CFD INVESTIGATION OF DEVELOPING AND REDEVELOPING LAMINAR FLOW AND HEAT TRANSFER CHARACTERISTICS IN COILED TUBE SYSTEMS FOR CONSTANT WALL TEMPERATURE HEATING AND COOLING (PART II: SPIRAL CONFIGURATIONS)

摘要

In this paper developing and redeveloping laminar fluid flow and heat transfer performance in spiral coiled tube heat exchanger systems with specified coil-to-tube radius ratios (5 to 45), spiral pitch and inside-to-outside flow are investigated using appropriate numerical modeling techniques in the CFD package (Fluent v6.2). The CFD models employ variable thermo-physical properties in the analysis of uniform wall temperature heating and cooling of common working fluids such as air and water. The CFD models developed in Part I of this investigation are extended to analyze spiral tube configurations. Appropriate dimensionless variables used to describe the (re)developing hydrodynamic and thermal flow for coiled tube systems are defined and discussed. The spiral configuration is unique in that fully developed flow is not attained for the coil-to-tube radius ratios used in this work. In spiral configurations, the centrifugal effects are not constant as the radius of curvature is not constant. Flows within the spiral flow passages are analyzed as re-developing or re-establishing hydrodynamic and thermal flows. As with the helical flow configuration, it has been shown that in addition to the radius ratio and the Dean and Prandtl numbers, the heat transfer performance also depends upon the interactions (expansion and suppression) between the viscous and thermal boundary layers due to secondary flows caused by the centrifugal forces inherent in coiled tube systems. The predictions of the numerical simulations also showed that the local friction factor and heat transfer performance continuously vary along the length of the tube. The results obtained from this work indicate the limits of application of the friction factor and heat transfer correlations reported in the literature.