Entrainment Performance Research Articles

Full-operating-condition performance improvement strategy and optimal design of a two-stage ejector with auxiliary secondary flow for MED-TVC systems

Periodic fluctuation of steam-source pressure occurs in the thermal vapor compression multi-effect distillation (MED-TVC) systems. Because ejectors are sensitive to the boundary conditions, as a key component for recovering excess steam in systems, ejectors experience a rapid decline in performance when the steam-source pressure fluctuates and does not satisfy the design condition. This decline negatively impacts the overall system performance and reduces the energy utilization efficiency. Therefore, a two-stage ejector with auxiliary secondary flow (ASF-TSE) and a full-operating-condition performance improvement strategy were developed in this study to enhance the ejector performance under full working conditions and broaden the efficient operation range of ejectors. The flow field characteristics and performance of the ASF-TSE, single ejector (SE), and two-stage ejector (TSE) in the low and design-pressure ranges were compared and investigated. The results showed that within the design-pressure range, the maximum and average increase ratios of the ASF-TSE for entrainment performance compared with the TSE are 17.53 % and 13.43 %. Moreover, the entrainment capacities of the three ejectors were compared and analyzed in the full operating range. Simulation results show that compared with the SE and TSE, the entrainment performance of the ASF-TSE is improved by 87.5 % and 8.0 %, respectively.

Numerical studies on structure optimization and flow characteristics of a hydrogen recirculation ejector under multiple load conditions

Ejectors are ideal hydrogen recirculation devices for PEMFC systems but typically exhibit poor recirculation performance at low loads. For purpose of extending ejector's operating range in vehicular applications, the entraining performance across multiple loads should be focused. In this study, A 40 kW ejector was designed with a 1D theory and optimized using a 2D CFD model. Simulations were conducted to study the influence of various diameter and axial dimensions on the ejector's performance and flow characteristics. The results reveal that as the load increases from 20% to 100%, the suboptimal diffuser exit diameter decreases from 2.5 to 1.5 times the mixing chamber diameter, the optimal nozzle exit position increases from 0.59 to 0.63 times, and the optimal mixing chamber length increases from 2.2 to 4.3 times. A convergent nozzle with a short throat significantly improves low-load entraining performance and achieves the highest performance growth in the variable-nozzle design. Additionally, an optimization method for axial dimensions based on the proposed parameter flow uniformity was established, allowing for precise determination of the optimal dimensions through a few simulations.

Enhanced performance of two-stage ejector based on flow-field coupling effect in MED-TVC systems

Steam ejectors serve as prevalent flow devices in multi-effect distillation systems, collecting excess steam and energy to promote sustainable energy utilization. In contrast to traditional ejectors, two-stage ejectors exhibit superior pressurization and vacuum capacities. However, the flow-field coupling effect, which is closely related to the entrainment performance of the two-stage ejector, is rarely explored. In this study, the flow-field coupling effect of the two-stage ejectors is investigated, and two new strategies are proposed to optimize the ejector performance. A concept of two-stage choking and a design principle for a two-stage choking structure is proposed by analyzing the size matching between two stages based on the flow-field coupling effect. Based on the design principle, the optimal structure parameter can be obtained by evaluating the two-stage choking state. Subsequently, a dynamic pressure control criterion based on the flow-field coupling effect is proposed to improve the entrainment performance. The results show that the entrainment performance of the two-stage ejector based on this control criterion is increased by 29.31 % and 4.15 % respectively when the second-stage primary pressure is 300 kPa and 450 kPa. Moreover, simulation results demonstrate that the upper limit of the entrainment performance is determined by the second-stage primary flow pressure.

