Effect of Inlet Turbulence Intensity and Pipe Length on the Nusselt Number Distribution of an Impinging Jet

Abstract

This study investigates the influence of inlet turbulence intensity and pipe length on the heat transfer characteristics of a circular impinging jet, encompassing both developing and fully developed flow regimes. There have been few studies on heat transfer enhancement through a jet in a traditional flow regime before jet impingement. This investigation aims to provide insights into heat transfer enhancement with different pipe lengths and inlet turbulence intensity using a combined experimental and numerical approach. Computational fluid dynamics (CFD) simulations are performed over a wide range of operating conditions, including pipe length ratios (L/D = 20–130), inlet turbulence intensities (Tu = 2–10%), Reynolds numbers (Re = 5000–40,000), and nozzle-to-surface spacings (H/D = 1–8). Complementary experiments are conducted for selected operating conditions, including two Reynolds numbers (Re = 5000 and 10,000), a fixed inlet turbulence intensity (Tu ≈ 5%), three pipe length ratios (L/D = 20, 40, and 60), and the same range of nozzle-to-surface spacings (H/D = 1–8), to validate the numerical predictions and elucidate the underlying flow and heat transfer mechanisms. Experimental results show that, for short pipe lengths, the stagnation Nusselt number increases with H/D, reaching a maximum at H/D = 6. Increasing the H/D weakens both the magnitude and gradient of the secondary Nusselt number peak, irrespective of pipe length. Turbulent kinetic energy plots, obtained for CFD analysis, confirm that localized heat transfer in the wall jet region is enhanced due to stronger mixing occurring at low H/D and is responsible for the secondary Nusselt number peak. The simulations demonstrate that the influence of pipe length on the Nusselt number at low L/D is evident only at low turbulence intensities and diminishes at higher Tu, as the pipe entry length becomes increasingly governed by inlet turbulence. At low Tu, increasing the Reynolds number causes the secondary peak to vary across all pipe lengths due to an upstream shift in the transition onset within the pipe. Additionally, at higher Tu values, the impact of pipe length on flow development and heat transfer rate becomes less significant, as higher Tu accelerates flow development. The average Nusselt number plots reveal that, for Tu = 2%, the peak Nusselt number at L/D ≈ 60 is approximately 16% higher than that at L/D = 130 at H/D = 6. This study demonstrates that heat transfer performance can be improved by more than 11.5% with impinging jets at a shorter pipe length. This study shows that jet impingement cooling in compact configurations can be achieved with short pipe arrangements across a range of turbulence levels. Copyright © 2026 by ASME.