Spatially resolved heat transfer measurements of impinging jets on statistically rough surfaces using an infrared-based gradient sensor

Heat-transfer measurements of jets impinging on rough surfaces are scarce because conventional heatedthin-foil methods confine high-resolution measurements to smooth walls. This study presents an infraredthermography-based gradient sensor that enables spatially resolved wall-heat-flux measurements on thermally thick plates with embedded, statistically homogeneous roughness. The method is validated against direct numerical simulation and established smooth-wall datasets for Reynolds numbers in the range 5000 ≤ 𝑅𝑒 ≤ 30 000 and nozzle-to-plate distances in the range 2 ≤ 𝐻∕𝐷 ≤ 5, showing good accuracy and repeatability. Using two roughness scales (𝑘$_{99}$∕𝐷 = 0.04 and 0.12), we provide the first systematic dataset of local Nusselt-number distributions for turbulent impinging jets on rough surfaces at Reynolds numbers relevant for practical applications. Roughness is shown to have little effect at low 𝑅𝑒, but a pronounced heat-transfer enhancement appears once a combined threshold in roughness height and Reynolds number is exceeded. For the larger roughness, strong augmentation is already observed for 𝑅𝑒 ≥ 10 000, especially near 𝑟∕𝐷 ≈ 1 where wall-jet shear is highest. ... mehrAt higher Reynolds numbers, roughness appears to fundamentally modify the radial heat-transfer pattern: the secondary peak, typically observed for smooth-wall impingement, disappears, giving way to a bell-shaped Nusselt profile, likely governed by altered near-wall flow dynamics. These findings offer new insight into heat transfer of rough-wall jet impingement at high 𝑅𝑒 and establish a validated measurement technique suitable for future studies integrating heat transfer, flow-field diagnostics, and wall-shear measurements.