MULTIPHASE FLOW
Dynamic slugs in gas–liquid horizontal pipes
Long liquid slugs, with sizes reaching 500 pipe diameters or more, may form in gas–liquid horizontal pipe flow at intermediate liquid loadings. Such slugs cause serious operational upsets due to the strong fluc- tuations in flow supply and pressure. Therefore, predicting the transition from short (hydrodynamic) to long slug flow regimes may play a significant role in preventing or reducing the negative effects caused by the long slugs.
We introduce a growth model for calculating the average slug length in horizontal and near horizontal pipes. The model applies a volumetric balance between the front and tail of the slug in order to calculate the slug growth rate. The dynamic behaviour of the liquid at the tail is described by a linear kinematic relation between the slug downstream and the wave upstream.
For the validation of the model we performed measurements in a 137 m length air–water horizontal pipe flow of an internal diameter (i.d.) of 0.052 m. The measurements provide a detailed flow map of the long slug regime and sub-regimes. Furthermore, we compared predictions by the model with avail- able data for a range of 0.019–0.095 m i.d. pipes to investigate the effect of varying operation pressures, different inlet conditions, different fluid properties and slight inclinations. The model predicted the tran- sitions from hydrodynamic to long slugs with satisfactory agreements, however it underpredicts the average slug length at relatively large mixture velocities.
In stratified gas–liquid horizontal pipe flow, growing long wavelength waves may reach the top of the pipe and form a slug flow, or evolve into roll-waves. At certain flow conditions, slugs may grow to become extremely long, e.g. 500 pipe diameter. The existence of long slugs may cause operational upsets and a reduction in the flow efficiency. Therefore, predicting the flow conditions at which the long slugs appear contributes to a better design and management of the flow to maximize the flow efficiency. In this study, we introduce a wave transition model from stratified flow to slug flow or roll-wave regimes. The model tracks the wave crest along the pipe. If the crest overtakes the downstream wave end before hitting the top of the pipe, a roll-wave is formed, otherwise a slug.
In this study, we present a model for predicting the average slug frequency in horizontal gas/liquid pipe flow. The model considers the probability of slug formation if slugs are triggered at the antinodes of a sinusoidal perturbation, along the pipe at the frequency of oscillation of the interface. A slug is assumed to form if and only if triggered at a space-time far enough from existing slugs. The probability of forming slugs is found to decrease with distance from the inlet, since the downstream passage of existing slugs prevents the formation of new slugs. Predictions by the model are compared with air/water, freon/water and air/oil measurements found in literature, with a satisfactory agreement. However, a deviation from measurements is observed when considering high viscosity liquid. The model contributes to the prediction of slug flow regime and can act as a guideline toward the design of gas/liquid horizontal pipe flow.
