Mixed convection phenomena in tubes with wire coil inserts

Abstract

A numerical investigation of the flow and heat transfer in a helical coil in laminar and transitional flow regime under mixed convection conditions is presented. The continuous separation and reattachment of the boundary layer caused by the wire coil, that interrupts the flow near the tube all, and the swirl induced by the helical shape of the insert, are identified as the mechanism for heat transfer enhancement. Simulations in the Reynolds number range 200<Re<1500 show the role of mixed convection on heat transfer, as the flow dynamics is sensitive to the buoyancy forced induced by the tube wall heating. The interaction of the secondary swirl flow with the buoyancy motion is investigated in this work, with the aim of clarifying the range where heat transfer could be worsened by the presence of the helical coil. The investigation has been undertaken considering a helical coil of geometrical characteristics e/D=0.071 and p/D=1.5 and water as working fluid. Validation with experimental results of friction factor and Nusselt number is also presented.