Flow visualization of supersonic two-phase transcritical flow of CO2 in an ejector of a refrigeration system

Supersonic two-phase flow of CO₂ in an ejector was investigated in flow visualization experiments. The ejector was installed in a transcritical CO₂ ejector-expansion refrigeration system with a convergent primary nozzle and a rectangular mixing chamber. The flow fields in the suction chamber and the mixing chamber of the ejector were visualized by direct photography for various operating conditions. The results showed that the liquid in the primary flow after the primary nozzle exit increased with increasing primary flow and secondary flow pressures. The expansion angle of the primary flow at the nozzle exit decreased with increasing secondary flow pressures. The primary flow was blocked in some cases at the mixing chamber entrance due to the large expansion angle which reduced the entrainment performance. The entrainment ratio was inversely related to the expansion angle. The primary and the secondary flows had a short mixing region in the mixing chamber with the mixed flow quickly becoming uniform.     

An experimental study and 1-D analysis of an ejector with a movable primary nozzle that operates with R236fa

The nozzle exit position is an important structural parameter of an ejector. Research shows that movable primary nozzle enhances performance of ejectors. The present work experimentally studied the critical entrainment ratios of an ejector at different nozzle positions, finding that the critical entrainment ratio first increases with the nozzle exit position and then remains constant. A 1-D model proposed to explain this phenomenon found that in a certain range, the critical entrainment ratio is determined by choking of the secondary flow in the converging cone. When the nozzle moves upstream, the mixed flow reaches sound speed at the constant area section, and the critical entrainment ratio remains unchanged.     

Numerical assessment of steam ejector efficiencies using CFD

Ejector efficiencies for the primary nozzle, suction, mixing and diffuser were determined for the first time, according to their definitions, using an axi-symmetric CFD model. Water was considered as working fluid and the operating conditions were selected in a range that would be suitable for an air-conditioner powered by solar thermal energy. Ejector performance was estimated for different nozzle throat to constant section area ratios. The results indicated the existence of an optimal ratio, depending on operating conditions. Ejector efficiencies were calculated for different operating conditions. It was found that while nozzle efficiency can be considered as constant, the efficiencies related to the suction, mixing and diffuser sections of the ejector depend on operating conditions.     

Maximum mass flow ratio due to secondary flow choking in an ejector refrigeration system

The occurrence of flow choking in an ejector of an ejector refrigeration system (ERS) was analysed and a model for predicting the maximum flow ratio of the ejector was developed. The multi-parameter equation to calculate the mass flow ratio takes into account the performance of the primary nozzle, the flow entrainment and mixing relating to ejector geometry and operating conditions. We validated the model using the reported experimental data of refrigerant R113, R141b and steam ERS. The present model was shown to provide better accuracy compared with results obtained by applying the existing 1-D ejector theory. We discussed the application of the model and highlighted the significance of the parameters for future work.     

Numerical study on optimization of ejector primary nozzle geometries

In an ejector refrigeration system (ERS), ejector acts as a compressor but without using any moving parts. To some extent, ejector performances are subjected to primary nozzle's geometries with the action of shock waves. In this paper, CFD simulation was conducted to improve the ejector performance by varying the following ejector primary nozzle's geometries and surface roughness: two angles of convergent and divergent portion, three lengths and surface roughness of throat, convergent and divergent portion. The CFD model was validated with the test results of an ERS experimental rig with working fluid of R134a. The optimum geometric parameters and surface roughness of the primary nozzle were obtained with the CFD analysis. The simulation results revealed that the throat and divergent portion of the primary nozzle should be paid more attention when designing ejector since the entrainment ratio of the ejector is rather sensitive to the length and surface roughness of these two portions.     

A dislocation and point force approach to the boundary element method for mixed mode crack analysis of plane anisotropic solids

Abstract

This study investigates the feasibility of enhancing steam‐driven ejector performance. Initially, a one‐dimensional ejector theory is used to examine the effects on ejector performance of three isentropic efficiencies: nozzle efficiency η_m , mixing efficiency η_m, and diffuser efficiency η_m . Theoretical analysis demonstrates that mixing efficiency profoundly affects ejector performance, but that the other two efficiencies have slightly influenced ejector performance. This finding suggests that efficient mixing can promote ejector performance. This study also attempts to improve mixing efficiency using a petal nozzle. The behavior and characteristics of a petal nozzle are investigated by testing the nozzle under various operating conditions, i.e. primary pressure, secondary pressure, and back pressure. In addition, the study compares the experimental and theoretical results. These results prove that using a petal nozzle can improve ejector performance. The shadowgraph method was used to visualize the inner flow field of an ejector. The flow patterns observed should help to further improve ejector performance.     

