Study on Flow Characteristics of Small Tube Bundles with a Longitudinal Pitch Ratio of 1

Small tube bundles are widely used in the field of aircraft thermal management due to their excellent characteristics such as lightweight, high efficiency, and strong reliability. However, the traditional tube bundles face greater pressure loss under the larger mass flow, so it is urgent to develop novel ultra-low resistance heat exchange units. This study proposes a tight tube bundle structure with the longitudinal pitch ratio of 1. Starting from the drag reduction characteristics of the structure, the flow characteristics under different transverse pitch ratios were studied by numerical and experimental methods. The values of the flow resistance coefficient calculated by the turbulent k-ε model is closed to the experimental results and has the same trend for the entire range of Reynolds numbers, and the maximum error is 5 %. The experimental results indicate that the flow resistance coefficient of tube bundle with the longitudinal pitch ratio of 1 is lower about 80% than that of non-compact tube bundles with different longitudinal pitches. The numerical results prove that the reduction of flow separation generated by gaps between tubes is the dominate factor to the change of flow resistance coefficient. Compared with the non-compact tube bundles, the novel tube bundle only has once flow impact and separation, located in the leading edge of first pipe tube and the in the tail of last pipe tube respectively, which fully avoids the pressure losses caused by the wake recirculation zones between different tube pipes. Besides, there is no transverse airflow channel between tightly arranged tube bundles, which reduces the mixing of airflow and enhances the transverse symmetry of flow. Furthermore, it can be observed that the pressure variation pattern of the compact pipe arrangement structure is simple. The pressure gradually decreases along the flow direction. The low-pressure area is only formed by the fluid detachment from the tube wall surface exists. The vortex structure has a small impact on the flow, and the pressure gradient is reduced by 35% than the non-compact structure, resulting in a significant reduction in the flow resistance of the structure. For the tight tube bundle itself, the flow resistance coefficient still follows the pattern of increasing with decreasing transverse pitch ratio. In addition, based on the experimental data under different transverse pitch ratios, the resistance empirical correlation for tight tube bundles with transverse pitch ratios ranging from 2.31 to 9.25 is proposed, which provides an insight into developing the efficient and low resistance heat exchange structure.