A full operating conditions ejector model for refrigeration systems driven by low-grade heat sources

The ejector refrigeration technology driven by low-grade heat sources constitutes as one of the crucial means to solve the energy shortage in present-day society and to reduce the global environmental pollution. Thermodynamics ejector models are the basis of performance prediction, system control and structural design in ejector refrigeration systems driven by low-grade heat sources. However, the majority of present ejector models are unable to simultaneously meet the general feasibility of full operating conditions and the level of prediction accuracy for improving the energy efficiency of the refrigeration system. In this study, a novel theoretical model of ejectors is successfully derived based on the foundation of compound-choking theory and gas dynamic relations, which easily predicts the entrainment performance under the on-design and off-design conditions in ejector refrigeration systems. Indeed, a direct solution algorithm is presented to quickly and accurately solve this model and a linear correlation is employed between the mixing loss coefficient and the back pressure at the subcritical mode. The developed model is verified with experimental results in R245fa, R134a and R600a ejector refrigeration system. The results show that the model provides a precise estimation of the entrainment performance with the mean relative errors of 2.45 %, 5.49 % and 3.67 % for R245fa, R134a and R600a refrigerants at full working conditions, while the prediction of critical condensing temperatures is considerably approximated by the experimental measurements. In the comparative analysis with the previous model, this model demonstrates a superior prediction performance, especially in the subcritical mode using the linear function mixing loss coefficient. This study will be a valuable aid in the improvement of energy efficiency in ejector refrigeration systems.

Theoretical studies on wide-operating-range ejector based on operating conditions and multi-parameter coupling effect in proton exchange membrane fuel cell system

When the ejector in a fuel cell system operates under off-design conditions, its performance will significantly decrease. To improve the performance of the ejector under all operating conditions, this paper proposes a theoretical model for predicting performance and optimizing the structure of the ejector that has not been studied before. Based on the jet theory, a two-dimensional three-segment velocity distribution is established, and the exponent of the velocity function is modified by simulation and experimental data, reducing the model’s calculation error from 15% to 6.5%. By combining the Lagrange algorithm, we investigate the influence of operating conditions and multi-parameter coupling effects on ejector performance, obtaining optimal values for structural parameters under different load conditions. The results show that the optimal value of main nozzle exit position (Lx) increases with increasing current and the optimal value ranges from 0.5 to 6 mm. The influence of dimensionless parameter (λc), dimensionless parameter (λd) and diffusion angle (αd) on ejector performance is significant and optimal values exist. Reducing the nozzle throat diameter (dnt) can effectively promote the entrainment performance at low power, and four adjustment conditions are proposed. When dnt decreases from 3.2 to 2.0 mm, the operating range of the current is increased by 38.42%, the power range is increased by 42.12%, and the pressure drop of the ejector (pd) decreased by 9.66 bar. The theoretical model not only improves the accuracy of predicting the entrainment ratio, but also realizes the study of the influence of multi-structural parameter coupling effect on the performance of the ejector, which leads to a significant improvement in the optimization accuracy and efficiency, and extends the applicable power range of the ejector. This provides a new direction for the future performance optimization of wide operating range ejectors considering multi-parameter coupling effects.

Performance prediction and geometry optimization of ejector in PEMFC system using coupled CFD-BPNN and genetic algorithm

The ejector is a promising hydrogen recirculation device in proton exchange membrane fuel cell (PEMFC) systems. However, the limited entrainment performance of the ejector at wide operating conditions hinders its development and widespread application in PEMFC systems. To address this challenge and design a high-performance ejector adapted to the wide power range of PEMFC systems, a backpropagation neural network (BPNN) model with computational fluid dynamics (CFD) simulation data is developed. The sensitivity analysis of geometric parameters based on the CFD model reveals that the nozzle throat diameter and the mixing chamber diameter exert the most pronounced impact on the entrainment performance, with average influence rates of 24% and 57%, respectively. Furthermore, two advanced multi-objective genetic algorithms are applied to improve ejector performance. The linear weighted genetic algorithm (LWGA) method proves effective in elevating the overall ejector performance, achieving a remarkable enhancement in entrainment performance of up to 7.9%. On the other hand, the non-dominated sorting genetic algorithm (NSGA- II) method is favorable for expanding the operational power range of the ejector by 30%, as well as a 4.5% increase in overall ejector performance. This work provides a robust framework for designing and optimizing high-performance ejectors in high-power PEMFC systems.

