixing elements aremostly pipes cut at an angle of 45 , at the end of which—atsuitable speed—a pressure drop occurs, sucking in the gasusually through the hollow tubular shaft. The gas isatomized by the shear forces generated in the liquid by thesharp pipe ends. The two paddles in the new impellerreduce the shear stress and weaken gas dispersion.