Mixing Of non-Newtonian Fluids Research Articles

Numerical study of non-Newtonian power-law fluids under low-frequency vertical harmonic vibration

Resonance Acoustic MixingⓇ(RAM) technology applies an external low-frequency vertical harmonic vibration to mix ultrafine granular materials and subsequently non-Newtonian fluids. Although this system is used for various applications, its mechanism is yet not well understood, especially in the mixing of non-Newtonian fluids. To address this gap in knowledge, a phase model of the shear-thinning and shear-thickening non-Newtonian power-law fluid in a low-frequency vertical harmonic vibration container is established in this study, and the different power-law index is also considered. During the initial mixing process, there is Faraday instability at the gas–liquid interface, and Faraday waves are related to the power-law index. With the continuous input of external energy, the flow field is further destabilized, so that the uniform mixing is finally completed. In addition, the rheology of non-Newtonian fluids is consistent with the constitutive relation of power-law fluids. The dynamic viscosity of shear-thinning non-Newtonian fluid decreases rapidly with the increase of mixing time, while the shear-thickening non-Newtonian fluid decreases rapidly with the increase of mixing time. The variation of shear rate for Newtonian and non-Newtonian fluids is identical. Finally, a proper vibration parameter for the high mixing efficiency of RAM is proposed.

Mixing of non-Newtonian fluids in a cylindrical stirred vessel equipped with a novel side-entry propeller

A side-entry propeller was designed and introduced in this study. The mixing performance of shear thinning fluids in a cylindrical stirred vessel equipped with this propeller was experimentally investigated via an ultrasonic Doppler anemometer (UDA, experimental results were consistent with theoretical findings. The power number and Reynolds number of this propeller were evaluated by using Chhabra, Metzner and Reed equations. Results showed that the power number versus the Reynolds number curves were highly comparable with Metzner and Reed equations. The velocity jet vectors flow field of 320, 380, and 440 rpm were described in detail. These findings demonstrated that the circulation loops, cavern size, and shape were highly influenced by shear thinning parameters and operating conditions. The average of the velocity profiles from five sample lines in front of the propeller was utilized to analyze the effect of rheological properties and operating conditions on the propeller. The axial, radial and tangential 2D velocity profiles located at one sample line (200 mm) in front of the propeller at the design rotation speed were evaluated.

Experimental and CFD studies of power consumption in the agitation of highly viscous shear thinning fluids

Mixing of non-Newtonian fluids is widely encountered in the process industries. In this research, we obtained a constitutive expression that relates the viscosity of a mixture of glycerol and a gel formed of polyethylene glycol and Carbomer to the shear rate, temperature and mass fraction of one of the two components. We found that the mixtures of these two fluids were well characterized by a non-Newtonian power law model. We then used a number of homogeneous mixtures of the two fluids at different temperatures and mass fractions in a simple stirred tank agitated mechanically by a Rushton turbine to derive experimental power curves, which we then derived numerically in a CFD model by replicating the experimental conditions. We used a combination of an air bearing and a load cell to precisely measure the power required by the impeller to agitate the non-Newtonian mixtures. The computational and experimental results are in good agreement, indicating that the rheological data and the CFD model are accurate.

The effect of chaotic advection on mixing degree and pressure drop of non-Newtonian fluids flow in curved micromixers

Since the majority of fluids in engineering and biology applications are non-Newtonian, a numerical investigation was performed on the mixing of non-Newtonian fluids. Chaotic advection is one of the useful mechanisms to enhance the mixing performance which can be generated using curved micromixers. Therefore, mixing of non-Newtonian fluids in curved micromixers was studied in this study. The power-law fluids with power-law indices between 0.49 and 1 and Reynolds numbers between 0.1 and 300 were investigated. Open source CFD code of OpenFOAM was utilized to simulate mixing process in micromixers. The results showed that the mixing index decreases and dimensionless pressure drop increases by decreasing the power-law index. The chaotic advection and mixing index increase by increasing the Reynolds number for all values of power-law index. In addition, micromixers with sharp turns in their geometries yielded efficient mixing performance for low power-law indices and low Reynolds numbers.

Using a tracer technique to identify the extent of non-ideal flows in the continuous mixing of non-Newtonian fluids

The identification of non-ideal flows in a continuous-flow mixing of non-Newtonian fluids is a challenging task for various chemical industries: plastic manufacturing, water and wastewater treatment, and pulp and paper manufacturing. Non-ideal flows such as channelling, recirculation, and dead zones significantly affect the performance of continuous-flow mixing systems. Therefore, the main objective of this paper was to develop an identification protocol to measure non-ideal flows in the continuous-flow mixing system. The extent of non-ideal flows was quantified using a dynamic model that incorporated channelling, recirculation, and dead volume in the mixing vessel. To estimate the dynamic model parameters, the system was excited using a frequency-modulated random binary input by injecting the saline solution (as a tracer) into the fresh feed stream prior to being pumped into the mixing vessel. The injection of the tracer was controlled by a computer-controlled on-off solenoid valve. Using the trace technique, the extent of channelling and the effective mixed volume were successfully determined and used as mixing quality criteria. Such identification procedures can be applied at various areas of chemical engineering in order to improve the mixing quality.

