Developing adiabatic two-phase flow after an expansion valve: Flow development in an 8 mm tube

Abstract

This paper presents the developing two-phase flow after an expansion valve. R134a with 1.3% POE32 is used as a working medium. The flow patterns are visualized from 15 mm until 595 mm after the expansion valve in a straight tube with an inner diameter of 8 mm. The experiments cover the mass flux from 100 to 480 kg m−2 s−1 and the range of quality from 0.002 to 0.500. The observed flow patterns are classified into four regions as two-phase flow progresses along the tube: well-mixed, separating, separated but developing, and fully developed. Due to the sudden expansion in TEV, the flow structure in the well-mixed region is similar to homogeneous flow, with bubbles or droplets dispersed uniformly. Then, the gravity/buoyancy force becomes more dominant over inertia and surface tension because of the reduction of liquid velocity and the growth of bubbles. Consequently, bubbles migrate upwards while droplets fall downward, characterizing the separating region. After phase separation, the flow characteristics may still change for a certain distance before the flow is fully developed. Based on the observed flow regimes at various working conditions, a 3D flow pattern map is built, adding the distance from the expansion valve as the third dimension besides the mass flux and quality. In a following paper, the authors will discuss the effect of tube diameter, oil, and structure of expansion devices on developing two-phase flow. The developed flow patterns will be compared with the existing flow pattern map in steady conditions as an asymptote.