|Ni'am Nisbatul Fathonah||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
|Tantular Nurtono||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
|Kusdianto||Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS, Sukolilo, Surabaya 60111, Indonesia|
|Wahyudiono||Department of Materials Process Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan|
The main objective of this work is to study the effect of stirrer entry angle on the hydrodynamic characteristics in an agitated tank with side-entry mixer (side-entry mixing tank) using the CFD simulation method. For validation purposes, the simulation results were compared with the experimental results. Qualitatively, it was found that there was a similar fluid flow in the simulation and experiment results. The agitated tank system consisted of a 40 cm diameter cylindrical tank and a three-blade marine propeller with 4 cm diameter. The working fluid was water, with a liquid height of 40 cm. The rotational speed varied between 100-400 rpm, with the stirrer entry angle () set at 0o, 10o and 15o (right-hand side). The modelling configurations used in the simulation were an RNG Standard k-? model as a turbulence model, coupled with a Multiple Reference Frame (MRF) for the propeller motion approach method in transient conditions. The results show that simulation configuration MRF-RNG k-? produced realistic results to describe the hydrodynamic characteristics in the side-entry stirred tank. This is supported by the simulation results, which qualitatively produced similar flow patterns in the simulation and experiment. In the quantitative analysis, at higher rotational speeds the circulation flow formed tended to be pushed further from the impeller discharge, which is supported by the average velocity experimental data. Average velocity in the tank had a tendency to increase as the ? increased. The predicted average velocities (in m/s) were 0.0175, 0.0185 and 0.0197 at ? 0o, 10o and 15o respectively, at a constant rotational speed (400 rpm). Larger ? produced high tangential velocity, leading to a strong circulation flow. Applications of this side-entry mixing tank include those in large scale reactors and storage tanks to maintain the homogeneity of the material inside.
CFD; MRF; RNG k-?; Side-entry angle; Side-entry mixer
Agitated tanks with side-entry mixers are widely used in the pulp and paper industries, flue gas desulphurization, the petroleum industry, and as bio-digesters in biogas plants. These agitators provide rugged reliability, application versatility, and easy, economical installation, operation and maintenance. The main feature of an agitated tank with a side-entry mixer is the large tank size, with a small impeller diameter. The ratio of the impeller diameter to the tank diameter is relatively small compared to an agitated tank with top-entry mixer.
The types of stirrer used depend on the desired process outcome, process requirements, and the scale or volume of the material to be processed. The greatest advantage of the type of stirrer employed in the mixing process using a side-entry stirred tank is that it generates axial flow, since the desired flow pushes the fluid toward the front of the impeller shaft, allowing the fluid to circulate to all parts of the tank after colliding with the tank wall. This axial-type stirrer is also highly recommended for controlled flow operations, such as blending/mixing operations, to suspend solids in the liquid, and to accelerate heat transfer (Joshi et al., 2011). Based on previous research, the type of stirrer most widely used in side-entry stirred tanks is a type of propeller and axial turbine (IBT or PBT). These axial impellers are also highly recommended for controlled flow operations, such as blending/mixing operations, to suspend solids in the liquid, and to accelerate heat transfer.
The performance of an agitated tank with a side-entry mixer, however, will depend on the location, position, the speed, and number of impellers used. The location and position of the impeller are determined by the impeller entry angle (?); i.e. the angle between the impeller shaft and the center line of the cylindrical tank, and the distance of the impeller from the bottom and wall of the tank. As a result, the flow pattern produced becomes very complicated and has unstable characteristics in space and time.
