Supersonic Ejector Research Articles

Numerical investigation of the flow characteristics inside a supersonic vapor ejector

Integrating a vapor ejector with an air-cooled absorption cooling system (ACS) requires understanding how the ejector responds to varying condenser conditions and how the geometrical parameters affect the system's performance. This study provides a numerical investigation of the flow characteristics inside supersonic vapor ejectors. The primary objectives were identifying the best nozzle design for ACS and explaining how the secondary flow responds to different back pressures. The developed model was validated against experimental data and a one-dimensional model. Despite exhibiting increased flow fluctuations, the convex nozzle achieved an entrainment ratio of 0.4. This value was 4.9 % and 7 % higher than the values obtained by the straight and the concave nozzles, respectively. In contrast, the concave nozzle exhibits better flow stability and pressure recovery, which are considered appealing for the air-cooled ACS. The straight nozzle emerged as a balanced alternative, offering moderate entrainment alongside favorable flow stability. Moreover, secondary flow behavior at different operating modes was elaborated. Secondary flow choked at back pressures between 60–70 kPa, indicating optimal entrainment. However, at 75–80 kPa, while the secondary flow was entrained, it failed to reach sonic speed due to high-pressure waves, resulting in the sub-critical condition. Further increases in back pressure to 85–90 kPa induced back-flow due to elevated local static pressure. Mach number profiles at the mixing tube entrance remained consistent under critical operation but deviated post-critical back pressure, reflecting altered flow characteristics downstream of the mixing tube. Such elaboration of flow dynamics within ejectors paves the way for innovative designs of vapor ejectors, potentially developing ACS.

Spatiotemporal evolution and instability of the mixing layer in supersonic ejector based on large eddy simulation

Spatiotemporal evolution and instability of the mixing layer in supersonic ejector based on large eddy simulation

Experimental Investigation of the Performance of a Novel Ejector–Diffuser System with Different Supersonic Nozzle Arrays

The supersonic–supersonic ejector–diffuser system is employed to suck supersonic low-pressure and low-temperature flow into a high-pressure environment. A new design of a supersonic–supersonic ejector–diffuser was introduced to verify pressure control performance under different operating conditions and vacuum background pressure. A 1D analysis was used to predict the geometrical structure of an ejector–diffuser with a rectangular section based on the given operating conditions. Different numbers and types of nozzle plates were designed and installed on the ejector to study the realizability of avoiding or postponing the aerodynamic choking phenomenon in the mixing section. The effects of different geometrical parameters on the operating performance of the ejector–diffuser system were discussed in detail. Experimental investigation of the effects of different types of nozzle plates and the back pressures on the pressure control performance of the designed ejector–diffuser system were performed in a straight-flow wind tunnel. The results showed that the position, type and number of the nozzle plates have a significant impact on the beginning of the formation of aerodynamic choking. The geometry of the ejector and the operating conditions, especially the backpressure and inlet pressure of the ejecting stream, determined the entrainment ratio of the two supersonic streams. The experimental results showed that long nozzle-plate had a better performance in terms of maintaining pressure stability in the test section, while short a nozzle-plate had a better pressure matching performance and could maintain a higher entrainment ratio under high backpressure conditions.

Experimental and numerical studies on the performance of a precooled supersonic ejector

Precooling the secondary flow enhances the performance of the supersonic ejector. However, reducing the temperature of the secondary flow without experiencing significant pressure loss presents a challenge, particularly when the secondary flow is at high temperature and negative pressure. In response to this challenge, an original design for a two-strut supersonic ejector incorporating a plate-fin precooler is developed. Experimental and numerical calculation methods are employed to analyze the system’s performance under various secondary flow conditions, including ambient air, precooled gas, and non-precooled gas. Moreover, the impact of the secondary flow, including total temperature and mass flow rate on the flow structure and mixing process of the supersonic ejector is analyzed. The results show that the shock waves, reflected shock waves and shock trains exist in the flow channel of supersonic ejector simultaneously. The shock waves intensity decreases with the rise of the total temperature and mass flow rate of the secondary flow. Moreover, this leads to the static pressure behind the nozzle changing from a ω−shape to a U-shape along the flow direction. Meanwhile, the precooler has minimal impact on the pressure loss of the secondary flow while achieving a remarkable pressure recovery coefficient of over 97% and a temperature drops of above 40%. Thus, precooling the secondary flow helps decrease the static pressure in the suction chamber, which results in a minimum 29% increase in compression ratio, enhancing the pumping capacity of supersonic ejector. The study also observed that the drop of secondary flow temperature leads to a lower velocity ratio between the secondary and primary flows, which is beneficial for their mixing process.

