Evaluation of Various Ejector Profiles on CO2 Transcritical Refrigeration System Performance

This study examines the potential impact of the different ejector profiles on the CO₂ transcritical cooling system to highlight the contribution of the multi-ejector in the system performance improvement. The research compares the implementation of an ejector-boosted CO₂ refrigeration system over the second-generation layout at a motive flow temperature of 35 °C and discharge pressure of 90 bar to account for the transcritical operation mode. The result revealed a significant energy saving by reducing the input power to the maximum of 8.77% when the ejector was activated. Furthermore, the multi-ejector block could recover up to 25.4% of the expansion work losses acquired by both ejector combinations VEJ1 + 2. In addition, the behavior of the multi-ejector geometries and operation conditions greatly influence the system exergy destruction. The analysis shows a remarkable lack of exergy destruction during the expansion process by deploying the ejector in parallel with the HPV.     

Turbulence Model Comparative Study for Complex Phenomena in Supersonic Steam Ejectors with Double Choking Mode

Scholars usually ignore the non-equilibrium condensing effects in turbulence-model comparative studies on supersonic steam ejectors. In this study, a non-equilibrium condensation model considering real physical properties was coupled respectively with seven turbulence models. They are the k-ε Standard, k-ε RNG, k-ε Realizable, k-ω Standard, k-ω SST, Transition SST, and Linear Reynolds Stress Model. Simulation results were compared with the experiment results globally and locally. The complex flow phenomena in the steam ejector captured by different models, including shock waves, choking, non-equilibrium condensation, boundary layer separation, and vortices were discussed. The reasons for the differences in simulation results were explained and compared. The relationship between ejector performance and local flow phenomena was illustrated. The novelty lies in the conclusions that consider the non-equilibrium condensing effects. Results show that the number and type of shock waves predicted by different turbulence models are different. Non-equilibrium condensation and boundary layer separation regions obtained by various turbulence models are different. Comparing the ejector performance and the complex flow phenomena with the experimental results, the k-ω SST model is proposed to simulate supersonic steam ejectors.     

Experimental Study on a Multi-Evaporator Refrigeration System Equipped with EEV-Based Ejector

This study presents an experimental rig of a multi-evaporator refrigeration system, in which the pressure difference between two evaporators can be maintained by using both the pressure-regulating valve (PRV) and electronic expansion valve (EEV)-based ejector. The proposed EEV-based ejector that is used to partially recover the throttling losses of the PRV consists of an EEV and the main body of an ejector. The established experimental system can work in both PRV-based mode and ejector-based mode by switching the valves. Via experimental means, the performances of both modes were evaluated by varying the cooling loads. Moreover, the effects of the spindle-blocking area percentage of the EEV-based ejector and the condensing temperature on the system performance were identified. The results showed that: (1) the system performance of the ejector-based mode was 3.6% higher than the PRV-based mode; (2) both entrainment ratio and coefficient of performance dropped along with the increase in ejector spindle-blocking area percentage; (3) compared to ejector spindle-blocking area percentage, the condensing temperature had a more evident influence on the system performance.     

变截面超-超引射器启动特性数值研究

为研究不同结构参数与来流参数下变截面超-超引射器的启动特性,用于指导超-超引射器设计与工况调试,采用二维雷诺平均Naiver-Stokes方程,数值研究了引射器混合室不同收缩比、一次流和二次流的不同总压比、总温比下超-超引射器的启动规律,并定义了“启动系数”来判别超-超引射流场是否建立。研究结果表明：随着收缩比（范围0.7～0.9）的增加,超-超引射器启动的临界总压比、总温比均先降低后升高,收缩比0.8时,存在最佳总压比5.88,最佳总温比0.21。结构参数一定,超-超引射器随总温比升高启动难度增加。当超-超引射器处于启动状态下,室压不随总温比、总压比变化而变化,引入的启动系数较引射系数可不依赖具体工况而直接判别超-超引射器是否启动。     

引射系统排气装置的数值模拟

为削弱引射系统尾气对光传输的影响,依据将尾气排放到光传输区域以外的设计思路,运用数值模拟的方法,对引射系统排气管道内的流场特性及尾气射流影响区域开展了研究。通过对直管道以及弯管道内流场特性的数值分析和比较,给出了一种排气管道的结构设计方式。在给定系统出口速度的情况下,运用多组分输运模型,分别对无自然风和有侧向自然风状态下的尾气影响范围进行了分析,初步确定了尾气射流排出系统后的影响区域,并依据尾气影响区域,对管道长度及布置方式进行了研究。     

