Ejector Performance Research Articles

A comparative study of single and dual ejector concepts for anodic recirculation system in high-performance vehicular proton exchange membrane fuel cells

The fuel supply system is a critical element in Proton Exchange Membrane Fuel Cell (PEMFC). In vehicular applications, an anode recirculation system (ARS) is required to return residual hydrogen to the fuel line. A 1D model of an ejector-based ARS was created and integrated with a vehicular PEMFC in this study. Therefore, two configurations, single-ejector-based and dual-ejector-based ARS, were developed to regulate the fuel supply and, thus, utilized to examine and conduct a comparative study of their advantages and disadvantages to the dynamic behavior of the stack. The use of ARS was found to improve stack performance and fuel utilization by delivering high stoichiometry, recirculation rate, and relative humidity. Contrary to a single-ejector-based ARS, whose use is restricted due to its limited working range, dual-ejectors can cover all fuel cell stack operating ranges. Sensitivity analysis revealed that primary pressure was the most significant parameter affecting ejector performance and flow characteristics.

Optimization of geometric parameters of ejector for fuel cell system based on multi-objective optimization method

ABSTRACT In this study, the multi-objective orthogonal experiment is employed to optimize the geometric parameters of the ejector. The optimization objective is determined by applying linear weighting to the entrainment ratios for 100 SLPM and 990 SLPM operating conditions. Four geometric parameters are optimized, including the nozzle exit diameter, nozzle exit position, mixing chamber length, the mixing chamber diameter. Moreover, Computational Fluid Dynamics simulations are performed, and the effects of each geometric parameter and the interaction between parameters on the performance of the ejector are ranked by range analysis. The results show that the mixing chamber diameter has the most significant impact on the optimization objective, and the degree of influence of the other factors and interactions on the entrainment ratio varies for different operating conditions. Compared with the initial parameters, the parameters obtained by the multi-objective optimization method have an average improvement of 96% in entrainment ratio over the full operating range, and the experimental results are in general agreement with the simulation results. The adaptability analysis of the ejector performance indicates that the ejector optimized by the multi-objective optimization method can meet the hydrogen excess ratio requirements for the full operating range of the fuel cell system.

Study on the ejector-expansion refrigeration system for low-temperature freezer application: Experimental and exergetic assessments

In this study, the authors propose applying the ejector-expansion refrigeration system (ERS) to develop a -50 °C low-temperature freezer. The main performance of the ERS based prototype freezers were investigated by experimental and exegetic methods. The influences of nozzle throat diameter and compressor displacement on ejector performance were discussed in detail. The higher efficiency and transiting exergy efficiency of ejector could be obtained with a larger nozzle throat diameter and compressor displacement. When the nozzle throat diameter rises from 0.4 to 0.7, ejector efficiency and transiting exergy efficiency increase from 16.1% to 20.1% and 26.1% to 30.2%, respectively. Moreover, these results demonstrated that the ERS-based freezer could operate stably with appropriate nozzle throat diameter and compressor displacement. Compared with the conventional single-stage vapor compression refrigeration system (CVRS), the compression ratio of ERS was reduced by 15.8%. In addition, the system performance decreased with the increase of capillary tube length. When capillary tube length varied from 40 mm∼ 80 mm, the COP of the freezer decreased from 0.59 to 0.52, and the overall exergy efficiency of freezer decreased from 22.1% to 17.1%. This research confirms that the ERS could provide a novel way for developing a -50 °C low-temperature freezer.

Improving the performance of a commercial absorption cooling system by using ejector: A theoretical study

Absorption cooling systems (ACS) have lower coefficients of performance (COP) compared to direct expansion (DX) cooling systems. Nevertheless, ACS offers a green alternative to typical DX systems. In this study, a numerical model was developed for the commercial low-capacity Robur® absorption cooling system (RACS). The model was developed based on mass, concentration, and energy balance equations, in addition to heat transfer equations. The model results were validated against experimental data available in the literature for the same cooling unit yielding a good agreement. Hence, to improve the COP of the RACS, a vapor ejector was introduced between the generator and the condenser. An improvement of 70.6% in the COP was obtained at the design condition. A parametric analysis was implemented to study the significance of the key parameters in the RACS performance. It was found that the increase in the ambient temperature not only increased the activation temperature, but it also decreased the COP and increased the circulation ratio (CR). Consequently, in hot environments, lowering the evaporator temperature is recommended to avoid the need for higher CR. Optimizing the nozzle throat and the mixing tube diameter improves the ejector performance, and hence the RACS performance, as long as the ejector operates under critical conditions. Finally, the absorber coil was found to have the most significance on the RACS performance in comparison with the rectifier coil and the refrigerant heat exchanger.

