We consider the apparent roughness and increased pressure drop associated with the presence of a thin liquid film between the gas and the pipe wall in two- and three-phase stratified-annular gas-liquid flow. The main objective is to improve the pressure drop predictions for near-horizontal gas-condensate flows with low liquid loading and high gas flow rate. To support the model development, SINTEF conducted experiments in 8- and 12-inch 2.5-degree upward-inclined pipes at the Tiller test facility in Norway. A model for the film roughness developed in a previous study was generalized to account for the effect of gravity, which limits the film thickness for lower gas rates. Oil droplets are more effectively transported by the turbulence in the gas, so the liquid film on the pipe wall tends to have a lower aqueous fraction than the liquid layer in the bottom of the pipe. In the general case, the model covers all stages from a pure stratified flow at low gas rates to stratified annular flow for intermediate gas rates to (in the case of low liquid loading) pure annular flow at high rates. Model predictions are in good agreement with the SINTEF data.
Annular stratified flow often occurs in pipeline transport of gas-condensate fluids at high rates. The key difference between pure stratified flow and annular stratified flow is that the latter includes a thin film of liquid between the gas and the pipe wall, which is held in place by the turbulent fluctuations. The thin film can become very rough, significantly increasing the frictional contribution to the pressure gradient; this is particularly significant for flows with low liquid loading, where other contributions to the pressure gradient are relatively small.