在单相和两相流换热器（尤其是“盘管”）中，除了直管外，还有许多奇点，尤其是180°回流弯管（也称为回流弯管或U形弯管）。然而，与关于直管内压降和传热的文献（其中有许多实验数据和预测方法）相反，关于U形弯管的研究数量有限。几乎没有可靠的实验数据，也没有公认的预测方法。事实上，U形弯管中的两相流是传热行业中一个尚未解决的老设计问题，它会影响蒸发和冷凝器。因此，本研究的目的是提供一个详细的洞察两相压降和流动模式在美国- 弯曲。基于一种新型U形弯曲试验段，进行了广泛的试验研究。实验活动包括五个试验段，三个内径（7.8、10.8和13.4mm），五个弯曲直径（24.8、31.7、38.1、54.8和66.1mm），三个流动方向（水平、垂直上流和垂直下流），两种流体（R134a和R410A），两个饱和温度（5和10°C），质量速度范围为150到1000kg s-1m-2。

In single- and two-phase flow heat exchangers (in particular "coils"), besides the straight tubes there are also many singularities, in particular the 180° return bends (also called return bends or U-bends). However, contrary to the literature concerning pressure drops and heat transfer in straight tubes, where many experimental data and prediction methods are available, only a limited number of studies concerning U-bends can be found. Few reliable experimental data and no generally proven prediction method is available. Indeed, two-phase in U-bends is an old unresolved design problem in the heat transfer industry influencing evaporating and condensers.Thus, the present study aims at providing a detailed insight into two-phase pressure drops and flows patterns in U-bends. Based on a new type of U-bend test section, an extensive experimental study was conducted. The experimental campaign covered five test sections with three internal diameters (7.8, 10.8 and 13.4mm), five bend diameters (24.8, 31.7, 38.1, 54.8 and 66.1mm), three flow orientations (horizontal, vertical upflow and vertical downflow), two fluids (R134a and R410A), two saturation temperatures (5 and 10°C) and mass velocities ranging from 150 to 1000kg s-1m-2.