Mixing of highly viscous fluids is widely used in many industrial processes, e.g., for the production of foods, polymers or paints. Obtaining an efficient mixing with high quality of homogenization at low impeller rotational speeds is a challenging task, due the high power consumption required for moving the viscous fluid.
Among the several stirrers available in industries, the helical ribbon and screw impellers are efficient for the mixing of highly viscous liquids. These impellers are often operated in the laminar regime of fluid flows. Depending on the purpose of the mixing operation, the helical ribbon impeller may be used with one or more helical ribbons mounted on a central shaft, supported or not by cross-beams [1
In a stirred tank fermenter, Mohd et al. [3
] compared the performance of a Rushton turbine and a double-flight helical ribbon impeller (HRI). They reported that the HRI operating at moderate conditions (250 rpm) performs better and requires less energy than a double Rushton turbine operating at an optimum condition (600 rpm). Yao et al. [4
] and Ameur [5
] studied numerically the total and local dispersive mixing performance of double helical ribbons (DHR) and Maxblend impellers. Their results showed that the DHR can achieve a good total circulation throughout the agitated vessel, but it can’t give a promising local mixing performance.
Via numerical simulations and using the smoothed particle hydrodynamics (SPH) method, Robinson and Cleary [6
] explored the mixing performance of helical ribbon impellers with a Newtonian fluid. They reported that the addition of an extra ribbon to the single helical ribbon (SHR) doubles the number of smaller circulation cells of fluid; thereby improving the overall mixing rate.
Delaplace et al. [7
] proposed an approximate analytical model to predict the power required for stirring shear-thinning fluids with helical ribbon impellers at low Reynolds numbers. Zhang et al. [8
] simulated the 3D non-Newtonian flows generated by a double helical ribbon (DHR) agitator and they used three CFD (Computational Fluid Dynamics) methods to calculate the Metzner constant. Ameur et al. [9
] studied the effect of blade pitch and other operating conditions on the flow patterns and power consumption with helical screw impellers. Maingonnat et al. [11
] studied the link between the power consumption, operating conditions and the rheological behavior of fluids stirred by a double ribbon impeller (Ekato-Paravisc). Driss et al. [12
] simulated the Newtonian laminar flows in vessels stirred by double helical ribbons (DHR) and double helical screws (DHS) impellers. Their results revealed that the double helical ribbons impeller yields more active velocity field and pumping flow. Also, the energy efficiency reaches the highest values at the same Reynolds number with DHR impeller.
Ameur et al. [13
] compared, via numerical simulations, the performance of a Simple Helical Ribbon (SHR) and Double Helical Ribbon (DHR) impellers for mixing shear thinning fluids. They found that the DHR impeller enhances the fluid circulation but an additional power is consumed.
Anne-Archard et al. [14
] discussed the distributions of shear rates and their link to power consumed when mixing power-law fluids with helical and anchor impellers. They established a Metzner–Otto correlation for mixing in power-law fluids (Bingham, Herschel-Bulkley and Casson fluids). The comparison made by Iranshahi et al. [15
] between the anchor, DHR and EkatoParavisc impellers in term of mixing characteristics revealed that the DHR is more efficient than the EkatoParavisc. The simulation results given by Yu et al. [11
] suggest a great potential for using the helical ribbon impeller for the mixing of high solids digester. Dieulot et al. [16
] used a non standard helical ribbon agitator fitted with an anchor at the bottom in unsteady agitation conditions. They found that energy savings can reach up to 60% compared to the energy required to obtain the same mixing time with a constant agitation speed.
Kuncewicz et al. [17
] used a two-dimensional numerical model describing the Newtonian laminar fluid flows with a screw impeller. They proposed an optimization criterion of the tank-impeller system regarding mixing time. Masiuk and Rakoczy [18
] modeled the mixing of granular materials by a multi ribbon blender. They showed in their study that the theory of information can be employed to describe the random process of mixing of granular materials by this kind of impellers. Rivera et al. [19
] mounted a Maxblend impeller and a double helical ribbon agitator on two independent coaxial shafts rotating at different speeds, for obtaining the so called Super blend coaxial stirrer. This mixer configuration is found as a good alternative for tough mixing applications.
Practical designs of mixers are generally performed based on experiments and measurements, which are often expensive and not an easy task [20
]. However, CFD (Computational Fluid Dynamics) simulations may be an efficient alternative. The analysis of mixing characteristics may be achieved in less time and with a greater flexibility in visualizing the fluid motions in the whole vessel volume [23
The present paper investigates via numerical simulations the mixing characteristics of this class of industrial stirrers. We focus on the effect of impeller geometry on the flow fields and power consumption. Effects of the blade width of a helical ribbon are explored. Also, a comparison between the performance of a Helical Ribbon (HR), a Small Screw (SS), a Large Screw (LS) and a Helical Ribbon-Small Screw (HR-SS) is made.
As stated above, some papers have been published on the performance of double helical ribbon (DHR) and double helical screw (DHS) impellers. However, in this paper, we compare the efficiency of HR and HS impellers used alone and in combination.
The present contribution focuses on the performance of helical ribbon mixers in unbaffled stirred tanks. The problem is analyzed under numerical simulations. Visualization of the 3D flow fields and power consumption of a Newtonian fluid within the laminar regime are illustrated.
Effects of the blade diameter and the design of the lower part of impeller on the flow patterns and power input are examined. The obtained results proved the necessity of close clearance impellers for mixing highly viscous fluids at low Reynolds numbers. The shape of the lower part of impeller blade should follow the design of the tank to ensure mixing near the vessel base.
A comparison between the performance of simple helical ribbon (HR), a simple small screw (SS), helical ribbon-small screw (HR-SS) and a large screw (LS) is made. At the same impeller rotational speed, the large screw impeller yielded the widest well-stirred region, followed by the SS, HR and then by the HR-SS impeller, respectively.
In terms of less power consumption and when these impellers are rotating at the same rotational speed, the following classification is obtained: SS < HR < HR-SS < LS. Compared to the power required by small screw impellers, the increase in Np is estimated to be 31.92%, 35% and 35.5% for HR, HR-SS and LS impellers, respectively. Also, the increase in impeller rotational speed yields a decrease in power consumption, due to the decrease of viscous dissipation rates.
However, further studies are needed for other fluids and especially for rheologically complex fluids. Also, the case of unsteady flows may give further insight on the performance of these impellers, especially the mixing time at the same power input.