While past research has primarily focused on heat transfer in larger diameter tubes, the capabilities of small-diameter tubes have often been estimated using correction factors. However, with the evolution of compact heat exchangers, conducting dedicated studies on small-diameter tube bundles has become increasingly crucial to achieve more precise conclusions. An experimental research on flow and heat-transfer characteristics of staggered tube bundles with different tube diameters (2, 3, and 5 mm) is conducted. In the experiment, the number of rows (4−12), the mass rate of the air (0.06−0.18 kg/s), and the transverse tube pitch (S₁/d = 2, S₁/d = 3) are variables to study the characteristics of the airside flow resistance and heat transfer. The three main conclusions of the experimental results are as follows: (1) Under the same conditions, the smaller tube diameter leads to the larger airside convective heat-transfer coefficient. Besides, the deviation between the Nusselt number of the experiment and the empirical correlation of &Zcircukauskas is in the range between −14 and −10%; (2) The effect of transverse distance on heat transfer is not obvious, but the convective heat-transfer coefficient increases significantly with the increase of row number; (3) The external pressure drop of the tube exhibits an exponential increase with the air-flow rate. Particularly in the experimental samples with smaller diameters, the outflow resistance of the tube is noticeably higher compared to other tubes. Finally, new empirical correlations of the airside convection heat transfer for the small-diameter staggered tube bundles are fitted according to the experimental data, and it is hoped to provide a reference for the more accurate design of tube-bundle heat exchangers.

中文翻译：

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交错布局小直径管束的流动和传热特性：实验研究

虽然过去的研究主要集中在较大直径管中的传热，但通常使用校正因子来估计小直径管的能力。然而，随着紧凑型换热器的发展，对小直径管束进行专门研究对于获得更精确的结论变得越来越重要。对不同管径（2、3、5 mm）交错管束的流动与传热特性进行了实验研究。实验中，排数（4−12）、空气质量速率（0.06−0.18 kg/s）和横管节距（S ₁ / d = 2，S ₁ / d = 3）为变量来研究空气侧流阻和传热的特性。实验结果的三个主要结论如下：（1）相同条件下，管径越小，空气侧对流换热系数越大。此外，实验的努塞尔数与&Zcircukauskas的经验相关性之间的偏差在-14%到-10%之间；(2)横向距离对传热影响不明显，但对流换热系数随着排数的增加而显着增加；(3)管外压降随气流流量呈指数增加。特别是在直径较小的实验样品中，该管的流出阻力明显高于其他管。最后根据实验数据拟合出新的小直径交错管束空气侧对流换热经验关系式，以期为更精确的管束换热器设计提供参考。