局部曲率约束下超临界流体二次流动发展规律及其与传热耦合机理研究

Return bends are widely used in the heat exchangers of industrial systems where working fluids operate at supercritical pressures. The introduction of return bend leads to a discontinuous curvature effect within the flow channel, which subsequently changes the flow field and makes the heat transfer performance quite different from that of straight or curved tubes. This work aims to reveal the bend effect on flow and heat transfer of supercritical fluids. Effects of bend arrangement, tube/bend geometries and operating conditions (pressure, temperature, mass flux and heat flux) will be experimentally obtained under both forced and mixed convection, by performance comparison between horizontal tubes including/excluding U-bends. The integral heat transfer enhancement and deterioration of U-tubes are to be determined under different operation conditions. Following experimental studies, a numerical model, which can predict the curvature-induced secondary flow and the flow separation caused by sharp density variation, will be developed and validated. Then the supercritical flow and thermal field within and following U-bends will be analyzed in detail, to fully reveal the evolving process of secondary flow. Special attentions will be paid to the interaction of vortexes induced by centrifugal and buoyancy forces, and its effect on the supercritical heat transfer. Finally, based on non-dimensional analysis among effects of dramatic property variations, buoyancy and centrifugal forces, a semi-empirical model will be proposed to quantitatively describe the U-bend effect on heat transfer.

换热器是跨临界余热发电、制冷及热泵系统的关键设备。换热单元管中含有U型回弯等弯曲部件时，在局部曲率约束下管内存在沿程渐衰的二次流效应，导致使用现有超临界直管、连续弯管模型来预测其换热能力会出现明显偏差。为此，本项目围绕“局部曲率约束下超临界流体二次流动的演化规律及其与传热的相互作用机制”这一关键科学问题开展研究，通过实验获得超临界流体传热中局部曲率效应的定量基础数据，明确布置形式、管径、曲率等关键参数对传热的影响规律；建立并优化二次流动模型、异密度流体分层流动模型和湍流动力学模型，详细分析流场中漩涡生成、重组和再生过程，揭示各类U型回弯对超临界热流场演变的扰动机理，结合力间相互作用无量纲分析来量化回弯效应，最终形成U型管换热器的传热优化模型。本项目的实施在理论上可丰富和发展局部曲率约束下超临界流体的流场演变规律和传热机理，在实践上为先进跨临界系统中U型换热器的设计和优化提供科学依据。

换热器是跨临界余热发电、制冷及热泵系统的关键设备。回转弯头是换热器的常用管件，但目前关于回弯对超临界流体流动传热特性影响的研究仍然比较匮乏，缺乏回弯的引入对超临界流体传热特性影响程度的定量基础数据，回弯及其下游超临界流场结构演变规律和热量传递机理尚未得到足够重视。本项目首先通过直管和U型管的直接实验对比来确定局部曲率对超临界流体二次流动发展规律及传热的影响，发现了现有浮升力判据存在拟气区表现效果不佳、拟液区与拟气区数值无一致性、其峰值与壁温差峰值位置无相关性等局限性，随后对现有判据进行了修正，优化后的判别式较已有判据更准确且适应性更广。其次，指出了回弯明显影响弯内及下游传热的临界条件为无量纲格拉晓夫数大于20。高热流比、强浮升力条件下引入转向回弯后局部换热系数最大增幅可达240%，局部曲率的引入大大增强了下游的传热情况，使该下游区域的平均传热速率提高20%以上，影响区域可延伸至下游90倍管径处。与直管中相同加热部分相比，弯曲段内质量流速的降低或曲率的增加都会导致平均传热显著增强，研究范围内最高可达53%。随后，详细分析了流场中浮升力、离心力引起的漩涡生成、重构和再生过程，指出U形回弯的引入打乱了混合对流原本的持续发展状态。下游热流场的再发展类似于管道进口处的热边界层初发展，更大的曲率会更彻底地破坏原始的热流场，并使原先的混合对流流场进行更充分的重新发展，阐明了回弯对超临界压力流体热流场演变的扰动机理。最后，获得了对回弯下游传热强化具有较好预测能力的模型，验证了其在弯段绝热等典型换热器应用背景下的适用性。进一步建立了系统热经济性分析及优化模型，从系统层面研究了蒸汽发生器设计对系统性能评价指标的影响，发现对于边界不固定的系统优化问题，蒸发器设计引入的净功输出、单位发电成本等系统性能指标的偏差可达6%；对于固定系统边界问题，蒸发器预测偏差对系统热经济性影响则可忽略。本项目在含弯管道中超临界流体对流换热的实验和模型研究方面取得了较大进展，揭示了局部曲率约束下超临界热流场中二次流动演变与传热的耦合关系，丰富和发展了超临界流体对流换热理论，为跨临界能源系统中此类换热器的设计和开发提供了依据。