Review on two-phase flow distribution in parallel channels with macro and micro hydraulic diameters: Main results, analyses, trends

Abstract

Two-phase flows in parallel channels are found in various industrial applications employing heat exchangers, boilers and condensers, cooling systems, etc. Two-phase flow distribution in headers with multi-parallel channels has been studied over the past decade. Uneven phase distribution causes a reduction in both the thermal and fluid-dynamic performance and in many cases the failure of the device. The phase separation in these devices with several channels connected to a header is complicated. Thus, to date, there is no general way to predict the distribution of two-phase mixtures. The design of headers, in most cases, is still based on an empirical approach, due to the great number of variables which act together, that is, geometrical parameters, operating conditions and fluid properties. This paper aims to address the fundamental questions related to the main influences on two-phase flow distribution in devices with multi-parallel channels. This review summarizes the experimental, numerical and theoretical work carried out by various investigators over a period of several years, including works in micro (0.8 mm < dh ≤ 2.0 mm) and macrochannels (2.0 mm < dh < 30.0 mm). The investigation allowed us to identify the main geometrical and operating conditions which influence the two-phase flow distribution in parallel channels. A tentative assessment of the role of these parameters is also carried out.The header and the feeding tube positions were found to be the main factors influencing the mass flow rate distribution among parallel channels. An analysis of the action of the main forces (inertial and gravitational) in each case, due to the geometrical parameters, operating conditions and fluid properties, was essential to determine the two-phase flow distribution among the channels. It was verified that the mist flow pattern induced the best level of distribution among the parallel channels, independently of the header position and flow direction.The existing models and correlations developed for two-phase flow distributions are still preliminaries, and do not reflect the real complexity of two-phase flow inside the header. Further developments are needed to make these tools enough mature to predict the two-phase flows in such complex geometries. Modifying the geometry by inserting specific devices into the header or the feeding tube is one promising approach to improve the two-phase flow distribution in parallel channels.