Performance Evaluation and Degradation Mechanism for Proton Exchange Membrane Fuel Cell with Dual Exhaust Gas Recirculation

Fuel gas utilization and water management are particularly challenging integrated engineering problems in hydrogen–oxygen proton exchange membrane fuel cell (H2/O2 PEMFC) systems. Herein, a standardized process is adopted to evaluate the performance and investigate the degradation mechanisms of a PEMFC with dual exhaust gas recirculation. The purpose of incorporating recirculation subsystems in the fuel cell is to achieve a high fuel gas utilization rate and realize effective water management inside the stack, which consists of 3D‐printed ejectors and a customized recirculation pump. Evaluation of the electrochemical performance degradation and morphological characterization of the fuel cells under different operating strategies are performed after 50 h durability experiments. At a current density of 400 mA cm−2, the performance degradation rates of the stack decrease from 16.50% to 7.49% and 0.71% in the ejector and recirculation pump operation strategies, respectively. The results show that using exhaust gas recirculation devices (ejector/pump) in the fuel cell stack can help in effectively mitigating water flooding and chemical degradation of the membrane electrode assembly. The findings of the study provide a perspective on the exhaust gas recirculation behavior and contribute to the engineering application of H2/O2 PEMFCs.

New Adapted One-Dimensional Mathematical and Regression Model to Predict Ejector Performance

New Adapted One-Dimensional Mathematical and Regression Model to Predict Ejector Performance

Structural optimization of hydrogen recirculation ejector for proton exchange membrane fuel cells considering the boundary layer separation effect

Ejectors can greatly improve the efficiency of proton exchange membrane fuel cells (PEMFCs) by pumping unreacted hydrogen, and boundary layer separation (BLS) significantly impacts the recirculation performance of an ejector. However, current studies usually focus on improving ejector performance without considering the effect of the BLS. Therefore, this paper focuses on the influence of the key structural parameters of the ejector on the BLS and the recirculation performance of the ejector. A mathematical model of the hydrogen recirculation ejector is established. Considering the BLS effect, the influence of the ejector structure parameters on the BLS and the performance of the ejector are studied under the design condition. To ensure the high performance of the ejector in a wide working range, the structure of the ejector is optimized in the full working range. The results show that when the backpressure is less than the reflux point (3.47 bar), the ejector works in the subcritical mode, and the hydrogen recirculation ratio decreases rapidly with increasing backpressure. When the backpressure is 3.35 bar, the BLS appears, and the separation area increases with increasing backpressure. The optimized ejector has good hydrogen recirculation performance in a specific range of throat diameters (6.2–7.2 mm), constant-area mixing chamber lengths (26.8–33.5 mm), and nozzle exit positions (−8–2 mm). When the throat diameter is less than 13.4 mm or greater than 33.5 mm, the degree of BLS is intensified, and the recirculation performance of the ejector decreases sharply. When the primary flow pressure is 6 bar, the efficiency of the optimized hydrogen recirculation ejector is significantly improved by 33.2%.

Performance analysis of dual ejector-expansion air source heat pump cycle with three-stage evaporation for cold region application

In the present study, a modified dual ejector-expansion heat pump cycle with three-stage evaporation has been proposed for air source heat pump applications in cold region. This cycle layout could lead to an improvement in cycle performance due to much expansion work recovery and better temperature variation matching between cooled air and refrigerant. The performance analyses of the proposed cycle and ejectors using refrigerant R290 have been conducted at different operating conditions. In addition, the performance characteristics of the cycle are compared to those of the baseline cycles using a conventional expansion device, as well as a single ejector and a single evaporator. Through thermodynamic analyses, it is found that the improvements in the coefficient of performance of the proposed cycle can reach up to 26.1% and 10.5%, respectively, as compared to the basic vapor-compression heat hump cycle and ejector-expansion heat hump cycle. The exergy destruction of the proposed cycle can be reduced by 28.6% and 13.6% than those of the other two basic cycles. In addition, the performance of ejectors under different operating conditions is studied. The pressure lift ratio of the ejector in the new cycle varies from 1.36 to 1.69, which shows 14.3%∼21.6% improvement compared to the basic ejector-expansion heat pump cycle. Overall, this paper confirms the performance advantages of the proposed new heat pump cycle and its potential application in an air-source heat pump system.

