Thermal hydraulic characteristics of air-water two-phase flows in helical pipes

Helically coiled heat exchangers, where one of the working fluids is flowing through helical coil, are used in various process industries due to better heat transfer characteristics and the resulting compact layout. Out of these, process requirements make some of the heat exchangers to operate in air-water two-phase region. Even though the characteristics of their operation with single-phase working fluids are well documented, it is not so for the case of two-phase flows. There do exist few experimental results on hydrodynamics of air-water flow through helical pipes. However numerical investigation, which can give much insight into the physics of the problem, is lacking and this is the subject matter of this paper. Two-fluid Eulerian-Eulerian scheme available in FLUENT 6.3 is used in this analysis. Pitch circle diameter and pitch of the coil influence the nature of flow through helical coils. Another factor affecting the nature of two-phase flow in helical pipes is the diameter of the pipe. CFD analysis has been carried out by varying these parameters and their influence on thermal hydraulic characteristics of the two-phase flow are brought out. Subsequently analysis has been carried out by changing the inlet void fraction for a given value of the flow velocity. From these analyses, it has been established that correlations for heat transfer and pressure drop shall take into account the pitch circle diameter, pipe diameter and void fraction at the inlet. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.