高超声速飞行器及发动机的热环境非常恶劣，结构防热是关键技术。再生冷却是目前最常用的主动热防护方式之一。然而随着飞行马赫数以及发动机性能的不断提高，飞行器及发动机的总温与热载荷不断增加，结构防热难度持续增大。因此需要在有限的冷却剂流量条件下尽可能地提高再生冷却通道的对流传热效率。提高传热效率的有效途径之一是对冷却通道结构进行优化，在其壁面上构建凹陷窝、微肋等微小结构，通过产生旋涡结构，改变冷却通道的流动形态，达到提高对流传热性能的目的。凹陷窝和微肋是两种常见的在壁面上构建的微小结构，其形状简单可以通过精密机械加工、3D打印等先进制造技术实现。流体流过凹陷窝和微肋时会形成复杂的三维旋涡结构，增强了湍流脉动，提高了对流传热性能，已被应用于能源、电子和制冷等领域。

目前国内外关于微结构流动与传热的研究，绝大多数针对空气或水等简单物质，并且研究对象的通道尺度较大，多为厘米量级。随着燃料再生冷却技术在高超声速飞行器与发动机领域的应用需求，冷却通道微结构的强化传热特性越来越受到关注。此时，冷却介质是复杂的碳氢燃料，如航空煤油，其物性复杂多变，并且经历液态、气态、超临界态等多物态变化。航空煤油的物性参数在临界点附近会发生剧烈变化，其超临界流动与传热出现很多特殊变化，如流动失稳和传热恶化。因此，本文以再生冷却的微结构强化传热为研究背景，以航空煤油为研究对象，采用数值仿真与实验研究相结合的方法，研究凹陷窝、微肋及其组合结构对毫米级小尺度矩形再生冷却通道内煤油流动及传热特性的影响规律。

本文首先采用雷诺平均方法对两种典型的微结构进行了参数化研究，分别揭示了凹陷窝和微肋导致的旋涡演化特性以及强化传热机理，得到了关键几何参数、煤油流动参数以及壁面热流参数对流阻与传热性能的影响规律。研究表明，随着窝深的增加，凹陷窝结构的传热增益和流阻增益均呈现非单调的变化规律，综合传热因子也随窝深先增大后减小。而微肋结构的传热增益和流阻增益均随着肋高逐渐增大，但综合传热因子随肋高不断下降。尽管超临界煤油会发生传热恶化，但是凹陷窝结构可以减弱超临界传热恶化的影响，而微肋结构可以完全消除传热恶化，并且这两种微结构都可以明显增强超临界煤油的传热性能。基于参数化数值计算结果，进一步分析带有凹陷窝结构和微肋结构的矩形通道煤油流动与传热关系，获得努塞尔数和摩擦因子随几何参数以及流动参数的变化关系式。

采用煤油流动与传热的辐射加热系统，对不同微结构的煤油流动与传热特性开展了实验研究，验证了凹陷窝、微肋及其组合结构均可以显著提高煤油的对流传热性能。通过实验数据，分析了不同微结构对矩形通道内煤油流动和传热的影响，比较了不同微结构的强化传热效果、压力损失增益以及综合传热性能。实验结果表明，凹陷窝结构的优势体现在压力损失更小，而微肋结构的增强传热特性更好并且具有更优的综合传热性能。凹陷窝和微肋的组合结构可以发挥两者各自的优势，其中微肋位于凹陷窝上游的组合方式具有更好的强化传热效果，而凹陷窝在上游的组合方式流阻增益更小。根据不同组合微结构通道的努塞尔数和摩阻系数的实验数据，采用多元线性回归的方法获得了不同组合结构的传热努塞尔数与摩阻系数的预测公式。

本文进一步综合考虑了煤油的湍流流动与传热、固体的热传导过程以及真实三维冷却结构所具有的进出口汇流特性，采用流/固耦合计算方法对不同实验件构型的煤油流动与传热特性展开了数值仿真。计算结果与实验数据很好地吻合，说明了计算方法的准确性。流/固耦合计算得到了不同微结构的壁面温度和压力分布，比较了不同横截面上的流线、速度和湍动能的变化规律，从而揭示了不同微结构的强化传热机理。研究发现，组合结构可以增加旋涡结构，改善速度和湍动能的分布，从而提高传热效果、降低壁面温度，而且还能够减少流动阻力的增加，故组合结构的传热性能优于凹陷窝结构，同时压力损失小于微肋结构。

Thermal protection is required for cooling combustion chamber, because hypersonic vehicle and engine are subjected to severe thermal loads from aerodynamic heating and heat releasing in combustion. Regenerative cooling is one of the most effective thermal protections for rocket and scramjet engine. However, as the flying Mach number and engine performance continue to improve, the total temperature and heat flux on the combustion chamber have further increased. Therefore, it is quite necessary to enhance the convective heat transfer efficiency in the cooling channel with limited mass flow rate of coolant. Micro-structures such as dimples and micro-ribs are constructed on the walls of the channel to optimize the structure in cooling channel structure. These micro-structures improve convective heat transfer performance by changing flow state and generating strong vortex in the cooling channel. Dimple and micro-rib can be processed by precision machining, laser cutting, 3D printing (additive manufacturing) and other technologies. The distribution caused by dimples or micro-ribs can enhance turbulence and cause complex three-dimensional vortex as well as improve the convective heat transfer performance on the walls. Micro-structure enhanced heat transfer technology has been widely used in energy, electronics and refrigeration and other fields.