Investigation of steam ejector parameters under three optimization algorithm using ANN

Several industries have adopted Steam ejectors as promising energy-saving devices. The steam ejector is used extensively in the HVAC, refrigeration, and desalination industries because it harnesses surplus heat. Numerous computational fluid dynamics (CFD) and thermodynamic models have been addressed in the academic literature. Predictions for steam ejector parameters using these models are fairly accurate. However, considerable discrepancies between experiments and models are sometimes as high as 20 %. Contrarily, CFD simulations for an ejector are computationally expensive and necessitate careful selection and tweaking of the turbulence model. ANN-based data-driven techniques are now used for complicated mechanical system modeling. The advantages of a well-trained ANN model include low computational costs and adaptability for integration with real-time control systems. In this study, we developan artificial neural network (ANN) model of a steam ejector and analyzes how different training strategies affect the ANN model's ability to make accurate predictions. The model is trained using a sizable experimental library with various geometrical and operational conditions. Using a typology of four input factors (Nozzle Throat Diameter (Ntd), Diffuser Outlet Diameter (Dod), Ejector Area Ratio (σp), and Entrainment Ratio (ER)) that capture geometry and operating conditions characteristics, the framework predicts two output parameters: (a) Coefficient of performance (COP) and (b) Pressure reduction ratio (PRR) of the steam ejector. Among the three optimizers tested for algorithm training, the ANN model trained using the ADAM optimizer performed the best. The model's hidden layer contains 102 neurons, and a split ratio of 0.2 is used during dataset training. The average error for COP and PRR predictions made by the ANN model is ± 1.8 % and ± 2.4 %, respectively. As a result, the ANN model can be used to improve the steam ejector system's configuration and operation.

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.

Multi-Objective Optimization of a Solar Combined Power Generation and Multi-Cooling System Using CO2 as a Refrigerant

This paper proposes a new combined multi-cooling and power generation system (CMCP) driven by solar energy. Carbon dioxide is used as a refrigerant. A parabolic trough collector (PTC) is employed to collect solar radiation and convert it into thermal energy. The system includes a supercritical CO2 power system for power production and an ejector refrigeration system with two ejectors to provide cooling at two different evaporating temperatures. The CMCP system is simulated hourly with weather conditions for Tunisia. The PTC mathematical model is used to calculate the heat transfer fluid outlet temperature and the performance of the CMCP system on a specific day of the year. A 1D model of an ejector with a constant area is adopted to evaluate the ejector performance. The system’s performance is evaluated by an energetic and exergetic analysis. The importance of the system’s components is determined by an exergoeconomic analysis. The system is modeled using MATLAB software. A genetic algorithm is used for multi-objective optimization to determine the best values and solutions for the system’s design parameters. The optimal energy and exergy efficiencies were found to be 13.7 percent and 37.55 percent, respectively, and the total product unit cost was 31.15 USD/GJ.

Simulation and experimental measurements of 10-kW PEMFC passive hydrogen recovery system

Simulations are performed to examine the performance of a vacuum ejector in the hydrogen recovery loop of a 10-kW PEMFC system. The simulations commence by examining the effects of the primary flow fluid pressure and secondary flow temperature on the recirculation ratio and hydrogen stoichiometric ratio. Further simulations are then performed to investigate the temperature, pressure, velocity and Mach number distributions within the ejector for various values of the primary flow inlet pressure and temperature. A prototype ejector is fabricated using a 3D printing technique. Experiments are performed to evaluate the gas tightness and gas recovery performance of the ejector under realistic operating conditions. The simulation results show that the recirculation ratio and hydrogen stoichiometric ratio increase with a decreasing primary flow inlet pressure and secondary flow inlet temperature. As the primary flow inlet pressure increases, the pressure, velocity, and Mach number in the mixing chamber increase, and the hydrogen recovery performance decreases. Furthermore, as the temperature of the primary fluid flow increases, the stability of the isentropic flow condition within the ejector is enhanced. The experimental results show that the prototype vacuum ejector has a maximum gas leakage of just 0.7 psi and a minimum hydrogen recirculation rate of 59.3%. Consequently, it has significant potential for passive hydrogen recovery in large-scale fuel cell systems.

