General characteristics of two-phase flow distribution in a multipass tube

An experimental study has been performed using two different types of multipass tubes, one with four vertical upward passes attached to a horizontal main pipe and the other with five horizontal passes placed to a vertically oriented main pipe. Passes had 6 mm inside diameter, and the inside diameter of main pipes was 20 mm for the vertical type and 6 mm for the horizontal type. Multipass tubes simulated evaporators for air-conditioning systems, and refrigerant R11 was used as the working fluid. Vapor and liquid flow rates at each pass were measured under various conditions of two-phase mixture at the main pipe inlet, that is, inlet flow rate and quality. Results suggested that measured values of flow distribution ratio presented a marked distinction between the vertical and horizontal types. On the other hand, in spite of different pass orientations, the same data trend that the flow distribution is dominated by the main tube inlet flow condition was observed, and the flow division ratio was estimated in terms of both phase flow rates before the junction. Based on these results, a flow split model was suggested and an attempt for the prediction of flow distribution was made.