Modeling and Control of Ejector-Based Hydrogen Circulation System for Proton Exchange Membrane Fuel Cell Systems

Ejector-based proton exchange membrane fuel cells (PEMFCs) are of great interest due to their simplicity and feasibility. Thus, proton exchange membrane fuel cells are considered the most suitable technology for in-vehicle systems, industrial applications, etc. Despite the passive characteristics of the ejector, active control of the hydrogen supply system is needed to ensure sufficient hydrogen, maintain the stack pressure, and ensure effective entrainment. In this research, a novel semi-empirical model is proposed to accurately predict the entrainment performance of the ejector with an 80 kW fuel cell system. According to the precise semi-empirical model, the hydrogen supply system and the anode channel are modeled. Then, a fuzzy logic controller (FLC) is developed to supply sufficient and adequate gas flow and maintain the rapid dynamic response. Compared to the conventional proportional–integral–derivative controller, the fuzzy logic controller could reduce the anode pressure variability by 5% during a stepped case and 2% during a dynamic case.

Effects of primary flow temperature on phase change characteristics in hydrogen recirculation ejector for PEM fuel cell system

The hydrogen recirculation ejector, known for its low cost and simple structure, has garnered widespread attention in PEM fuel cell applications. As a fluid-driven passive component, the fluid dynamics inside the ejector play a pivotal role in determining its entrainment performance. This study explores the impact of the primary flow temperature on phase change characteristics based on the Euler-Lagrange model. Results indicate that at a primary flow temperature of −40 °C, the mass flow rate of droplets at the outlet reaches 285.2 mg/s, representing a condensation efficiency of 24.1%. Concurrently, condensation elevates the outlet temperature by 31.2 °C, resulting in an 8.8% reduction in the entrainment ratio. The mass flow rate of droplets decreases significantly as the primary flow temperature increases. As the temperature rises to 60 °C, homogeneous droplets vanish entirely. Simultaneously, foreign heterogeneous droplets evaporate by 42.0%, attributed to the negative subcooling degree at the diffuser and outlet pipe. This study emphasizes the significance of controlling the primary flow temperature for the hydrogen recirculation ejector.

Study on the effect of pulsed gas flow on the entrainment performance of hydrogen ejector

The ejector is a crucial component of the hydrogen circulation system of proton exchange membrane fuel cell (PEMFC), which is widely used because of its simple structure, reliable operation, and no additional power consumption. However, the current research on pulsed ejectors is still lacking and in-depth. In this study, CFD modeling is conducted on the ejector, and the pulsed gas flow is simulated by user define function (UDF), and the influence of the pulsed gas flow on the entrainment performance of the ejector is analyzed. The results show that, compared with the steady-state condition, the entrainment performance of the ejector using pulsed gas flow under different primary flow pressures is improved, and the maximum entrainment ratio is 3.195 under steady-state conditions and 3.507 under transient conditions, which is increased by 9.8%. The effect is especially pronounced at low pressure, where the entrainment ratio increases by 16.6%. And by comparing the gas flow of different pulse frequencies, it is found that when the pulse frequency is 1Hz, the ejector has the best entrainment performance.

Numerical investigation in effects of refrigerants’ thermodynamic properties on performance of supersonic ejector

ABSTRACT Using zeotropic mixtures with unique temperature glide characteristics as the working medium in heat-driven ejector refrigeration system will potentially reduce the irreversible losses in heat exchange process. However, the intrinsic link between the pure/mixture refrigerants’ thermophysical properties and the entrainment performance of supersonic ejector has not yet been elucidated. In this study, a novel ejector CFD model is established by coupling the real-gas thermodynamic equation and species transport equation, then the relationships between thermophysical properties of pure/mixture refrigerants and ejector’s dimensionless operational parameters were thoroughly investigated. The results indicated that the ejector expansion ratio rises as primary flow inlet temperature or refrigerant’s normal boiling point increases, while the dimensionless parameters of primary nozzle are relatively insensitive to the variation of primary flow inlet temperature and refrigerant’s boiling point, resulted in changes of supersonic jet expansion behaviour. Moreover, the specific volume and adiabatic index change characteristics resulted in minor fluctuations of primary nozzle’s dimensionless parameters. When the bubble point temperature is fixed at 30℃ in the condenser, the saturated vapor/liquid phase pressure glide characteristics of R600a/R1234ze mixture may lead to an increasement of ejector’s compression ratio.

