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| 超临界二氧化碳垂直管内流动传热理论与实验研究 摘 要 超临界流体广泛应用于工程技术领域，例如，CO2、H2O和有机工质， 其流动传热特性对工程设计具有重要意义， 但由于超临界流体物理微观和宏观行为机理尚不清晰， 所以其异常的流动传热特性并未得到很好的解决，尤其是传热恶化现象。 按照传统热力学思路，超临界流体是绝对均匀的单相流体， 普遍认为仅在微观上可分为类气和类液两种不同的特性。 但是，最近的实验研究表明，超临界流体在宏观上仍存在类气和类液之间的转换， 且这一过程与拟沸腾理论一致。 然而，拟沸腾现象还未引起足够的重视， 拟沸腾理论也同样没有系统的用于超临界流体流动和传热过程研究。 目前，关于超临界流体的传热恶化现象研究已进入了瓶颈期， 难以有进一步的较大突破。 本文重点基于拟沸腾理论，对超临界流体流动传热过程进行理论分析和实验研究， 围绕超临界流体垂直管内向上加热流动过程中 的换热、流动压降、以及流动不稳定特性三个方面展开研究， 阐述拟沸腾作用超临界流体流动传热机制，并提出新的无量纲数对其定量分析。 通过理论分析了浮升力和流动加速效应对流动结构和传热过程的影响， 并将常用的浮升力和流动加速效应准则与实验数据进行了比较， 进一步揭示了传统单相流体概念对超临界流体传热恶化研究的不足。 着重基于拟沸腾理论， 研究了超临界流体强制对流换热过程中内部流 动传热场的特性与已知参数和物性之间的联系和规律， 提出了一个新的传热分析方法。 假设类液和类气转换过程不均匀的情况下， 从经典的量纲分析和亚临界过冷沸腾理论模型出发， 对超临界流体传热过程进行了量纲分析，得出了一系列无量纲数， 其中π表征了拟沸腾如何作用传热恶化过程，当类气膜在近壁区生长的较快时， 大量的热量聚集，而π13表征了较大的壁面附近的温度梯度， 使类气膜覆盖在壁面上，流动接近亚临界过冷沸腾， 当核心的冷类液不能充分润湿热壁面时，传热恶化。 新无量纲数较好的诠释了拟沸腾诱导传热恶化机制， 解释了超临界压力下的流体传热恶化和亚临界压力下过冷沸腾之间的相似性， 为超临界拟沸腾传热研究提供了理论依据。 超临界流体的传热恶化起始点和局部壁温预测是相关动力循环应用中常常遇到的问题， 但没有完全被突破。 对垂直加热管内超临界CO2强制对流换热过程 中的传热恶化起始点和局部壁温分布特性进行了实验研究， 包括8 mm、10 mm和12 mm，全面揭示了管径对超临界流体过程传热的影响， 实验表明管径对传热恶化起始点没有影响，但是对恶化程度有影响， 影响程度与其他参数有关。 将超临界CO2传热恶化起始点判断准则推广到H2O、R134a和R22， 三种工质的临界值分别为2.018×10-4、1.653×10-4和1.358×10-4， 保证了不同超临界流体用于相关动力循环中的安全。 依据拟沸腾作用传热恶化机制， 提出可以预测传热恶化发生时壁温峰值位置和飞升值的新方法，实验数据发现， 以主流温度为定性温度的Pr对超临界流体传热过程影响很重要， 而非考虑壁面温度的平均概念。 除此之外，系统的研究了超临界CO2强制对流过程中的壁温多峰现象。 近年来，对超临界压力下的流体通道内的传热性能关注比较多， 而流动压降特性报道相对较少。 对于超临界流体的流动阻力研究，通常是考虑物性对其流动过程的影响，但是， 对传统的等温流体经验关联式简单的修正并不是有效的方法。 本文通过实验调查了高温、高压下超临界CO2 在管内垂直向上流动过程中的流动压降特征， 压力p、质量流速G和热流密度qw分别为7.523 MPa、5001500 kg/(m2•s)和15400 kW/m2， 采用8、10和12 mm三种内径。 依据拟沸腾理论对超临界流体的流动压降特性进行分析， 超临界流体的摩擦压降pf可以表达为（dpf/dx）/（dpf,iso/dx）=f（p，qw，G， Tb/Tw）的形式，Tb和Tw分别为主流温度和壁面温度，pf,iso为等温流体摩擦压降， 提出了超临界流体传热与摩擦压降的新比拟准则， 并发展了预测超临界流体摩擦压降的新关联式。 虽然超临界压力下的流体不存在气、液一级相变过程， 但由于物性在拟临界点附近剧烈非线性变化，使其仍可能发生剧烈不稳定流动。 目前，关于超临界流体不稳定流动类型和发生机理没有统一的认识， 主要原因是超临界流体的流动不稳定影响因素较多。 本文通过实验研究了超临界CO2在垂直加热单管内的流动不稳定现象， 详细调查了流动不稳定的基本特征、影响因素、以及发生条件， 根据流动传热特征和通道内出口温度与拟临界温度Tpc相对大小， 将流动传热分为5种类型，给出了流动和传热稳定边界， 实验观察到的超临界CO2流动不稳定仅发生在传热恶化后， 且出口温度大于和靠近拟临界温度时，流动不稳定与拟沸腾传热有关， 根本原因是由通道内的径向和轴向密度分布不均导致的。 关键词：超临界流体； 拟沸腾； 传热恶化； 摩擦压降； 流动不稳定； 临界热流密度 Abstract Supercritical fluids such as CO2, water and organic fluids are frequently applied in power systems. The accurate prediction of flowand heat transfer is important to keep the safety operation of advanced power systems such as solar driven supercritical carbon dioxide Brayton cycle. However, due to that the physical micro- and macroscopic behaviors of supercritical fluids are still open, the heat transfer and flow mechanism of supercritical fluids has not been well revealed. It is widely believed that liquid-like and gas-like supercritical fluid are two phases distinguishable at molecular scale. Only recently, it became clear that the transition from liquid-like to gas-like supercritical states macroscopicly when crossing the Widom line (WL) is detected by experiment successfully, and explained based on the pseudo-boiling concept. However, the pseudo-boiling phenomenon has not attracted enough attention, and the pseudo-boiling theory has also not been comprehensibly used to study the flow and heat transfer of supercritical fluids. At present, the research on heat transfer deterioration (HTD) of supercritical fluids has encountered a lot of bottlenecks, and it is difficult to make further breakthrough. This paper, based