At present, previous research on micro-structure mostly focused on the heat transfer characteristics in simple fluid such as water or air. The heat transfer properties are the main research objects and the details of the flow field have not been sufficiently studied. The parameters of the micro-structures are mostly in the order of centimeters. With the application of fuel regenerative cooling technology in engines, the enhanced heat transfer characteristics of the cooling channel using micro-structures are paid more and more attention. At this time, the fluid medium is complex hydrocarbon fuel, such as aviation kerosene, whose physical properties change with temperature and will undergo various state changes, such as liquids, gases and supercritical states. The physical properties of aviation kerosene change drastically near the critical temperature, and there are many special changes in its supercritical flow and heat transfer, such as flow instability and deterioration of heat transfer. Therefore, it is necessary to study the aviation kerosene heat transfer performance in channels with micro-structure. In the present paper, the micro-structure and thermal properties of kerosene on the flow and heat transfer characteristics in millimeter-scale small rectangular channels are studied by combining numerical simulation and experimental research.

Two typical micro-structures, dimples and micro-ribs, are parametrically studied by the Reynolds averaging method. The evolutionary characteristics of vortex and the mechanism of enhanced heat transfer are studied. The influence with different geometric structure parameters, different kerosene flow parameters and different wall heat flux parameters on heat transfer performance and flow field is obtained. The results show that the heat transfer and flow resistance of the dimple show non-monotonic changes with the increase of depth. While with the increase of height, the enhanced heat transfer effect of the micro-ribs gradually increases, and the friction resistance also increases accordingly, leading to a decrease in the comprehensive heat transfer performance. Although the heat transfer performance of supercritical kerosene will deteriorate with the increase of wall heat flux, dimples can partially weaken the influence of supercritical heat transfer deterioration, while the micro-rib can completely offset the impact of heat transfer deterioration, and both micro-structures can significantly enhance the heat transfer of supercritical kerosene. Through the analysis of numerical results, the heat transfer performance of kerosene in rectangular channels with dimple structure or micro-rib structure is focused, and fitting formulas of Nusselt number and friction coefficient with geometric parameters and Reynolds number are obtained.

The enhanced heat transfer characteristics of different micro-structures are studied experimentally by using radiant heating based on a kerosene flow and heat transfer experimental system. The effects of different micro-structures on kerosene flow and heat transfer in rectangular channels are analyzed, and the changes of related flow and heat transfer parameters with Reynolds numbers are obtained. The enhanced heat transfer, increased pressure loss and comprehensive heat transfer performance of different micro-structures are also compared. Experimental results indicate that the micro-structure can strengthen the convective heat transfer in kerosene flow, but different micro-structures have different characteristics. The dimple structure cause smaller pressure loss and the micro-rib structure further enhance heat transfer performance. Composite micro-structures can combine the different advantages of dimple and micro-rib. The arrangement of micro-ribs in the upstream of dimples has stronger heat transfer effect. According to the experimental results of Nusselt number and friction coefficient with different composite micro-structure in cooling channels, the existing data are fitted by multiple linear regression methods, so as to obtain prediction formulas of the heat transfer and kerosene flow for the micro-structure with specific geometric parameters.

Based on the existing experimental configuration and typical working conditions, taking into account the turbulent flow of kerosene, the convective heat transfer of micro-structure and the solid heat conduction, the kerosene flow and heat transfer characteristics in the cooling channel with different micro-structures are numerically simulation by using the fluid-solid coupling analysis method. The calculated results are in good agreement with the experimental data, which further demonstrates the accuracy of the numerical simulation. In addition, the wall temperature and pressure distributions of different micro-structures are obtained, and the variation of streamline, velocity and turbulent kinetic energy on different cross-sections are compared, so as to analyze the evolution characteristics of flow field and vortex structure and reveal the mechanism of enhanced heat transfer in different micro-structures. It is found that the combined structure can increase vortex structure and improve the distribution of velocity and turbulent kinetic energy, which can reduce the wall temperature and reduce flow resistance. Therefore, the combined structure obtains a better heat transfer performance and lower pressure loss compared with dimple structure and the micro-rib structure.