两级蒸发对跨临界CO_2引射制冷系统影响的实验研究

两级蒸发引射制冷循环中通过二级蒸发器不仅能调节引射器出口干度还能提高系统效率。通过改变第二蒸发器冷冻水流量对两级蒸发引射制冷系统进行实验研究,并与改变引射器面积比的调控效果进行比较。结果表明:在实验工况范围内,气冷器压力、第一蒸发器压力和压缩机流量都随第二蒸发器冷冻水流量的增加而增大;而且引射器面积比越大,气冷器压力越高而蒸发器压力和压缩机流量越低。同时,系统引射系数随第二蒸发器冷冻水流量的增加而降低,而制冷量和COP则升高,尤其是在小引射系数下,系统制冷量和COP提高的更为明显。本研究为引射循环提供了另外一种良好的调控思路。     

基于延迟均衡的CO_2两相引射器模型研究

引射器对跨临界CO_2引射制冷系统性能有极大的影响。本文考虑CO_2两相引射器中存在的非平衡相变、超音速和壅塞等复杂流动现象,构建了CO_2两相引射器的1D分布模型,并采用延迟均衡理论分析喷嘴中的非平衡相变过程。与实验结果比较显示,所建立的延迟均衡模型能够很好的预测引射器的性能。此外,通过与均衡模型的相比显示,在本文所选工况下,延迟均衡模型计算所得的主动流流量比均衡模型预测值低12.39%~25.30%,同时非平衡现象将延缓喷嘴中的膨胀过程,使得喷嘴出口压力比均衡模型预测值高。本文采用所建模型进一步分析了引射器的结构对性能的影响,结果显示在一定的工况下存在最优的混合室直径使得引射系数和升压比都较高;而当混合室直径一定时,较长的混合室有利于提高引射器的升压比。     

Experimental and numerical analysis of a variable area ratio steam ejector

In the present paper, experimental and CFD results for a 5 kW capacity steam ejector with variable primary nozzle geometry are presented and compared. The variable geometry was achieved by applying a movable spindle at the primary nozzle inlet. Operating conditions were considered in a range that would be suitable for an air-conditioning application, with thermal energy supplied by vacuum tube solar collectors. The CFD model was based on the axi-symmetric representation of the experimental ejector, using water as working fluid. The experimental entrainment ratio varied in the range of 0.1–0.5 depending on operating conditions and spindle tip position. It was found that the primary flow rate can be successfully adjusted by the spindle. CFD and experimental primary flow rates agreed well, with an average relative error of 7.7%. CFD predicted the secondary flow rate and entrainment ratio with good accuracy only in 70% of the cases.     

Novel ejector model for performance evaluation on both dry and wet vapors ejectors

A novel ejector model is proposed for the performance evaluation on ejectors with both dry and wet vapor working fluids at critical operating mode. A simple linear function is defined in order to approach the real velocity distribution inside the ejector. Mass flow rates of the primary flow and secondary flow are derived by integrating the velocity function at the inlet section of the mixing chamber. By considering the flow characteristics of the critical-mode operating ejector, the developed model contains only one energy conservation equation and is independent of the flow in the mixing chamber and the diffuser. Experimental data from different ejector geometries and various operation conditions reported earlier are used to verify the effectiveness of the new model. Results show that the model has a good performance in predicting the mass flow rates and the entrainment ratio for both dry and wet vapor ejectors.     

