Heat Transfer and Pressure Drop Characteristics of Spirally Fluted Annuli: Part I—Hydrodynamics

Abstract

This paper is the first of two papers that present the results of a comprehensive study of heat transfer and pressure drop in annuli with spirally fluted inner tubes for the laminar, transition, and turbulent flow regimes. To understand the underlying physical phenomena responsible for heat transfer enhancement, flow mechanisms documented elsewhere are combined with pressure drop measurements to propose friction factor correlations for the low Re range (Re < 800) and the high Re range (800 < Re < 43,500). Friction factors were found to be functions of the flute depth, pitch and angle, and the annulus radius ratio. These correlations are used in the second paper to develop Nusselt number correlations in terms of the fluted annulus friction factor. Transition in these annuli occurred in the 310 < Re < 1000 range. In Part II of this study, this early transition is shown to be responsible, in part, for significant heat transfer enhancement. Friction factor increases were typically between 1.1 and 2.0 in the laminar regime, and up to 10 in the turbulent regime. These enhancement values can be used in conjunction with heat transfer enhancement values reported in Part II to determine appropriate ranges of applicability for spirally enhanced annuli.