on the theory of pseudo-boiling, heat transfer and flow of supercritical fluids is researched by theory and experiment and focusing on heat transfer, pressure drop, and flow instabilities. The heat transfer mechanism of pseudo-boiling is described, and a set of dimensionless numbers are proposed to analyse supercritical fluid heat transfer quantitatively. The effects of buoyancy and flow acceleration on the flow structure and heat transfer process are analyzed theoretically, and the criteria commonly used of buoyancy and flow acceleration are compared with the experimental data, which further reveals the shortcomings of the traditional concept of single-phase fluid in the study of supercritical fluid heat transfer deterioration. Based on the pseudo-boiling theory, the relationship between the characteristics of heat transfer field, known parameters and physical properties in the forced convection heat transfer of supercritical fluids is revealed. On the assumption that the transition from liquid-like to gas-like heterogeneous, an analysis method for pseudo-boiling heat transfer is developed from classical dimensional analysis and subcritical subcooled boiling theory of models. To analyze the pseudo-boiling resulting in HTD of supercritical fluid, a set of dimensionless numbers respectively are proposed to explain the anomalous heat transfer characteristics in vertical upward heating flow. The larger gas-like conversion rate promotes the rapid production of more high-temperature fluid in the near-wall region is characterized by π, and the larger temperature gradient causes gas film to cover the wall surface is characterized by π13. Heat transfer deterioration may be occuring when the cooler liquid-like fluid of the core region cannot rewet the hot wall adequately, and approaching subcritical supercooled boiling. The new dimensionless numbers can explain heat transfer deterioration of supercritical fluid flow induced by pseudo-boiling successfully, and the similarity between heat transfer deterioration of supercritical fluid and subcritical subcooled boiling, which provides a theoretical analysis method for the study of supercritical pseudo-boiling heat transfer. The onset of heat transfer deterioration and the prediction of local wall temperatures of supercritical fluids are often encountered in the applications of related dynamic cycles, but have not been completely solved. The onset of heat transfer deterioration and the distribution of local wall temperatures in forced convection of supercritical CO2 in a vertical heating tube are investigated experimentally, especially the effect of tube diameter, including 8 mm, 10 mm and 12 mm. The effects of pipe diameter on heat transfer is fully revealed, and the experiment shows that the pipe diameter has no effect on the onset of heat transfer deterioration, but has effect on the degree of heat transfer deterioration, which is related to other parameters. The onset criterion of supercritical CO2 heat transfer deterioration are extended to H2O, R134a and R22 to ensure the safety of them used in related dynamic cycles, which is 2.018×10-4、1.653×10-4和1.358 ×10-4. Based on the mechanism of heat transfer deterioration induced by pseudo-boiling, a new method is proposed to predict the location and value of wall temperature peaks when heat transfer deterioration