核电是解决能源紧缺和环境污染问题的一项重要技术。高温气冷堆属于第四代核反应堆，相比上一代核反应堆具有更高的堆芯出口温度，发电效率更高，安全性更好。在高温气冷堆堆芯出口温度的工作范围内，超临界CO2布雷顿动力循环的效率高于蒸汽动力循环和氦气布雷顿循环，此外，超临界CO2布雷顿动力循环系统结构简单，设备紧凑，适合中小型反应堆、模块化球床反应堆等应用场所。超临界CO2布雷顿循环系统透平出口温度较高，因此需要在系统中设置中间换热器、回热器以及预冷器以提高系统热效率，并且在系统中这些换热器是体积最大、成本最高的设备。因此，换热器的性能直接影响到超临界CO2布雷顿循环系统的效率，是该系统的关键设备之一。印刷电路板换热器（Printed Circuit Heat Exchanger，简称PCHE）具有高效、紧凑以及耐受高温高压的特点，具有替代传统换热器成为超临界CO2布雷顿循环系统中换热器选型的重大潜力。本文主要对PCHE中超临界CO2冷却条件下的流动换热问题开展研究，分别针对水平直流道PCHE和水平Z字形流道PCHE，通过数值模拟的研究方法揭示了超临界CO2在冷却条件下变物性、浮升力以及流道结构对其流动换热的影响机理和规律。研究结果表明，超临界CO2物性的变化影响局部的换热特性，浮升力会增强水平流道内的换热。Z字形流道的折角在提高换热能力的同时也会增大压力损失。在数值模拟结果的基础上，本文建立了PCHE中冷却条件下超临界CO2流动换热的半经验理论模型（准则关系式），为PCHE的设计和优化提供了理论基础和设计依据。此外，本文采用一般化平均温差（Generalized Mean Temperature Difference，简称GMTD）计算方法对PCHE进行设计计算，该方法相比于传统的对数平均温差法（Logarithmic Mean Temperature Difference，简称LMTD）更适用于变物性工质换热器的设计计算和评价。本文验证了GMTD方法对超临界CO2换热器设计计算的准确性，并进一步对比分析了GMTD法和LMTD法用于超临界CO2换热计算的差异。基于GMTD方法，本文还进行了典型预冷器工况下的直流道、Z字形流道PCHE以及管壳式换热器设计计算，结果表明PCHE相比传统的管壳式换热器具有更高的紧凑度和更好的综合换热性能。
Nuclear power is an important technology to solve the problems of energy shortage and environmental pollution. High Temperature Gas-cooled Reactor (HTGR) is one of the alternative reactor types of the fourth generation advanced nuclear power system. HTGR has higher core outlet temperature than the third generation reactors, such as PWR and BWR, which can effectively increase power generation efficiency, while also being safer and more economical. The supercritical carbon dioxide (S-CO2) Brayton power cycle is more efficient than the steam Rankine cycle and the helium Brayton cycle within a specific operating temperature range of the HTGR core outlet. In addition, the S-CO2 Brayton power cycle system has simple structure and compact equipment. It is suitable for small and medium-sized reactors, modular pebble-bed reactors and other applications. The turbine outlet temperature of S-CO2 Brayton cycle is high due to the low compression ratio, so it is necessary to provide intermediate heat exchangers, recuperators and pre-coolers in the system to improve the thermal efficiency. These heat exchangers are the largest and most costly equipment in the system. Therefore, the performance of the heat exchangers directly affects the efficiency of the S-CO2 Brayton cycle system and they are key devices of this power system. Printed circuit heat exchanger (PCHE) is efficient, compact and with wide operating pressure and temperature ranges. It has the potential to replace traditional heat exchangers, such as shell-and-tube heat exchanger, to be the most promising candidate for the heat exchangers in the S-CO2 Brayton cycle system.The flow and heat transfer characteristics of S-CO2 under cooling condition in PCHE with horizontal seimicircular straight channels and zigzag channels were investigated in the present thesis. The numerical simulation reveals the effect of variable thermophysical properties, buoyancy and channel structure on the flow and heat transfer during cooling. Results show that the of thermophysical property variations affect the local heat transfer performance and the buoyancy enhances the heat transfer in the horizontal semicircular channel. The bending angle of the zigzag channel increases the heat transfer performance but also increases the pressure drop along the channels. Based on the numerical simulation data, semi-empirical theoretical models (correlations)for the flow and heat transfer performance of S-CO2 under cooling conditions were established, which provides the basis for the design and optimization of PCHE. In addition, the generalized mean temperature difference (GMTD) method is adopted for the PCHE design. The GMTD method is more suitable to working fluids with variable thermophysical properties sucha as S-CO2. The applicability and the accuracy of GMTD method for heat transfer calculation of S-CO2 are verified. The differences between heat transfer calculation results using GMTD and LMTD methods for S-CO2 are further analyzed. Based on the GMTD method, design calculations of PCHE with straight channels, zigzag channels and shell-and-tube heat exchanger are carried out and compared. Results show that PCHE has higher compactness and better heat transfer performance than traditional shell-and-tube heat exchangers. Moreover, the effects of the operating pressure and temperature, as well as the flow channel structures are analyzed based on the design calculation results.
