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Laminar Mixing of High-Viscous Fluids by a Novel Chaotic Mixer

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Laminar Mixing of High-Viscous Fluids by a Novel Chaotic Mixer Laminar mixing of high-viscous fluids by a novel chaotic mixer S. M. Hosseinalipour*, P. R. Mashaei, M. E. J. Muslomani School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran Article info: Abstract Received: 08/07/2016 In this study, an experimental examination of laminar mixing in a novel chaotic mixer was carried out by blob deformation method. The mixer was a circular Accepted: 17/11/2018 vessel with two rotational blades which move along two different circular paths Online: 17/11/2018 with a stepwise motion protocol. The flow visualization was performed by Keywords: coloration of the free surface of the flow with a material tracer. The effects of Chaotic mixer, significant parameters such as rotational speed of blades, blades length, and rotational speed amplitude on mixing efficiency and time were analyzed by Laminar flow, measuring of the area covered by the tracer. The results revealed that increasing Average rotational rotational speed intensifies stretching and folding phenomena; consequently, speed, better mixing is obtained. Also, a better condition in flow kinematic was Mixing index. provided to blend, as stepwise motion protocol with wider amplitude applied. A reduction in mixing time could be observed while the blades with longer length were used. In addition, it was also found that the promotion of mixing by rotational speed is more effective than that of two other parameters. The quantitative data and qualitative observations proved the potential of the proposed chaotic mixer in the wide range of industrial processes including chemical reaction and food processing in which laminar mixing is required. 1. Introduction such condition, turbulent flows are rarely adopted despite their mixing benefits because There is no doubt that high-viscous fluids like the large shear stress of high-speed blades in honey, gel, corn syrup, chocolate, castor oil, turbulent flow can adversely affect the quality of glycerol, etc., are involved in different fields products. Furthermore, providing turbulent flow such as food and grain processing, chemicals for fluids with high viscosity consumes a large production, pharmaceuticals, petrochemical and amount of energy due to this fact that the motion refining, polymers, plastics, and paints. On the of blades with high rotational speed in these other hand, the mixing process is necessary to fluids requires huge power. Chaotic advection is achieve a desirable product in such industries. a new approach known to overcome this Regarding the high viscosity of mentioned problem. Aref [1, 2] showed that even laminar working fluids, the mixing procedure is laminar. mixing can cause an improved mixing while the As a result, laminar mixing plays an important chaotic behavior of particles is detected. This role in almost all such industrial areas. Under efficient method has been applied by many *Corresponding author 199 email address: [email protected] JCARME S. M. Hosseinalipour, et al. Vol. 8, No. 2 researchers to obtain more homogeneous mixing significant influence on mixing efficiency in laminar flow. improvement at low Reynolds numbers. Zhang Chein et al. [3] investigated experimentally and Chen [12] discussed a new design and laminar mixing in several classes of two- implementation of a liquid mixer which could dimensional cavity flows by means of material work under different impeller/tank velocity line and blob deformation. Their experimental control scheme such as DC signals, periodic system comprised of two sets of roller pair signals, and chaotic signals. Comparable connected by a belt which could produce a range experiments showed that the chaotic of Reynolds number of 0.1-100 and various perturbations injected into the stirred tank can types of flow field with one or two moving improve the liquid mixing efficiency boundaries. They found that the mixing remarkably. Kato et al. [13] used three non-circle efficiency is a function of the velocity of walls cams to generate the unsteady speed in an and an optimum value can be found for it. impeller revolution and compared mixing Lemberto et al. [4] conducted several performances of this impeller with those of a experiments to analyze the mixing performance conventional impeller operated under steady in a stirring tank under low to moderate speed. It was demonstrated that the doughnut Reynolds number conditions. The results rings above and below the impeller are demonstrated that the mixing performance can disappeared when unsteady mixing is employed be enhanced under low Reynolds number using because the unsteady motion moves the center of a controlled periodic motion of impeller. Yao et vortexes. Takahashi et