The effect of ? on fluid flow in agitated tanks with side entry angle was first investigated experimentally by Kipke (1984) in cylindrical tanks with diameters of 0.7 m and 1.4 m. He showed that the most effective agitation process to achieve good homogeneity was obtained with ? = 7-10o, depending on tank diameter. Subsequently, several studies have further analysed fluid flow characteristics in industrial-scale agitated tanks with side-entry mixers using CFD-based simulations. Dakhel and Rahimi (2004) examined a side-entry stirred tank used for the storage of crude oil with a diameter of 44 m. The model configuration used in the simulation was MRF and RNG k-?., Fang et al. (2011) investigated a side-entry stirred tank with multiple agitators with the same configuration of MRF and RNG k-?. Wu (2012) established an optimum angle in the 30o-40o range for a cylinder tank with diameter >40 m. The effect of impeller entry angles on mixing performance in large-scale biogas reactors (D = 16 m) was investigated by Xinxin et al. (2018). Validation of the results of the simulation in these works can only be carried out qualitatively and globally, using the limited data that can be obtained at the plant site.
In a simulation, an appropriate approach is needed to obtain more realistic results. In previous works, Lane et al. (2000) compared the sliding mesh (SM) and multiple reference frame (MRF) models to simulate fluid flow in a top-entry stirred tank with a disc turbine as a stirrer. Their results show that the MRF provides a more realistic flow pattern and kinetic energy value, and that its energy dissipation had a smaller error. Winardi et al. (2013) used a combination method of two configurations of turbulence and impeller motion modelling, and MRF with k-? in steady mode followed by SM-LES in transient mode, to characterize the flow patterns that occur in side-entry stirred tanks using PBT. In the following year, Winardi et al. (2014) conducted simulations with the same method for the same agitated vessel geometry, but with a different type of impeller, namely a marine propeller. Fathonah et al. (2017; 2018) used an MRF modelling configuration with the k-? model to simulate a side-entry stirred tank with an IBT. Their results showed qualitatively that the MRF was able to describe a good flow pattern clearly and was almost similar to the experimental results. Dakhel and Rahimi (2004) examined an agitated tank with a side-entry mixer used as crude oil storage tank with a diameter of 44 m, using MRF and RNG k-?. Fang et al. (2011) also investigated an agitated tank with a side-entry mixer and multiple impellers, with MRF and RNG k-?. Madhania et al. (2018) obtained good simulation results from the mixing of liquid-liquid with a very large viscosity difference in an agitated tank with a side-entry mixer and conical bottom. The turbulence model used in CFD simulation plays an important role in predicting the exact flow conditions in a side-entry stirred tank, because not all turbulence models can be used in all operating conditions (Daryus et al., 2016). This research is a simulation study, validated with experimental data. Observation of the flow pattern that occurs as a result of changes in the horizontal entry angle of the propeller, rotational speed and variations in flow patterns, including the phenomenon of MI, are examined. The effect of impeller entry angle on hydrodynamic characteristics in an agitated tank with side entry marine propeller in laboratory conditions was studied using the CFD simulation approach with the MRF-RNG k-? model.
This work has presented a three-dimensional and transient CFD simulation of the hydrodynamic characteristics in an agitated tank with side-entry marine propeller. It has been found that the simulation configuration MRF-RNG k-? provided realistic results to describe these characteristics with different impeller entry angles (). For validation purposes, the simulation results were compared with the experimental results. Qualitatively, similar fluid flows were found in the simulation and experiment results. In the quantitative analysis, at higher rotational speeds the developed circulation flow tended to be pushed further from the impeller discharge stream; this was validated by the measured average velocity data. From the research results described in the results and discussion section, the horizontal slope of the propeller entry angle has a significant effect on changes in the hydrodynamic characteristics and flow patterns in the side-entry mixing tank. Average velocity in the tank tends to increase as the impeller entry angle ? increased. The predicted average velocities were 0.0175, 0.0197 and 0.0182 m/sec at ? = 0o, 10o and 15o respectively, at constant rotational speed. A larger impeller entry angle ? produced high tangential velocity, leading to a strong circulation flow. Such a flow is good for the mixing process because it can reach all parts of the vessel.
The authors are grateful for the financial support provided by the PMDSU research and scholarship grant 2018 from the Directorate of Research and Public Service, Directorate General of Research Strengthening and Development, Ministry of Research, Technology and Higher Education of the Republic of Indonesia, with contract number 818/PKS/ITS/2018. We also extend our gratitude to Mr. M. Murtadho and Ms. Yukh Ihsana for their assistance in the experiment.
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