喷射器极限工况特性实验研究

喷射器作为热驱动喷射式制冷系统的核心部件,其性能会影响整个制冷系统的运行效率。极限工况是指喷射器从可以工作状态到不能工作状态的极端工况,对该工况下喷射器的特性研究具有重要意义。本文自行设计并搭建了以R134a为制冷剂的喷射式制冷系统极限工况的实验装置,分别对引射流体质量流量为零的极限工况下不同喷射器工作流体压力及喷射器出口背压对缩放喷嘴出口背压的影响规律进行了实验研究。结果表明:极限工况下,喷嘴出口背压同时受工作流体压力和喷射器出口背压的影响,随工作流体压力升高而降低,随喷射器出口背压升高而升高。同时,得到该喷射器在工作流体压力为1.5~3.2MPa,且喷射器出口背压在0.66~0.96 MPa范围内的最低引射流体压力,为工程应用提供参考。     

Influence of geometrical factors on steam ejector performance – A numerical assessment

A number of factors influence the performance of an ejector. In this work, three geometrical factors – the area ratio between the nozzle and constant area section (r_A), nozzle exit position (NXP) and constant area section length (L_m) – were considered. The theoretical analysis was carried out by a CFD model of a steam ejector using FLUENT. The results indicated the existence of an optimal area ratio, depending on operating conditions. Therefore, a new feature, a spindle in the primary nozzle is suggested to fine tune the primary flow rate depending on operating conditions. The location of NXP influenced both entrainment ratio (λ) and critical back pressure (p_c,crit), showing an optimum at 60 mm from the inlet plane of the ejector mixing section. L_m had little influence on λ; however, longer constant area sections resulted in higher p_c,crit. For an optimal design, a value of L_m = 155 mm was identified.     

Effect of throat diameters of the ejector on the performance of the refrigeration cycle using a two-phase ejector as an expansion device

This paper is a part in a series that reports on the experimental study of the performance of the two-phase ejector expansion refrigeration cycle. In the present study, three two-phase ejectors are used as an expansion device in the refrigeration cycle. The effects of throat diameter of the motive nozzle, on the coefficient of performance, primary mass flow rate of the refrigerant, secondary mass flow rate of the refrigerant, recirculation ratio, average evaporator pressure, compressor pressure ratio, discharge temperature and cooling capacity, which have never before appeared in open literature, are presented. The effects of the heat sink and heat source temperatures on the system performance are also discussed.     

Performance optimization of a transcritical CO2 refrigeration system using a controlled ejector

An on-line optimal quasi cascade controller for an ejector with variable nozzle throat area is proposed to improve the operating performance of the transcritical CO₂ ejector refrigeration system. The optimal gas cooler pressure is tracked in real time by the controller including a tracker and a predictor. Using the system dynamic model, the dynamic responses of the system performance and ejector efficiency under variable nozzle throat area are first analyzed. Then the parameters of the tracker and predictor are determined by simulation respectively which exhibits a good dynamic characteristic with an acceptable settling time. Besides, the controller presents a good robustness under variable compressor speeds and mass flow rate of cooling water. Furthermore, the system performance is actually increased to the maximal value by the controller even at the variable operating conditions. Finally, the optimal controller is verified by experiments to be an effective way to improve the system performance automatically.     

负压钻井液振动筛气液喷射器性能的数值模拟研究

随着石油钻井技术的发展,传统钻井液振动筛不能满足现有工程需求,因此提出了一种新型负压钻井液振动筛。气液喷射器是负压钻井液振动筛的核心设备,通过它在筛网下方形成负压区域,使钻井液受振动和负压的复合作用,增大钻井液透过筛网的能力,因此其性能直接影响负压振动筛的处理效率。为了提高负压振动筛的工作效率,需要对气液喷射器的结构和工况进行合理设计。运用流体动量守恒方程,推导出恒定流动状态下气液喷射器混合室的动量方程及其性能计算方程。运用计算流体力学方法对气液喷射器内部复杂的两相流动过程进行数值模拟,并通过对比数值模拟结果与理论计算结果来验证数值模型的合理性。对不同引射流体液体体积分数、工作气体压力、喷嘴距和喷管面积比下气液喷射器的喷射效果进行数值模拟,结果表明气液喷射器的工作参数和结构参数对其喷射系数和真空度有极大的影响。根据模拟结果可知,在引射液体体积分数为30%、工作气体压力为300 kPa、喷嘴距为60 mm、喷管面积比为3.484的情况下,气液喷射器的性能达到最佳。研究结果为负压钻井液振动筛中气液喷射装置的设计和现场应用提供了理论依据。     