Performance investigation on the bypass ejector for a proton exchange membrane fuel cell system

The poor entrainment performance of the conventional ejector is a significant problem that makes it hard to use in proton exchange membrane fuel cell (PEMFC) systems. This study aims to evaluate the entrainment performance of a bypass ejector for a 100 kW PEMFC system. The effects of three critical geometric parameters, namely the axial position, width, and angle of the bypass inlet, on the entrainment performance are thoroughly investigated. The results demonstrate that the bypass flow exhibits a significant performance improvement in the critical mode. In contrast, the performance improvement is negligible and even negative in the subcritical mode. After careful evaluation of the entrainment performance across various stack powers, the optimal axial position, width, and angle of the bypass inlet are found to be 1.1, 2 mm, and 10°, respectively. A comparative analysis between the bypass ejector and the conventional ejector underscores a significant advantage for the former, exhibiting a remarkable 22.1 % increase in the hydrogen entrainment ratio at the stack power of 101 kW. Nevertheless, the entrainment performance of the bypass ejector diminishes when operating at low stack powers below 24 kW.

Working condition expansion and performance optimization of two-stage ejector based on optimal switching strategy

Ejectors are widely used as steam power cycle components to recover superfluous water vapor. However, in a system for preparing distilled water used for pharmaceutical injection, the ejector periodically deviates from the design condition owing to fluctuations in steam source pressure. In particular, when the pressure is reduced to approximately 75% of the design value, performance is significantly degraded, affecting water production efficiency. To solve this problem, a two-stage ejector with control switching strategy is proposed in this paper. The performance of a series of two-stage ejector structures with different scale ratios was analyzed, and the optimum scale ratio was determined. Furthermore, an optimal switching strategy is devised to ensure that the two-stage ejector maintains stable performance under different primary flow pressures. The numerical results indicate that the entrainment ratio of the two-stage ejector is 79.4% higher than that of the single-stage ejector when the pressure is 60% of that of the designed primary flow. Moreover, the two-stage ejector can maintain satisfactory entrainment performance when the traditional ejector enters reversed mode under 50% of the designed pressure. The experimental results show that the entrainment performance of the proposed two-stage ejector can be improved significantly when power fluid pressure is insufficient.

Performance and mixing process investigation of a novel mixing-enhanced ejector

Ejector refrigeration systems rarely require electrical energy, satisfying the requirements of sustainable development and low carbon emissions. However, owing to the limitations of the operating conditions, the ejector performance is not sufficient, and there is an urgent need to improve the entrainment performance of the ejector. In this study, experiments and numerical simulations were performed on a conventional ejector, and the numerical simulation results were fitted with the experimental data. A numerical simulation was used to investigate the mixing process and entrainment performance of the novel mixing-enhanced supersonic steam ejector. The optimal values of lm = 120 mm and dc = 57.5 mm achieved the best entrainment ratio (0.778) for the novel ejector under 5%-tab blockage ratio. The results showed that the entrainment performance of the novel ejector improved by 51.1–54.7% and 10.5–12.3% compared to those of the conventional ejector and the conventional ejector with an adjusted mixing chamber, respectively. This was because the generated streamwise vortices entrained more secondary flow into the mixing chamber, and the mixing process between the primary and secondary flow avoided unnecessary kinetic energy loss, increasing the mixing efficiency and accelerating the secondary flow crashing out of the diffuser.

