Introduction
Agitator is one of the core components of the stirred tank reactor, with which efficient homogeneous mixing of fluid or uniform dispersion of multiphase fluid is always expected. Although many kinds of agitators have been proposed, there is still no universal agitator for any working condition. Therefore, it is still an important task to explore new high efficiency agitator in the field of chemical engineering.
Traditional agitator, e.g. propelling agitator or Rushton turbine (RT) impeller, has the characteristics of simple structure, easy operation and relatively mature design principles. Nevertheless, these agitators are only suitable for a specific process, e.g. the propelling agitator is mainly used for heat exchange due to its large amount of axial flow, the RT impeller is mainly used for the process that requires high values of turbulent kinetic energy (TKE). However, the TKE generated by the RT impeller is usually larger only around the impeller. In addition, the mass exchange between two loops generated by the radial pumping of the blades is very poor due to the existence of the disk. To improve the distribution uniformity of the TKE and its dissipation rate, as well as to intensify the mixing performance in the case of a wide range of fluid viscosity, three typical large cross-section impellers, i.e. the Maxblend impeller, Fullzone impeller and Sanmeler impeller, have been proposed. These agitators have shown excellent mixing performance and flexible adaptability in the practical applications1,2, in particular, in the case of medium viscosity system. However, to our knowledge, no significant increase in the magnitude order of the TKE has been reported. Recently, our group has proposed a new design of the multi-blade combined (MBC) agitator3, with which the mixing can be intensified significantly with higher TKE and narrower distribution of the TKE. Thus, it is necessary to carry out a fundamental study of the flow characteristics in the stirred tank equipped with the MBC agitator.
Velocity distribution is the basic flow characteristics of the stirred tank. Different velocimetry measurement techniques like laser-doppler velocimetry (LDV)4,5and particle image velocimetry (PIV)6,7have been widely adopted to study the flow patterns and turbulent characteristics. Traditional PIV technique uses two cameras to capture the consecutive image pair illustrated by the double pulse laser. Thus, an external trigger is required to match the image capture to the shot with the pulsed laser, resulting in a comprehensive measurement system. In recently, with the development of high-speed CMOS cameras and continues laser, time-resolved PIV (TR-PIV), which is a simple velocimetry system, has been gradually used for the flow field measurement 8. The image capture frequency can reach thousands of frames per second, allowing a very short interval time between two adjacent images for calculating the velocities directly. For example, a capture frequency of 500 fps corresponds to 2000 μs interval time, which is similar to that of traditional PIV 9. Therefore, the TR-PIV system provides a simpler and easier to operate experimental tool for the dynamic measurement of unsteady flow without using comprehensive external trigger system.
In addition to the measurement techniques, computational fluid dynamics (CFD) technology has been flourishing in predicting flow characteristics in different types of the reactors, mainly using Reynolds Averaged Navier–Stokes (RANS) approach, large eddy simulation (LES) or direct numerical simulation (DNS). RANS approach solves Reynolds Averaged Navier–Stokes (RANS) equations with a closure turbulent model, e.g. the standard k-ε model, however, the predicted turbulent kinetic energy is generally under-predicted because the flow is extremely anisotropic especially near the impeller region10. DNS can give the most accurate predictions of the flow fields. However, it needs very high-quality grids and a lot of computing resources. LES is a filter-based approach in which larger eddies are solved directly and smaller eddies are modeled by the sub-grid scale (SGS) model. Murthy and Joshi 11 have summarized the published literatures on the predictions of the flow pattern using LES approach. Several works have proved that the LES approach lies between RANS and DNS in terms of accuracy and computational cost.
The objective of this work is to study the flow characteristics in the stirred tank equipped with our newly developed MBC agitator by using TR-PIV technique and LES prediction. The CFD predictions are firstly assessed by the TR-PIV experiments. Then the turbulent characteristic, time-averaged flow patterns, flow rates and power number are analyzed by LES approach in detail, thus, providing a fundamental understanding of the flow characteristics and guiding its practical applications.
Experimental