Experimental and numerical investigation of the effect of shock wave characteristics on the ejector performance

The entrainment performance and the shock wave structures in a three-dimensional ejector were investigated by Computational Fluid Dynamics (CFD) and Schlieren flow visualization. The ejector performance was evaluated based on the mass flow rates of the primary and secondary flows. The shock wave structures in the ejector mixing chamber were captured by the optical Schlieren measurements. The results show that the expansion waves in the shock train do not reach the mixing chamber wall when the ejector is working at the sub-critical mode. Decreasing of the shock wave wavelength increases the secondary mass flow rate. A three-dimensional CFD model with four turbulence models was then compared with the experimental data. The results show that the RNG k-ε model agrees best with measurements for predictions of both the mass flow rate and shock wave structures.     

跨临界CO2两相流引射制冷系统性能实验研究

对跨临界CO2两相流引射制冷系统性能进行了实验,分析了工况及引射器几何参数对系统性能的影响,结果表明:在实验工况范围内,跨临界CO2两相流引射制冷系统制冷量和COP随气体冷却器压力的升高而升高,随气体冷却器出口温度的升高而降低。对于使用不同喉部直径喷嘴的系统,在相同工况下,引射器喷嘴喉部直径较大的系统的性能较好。对于使用不同直径混合室的系统,随着气体冷却器压力的升高,使用小直径混合室的系统COP变化较大;当气体冷却器压力较低时,使用大直径混合室的系统COP较高,而当气体冷却器压力较高时,使用小混合室直径的系统性能较好。在相同工况下,与传统跨临界CO2循环进行比较,两相流引射制冷循环系统COP最大可提高14%。     

Performance optimization for a variable throat ejector in a solar refrigeration system

In a solar vapor ejector refrigeration system, the solar heat supply may vary because of variations in solar irradiation intensity, making it difficult to maintain a steady generator temperature. To improve ejector performance, this study proposes a variable throat ejector (VTEJ) and analyzes its performance using CFD simulations. The following conclusions can be drawn. An ejector with a greater throat area and larger solar collector allows a wider operating range of generator temperatures, but may be overdesigned and expensive. Conversely, decreasing the throat area limits the operating range of generator temperatures. Thus the ejector with a fixed throat area may be unsuitable to use solar energy as a heat source. For a VTEJ, this study derives a curve-fitting relationship between the optimum throat area ratio and the operating temperatures. Using this relationship to adjust the throat area ratio, the ejector can consistently achieve optimal and stable performances under a varying solar heat supply.     

Experimental study on the improvement of CO2 air conditioning system performance using an ejector

Several preceding researches have evidenced that the transcritical air conditioning system using CO₂ as a refrigerant has an inherent inefficiency resulting in degraded steady-state system performance of a CO₂ air conditioning system compared with that of a conventional air conditioning system. As a practical improvement, two-phase ejector was considered in place of expansion device in this study. The two-phase ejector for CO₂ air conditioning system was designed and developed considering the non-equilibrium state for evaluating the sonic velocity and the critical mass flux. The experiments of performance with respect to variation of ejector geometry such as the motive nozzle throat diameter, mixing section diameter and the distance between motive nozzle and diffuser were carried out. There exist optimum design parameters in each test. Experiments showed that the coefficient of performance of the system using an ejector was about 15% higher than that of the conventional system.     

Ejector performance analysis under overall operating conditions considering adjustable nozzle structure

The variable-geometry ejector (VGE) is feasible for unstable heat-source utilization; the ejector can be adjusted to its design point to obtain high efficiency. Moreover, as the adjustable nozzle in the VGE significantly affects the performance of the ejector, a theoretical model is necessary to evaluate VGE performance. In this study, a two-dimensional theoretical model was proposed based on an adjustable-nozzle theory. Method of characteristics was employed to accurately predict the driving-flow development in the mixing section. In addition, the suction-flow velocity distribution on the effective area was considered. The proposed model was validated by employing the data from literature and additional experimental data obtained from a VGE test setup using R134a. The validation result shows that the proposed model predicts the ejector performance accurately; moreover, the model is more adaptive while the nozzle configuration changes. The theoretical model, proposed herein, is practical for the design and application of the VGE.