Research on performance of multi-nozzle ejector for aeroengine air system

Research on performance of multi-nozzle ejector for aeroengine air system

Experimental performance of ejector heat pump operating in the sub-critical mode

Ejector heat pump (EHP) is an efficient-energy technology with a promising potential to replace the vapor compression cycle in heating and cooling applications. It can be powered by low-grade waste heat and uses environmentally friendly working fluids. EHP has also become a viable solution in research seeking cooling applications. However, little attention has been paid to using EHP for heating purposes. In order to investigate this, a steam EHP for domestic water heating was designed and built. The coefficient of performance (COP) is evaluated at various operating and design conditions in sub-critical operational modes to achieve higher condensation temperatures. Two primary nozzles with a throat diameter of 1.5 mm and 2.0 mm were investigated. The primary nozzle is movable along the ejector’s axis, allowing investigation of its positional effects on the EHP’s COP. Experimental measurements revealed that using a smaller throat diameter results in a high COP and low back pressure. The EHP COP and back pressure increase when the LTE temperature increases. Using a throat diameter of 1.5 mm, the EHP COP increases as the nozzle exit position (NXP) becomes closer to the constant area section. A COP of 2.42 and a back pressure of 4.28 kPa are achieved at a high-temperature evaporator (HTE) temperature of 130 °C and a low-temperature evaporator (LTE) temperature of 30 °C using a primary nozzle with 1.5 mm.

Numerical Investigation on the Effect of Section Width on the Performance of Air Ejector with Rectangular Section

Due to its simple structure and lack of moving parts, the supersonic air ejector has been widely applied in the fields of machinery, aerospace, and energy-saving. The performance of the ejector is influenced by the flow channel structure and the velocity of the jet, thus the confined jet is an important limiting factor for the performance of the supersonic air ejector. In order to investigate the effect of the confined jet on the performance of the ejector, an air ejector with a rectangular section was designed. The effects of the section width (Wc) on the entrainment ratio, velocity distribution, turbulent kinetic energy distribution, Mach number distribution, and vorticity distribution of the rectangular section air ejector were studied numerically. The numerical results indicated that the entrainment ratio of the rectangular section air ejector increased from 0.34 to 0.65 and the increment of the ER was 91.2% when the section width increased from 1 mm to 10 mm. As Wc increased, the region of the turbulent kinetic energy gradually expanded. The energy exchange between the primary fluid and the secondary fluid was mainly in the form of turbulent diffusion in the mixing chamber. In addition to Wc limiting the fluid flow in the rectangular section air ejector, the structure size of the rectangular section air ejector in the XOY plane also had a limiting effect on the internal fluid flow. In the rectangular section air ejector, the streamwise vortices played an important role in the mixing process. The increase of Wc would increase the distribution of the streamwise vortices in the constant-area section. Meanwhile, the distribution of the spanwise vortices would gradually decrease.

Gas/particle flow and pressure variation characteristics of Ende pulverized coal gasifier equipped with ejector

AbstractEnde pulverized coal gasifier (EPCG), as a typical representative of circulating fluidized bed gasifier, has been widely used in gasification field. The return conduit of EPCG is generally not equipped with a feedback control device (FCD), which tends to result in a low solid circulation rate (Gs) and high unburned combustibles in fly ash. To address these issues, a new technique using the ejector as the FCD was designed to provide a stable pressure barrier and improve solid circulation. Preliminary thermal experiments demonstrated the feasibility of improving the EPCG with injector system. Currently, the specific mechanisms that contribute to further optimize this technology have not been systematically studied. Considering the large impact of ejector operation on solid circulation rate and stability as well as syngas production, this work adopts a gas/particle 1:15 cold EPCG bench to investigate the gas/particle flow and pressure variation characteristics before and after ejector operation and under different working medium pressures (Pp). When the ejector operates at the design pressure Ps = 0.187 MPa, the outlet pressure of the return conduit is slightly larger than the furnace bottom pressure, thus preventing the reverse gas flow at the furnace bottom. The particle velocity and concentration distribution in the furnace are basically consistent with that without the ejector, showing a similar central spout region and near‐wall annulus flow region. As Pp increases from 0.04 to 0.24 MPa, the average particle velocity in the return conduit increases from 0.95 to 2.4 m/s, the particle concentration decreases from 0.088 to 0.051, and the solid circulation rate increases from 0.37 to 0.53 kg/h. The ejector outlet pressure gradually increases, the inlet pressure decreases, and the pressure barrier provided by the ejector rises from 1052 to 3640 Pa. When Pp is 0.12 MPa, the ratio of GN2/Gair is 0.91%, which can achieve the purpose of continuous and stable pressurization with a small amount of nitrogen.

