Numerical characterisation of slug flow in horizontal air/water pipe flow

Abstract

In this work, the slug flow regime in an air-water horizontal pipe flow has been simulated using the CFD technique. The variables identified to characterise the slug regime are the slug length and slug initiation. Additionally, the pressure drop and the pressure distribution within the simulated pipe segment have been predicted. The volume of fluid method was employed assuming unsteady, immiscible airwater flow, constant fluid properties and coaxial flow. The model was developed in the STAR-CCM+ environment, and the grid was designed in the three dimensional domain using directed mesh. A grid independency study was carried out through the monitoring of the water velocity at the outlet section. 104,000 hexahedral cells for the entire geometry were decided on as the best combination of computing time and accuracy. The simulated pipe segment was 8 m long and had a 0.074 m internal diameter. Three cases of air-water volume fractions have been investigated, where the water flow rate was pre-set at 0.0028 m 3 /s, and the air flow rate was varied at three dissimilar values of 0.0105, 0.0120 and 0.015 m 3 /s. These flow rates were converted to superficial velocities and used as boundary conditions at the inlet of the pipe. The simulation was validated by bench marking with a Baker chart, and it had successfully predicted the slug parameters. The computational fluid dynamics simulation results revealed that the slug length and pressure were increasing as the air superficial velocity increased. The slug initiation position was observed to end up being shifted to a closer position to the inlet. It was believed that the strength of the slug was high at the initiation stage and reduced as the slug progressed to the end of the pipe. The pressure gradient of the flow was realised to increase as the gas flow rate was increasing, which in turn was a result of the higher mean velocity.

Mathematical Model for Predicting the Development of Two-Phase Flow in a Horizontal Pipe

“…The program resulted in two blue dots on the map in Figure 2, and they appear within the boundaries of slug flow as identical to the Mohammed results as it is illustrated in Figures 3a and 3b. The second validation was performed numerically on the current study using deploying the boundary conditions of Al-Hashimy et al [19] and Ban et al [20]. Al-Hashimy studied three cases of air-water volume fractions in a horizontal pipe of 0.074m diameter, the water flow rate was fixed to 0.0028 m 3 /s and the air flow rate was alternated three times at 0.015, 0.012, and 0.015 m 3 /s, as shown in Figure 3c.…”

“…The experimental results were plotted on a Baker chart revealed that the tested experimental cases fell within the slug flow regime. Al-Hashimy et al [19] studied three cases of slug flow of two-phase air-water flow in a horizontal pipe using the CFD technique. The model was generated in the STAR-CCM+ environment, and the grid was created in three dimensions (3D) using directed mesh.…”

Recovering waste energy of the combined gas turbine system using paraffin melting

“…The gradient of the scalars at the center of the cell is calculated by Least Squares Cell-Based Gradient method. For achieving the time-independent solution, the Courant-Friedrichs-Levy (CFL) number was set to be 1 [30] by setting the minimum mesh cell length to be 6x10 −3 and the time step to be 0.1s. journal.ump.edu.my/jmes ◄…”

Comparison of multiphase SPH and LBM approaches for the simulation of intermittent flows

“…A detailed comparison of both codes [9] concludes that although performing equally well on simple cases, they have trouble to simulate complex cases with a dominant gas phase. From an academic perspective, different models and methods have been used for slug flow modeling, for example volume-of-fluid [10,11], level-set [12,13], lattice Boltzmann method (LBM) [14], smoothed particle hydrodynamics (SPH) [15,16] or phase field [17,18], but these are applied on microfluidic problems for the most part. In this work, we will focus on two of them : SPH and LBM, because while they are very different numerical methods, both in their origin and in their nature, they have shown a strong potential to model multiphase flows [19,20,21].…”