Numerical Study on Flow and Noise Characteristics of High-Temperature and High-Pressure Steam Ejector

Based on the shear stress transfer (SST) k-ω model, Ffowcs-Williams and Hawkings (FW–H) equation, and Lilley sound source equation, the flow and sound field of high-temperature and high-pressure steam ejectors are simulated. The entrainment performance, near-field sound source, and far-field noise of the steam ejector are discussed. The influences of working parameters including the primary steam pressure, the secondary steam pressure, and the back pressure are analyzed. The results show that under the design conditions, the steam ejector has two shock waves and three sound source regions. A shear layer at the boundary of the first shock wave generates the Sound source-I, and the flow separation at the boundary of the second shock wave causes the Sound source-III. The Sound source-II is located near the mixing chamber wall and the sound pressure levels around the ejector depend on the distances from the Sound source-II. In terms of the entrainment performance, with the increasing primary pressure or the decreasing secondary pressure, as the driving pressure difference of the secondary steam decreases, so does the entrainment ratio. As the back pressure increases, the entrainment ratio firstly remains constant, and then rapidly decreases when the back pressure exceeds the critical value at pb = 5.5 MPa. In terms of the noise characteristics, the sound pressure level and the intensity of the second shock wave have a positive correlation. When the primary or secondary pressure increases, the sound pressure level increases. Moreover, with the increasing back pressure, the sound pressure level firstly decreases, reaches the minimum of 98.2 dB at the critical back pressure, and then slowly increases.

Experimental investigation on the performance of a novel resonance-assisted ejector under low pressurization

It is of great significance and value to apply different techniques to optimize the entrained performance of an ejector. In this paper, a novel resonance-assisted ejector is proposed by applying a mixing enhancement technique. Experimental studies were conducted to investigate the oscillation effect on the performance of the ejector under different geometries and operating conditions. The results show that adding high-frequency oscillations to the primary flow can provide up to a 14% improvement in entrainment performance. As the length of the resonance tube increases, the performance tends to increase and then decrease. The performance is sensitive to variations in the distance between the nozzle of the resonance generator and the inlet of the resonance tube. The shorter the distance, the greater the performance improvement. The critical pressure ratio of the resonance generator is approximately 5.5. The entrained performance enhancement rate increases gradually over the critical pressure ratio. Entrainment enhancement caused by resonance generator can be eliminated with small mixing cross-sectional areas. The oscillation effect enhances the entrained performance at different backpressures, and the higher the pressure the greater the enhancement rate. This paper provides new ideas for the optimization of ejector performance.

Optimization of the primary nozzle for design a high entrainment ejector in spacesuit portable life support system

In looking forward to the future of human space exploration, it is increasingly stringent for the ventilation circuit subsystem in terms of reliability, simplicity and energy-efficient. Considering this requirement, the ejector, with the advantages of miniature size, no parasitic energy consumption and high reliability, is considered an appropriate alternative to the ventilation fan because it simultaneously implements the functions of mixing, entraining, and lifting pressure. This paper proposed a high entrainment ejector for application in spacesuit portable life support systems. The optimizations of the ejector nozzle geometrical parameters and nozzle exit position (NXP) were performed using a validated three-dimensional numerical model under various operating conditions. The results show that the NXP has a more significant effect on entrainment performance than other geometrical parameters in spacesuit portable life support systems, and the optimum NXP are not impacted by changes of the primary flow pressure and mixing chamber diameter, which similarly obtained the optimal entrainment ratio at the nozzle exit position of 2 mm. The maximum increments of entrainment ratio with the NXP varying from −4 mm to 8 mm are 0.2945 and 0.7664 under different primary flow pressures and mixing chamber diameters, respectively. Compared with the nozzle exit position, the most effects of nozzle converging angle and length on entrainment ratio are just 0.0864 and 0.0445. The optimum ejector extends the oxygen service duration by 12 min compared to the original one for the same volume of an oxygen tank, which is vital for the astronauts to conduct extravehicular activities in outer space.

Air entrainment mechanism of chute bottom aerators in high-speed chute flows

In high-head dam projects, chute aerators are commonly applied as artificial aeration devices to prevent cavitation erosion damage in high-speed spillway and tunnel flows. These specific chute air–water flows are generated by jet lower-surface aeration above the bottom cavity and jet impact aeration on the bottom floor. These air entrainment processes determine the total air flux transport downstream of the chute flow. However, the differential air fluxes caused by the two different aeration mechanisms remain unclear. Based on physical model investigations, the two air entrained processes are observed by a high-speed camera, and detailed air flux data are measured in the aerator cavity and impact areas. The effects of the jet lower-surface disintegrating and then instantaneously reattaching to the chute floor are discussed. The measured air flux data indicate that the inertial jet impact entrainment dominates with increasing the Froude number, while the air flux proportion of the jet lower-surface aeration increases with the aerator height. A general prediction model for aerator cavity entrainment is proposed, considering the effects of the Froude number and jet lower-surface aeration. The scale effects of the coupling mechanism on chute aerator air entrainment highlight that aerator cavity air entrainment primarily depends on the inertial jet impact, and the main effects of aerator design on entrainment performance are manifested in the air cavity properties.

