Ring flow is a method of flowing liquids and gases in a pipeline where the lighter material flows down the center and the heavier material forms a thin film along the pipe wall. It is often seen in the petroleum industry and has variations such as wavy flow and annular tufted flow. Calculating ring flow is difficult due to variations in the inside diameter of the pipe. Two sets of equations are commonly used to determine ring flow, wet perimeter flow, and the Petroleum Engineering method. Friction factors also have to be taken into account in ring flow. There are various stages in piped gas-liquid flow where transitions occur between various types of flow regimes.
Ring flow is a method of flowing liquids and gases in a pipeline where the lighter molecular weight material flows down the center of the pipe and the heavier molecular weight material forms a thin film which 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 pipes. The lighter mass fluid or gas may also be in the form of a mist or colloidal suspension known as an emulsion. The interface between flowing materials may not be precise and may involve mixtures of gases and liquids.
Variations in ring flow are classified as wavy flow, where irregularities occur, or thin ring flow. In the annular tufted flow, as the flow rate increases, the colloidal suspension of the gas globules in the core increases and causes the globules to spread out into strips and clumps. There are several other types of flow regimes as well, including bubble, slug, and churn flow in vertical pipe, as well as layered and corrugated layered in horizontal pipe.
Calculating ring flows can be difficult, as the equations require an accurate measurement of the inside diameter of the pipe. This varies because the annular flow has a no-flow boundary within it which changes the effective diameter of the inside of the pipe. It is difficult to obtain precise values depending on the calculation method used.
Two sets of equations are commonly used to determine ring flow. The first is known as wet perimeter flow, where the effective diameter of the pipe is divided by a square product of the internal and external flow areas. Wetted perimeter calculations are not ideal, as they are based entirely on internal flow subtracted from external flow with no tolerance for the no-flow region. The Petroleum Engineering method uses a more complex method to compare internal and external flow and is known to produce flow rate results that are approximately 40% higher than the Wetted Perimeter method. Petroleum engineering equations appear to reflect actual measured flow rate better than the wet perimeter method, however the wet method is the standard used in academic engineering.
Friction factors also have to be taken into account in ring flow. Using the outside surface area of the pipe to estimate friction is one method. An average attrition based on weighted data is also created and both are considered legitimate approaches.
There are also various stages in piped gas-liquid flow where transitions occur between various types of flow regimes. Transitions can include annular to tuft-annular and plug-to-annular flow shifts in the vertical pipe. In the top tube, common transitions in flow patterns include the transition from slug to annular. 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.
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