Effects of expansion pipe length on heat transfer enhancement of impinging jet array

Abstract

We experimentally and numerically investigated the effect of expansion pipe length on heat transfer enhancement and flow characteristics of impinging jet array with mounting expansion pipe. The inner diameter (d) and the length of each main pipe nozzle was d = 17.2 mm and 200 mm, respectively. Nozzle arrangement was distributed in 5 rows × 5 columns with an in-line configuration. The jet-to-wall distance (H) was fixed at H/d = 8, while the jet-to-jet spacing (S) was varied at S/d = 6 and 8. The expansion pipe length (L) was varied at L/d = 4, 6 and 8, and the inner diameter of the expansion pipe (D) was fixed at D/d = 4. Reynolds number of the jets was kept constant at 30000. The conventional impinging jets were also performed to compare with the case of jets with mounting expansion pipe. The 3-D numerical simulation with v2-f turbulence model was applied to simulate the flow characteristics. Results showed that the surrounding air was induced into the expansion pipe, and an entrainment of induced air became greater when expansion pipe was longer. This influenced more markedly on enhancement of Nusselt number at stagnation point for narrow jet-to-jet spacing (S/d = 6).