Mixing with helical ribbon agitators: Part III. Non‐Newtonian fluids

Abstract

The use of helical ribbon agitators to mix viscous non-Newtonian fluids has been investigated. A generalized model, based on an appropriate definition of effective viscosity, is proposed to predict power consumption. This model is most successful with fluids that do not have a high degree of elasticity.It was found that the efficiency of mixing of pseudoplastic fluids was about half of that of Newtonian fluids in the same mixer, while the efficiency of mixing viscoelastic fluids was still lower and approximately independent of the mixer geometry. Blade width was the primary variable affecting the mixing efficiency on inelastic fluids. SCOPEThe mixing of very viscous fluids can be efficiently accomplished with a helical ribbon agitator (HRA). A model to predict the power consumption of the HRA mixing Newtonian fluids has been reported by Patterson et al. (1979); however, very viscous fluids are often non-Newtonian or viscoelastic in nature.

Effect of rheological properties on power consumption with helical ribbon agitators

“…Also included in the same table are the calculated value of Kp using the semi-analytical methods due to Chavan and Ulbrecht (1973), Yap et al (1979) and the empirical correlation proposed by Shamlou and Edwards (1985). While the agreement between the predicted and experimental values of Kp is seen to be moderately good for the models proposed by Chavan and Ulbrecht (1973) and by Yap et al (1979), the correspondence with the results of Shamlou and Edwards (1985) is rather poor. Further examination of this table shows that the value of Kp increases with the decreasing agitator pitch and with the increasing blade width, albeit the latter is a rather small effect.…”

“…This difficulty was circumvented by Metzner and Otto (1957) who introduced the concept of the effective shear rate, that is, where k, is an experimentally determined constant and it is a function of geometry (see, for example, Takahashi et al, 1984). Though originally postulated to be independent of the liquid rheology, recent work, however, suggests that for shear thinning fluids its value also depends upon the power law index (Beckner and Smith, 1966;Yap et al, 1979;Brito et al, 1991). While Eq.…”

“…The torque acting on the agitator shaft was measured using a torquemeter whereas the rotational speed was measured using a tachometer. More detailed descriptions of the experimental setup and procedure are available elsewhere (Carreau et al, 1976;Yap et al, 1979). It is sufficient to add here that each power data point represents an average of several repeated measurements in order to minimize the experimental uncertainty; the torque measurement is believed to be reproducible within *2.0%.…”

“…Admittedly, some attempts have also been made at elucidating the effect of viscoelasticity on power consumption but conflicting conclusions have been reported in the literature. For instance, both Chavan and Ulbrecht (1973) and Yap et al (1979) concluded that the shear thinning effects completely overshadowed viscoelastic effects for helical ribbon impellers. Whereas Nienow et al (1983) reported a slight increase in power consumption for turbine impellers in viscoelastic xanthan gum solutions.…”

Calculation of Metzner Constant for Double Helical Ribbon Impeller by Computational Fluid Dynamic Method

“…Of all close clearance impellers, the helical ribbon impeller, due to its high mixing efficiency [3] and its unique feature of producing axial flow in a stirred tank [4], is widely chosen in mixing highly viscous Newtonian fluids and non-Newtonian fluids. In industrial applications, power consumption is one of the most important factors concerning designers [5][6][7][8]. For Newtonian fluids, theoretical and experimental studies [9,10] have shown that the power number N p is inversely proportional to the Reynolds number Re in the laminar flow:…”