含蜡原油管道停输过程中的管内复杂自然对流换热机理研究

The heating energy cost in pipelining waxy crude oil is dramatic, which can be greatly reduced if the pipelining temperature of the crude oil can be decreased. However, the heat transfer mechanism during the shutdown process is still not clarified because it involves many complex factors such as strongly nonlinear variation of crude physical properties, phase transition, moving phase interface as well as the coupling of natural convection and conduction. The target of the present study is to discover the heat transfer mechanism of waxy crude oil pipeline after its shutdown, and provide the theoretical basis to decrease the crude oil pipelining temperature. Distinct from the current methods, this study divides the shutdown process into four different stages in sequence, i.e., Newtonian fluid, solid-liquid disperse system, coexistence of liquid crude oil and gel, coexistence of solid, gel and liquid crude. This division is based on the dramatic variation of the rheological properties of the waxy crude oil during the shutdown process, which is the key factor affecting its heat and fluid flow behavior. The natural convection models for each stage will be established, the whole heat transfer process during the shutdown period can be revealed by combining these models. Through experimental and numerical studies, the mechanism of natural convection of each stage will be discovered, especially the effect of wax-crystal structure and latent heat of wax precipitation. Moreover, the empirical correlations of natural convection for each stage will be obtained. This study will enrich the natural convection of non-Newtonian fluid as well as phase change heat transfer, improve waxy crude transportation technology and bring enormous social and economic benefits.

含蜡原油管道加热输送能耗高，如能降温输送，节能潜力巨大。由于停输温降过程涉及原油物性强非线性变化、相变、相界面移动及自然对流与导热耦合等多方面复杂因素，其传热机理尚未得到有效地揭示。本研究旨在揭示热油管道停输后的传热机理，为实现降温输油技术奠定基础。有别于现有研究，本研究从影响原油流动和换热性能的关键因素，即含蜡原油复杂流变性质出发，将停输自然对流过程细分为牛顿流体、固-液分散体系、胶凝状态和液态原油共存及凝固状态、胶凝状态和液态原油三者共存四个阶段，并分别建立相应对流换热模型，综合得到全面反映各种传热驱动机制的数理模型。通过实验和数值模拟揭示各阶段自然对流换热规律，尤其是不同状态蜡晶结构和析蜡潜热产生的影响，得到各阶段的自然对流换热关联式。研究成果将丰富非牛顿流体自然对流和相变传热理论，提高含蜡原油输送水平，产生良好的经济效益和社会效益。

含蜡原油管道停输温降是一个涉及复杂相变、流体非牛顿性、固液相间作用的复杂非稳态自然对流换热过程。掌握停输温降规律对含蜡原油管道的安全运行具有重要意义。本项目采用实验与数值计算相结合的方法研究了含蜡原油管道停输过程中的管内复杂自然对流换热规律。在实验方面，首先设计、搭建了一套停输温降实验装置并进行了校准。然后，进行了含蜡原油的停输温降实验，揭示了含蜡原油管道的停输温降规律。在数值计算方法方面，对含蜡原油停输温降数学模型进行了研究，并对含蜡原油停输温降规律及其影响因素进行了分析。首先根据停输温降过程中不同阶段传热机制的差异，将停输温降物理过程划分为纯液相自然对流换热、固液分散体系自然对流换热、固液分散体系、多孔介质自然对流换热共存以及多孔介质自然对流换热四个阶段。在综合考虑相变和流体非牛顿性等重要影响因素的基础上，建立了能够描述各传热阶段换热形式的含蜡原油管道停输温降数学模型。对析蜡量、蜡晶骨架渗透性、原油非牛顿性、管径、埋深、结蜡层厚度、土壤物性、气温等影响因素进行了深入的分析，阐明了相关规律。相关研究成果可以为管道日常运行以及安全、节能方案的制定提供一定的参考。