超声速转子叶片非定常引射器流场特性数值模拟

采用计算流体力学方法,结合适当的边界条件,对超声速转子叶片非定常引射器进行了模拟。从结果可以看出:此类引射器内流态复杂,主气流出口斜激波干扰现象明显,叶片设计参数对引射器性能影响很大。叶片尾部的膨胀波有效诱导了被引射气流,在短距离内增强了气流混合,湍流效应对引射器性能的影响较小;叶片可维持自旋转,提升叶片转速可增强引射效率。最后,对引射器内的流动机理进行了探讨。     

过氧化氢加酒精补燃气体发生器实验研究

介绍了过氧化氢催化分解气体加酒精补燃以提高气体发生器排气温度的理论计算和分析方法,并建立了一套实验研究装置。 装置中质量分数为90%的过氧化氢溶液由高压气挤推,通过催化床完成催化分解,产生高温气体;质量分数为97%的酒精溶液经由分布喷嘴喷入过氧化氢分解气体中发生自燃,从而提高气体温度。实验研究了采用不同酒精喷入形式和参数对气体发生器排气温度均匀性的影响。结果表明：利用沿气体发生器燃烧室周向分布的多喷嘴进行酒精喷注,并与高浓度过氧化氢催化分解气体进行混合燃烧的方法,能够有效提高气体发生器的排气温度,并得到均匀的排气温度场。     

超临界二氧化碳类液-类气区边界线数值分析

超临界二氧化碳(S-CO₂)因在萃取、沉淀、热力循环及化学反应等方面有着十分广阔的应用前景,逐渐成为学术界的重要研究课题.由于在近临界区,可以观察到随温度或压力变化出现大量的物性异变现象,使得各国学者对流体临界点附近区域的研究产生了浓厚兴趣.随着分子动力学模拟技术的快速发展,该技术可辅助传统实验方法用于研究近临界流体的相关物性.为确定S-CO₂在近临界区Widom线范围及类液-类气区的分子结构特征,本文通过分子动力学模拟技术结合聚类分析,研究了温度和压力范围分别在300—350 K和5.5—18.5 MPa下,CO₂密度时间序列变异系数及偏度同Widom线和类液-类气区间的关系.结果表明:S-CO₂在近临界区Widom线的确定可通过连接密度时间序列曲线变异系数极大值点来确定,Widom线沿着临界点开始延伸直到350 K时停止;S-CO₂类液区和类气区的分子分布结构可以用数密度分布的偏度来区分,偏度在类气态时为正值,在类液态时为负值,而在Widom线上达到最大值.     

基于两相喷射器的压缩/喷射制冷系统性能分析

研究了两相喷射器中流体的流动过程,并应用质量守恒、动量守恒、能量守恒对两相喷射器建立了热力学模型,以R134a/ R1234yf为工质,分析了喷射器结构参数以及工况参数对压缩/喷射制冷系统性能的影响。计算结果表明：喷射器存在最佳喷嘴出口面积和最佳喉部面积比使压缩/喷射制冷系统的性能最佳;性能系数(COP)随蒸发温度升高和冷凝降低而升高,而性能系数提高率(COPi)随着蒸发温度降低和冷凝温度升高而升高;相同工况下,以R134a为工质的系统性能系数、制冷量均高于R1234yf;当系统以R1234yf为工质,蒸发温度为5℃,冷凝温度为55℃时,压缩/喷射制冷系统的COP值较传统压缩制冷系统的COP值可提高26%。     

太阳能两级喷射式制冷系统性能的模拟及其分析

讨论了一种新型的太阳能制冷系统——带回热器的太阳能两级喷射式制冷系统。该系统最大的特点是将两个喷射器串联在一起,以提高整个太阳能制冷系统的性能系数。分别采用R134a、R152a、R245ca和R227ea为制冷工质下系统性能系数COP随两级间总压比分配度不同的变化关系。这四种制冷剂的两级压缩比的变化对总喷射系数的影响各有不同,并提出了气体喷射器的总压缩比1.2—2.8之间两级压缩比的不均匀分配原则。