【作者】 刘恒；

【导师】 黄惠兰；

【作者基本信息】 广西大学 ， 热能工程， 2018， 硕士

【摘要】 随着环境问题的日益严峻,具有温室效应的传统制冷剂CFCs及HFCs面临巨大的替代压力,自然工质CO2由于其高效及环境安全性被认为是最有前景的替代制冷剂之一,并且在跨临界循环中的性能表现突出。而使用喷射器来替换跨临界CO2制冷循环中的节流装置不仅能减少节流损失,还能有效提高系统循环效率。为进一步提高系统的性能,对其核心部件喷射器内部流动机理及几何结构进行了优化分析。首先,针对喷射器内部存在的激波效应对喷射器的性能有重要影响,而现有的一维理论模型无法实现对激波的捕捉这一不足。提出在考虑CO2两相喷射器内部存在的非平衡相变、超音速流和激波等复杂流动现象时,建立喷射器的CFD模型来捕捉喷嘴出口处的激波特性,采用空化理论和沸腾相变模型来模拟流体的相变过程。经与文献中的实验验证,沸腾空化模型能够有效分析喷射器的性能,喷射系数的最大误差为5.8%;进行五种湍流模型的对比,其中SSTk-ω模型能很好的预测内部流动。其次,进一步分析临界背压对喷射器性能的影响及其与激波特征间的关系,结果表明:喷射器存在临界背压,当背压低于临界值时,在扩散室入口处混合流体速度达到音速,出现壅塞,此时喷射系数维持在最大值不变,而背压的变化对激波链的影响不明显。随着出口压力的增大,激波链的长度逐渐变短且强度也随之减弱。最后,对设计工况下的喷射器几何尺寸混合段和扩散段进行优化,分析其与激波间的关系,结果表明:1)分析扩散角变化的影响,存在一个使摩擦损失和湍动能损耗都较小的最佳扩散角;2)在一定的工况下存在最佳的混合室长度和直径使喷射系数较高,当混合室直径一定时增加混合室长度有助于增加激波链的长度,但超过最佳混合室长度时,激波的强度逐渐减弱;而在混合室长度不变的条件下,减小混合室直径会使工作喷嘴出口处的激波幅度减弱,扩散室入口处的激波增强,而过多的增加混合室直径会使混合室的作用降低,失去升压效果。本文所建立的模型及方法有助于解释实验结果并可作为CO2两相喷射器的设计工具。更多还原

【Abstract】 With the increasingly serious environmental problems,traditional refrigerant CFCs and HFCs with greenhouse effect are facing a huge alternative pressure.The natural working fluid CO2 is considered as one of the most promising alternative refrigerants because of its high efficiency and environmental safety.And its performance in the transcritical cycle is outstanding.The use of ejectors to replace throttling devices in transcritical CO2 refrigeration cycles not only reduces throttling losses,but also effectively improves system cycle efficiency.In order to further improve the performance of the system,the internal flow mechanism,and geometry of the ejector of its core components were studied.First of all,the shock effect that exists inside the ejector has an important effect on the performance of the ejector,and the existing one-dimensional theoretical model cannot achieve the capture of the shock.For this deficiency,in the process of non-equilibrium phase change.supersonic flow and shock waves phenomena et al.,a CFD model of C02 two-phase ejector was built to capture the shock characteristics of the supersonic nozzle.The Cavitation and boiling phase transition models was used to simulate the phase change of fluids.Compared with the experimental results in the literature,the established model could accurately predict the performance of the ejector,the maximum error of the entrainment ratio was 4.3%.Five RANS turbulence’ models have been compared,as a result,the SST k-co model shows better performance.Secondly,the model was used to further analyze the influence of critical back pressure on jetting performance and its relationship with shock wave characteristics.The results show,the ejector has a critical back pressure,when the back pressure is lower than the critical value,the velocity of the mixed fluid reaches the speed of sound at the inlet of the diffusion chamber,and a fouling occurs.At this time,the entrainment ratio is maintained at the maximum value and the change of back pressure has no obvious effect on the shock wave chain.As the pressure of the outlet increases,the length of the shock chain gradually becomes shorter and the strength gradually weakens.At last,the model is used to study the geometry on the ejector performance and analyze its relationship with shock waves.The results showl)with the change of the diffusion angle,there is a balance between friction loss and turbulent kinetic energy loss,corresponding to the optimal value of the diffusion angle.2)Under certain conditions,there is an optimum mixing chamber length and diameter to maximize the entrainment ratio,increasing the length of mixing section with a fixed mixing section diameter would increase the length of the shock chain.But when the optimum mixing chamber length is exceeded,the intensity of the shock wave begun to decrease;when the length of the mixing chamber is constant,decreasing the diameter of the mixing chamber will weaken the shock intensity at the outlet of the working nozzle and increase the shock at the inlet of the diffusion chamber,while excessively increasing the diameter of the mixing chamber will reduce the function of the mixing chamber,losing pressure lift effect.The model and method helps to interpret the experiment results and shows clear potential for its utility as design instrument of two-phase flow of CO2 ejector.更多还原