Experimental and numerical analysis of airside thermal-hydraulic characteristics of small-diameter tube bundle under compressible flow conditions

High thermal loads in aviation necessitate compact, efficient, small-diameter tube bundle heat exchangers. However, the combined impact of high velocity and small diameter remains unclear. To address this gap, the present study investigates the airside thermal-hydraulic performance of small-diameter tube bundles in compressible cross-flow. Contrary to classical correlations, present experiments measuring Nusselt number (Nu) and friction factor (f_ac) reveal significant diameter dependence at high Reynolds numbers (Re). At Re = 10,000, 1 mm tube bundle exhibits an increase of 25.2 % in Nu and 17.5 % in f_ac compared with its 5 mm counterpart. To model tube conjugate heat transfer, a refined dimensionless parameter framework is developed to comprehensively incorporate the complex interactions among Re, Prandtl number (Pr), Mach number (Ma), and Eckert number (Ec). Numerical simulations reveal that the coupling between Ma and Ec plays a pivotal role in determining flow behavior and heat transfer characteristics. Specifically, higher Ma promotes flow separation, increasing pressure drop alongside enhanced heat transfer. In contrast, higher Ec suppresses flow separation, reducing pressure drop while still augmenting heat transfer. These findings elucidate the intricate interplay of thermal-hydraulic mechanisms in small-diameter tube bundles under compressible flow, providing a fundamental basis for designing advanced aerospace compact heat exchangers.