I. Introduction
Gas-liquid two phase flow is commonly encountered in oil and gas transportation, nuclear reactor, refrigeration, and air conditioning. Annular flow is a vital gas-liquid two phase flow pattern. Gas flows at a high speed in the center of the pipe to form a gas core with entrainment, and the liquid flows continuously along the pipe wall to form a liquid film. According to the flow direction, it can be divided into vertical annular flow and horizontal annular flow. Due to gravity, the liquid film in the upper and lower halves of the horizontal annular flow is asymmetric, which has a great effect on the heat and mass transfer performance. It is a characteristic of horizontal annular flow that the liquid film at the bottom of the pipe is thicker than that of the side and the top of the pipe.