occurs, and the mechanism of multiple wall temperature peaks in forced convection heat transfer is explained in vertical heating tube. The experimental data show that Pr, which is determined by bulk temperature, has an important effect on the heat transfer of supercritical fluid, rather than considering the average concept of wall temperature. In recent years, more attention has been paid to the heat transfer in the flow channel under supercritical pressure, but the flow pressure drop characteristics are relatively less reported. The effect of physical properties is usually considered in the study of the flow pressure drop of supercritical fluids. However, it is not an effective method to corrected the traditional empirical correlations of isothermal fluids simply. In this paper, the flow pressure drop of supercritical CO2 in the vertical pipe under high temperature and high pressure are investigated experimentally. The pressure p, mass flux G and heat flux qw were 7.523 MPa, 5001500 kg/(m2•s) and 15400 kW/m2, respectively, and inner tube diameter covering 8, 10, and 12 mm. The influences of various parameters on the frictional pressure drop are discussed. The friction pressure drop pf of supercritical fluid can be expressed in the form of (dpf/dx)/(dpf, iso/dx)=f(p，qw，G，Tb/Tw). Based on the pseudo-boiling theory, the flow resistance of supercritical CO2 is analyzed, and a new analogy criterion for heat transfer and frictional pressure drop of supercritical fluid is develop. A new correlation for predicting frictional pressure drop is proposed. When flow instability occurs, it is very disadvantageous to heat transfer and stable operation of equipment. Although there is no first-order gas-liquid phase transition under supercritical pressure, it is still possible for supercritical fluids to undergo violent unstable flow due to the dramatic nonlinear changes of its physical properties near the pseudo-critical point. At present, there is no unified understanding on the type and occurrence mechanism of the flow instability of supercritical fluids, the main reason is that there are many factors affecting the flow instability of supercritical fluids, and the comprehensively experimental research is less than the theoretical and numerical research. In this paper, the flow instability phenomenon of supercritical CO2 in the vertical heating single tube is studied experimentally. The essential characteristic, influencing factors and occurrence conditions of the flow instability are investigated in detail. The flow stability boundary are given, and the mechanism of the flow instability observed in the experiment is explained. Based on characteristics of heat transfer or flow, outlet temperature and the pseudo-critical temperature relative magnitudes, flow and heat transfer can be divided into five types, stable boundary are given. The experiment observed the supercritical CO2 flow instability occurs only after heat transfer deterioration, and the outlet temperature is greater than and near the pseudo-critical temperature. The flow instability associated with pseudo-boiling heat transfer, caused by radial and axial density inhomogeneous distribution in the channel. Keywords: supercritical fluid; pseudo-boiling; heat transfer deterioration; friction pressure drop; flow instabilities; critical heat flux |