Power Demand and Mixing Performance of Coaxial Mixers in Non-Newtonian Fluids

Experiments have been carried out in a transparent dished bottom stirred tank with a diameter T of 0.48 m and liquid level of 0.6T. The power consumption and mixing performance of a coaxial mixer consisting of a wall-scraping anchor and different inner dispersion impellers (Rushton turbine, 45° pitched blade turbine and CBY) operating in inner impeller-only, co- and counter-rotating modes have been experimentally characterized in viscous Non-Newtonian fluids (2% and 3% w/w CMC solutions) with different rheology behaviors. The results show that, for the co-rotating modes, the power consumption of the anchor could decrease up to 5% of that for the anchor rotating-only mode, whereas for the counter-rotating dispersion modes, it could increase to two times of that for the anchor rotating-only. However, the power consumption of the inner impellers is almost independent of the anchor rotation. We propose new correlations to give better fitted power curves between the generalized Reynolds number and the power number by considering not only the impeller geometry and the characteristic speed, but also the speed ratio. For each coaxial mixer, one reasonable power curve can be generated for different experimental speed ratios and different rotation modes. Impellers in co-rotating mode are more efficient than the inner impeller-only and the counter-rotating modes in the mixing of non-Newtonian fluids. Among the above three dispersion impellers, the power consumption of the CBY-anchor combination is the lowest compared with those for the other two combinations giving the similar mixing performance.

Flow regime analysis of non-Newtonian duct flows

Reoriented duct flows of generalized Newtonian fluids are an idealization of non-Newtonian fluid flow in industrial in-line mixers. Based on scaling analysis and computation we find that non-Newtonian duct flows have several limit behaviors, in the sense that such flows can become (nearly) independent of one or more of the rheological and dynamical control parameters, simplifying the general flow and mixing problem. These limit flows give several levels of modeling complexity to the full problem of non-Newtonian duct flow. We describe the sets of simplified flow models and their corresponding regions of validity. This flow-model decomposition captures the essential rheological and dynamical characteristics of the reoriented duct flows and enables a more efficient and systematic study and design of flow and mixing of non-Newtonian fluids in ducts. Key aspects of the flow-model decomposition are demonstrated via a specific, but representative, duct flow.

Stretching and mixing of non-Newtonian fluids in time-periodic flows

The stretching of fluid elements and the dynamics of mixing are studied for a variety of polymer solutions in nearly two-dimensional magnetically driven flows, in order to distinguish between the effects of viscoelasticity and shear thinning. Viscoelasticity alone is found to suppress stretching and mixing mildly, in agreement with some previous experiments on time-periodic flows. On the other hand, the presence of shear thinning viscosity (especially when coupled with elasticity) produces a dramatic enhancement in stretching and mixing compared to a Newtonian solution at the same Reynolds number. In order to understand this observation, we study the velocity field separately in the sheared and elongational regions of the flow for various polymer solutions. We demonstrate that the enhancement is accompanied by a breaking of time-reversal symmetry of the particle trajectories, on the average. Finally, we discuss possible causes for the time lags leading to this temporal symmetry breaking, and the resulting enhanced mixing.

Mixing of Non-Newtonian Fluids in Steadily Forced Systems

We investigate mixing in a viscoelastic and shear-thinning fluid-a very common combination in polymers and suspensions. We find that competition between elastic and viscous forces generates self-similar mixing, lobe transport, and other characteristics of chaos. The mechanism by which chaos is produced is evaluated both in experiments and in a simple model. We find that chaotic flow is generated by spontaneous oscillations, the magnitude and frequency of which govern the extent of chaos and mixing.

Mixing of non-Newtonian fluids in time-periodic cavity flows

Fluid mixing in two- and three-dimensional time-periodic cavity flows as a function of rheological fluid parameters is studied. Computational methods are applied to obtain accurate descriptions of the velocity fields, which form the basis of the mixing analysis. In addition to some classical techniques, like Poincaré maps and the analysis of periodic points, a recently developed mapping method is used to determine mixing efficiency over a wide range of different flow parameters. Within this framework, different mixing protocols can be evaluated with respect to their long term mixing behaviour and compared quantitatively. It is shown that local ‘optimal’ parameter settings for mixing of Newtonian fluids can result in a considerable worse mixing behaviour for non-Newtonian fluids, and vice versa, illustrating the importance of taking the rheology of the fluid into account.

The work of Warren Spring Laboratory in applied rheology

The policy and work of Warren Spring Laboratory in applied rheology is described. The basic research is directly related to industrial problems and is concerned with: (1) the determination of rheological properties for material characterization, quality control and the design of process equipment and plant, (2) research in unit operations such as pipeline transport, metering pumping and mixing of non-Newtonian fluids and pastes. and (3) the testing and development of on-plant instrumentation such as flow metering and process viscometry. As a service to industry the Laboratory provides an information service on the widely scattered rheological publications on commercial viscometers and rheometers and on process equipment and plant. Sponsored work is also carried out for individual or groups of companies and the range of topics studies is indicated.