Dedicated experiments and numerical simulations have been conducted to investigate the splitting characteristics of a gas-liquid two phase flow at a T junction. The experiments were carried out for different gas-liquid velocities. The flow rates in the two branches were measured accurately to determine how the two considered phases distribute in the two outlets. The experimental results have shown that when the two outlet pressures are asymmetric, the two-phase flow always tends to flow into the outlet which has a lower pressure. As the inlet liquid velocity increases, however, the two-phase flow gradually tends to split evenly. Compared with the experiment results, the pressure difference between the two outlets can be determined more accurately by means of numerical simulation. The trends of experimental results and simulations are in very good agreement.

Two-phase gas–liquid flows are usually observed in many systems, including oil recovery production, chemical industry, refrigeration systems and nuclear plants [

Up to now, scholars have carried out a lot of research work on the gas-liquid two-phase flow split at the T-junction. El-Shaboury et al. [

Previous studies showed that lots of experimental studies and CFD simulations have been done. However, there are relatively few studies on the influence of the outlet pressure of the branch pipeline on the split of two-phase flow at the T-junction. Therefore, both numerical simulation and experimental research was carried out to study the influence of the branch outlet pressure on the gas-liquid two-phase flow split at the impacting T-junction.

The experiment flow loop was established in the laboratory.

During the experiment, superficial liquid velocity was 0.1 to 0.5 m/s, and the air was 0.1 to 5 m/s. For gas-liquid flow splitting at the test loop, _{L0} is the liquid total mass flow rates, _{G0} is gas total mass flow rates. The flow rates in two branch pipelines (_{L1}, _{G1}, _{L2}, _{G2}) are measured respectively. The mass flow relationship is as follows:

Two parameters, _{G1} and _{L1} are defined to describe the split results. Here _{G1} is the fraction of the total gas mass flow that passes through one outlet of the tee junction. Similarly, _{L1} is the fraction of the total liquid mass flow that passes through one outlet of the tee junction. That is:

For an absolutely even split (symmetrical split), _{G1} = _{L1} = 50%. For uneven split, the flow rates in the two branch is >10% discrepancy from even splitting.

Before the experiment, we tested the symmetry of the pipeline use the single-phase. Keep all valves V6-V9 fully open during the test. The results are shown in _{G1} and _{G2} respectively represent the percentage of gas flow in the two branches of the total gas flow. _{L1} and _{L2} respectively indicate the percentage of the liquid flow in the two branches of the total liquid phase. From the test of single-phase flow, we can get that the test loop is basically symmetrical.

During the experiment, keep the valves on the test loop completely open to make the pressure at the outlet of the two pipes remains the same, which ensure the two branch pipelines symmetrically. In the experiment, at low gas and liquid velocity, the stratified flow was observed. The splitting result is shown in

As the gas and liquid velocity increases, intermittent flow was observed in the inlet pipeline. The liquid accumulated mass curve is shown in the

Adjust the opening degree of the separator valve V9 and V13 to make the pressure different in each separator. The splitting results of the single gas was shown in _{L2} in the outlet which has the higher pressure. From the results we can see that when the liquid velocity is low, the gas velocity has a greater impact on the splitting results of the two-phase flow. As the gas velocity decreases, the liquid phase tends to be evenly distributed in the two pipelines. At different gas velocities, the uneven distribution of the liquid phase gradually decreases with the increase of the liquid velocity, and finally tends to be evenly distributed. From the above results, we can conclude that the gas phase plays a leading role in the split of the two-phase flow at a lower liquid velocity. Because the outlet pressures of the two branch pipes are different, the gas phase carries more liquid phase into the side with a lower pressure. As the liquid velocity increases, the momentum of the liquid phase increases, and the carrying effect of the gas phase gradually weakens. Therefore, the pressure at the outlet of the two branch pipelines has a greater impact on the split of the two-phase flow if the liquid velocity is low. As the liquid velocity increases, the outlet pressure of the branch pipeline has little effect, and the liquid flow rates tends to be evenly distributed.

Due to the flow state in the horizontal pipeline is stratified flow or slug flow in the experiment, therefor the multiphase flow was modelled using the Volume of Fluid interface-tracking method. The equation solved by the VOF method is as follows:

Here

The flow of the gas in the control body is continuous and regarded as incompressible fluid. The velocity of the fluid in the control body is independent of time, and the fluid is steady flow. The convection term of the governing equation was discretized by second-order upwind scheme, and the turbulence model was treated by RNG k-ε Model [^{−5}.

The 3D model is filled with regular hexahedral meshes. Five different grid blacks were created for the model. The partial map of the grid is shown in

In order to verify the accuracy of the numerical calculation method, it was compared with the gas-liquid two-phase flow splitting experiment carried out above. The experimental conditions: the range of the superficial liquid velocity was 0.1 to 0.5 m/s, and the air was 0.1 to 5 m/s, the pressures of the two outlet are the same. The experimentally measured fraction of liquid phase from outlet 1 was shown in the

In the initial state, the gas volume in the pipeline is set to 0, that is, the pipeline is filled with liquid at the initial time. The inlet condition of the model is velocity inlet, gas velocity is 1 m/s, liquid velocity is 0.1 m/s. The outlets of the two pipelines are set as pressure outlets and the pressure at the outlets of the two pipelines is kept the same. The pressure of the two outlets is maintained at 1 Kpa. As shown in the

Keep the inlet conditions unchanged and change the outlet pressure of the two pipes. Make the pressure of outlet 2 (the right is pipeline 2) greater than the pressure of outlet 1. The pressure of the outlet 1 is maintained at 1 Kpa and the pressure of outlet 2 is maintained at 2 Kpa. During the beginning of the simulation, the two-phase flow did not produce obvious uneven distribution at the T-junction. But as time progresses, the gas-liquid two phases tend to flow into the side with lower pressure. Two-phase distribution cloud diagram was shown in

The outlet conditions of the two pipelines remain unchanged, the original pressure difference is maintained, and the inlet liquid velocity is changed to change from 0.1 m/s to 0.5 m/s. As shown in the

The aim of this research in this paper is to analyze two-phase flow splitting at the T-junction with different pressure at the two outlets. In this paper, both experimental research and numerical simulation are conducted. The results of the two methods are basically the same. It shows that the established numerical calculation method has high accuracy, and compared with the experiments, the simulation can more accurately control the pressure difference between the two outlets. When the pressure of the two outlets are same, regardless of the gas and liquid velocity, the two-phase flow splitting at the T-junction is always evenly. When the outlet pressure of the two outlets is different, the two-phase flow through the T-junction usually split unevenly, and the liquid phase tends to the branch with lower pressure, but with the increase of liquid velocity, this trend gradually decreases. Due to the increase of the momentum of the liquid phase, it occupies a dominant position in the splitting of the gas-liquid two-phase flow, and finally the two-phase flow is uniformly distributed at the T-junction.

In summary, if the pressure at the two outlets is symmetrical, the two-phase flow will be always splitting evenly at the T-junction. When the outlet pressure of the pipeline is asymmetrical, the increase of the inlet liquid velocity makes the two-phase flow tends to be evenly distributed.

The authors gratefully expressed their thanks for the financial support for these researches from the National Science and Technology Major Project of China (No. 2016ZX05028-004-003).