Annular flow is a method of liquid and gas flow in a pipeline where the material of lighter molecular weight flows down the center of the pipe and the heavier molecular weight material forms a thin film that flows along the pipe wall. It is often seen in the petroleum industry where flow rates are high, and can occur in both horizontal and vertical pipe. The lighter mass fluid or gas can also be in the form of a mist or colloidal suspension known as an emulsion. The interface between the flowing materials may not be precise, and can involve gas and liquid mixtures.
Variations in annular flow are categorized as wavy flow, where irregularities occur, or wispy annular flow. In wispy annular flow, as the flow rate is increased, the colloidal suspension of gas globules in the core increases and leads to the globules spreading out into wispy streaks and lumps. Several other types of flow regimens also exist, including bubble, slug, and churn flow in vertical pipe, as well as stratified and stratified-wavy in horizontal pipe.
Calculating annular flow rates can be difficult, as the equations require a precise measure of the inside diameter of the pipeline. This varies because annular flow has a no-flow boundary within it that changes the effective diameter of the pipe interior. Accurate values are hard to come by depending on the method of calculation used.
Two series of equations are commonly used to determine annular flow. The first is known as Wetted Perimeter flow, where the effective diameter of the pipe is divided by a square product of inner and outer flow areas. Wetted Perimeter calculations are not ideal, as they are based entirely on inner flow being subtracted from outer flow with no allowance for the no-flow region. The Petroleum Engineering method uses a more complex method of comparing inner and outer flow, and it is known to produce flow rate results that are about 40% higher than the Wetted Perimeter method. The Petroleum Engineering equations seem to reflect the actual measured flow rate better than the Wetted Perimeter method, yet the Wetted method is the standard used in academic engineering.
Friction factors also have to be accounted for in annular flow. Using the outer surface of the pipe to estimate friction is one method. Creating an average friction based on weighted data is also done, and are both considered legitimate approaches.
There are also various stages in gas-liquid flow in pipe where transitions occur between the various types of flow regimens. Transitions can include shifts from annular to wispy-annular, and plug to annular flow in vertical pipe. In horizontal pipe, common transitions in flow schemes include the slug to annular transition. These, as well as many other types of flow states and transitions, all have unique mathematical models for calculating what the current flow rate in the pipe actually is.