Abstract Correlations to predict the hydrodynamic and thermal entrance lengths in a smooth horizontal tube heated at a constant heat flux are often restricted to forced convection conditions. The reason being that most scholars do not distinguish very well between forced and mixed convection conditions. Furthermore, most correlations have been developed for hydrodynamically fully developed and thermally developing flows. The purpose of this study was to experimentally investigate the hydrodynamic and thermal entrance lengths for laminar flows in smooth horizontal tubes heated at a constant heat flux. Simultaneously hydrodynamically and thermally developing flows were investigated in both forced and mixed convection conditions. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, and maximum length-to-diameter ratios of 1 373 and 872, respectively, were used. Heat transfer and pressure drop measurements were taken at Reynolds numbers between 460 and 3 200 at different heat fluxes. A large database with a total of 513 mass flow rate measurements, 43 501 temperature measurements, and 1 665 pressure drop measurements were used. The test fluid was water and the Prandtl number ranged between 3 and 8. For the first time, correlations were developed to calculate the hydrodynamic and thermal entrance lengths for simultaneously hydrodynamically and thermally developing flow in smooth horizontal tubes. The correlations were valid for tubes heated at a constant heat flux in both forced and mixed convection conditions. For ease of use, the correlations were developed in terms of the local, inlet, and bulk fluid properties. Furthermore, the mixed convection correlations were developed in terms of both the Grashof and modified Grashof numbers.

中文翻译：

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在水平管中同时产生流体动力学和热力的强制和混合对流流动的层流流体动力学和热入口长度

摘要 在以恒定热通量加热的光滑水平管中，预测流体动力和热入口长度的相关性通常仅限于强制对流条件。原因是大多数学者没有很好地区分强制对流和混合对流条件。此外，大多数相关性是针对完全发展的流体动力学和热发展的流动而开发的。本研究的目的是通过实验研究以恒定热通量加热的光滑水平管中层流的流体动力学和热入口长度。在强制和混合对流条件下同时研究了流体动力学和热发展流动。两个内径分别为 4 mm 和 11.5 mm 的光滑圆形测试段，使用的最大长径比分别为 1 373 和 872。热传递和压降测量是在雷诺数介于 460 和 3200 之间的不同热通量下进行的。使用了一个包含总共 513 个质量流量测量值、43501 个温度测量值和 1665 个压降测量值的大型数据库。测试流体是水，Prandtl 数介于 3 和 8 之间。这是第一次开发相关性来计算光滑水平管中同时流体动力学和热发展流动的流体动力学和热入口长度。对于在强制和混合对流条件下以恒定热通量加热的管子，相关性是有效的。为便于使用，根据局部、入口和整体流体特性建立了相关性。