Heat transfer and pressure drop characteristics of laminar flow through a circular tube with longitudinal strip inserts under uniform wall heat flux

Heat transfer and pressure drop characteristics in a circular tube fitted with full-length strip, short-length strip, and regularly spaced strip elements connected by thin circular rods have been investigated experimentally. The strips have been rectangular square and crossed in cross-section with different aspect ratio. Laminar flow of water and other viscous liquids was considered. The rod diameter and length of the strip-rod assembly and the length of the strips were varied. Isothermal friction factor data has been generated. The heat transfer test section was heated electrically, imposing axially and circumferentially constant wall heat flux (UWHF) boundary condition. Reynolds number; Prandtl number; strip length, strip ratio, space ratio, and rod-diameter govern the characteristics. Smaller rod-diameter in the strip-rod assembly or pinching of the strips in place rather than connecting the strip elements by rods performs better thermohydraulically. Short-length strips (up to a limited fraction of the test section tube length) perform better than the full-length strip. The friction factor correlation and the correlation for Nusselt number under UWHF condition for full-length strip have been modified to make them suitable for short-length strip as well as regularly-spaced strip elements. Thermal entrance length in the correlations is represented by Graetz number Friction factor and Nusselt number correlations for short-length strips as well as regularly-spaced strip elements, in the limit, reduce to their full-length counterparts.