CFD Prediction of Gas-Liquid Flow in an Aerated Stirred Vessel Using the Population Balance Model

Abstract

The main topic of this study is the experimental measurement and mathematical modelling of global gas hold-up and bubble size distribution in an aerated stirred vessel using the population balance method. The air-water system consisted of a mixing tank of diameter T = 0.29 m, which was equipped with a six-bladed Rushton turbine. Calculations were performed with CFD software CFX 14.5. Turbulent quantities were predicted using the standard k-ε turbulence model. Coalescence and breakup of bubbles were modelled using the homogeneous MUSIG method with 24 bubble size groups. To achieve a better prediction of the turbulent quantities, simulations were performed with much finer meshes than those that have been adopted so far for bubble size distribution modelling. Several different drag coefficient correlations were implemented in the solver, and their influence on the results was studied. Turbulent drag correction to reduce the bubble slip velocity proved to be essential to achieve agreement of the simulated gas distribution with experiments. To model the disintegration of bubbles, the widely adopted breakup model by Luo & Svendsen was used. However, its applicability was questioned.

Modelling of the Bubble Size Distribution in an Aerated Stirred Tank: Theoretical and Numerical Comparison of Different Breakup Models

“…The captured bubbles were identified manually, and their size was evaluated via image analysis using the NIS-Elements software. A detailed description of the experimental techniques can be found in our previous work (Kálal et al, 2014 …”

“…It has been applied by many authors for modelling the BSD in stirred tanks (e.g., Ahmed et al, 2010;Kerdouss et al, 2008;Montante et al, 2008;Ranganathan and Sivaraman, 2011;Selma et al, 2010); other models have seldom been applied (Gimbun et al, 2009;Laakkonen et al, 2007). The model by Luo & Svendsen was introduced in our previous work (Kálal et al, 2014). This paper further explores the model's features and compares it with other breakup models.…”

“…The total breakup frequency of bubbles of size V i is then computed as an integral over all possible daughter bubble sizes. For more details please refer to Kálal et al (2014).…”

“…The most significant interphase force in stirred tanks is the drag force (Scargiali et al, 2007), which is mathematically described via the drag coefficient. Based on our previous results (Kálal et al, 2014), we have chosen the drag coefficient description by Montante et al (2007) as the most suitable for our simulation. It is simply expressed from the force balance applied to a particle:…”

“…Besides, the global gas hold-up in the system was monitored and the pseudo-steady-state was assumed to be achieved when this quantity remained constant. More details about the grid-sensitivity tests and numerical settings can be found in Kálal et al (2014 …”