CFD Investigation of Developing and Redeveloping Laminar Flow and Heat Transfer Characteristics in Coiled Tube Systems for Constant Wall Temperature Heating and Cooling: Part I — Toroid and Helical Configurations

Abstract

In this paper developing laminar fluid flow and heat transfer performance in toroidal and helical coiled tube heat exchanger systems with coil-to-tube radius ratios (5 to 45) and small helical pitch are investigated using appropriate numerical modeling techniques available in the CFD package (Fluent v6.2). Base CFD models were primarily developed, optimized and compared with available published friction factor and heat transfer data and correlations for the toroidal and helical coil systems. With the proven CFD modeling technique and the results obtained, the analysis was extended to the coil-to-tube radius ratios of interest and to the investigation of the effect of thermo-physical properties of working fluids on the system thermal performance. 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. Defining appropriate dimensionless variables to describe the developing and redeveloping hydrodynamic and thermal flow for coiled tube systems, the variations of friction factor and local Nusselt number along the coil are investigated. It has been shown that in addition to the common affecting parameters, i.e. the coil-to-tube 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 and torsional forces inherent in coiled tube systems. Upon investigation of the variations of the local dimensionless velocity and temperature along the coil length, it was found that for both heating and cooling conditions, fully-developed hydrodynamic and thermal conditions are not established in the coiled-tube system for the geometric constraints and system boundary and operating conditions used in this work. The case studies performed in this paper indicated approximately 20-30% higher for heating of water (20-30% lower for cooling of air and water) than values of heat transfer coefficients obtained from the reported correlations. The results obtained in this work can be used to correct/adjust the flow and thermal performance used in the design of toroidal and helical coiled tube systems.