Influence of geometric parameters on the performance of ejector used in aeroengine air system

By using an ejector to draw low-pressure, low-temperature air into the aeroengine's air system, it is possible to significantly enhance the cooling effect of the hot component of the engine. It is very necessary to clarify the sensitivity of ejector performance to different geometric parameters, especially the receiving chamber height, mixing chamber diameter, and diffuser diameter, which are of great significance to improve ejector performance and save limited aeroengine space. Therefore, this paper investigated the effects of these three geometric parameters on the ejector's performance by numerical simulation. The results show that increasing the mixing chamber diameter can significantly improve ejector performance. When the compression ratio is 1.17 and the mixing chamber diameter is increased from 3 mm to 9 mm, the entrainment ratio and the total pressure recovery coefficient are increased by 23.1 % and 7 %, respectively. Especially when the compression ratio is 1, simultaneously increasing the size of the receiving chamber, mixing chamber, and diffuser can greatly improve the ejector's performance. When the diameter of the mixing chamber is increased from 6 mm to 27 mm, the entrainment ratio and the total pressure recovery coefficient are increased by 211 % and 39 %, respectively. The change in ejector structure will influence its flow field and then affect the mixing process. When the size of the ejector is too small, the strong background shock wave suppresses the mixing of the primary flow and the secondary flow, which then causes an increase in the total pressure loss. When the size of the ejector is too large, the eddy current loss leads to ejector performance reduction.

Synergistic effect of adjustable ejector structure and operating parameters in solar-driven ejector refrigeration system

The ejector has a substantial impact on the performance of the solar-driven ejector refrigeration system. This work designs an adjustable ejector without disassembly and performs numerical modeling of the geometric model to enhance the ejector capacity to adapt to various operating conditions; the simulations dependability was also validated using experimental evidence. The findings indicate that within the given operating conditions, the adjustable ejector efficiency increased by an average of 82.4 %; the COP of the system increased by an average of 72.8 %. It is also found that the adjustable ejector has a saturation region, where the ejector performance rapidly declines as the nozzle exit position increases. Under the given operating conditions, the functional relationship between the optimal area ratio and nozzle exit position is obtained. By defining the operating condition coefficient (Γ), the function of the Γ and optimal area ratio is obtained, which provides the adjustable ejector control logic. From the energy perspective, the influence of structural changes on the vortex near the premixing chamber wall is analyzed, and the basic internal reasons for how structural alterations affect the ejector performance are initially revealed.

A streamlining method for ejector nozzle profile optimization based on polynomial function

In this study, ejector performance was optimized by streamlining the primary nozzle profile using polynomial functions. Thirteen streamline nozzle profiles with different initial and exit expansion half-angles which determined by quadratic and cubic functions were established, and their performance are discussed. The results indicate that the nozzle profiles designed by cubic function have the best performance than quadratic function. To improve the effective working range, the ejector should have an initial expansion half-angle that is as small as possible and an exit expansion half-angle that is as large as possible. To enhance the entrainment ratio, the ejector should have a constant exit expansion half-angle and a minimum initial expansion half-angle of 1°. An excessive initial expansion half-angle caused a sealed Mach number contour inside the nozzle, which wasted energy to reduce the velocity and increase the static pressure, decreasing the working efficiency of the primary flow. In addition, the entrainment ratio significantly decreased with a large exit expansion half-angle (greater than 17°).

Investigation of Fluid Characteristic and Performance of an Ejector by a Wet Steam Model

In this paper, a wet steam model is utilized to study the fluid characteristic and performance of a supersonic ejector. The condensation process, which has been ignored by most researchers, is analyzed in detail. It is found that the most intensive condensation happens at the primary nozzle downstream and nozzle exit region. Moreover, the impacts of primary flow pressure and back pressure on ejector performance are studied by the distribution of Mach number inside the ejector. Furthermore, the results show that the secondary mass flow rate first grows sightly then remains almost unchanged, while the primary mass flow rate rises sharply and ejector entrainment ratio drops dramatically with the increase in primary flow pressure.