Numerical analysis of two-stage vacuum ejector performance considering the influence of phase transition and non-condensable gases

Multi-effect distillation with thermal vapor compression system has not been able to penetrate the market because of the low coefficient of performance. The two-stage vacuum ejector is an effective vehicle to maintain system condensing temperature, discharge non-condensable gases and improve the system performance. Improving performance of two-stage vacuum ejector relies on the understanding of the fluid dynamic phenomena. Unfortunately, few studies paid attention to the effect of inner fluid phase transition and non-condensable gas mass fraction on the performance of vacuum ejector. Therefore, this paper contributes to proposing a Species transport and Phase transition model to investigate the two-stage vacuum ejector numerically considering the physical properties of non-condensable gases, thermodynamic transformation caused by condensation and evaporation with experimental validation. The results indicate that the entrainment performance of the ejector increases by about 14.95% when the non-condensable gases mass fraction increases from 0% to 100%. Meanwhile, with the consideration of heat and mass transfer in the phase change process, the mean error is 4.9%, which is much lower than single-phase model. Meanwhile, the internal fluid pressure, Mach number, shock waves and expansion degree can be largely influenced by the consideration of Species transport and Phase transition. The results of this paper proved that influence of non-condensable gases and phase transition cannot be ignored in the design and prediction of vacuum ejector and it can contribute to ejector optimization and enhancing the stability and efficiency of multi-effect distillation with thermal vapor compression system.

Transient characteristics investigation of the integrated ejector-driven hydrogen recirculation by multi-component CFD simulation

The hydrogen supply of the fuel cell system is realized by the cooperation of multiple components. Transient characteristics of a single component can affect the performance of other components. In this study, a three-dimensional multi-component computational fluid dynamics (CFD) model was developed to investigate the synergistic transient characteristics of the hydrogen recirculation components such as hydrogen injector, ejector, and purge valve in an 80 kW PEMFC. The results show that the entrainment performance of the ejector is reduced under unsteady purge conditions compared with steady conditions. The pressure fluctuation of the secondary flow is significant even under purge closed durations. There are drastic changes in velocity and pressure in the ejector, especially in the mixing chamber. Moreover, an abundant hydrogen supply capacity of the injector is necessary to deal with the excessive anode pressure fluctuation. The feedforward-feedback integrated control of the injector is a more efficient strategy to reduce pressure fluctuations compared with the feedback control.

Study on evolution laws of two-phase choking flow and entrainment performance of steam ejector oriented towards MED-TVC desalination system

In this study, a visualizable experimental platform of steam ejector oriented towards MED-TVC desalination system is constructed, and the evolution laws of two-phase choking flow and ejector's entrainment performance under various operating parameters are obtained. Further, a matching double-choking theory with consideration of the condensing flow is developed to explain the inherent reason behind these evolution laws. Key results revealed that the formed two-phase choking flows have great effects on ejector's entrainment performance. The primary jet flow choking area linearly increases as the primary steam pressure raises, and the entrainment choking area experiences a small reduction, while the extended flow area marginally increases. Accordingly, the entrainment ratio significantly decreases, while the choking cross-section is slightly shifted upstream. In addition, the primary jet flow choking area considerably decreases with increasing entrainment pressure, but the entrainment choking area is nearly unchanged and the extended flow area increases by more than 8 times. As a result, the entrainment ratio remarkably increases, the choking cross-section migration distances are much larger than that in the case of primary steam pressure varies. A meaningful guide evolves from fact that priority should be given to restrain the condensing flow for achieving a higher energy efficiency of ejector.