Heat transfer enhancement by multiple swirling impinging jets with twisted-tape swirl generators

The flow and heat transfer characteristics of multiple swirling impinging jets (M-SIJs) with 3 × 3 in-line arrangement, on impinged surfaces are reported. The experiments were conducted with four different jet-to-jet distances (S/D = 2, 4, 6 and 8) at the constant nozzle-to-plate distance of L/D = 4. The swirling jets with the swirl numbers of 0.4 were associated with twisted tapes. The multiple conventional impinging jets (M-CIJs) were also tested, for comparison. The flow patterns on an impinged surface were visualized using oil film technique while the distributions of temperature field and Nusselt number on impinged surface were evaluated via a thermochromic liquid crystal (TLC) sheet coupled with image processing technique. The experimental results showed that the M-SIJs offered higher heat transfer rate on impinged surfaces than the M-CIJs of all jet-to-jet distances (S/D).