合理利用可再生能源与余热资源是解决当前能源危机的有效途径之一。有机朗肯循环（Organic Rankine Cycle，ORC）是利用可再生能源与余热资源的有效手段，但在中高温条件下有机工质易发生热分解危及系统的安全。硅氧烷类工质临界温度较高，安全环保且成本较低，是合适的中高温ORC工质，已在实际机组中得到一些应用。但现阶段针对硅氧烷类工质及其混合物的热稳定性研究还不充分。本文主要以常应用于ORC的链状硅氧烷及其混合物为研究对象，对其热稳定性进行研究。本文利用反应釜系统实验台，对纯工质六甲基二硅氧烷（MM）和八甲基三硅氧烷（MDM）进行了热稳定性的实验研究。实验获得了两者的热分解基础数据，比较了两者的热分解差异，并根据热分解的部分产物，分析了链状硅氧烷的热分解机理。针对系统因素对热稳定性的影响，实验研究了空气、金属、水分对MM热稳定性的具体影响方式。结果表明：MDM热稳定性较MM差，分解反应的途径多样；系统因素能促进MM热分解。采用模拟计算的方式对硅氧烷混合工质进行研究。首先建立了ORC系统模型，以蒸发温度与冷凝温度为决策变量，热效率和输出功率为目标函数，基于遗传算法的多目标优化方法，对硅氧烷混合工质进行优选。研究表明：浓度比合适的混合工质，例如混合工质MM/MDM（0.8/0.2）和MM/MDM（0.6/0.4），不仅具有较优的性能，而且最优蒸发温度也相对较低，更不易发生热分解。基于实验与多目标优化的结果，选择混合工质MM/MDM（0.6/0.4）为研究对象，研究其热分解对ORC系统的影响。建立换热器模型，通过额定设计工况确定了换热器的结构参数。结合热分解规律，分别忽略和考虑气态分解产物，进行非设计工况的计算，分析了热分解对系统的影响情况。研究发现：混合工质MM/MDM（0.6/0.4）热分解对系统工况点、冷热源出口温度及换热量、系统的性能都有较大的影响。特别地，若分解产生了不凝性气体，将会对系统性能产生很大的影响。本文的研究可以为中高温ORC系统的设计和工质筛选提供理论基础，完善系统的安全使用要求，推动了有机工质热稳定性的研究进展。

Reasonable use of renewable energy and waste heat resources is one of the effective ways to solve the current energy crisis. The Organic Rankine Cycle (ORC) is an effective means to utilize renewable energy and waste heat resources, but organic working fluids are prone to thermal decomposition under medium and high temperature conditions and endanger the safety of the system. Siloxanes with high critical temperature, safety, environmental protection and low cost are suitable working fluids for medium and high temperature ORC, which have been used in some practical units. However, the research on the thermal stability of siloxanes and their mixtures is not sufficient at this stage. This article mainly studies the thermal stability of the chain siloxanes and their mixtures commonly used in ORC.In this paper, the experimental study on the thermal stability of pure working fluid hexamethyldisiloxane (MM) and octamethyltrisiloxane (MDM) was carried out by using the reactor system. The experiment obtained the basic data of thermal decomposition of the two, compared the thermal decomposition difference of the two, and analyzed the thermal decomposition mechanism of the chain siloxanes according to the partial products of thermal decomposition. Aiming at the influence of system factors on thermal stability, the experimental study on the specific influence of air, metals and moisture on the thermal stability of MM. The results show that the thermal stability of MDM is poorer than that of MM, the decomposition reaction pathways are diverse, and system factors can promote thermal decomposition of MM.The method of simulation calculation is used to study the mixtures of siloxanes. Firstly, the ORC system model is established, taking evaporation temperature and condensation temperature as decision variables, thermal efficiency and output power as objective functions. Based on the multi-objective optimization method of genetic algorithm, the mixed working fluids of siloxanes are optimized. The results show that the mixtures with proper concentration, such as MM/MDM(0.8/0.2) and MM/MDM(0.6/0.4), not only has better performance, but also the optimal evaporation temperature is relatively low, and it is less prone to thermal decomposition.Based on the results of experiments and multi-objective optimization, a mixed working fluid MM/MDM(0.6/0.4) was selected as the research object to study the effect of its thermal decomposition on the ORC system. The heat exchanger model was established, and the structural parameters of the heat exchanger were determined through the rated design conditions. Combining the law of thermal decomposition, the gaseous decomposition products are ignored and considered separately, and the non-design conditions are calculated, and the effect of thermal decomposition on the system is analyzed. The study found that the thermal decomposition of the mixed working fluid MM/MDM(0.6/0.4) has a greater impact on the system operating point, the temperature of the cold and heat source outlet, the amount of heat exchange, and the performance of the system. In particular, if non condensable gas is produced by decomposition, it will have a great impact on system performance.The research in this paper can provide theoretical basis for the design of medium and high temperature ORC system and screening of working fluids, improve the requirements for safe use of the system, and promote the research progress of the thermal stability of working fluids.