al. [14] investigated al. [5] adopted two unsteady stirring techniques, numerically and experimentally the mixing co-reverse periodic rotation and time-periodic performance in a vessel agitated by an impeller fluctuation of rotational speed, to promote that inclined itself to promote mixing laminar mixing of high viscosity fluids in a performance by spatial chaotic mixing. They stirred tank. The experimental findings proved found that the inclined impeller and eccentric that the method of co-reverse periodic rotation, position can reduce mixing time. Hwu [15] only when the Reynolds number is larger than a proposed a new conceptual apparatus that can be critical value, can cause significant employed to mix high-viscous fluids at low enhancements in mixing. Moreover, mixing time velocities. It consisted of a circular cavity decreases as the frequency of periodic co-reverse equipped with two blades arranged along the rotation and the amplitude of time-periodic perimeter. The numerical studies showed that the fluctuation increase. Several researchers [6-8] mixing efficiency depends on the length and investigated numerically and experimentally the rotational speed of blades. Mashaei et al. [16] laminar mixing in flow filled between two investigated experimentally laminar mixing in a eccentric cylinders driven periodically. The new type of chaotic mixer, proposed by Hwu results revealed that an efficient mixing is [15], by means of material line deformation. The achieved when cylinders are periodically rotated flow visualization was carried out by marking of with a larger period. Zambaux et al. [9] the free surface of the flow with a tracer in numerically showed that combining two working fluid. They demonstrated that the orthogonal secondary flows can enhance the goodness of mixing increases as the mixing in an annular configuration using the rotational speed of blades increases and the chaotic advection in flow. Galaktionov et al. [10] mixing performance strongly depends on the studied laminar mixing in a rectangular cavity length of the rotating blades. Robinson and with a circular cylinder placed in its center and Cleary [17] evaluated the effect of the reported that the presence of a rotational cylinder impeller on the chaotic mixing in a helical can be a simple method to eliminate the non- ribbon mixer and reported that an extra ribbon chaotic regions. Takahashi and Motoda [11] can improve mixing rate over other ribbon suggested a novel spatial chaotic mixer by geometries. Liu et al. [18] offered a mixer with inserting the objects into a vessel agitated by an double rigid-flexible combination impellers. impeller and stated that their method has a This type of impellers was able to intensify the 200 JCARME Laminar mixing of high-viscous . Vol. 8, No. 2 motion characteristics of the fluid by and food processing in which the rapid mixing of suppressing the symmetry structure of the fluid high-viscous fluids is required. flow. These authors also reported that the mixing efficiency increases with increasing width of the 2. Experiments flexible piece. Shirmohamadi and Tohidi [19] numerically and The liquid mixer is a circular vessel with two experimentally investigated a mixer equipped blades rotating along a circular path with a with two rotors with the circular cross-section. stepwise motion. The experimental system is The results revealed that the use of blades with designed to carry out two chief purposes. The sinusoidal motion reduces the weakly mixed first one is to be able to move two blades at regions. Jung et al. [20] suggested a barrier- various rotational speeds in separate circular embedded partitioned pipe mixer (BPPM) and paths. The second one is to take pictures of the numerically evaluated the mixing process. This free surface of flow by means of a digital camera. mixer was fitted out with orthogonally The experimental apparatus used in this work is connected rectangular plate and a couple of shown in Fig. 1. The upper surface of flow is barriers. Some methodologies such as the open to atmosphere. intensity of segregation, interface tracking, and The vessel diameter and height are 0.2m and Poincare section were applied to analyze the 0.3m, respectively. The mixer blades are chosen mixing performance. It was found that BPPM as parts from a pipe with the diameter and height significantly enhances mixing performance as of 0.12m and 0.3m, respectively. This kind of compared with traditional partitioned pipe blade is the main innovation of the present mixer. Jegatheeswaran et al. [21] used the non- mixer. When mixing is caused by pushing fluid, intrusive electrical resistance tomography (ERT) normal share rates and, thus, normal stresses technique to investigate the mixing of non- happen in the mixer. Higher viscosity leads to a Newtonian fluids in a chaotic static mixer. The more considerable
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