What Is Annular Flow?

Annular flow refers to a flow pattern of two-phase flow composed of gas and liquid. It is characterized by a liquid film along the inner wall of the tube, and most of the liquid moves along the tube wall in a film form, while the gas is in The central area of the tube entrains mist and flows at high speed. The condition for this flow pattern is that the velocity of the gas is greater than the velocity of the liquid phase. It is often found in oil and gas transportation in the petroleum industry, where fluid flow rates are high and can occur in horizontal and vertical pipelines. The gas can also flow in the form of a mist or colloidal suspension, called an emulsion. ^[1]

Annular flow is a kind of flow pattern in the gas-liquid two-phase flow with annular liquid film flow on the tube wall and cylindrical air flow in the center. At low temperature, there may be annular flow on the tube wall and liquid column flow in the center. When the central airflow contains small droplets, it is called ring-mist flow (ring mist flow), when it contains liquid strips or liquid filaments, it is called liquid filament annular flow, and when the liquid film contains small bubbles, it is called bubble annular flow. In the heating tube,

Gas-liquid two-phase annular flow is a flow pattern often encountered in engineering practice. It is a safe and economic flow pattern that is expected to be maintained or emerged. It is of great academic significance to study and predict its characteristics and transition mechanisms. Industrial application value. Butterworth analyzed the work achievements of the predecessors, and pointed out that the factors affecting the formation and stability of annular flow are: (1) liquid

Introduction to Circulation Research

When the gas phase velocity is higher and the liquid phase velocity is lower, an annular flow often occurs. In the annular flow, part of the droplets are entrained by the high-speed gas phase and flow in the center of the tube, while the liquid film surrounds the inner tube wall. The entrainment fraction is defined as the ratio of the liquid flow rate in the form of droplets to the total liquid flow rate in the gas core. The prediction of the entrainment fraction plays an important role in estimating the pressure drop, liquid holding rate, and dryness in the annular flow, and in designing and optimizing the separator.

The high-speed airflow in the annular flow causes a large shear velocity, which results in a large interface shear stress, which generates droplets on the liquid film and gas phase wave-shaped interfaces. These droplets can be combined in the gas phase and eventually re-deposited on the liquid film. When the gas flow rate is large enough, some droplets hit the top of the tube wall, forming a very thin liquid film. When the liquid phase velocity is large, the entrainment and deposition of droplets makes the liquid film on the side of the tube wall thick enough due to the large-amplitude perturbation wave, thereby generating intermittent flow.

At present, many theoretical studies have been performed on both gas-liquid two-phase horizontal and vertical annular flows. Annular flow mainly has three characteristics: a series of waves exist at the gas-liquid interface; entrained droplets entrained from the gas-liquid interface to the gas core; and some of the entrained droplets are re-deposited on the gas-liquid interface. ^[4]

Circulation research conclusion

(1) The droplet diameter distribution strongly depends on the gas phase flow rate and rarely depends on the liquid phase velocity. The droplet diameter decreases as the gas flow rate increases. By increasing the gas phase velocity, the droplet diameter distribution shifts to a smaller diameter and its distribution tends to be more evenly distributed.

(2) The droplet diameter is inversely proportional to the gas phase velocity, and for the apparent velocity of the liquid phase, the droplet diameter is increased in some cases, and the droplet diameter is decreased in other cases. But for horizontal and vertical flow, the droplet diameter is proportional to the apparent velocity of the liquid phase, because the relative velocity between the gas phase and the liquid phase determines the droplet diameter, and the droplet diameter increases as the relative velocity decreases, that is, Larger liquid velocities can produce larger droplets. At the same time, the droplet diameter tends to increase with the increase of the tube diameter.

(3) The study of droplet velocity distribution shows that when the droplet velocity is 80% of the gas phase velocity, the average droplet velocity increases with the increase of gas phase velocity. The experiment observed that the droplet velocity had a slight upward trend with the increase of the gas and liquid phase velocities, until the droplet velocity reached 80% of the gas phase velocity. At the same time, the dispersion of the droplet velocity distribution increases with the increase of the gas phase velocity. The larger the gas phase velocity, the smaller the resulting droplets, and these smaller droplets move at a larger range of speeds because of the smaller The liquid droplets are susceptible to the strong influence of gas-phase turbulence, causing lateral acceleration and deceleration, which causes the droplet velocity to change. Larger droplets, which are not easily affected by turbulence, exhibit a narrow speed range.

(4) The entrainment process is caused by the high-speed gas phase. The high-speed gas phase will shear the peaks of the perturbation wave at the gas-liquid interface due to the velocity difference between the liquid phase and the gas phase. The entrainment fraction is used to estimate the amount of liquid phase as it enters the gas stream as droplets. Studies have shown that the entrainment fraction increases with increasing liquid and gas flow rates, but is mainly affected by the apparent velocity of the gas phase. ^[4]