Flow transition in vertical gas liquid columns using Ansys Fluent.

Modeling gas-liquid multiphase flow in ANSYS Fluent is essential for understanding complex interactions in various industrial processes. Whether in chemical reactors, oil pipelines, or environmental systems, accurately simulating the behavior of gas and liquid phases is crucial for optimizing performance and ensuring safety. ANSYS Fluent provides powerful tools for capturing the dynamics of these flows, offering different modeling approaches like the Eulerian-Eulerian and Eulerian-Lagrangian methods. Eulerian Multi-Fluid VOF, AlAD (Algebraic Interfacial Area Density) and GenTOP (GENeralized TwO Phase) Models provide the key ingredients to systemically model mixed flow regimes and regime transitions in gas–liquid systems and gas–liquid flows. These flows often involve transient patterns, such as slug flow, annular flow, etc. These conditions often occur when more than one flow topology — droplet, bubbly or separated flows — exists, requiring unique special Eulerian Closure Modeling Techniques, sophisticated interface capturing methods and smart regime transition detection algorithms. This blog will showcase the setup of a bubble column problem using GenTOP (GENeralized TwO Phase) model.

The bubble column to be simulated is a 3D, rectangular bubble column as shown in figure 1. Air with a superficial velocity of 0.5 m/s is introduced through a sparger at the bottom of the column. The details of the sparger geometry are not modeled. Instead it is treated as a uniform inlet surface with an area of 2 x 10⁴ m². The liquid in the column is water, and a pressure outlet boundary condition is used at the outlet.

Figure 1: Schematic of the bubble column[1].

To setup the model select the pressure based transient solver and set the gravity acceleration of -9.81 m/s in the -Y direction. Copy the material properties of water-liquid and air from the material panel. Activate Eulerian multiphase model, select Generalized Two Phase Flow (GENTOP) under regime transition modeling. Use Implicit formulation for volume fraction equation. Make sure that the number of Eulerian phases is set to 3 as shown in figure 2. 

Figure 2. Multiphase model panel

Select liquid as the primary phase, air as the secondary phase and again select air as the their phase and enable GENTOP phase modeling (Figure 3). The diameter for the second and third phase will switch to sauter-mean as it will be derived from population balance model. 

Figure 3: Multiphase model panel- phase selection.

Enable the appropriate phase interaction for the three phase pairs. In this case, we will only activate drag coefficient for water : air and water : bubble pair and set the coefficient to universal-drag and ishi-zuber respectively. This is done to keep the model simple. Set Surface Tension Coefficient to 0.072 N/m as shown in figure 4. 

Figure 4: Phase interaction forces.

Setup the population balance model with 2 bins for the air phase with the ratio exponent (𝒒) of 1. Set the minimum diameter (𝒅_𝒎𝒊𝒏) to 0.01 m.  Fluent will automatically calculate the maximum diameter based on equation 1.

𝒅_𝒊=𝟐^{𝒊∙𝒒/𝟑}∙𝒅_𝒎𝒊𝒏                                                                         (1)

Where, i represents the bin number. Set the bubble phase minimum diameter to 0.03 m. Keep the default breakage and coalescence model active as shown in figure 5.

Figure 5: Population balance model panel

Set the turbulence model to k-omega SST with turbulence damping factor of 10. Enable production limiter and select mixture under turbulence multiphase model. Setup the air inlet with a velocity magnitude of 0.5 m/s and volume fraction of 1. The boundary value for bin-0 and bin-1 may be set to 0.5.  In this case, the air phase will be injected as droplets of two different sizes in a column of stationary water column. As the rising air droplets interact, they will coalesce or breakup. As they coalesce and the air droplets will become larger, and move into the GENTOP phase i.e. the phase Bubble.

Keep the default spatial discretization scheme and verify the higher order term relaxation factor is set to 0.7 for flow variables only. Retain the default under relaxation factors. Activate autosave options to save data files at every 0.1 sec flow time. Initialize the case with water turbulent kinetic energy 1e-5 m²/s² and water specific dissipation rate of 100 s^-1. Create a cell register for the top 0.1 m of the column. Patch this region with air phase and bin-1 fraction 1. Create a contour of volume fraction of water on a XY plane as shown in figure 6.

Figure 6: Initial volume fraction distribution inside the column

Create an iso surface of phase bubble with an iso-value of 0.5. Next add the above contour and the iso surface to a scene. Activate solution animation and setup the capture of the scene at every 0.05 sec flow time. Set the time advancement to adaptive and select multiphase specific method. Set the total solution time to 1 second and run the calculation. As the calculation proceeds, the animation will showcase the evolution of the gas droplets into the GENTOP phase bubble. As the gas interacts the flow regime shifts from bubbly flow to slug flow as shown in the animations below.

Animation 1: Flow regime transition in a vertical gas-liquid flow channel.

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