Compound-choking theory and artificial neural networks-based hybrid modeling for supersonic ejectors

As the core component of ejector refrigeration systems, the research of ejector mechanisms has always been a hot topic. Based on the compound-choking theory, this paper studies the relationship between the entrainment ratio and critical back pressure and designs a neural network-based online error compensation mechanism for supersonic ejectors. Firstly, a thermodynamic model of the ejector is developed by combining the ideal gas model, the compound-choking theory, and the constant-pressure mixing theory. Then, the thermodynamic relationship between the entrainment ratio and critical back pressure is derived by introducing the idea of a hypothetical mixing cross section. Compared with the existing results, our method is able to improve critical back pressure prediction accuracy by virtue of the actual entrainment ratio. Therefore, a new adaptive error compensation algorithm is proposed for supersonic ejectors based on neural networks. Thus, the proposed scheme can be regarded as a hybrid modeling method for supersonic ejectors, which combines thermodynamic laws and data-driven artificial intelligence algorithms. Experimental data from ejector refrigeration systems with R141b, R245fa, water vapor and R134a are used in this paper to verify the validity of the model. The simulation results show that our method can effectively predict the critical back pressure, and all the prediction errors are less than 10%.

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.

Adjoint optimization of a supersonic ejector for under-expanded, isentropic, and over-expanded primary flow modes

The geometric shape of an ejector’s walls plays an important role in the efficiency of the ejector. Depending on the outlet-to-throat area ratio of the primary nozzle, the flow exiting the primary nozzle of a supersonic ejector is categorized as under-expanded, isentropic, or over-expanded. Due to differences in the flow structures exiting the primary nozzle, the optimal shapes of the mixing chamber and secondary channel walls differ for these three flow modes. In this study, the geometric shape of a supersonic ejector was optimized numerically for these three modes. First, various geometric parameters were fixed, and the mixing chamber height was varied parametrically for different area ratios of the divergent part of the primary nozzle to determine the optimal mixing chamber height for the different supersonic flow modes. Then, the wall profiles of different components were optimized numerically for the parametrically optimized ejectors. The adjoint equations in the Ansys Fluent 2022 R2 software were solved, and the cost function was evaluated using a Reynolds-averaged Navier–Stokes flow solver with the k–ω shear-stress transport turbulence model. The results demonstrated that the entrainment ratio of the adjoint-optimized ejectors was 20.8%, 15.3%, and 16.5% higher than that of the corresponding parametrically optimized ejectors for the under-expanded, isentropic, and over-expanded modes, respectively. In addition, the adjoint-optimized ejector exhibited a wider performance range in the under-expanded mode than in the other modes.

Experimental and numerical studies on the performance of supersonic multi-nozzle ejector

The nozzle number influence the performance of the supersonic ejector. To figure out the different characteristic of the supersonic multi-nozzle ejector (MNE), this paper focused on two specific MNE (single nozzle and four nozzles), compared the starting characteristics, operating characteristics, and mixing characteristics based on the experiment and numerical simulation. Results revealed that the interaction between primary total pressure and the diffuser exit pressure played a major role in determining the starting states for the MNE. The four-nozzle ejector required a higher starting pressure than the single-nozzle ejector, due to the shock loss and friction loss, meanwhile, it showed priority when the compression ratio exceeded 8 or the entrainment ratio droped to below 0.068. The non-mixing length of the four-nozzle ejector is shorter than that of the single-nozzle ejector under the same conditions, and there is a linear relationship between the non-mixing length and the entrainment ratio for the single-nozzle ejector.

CFD analiza i ispitivanje geometrijskih parametara za dizajn visokoefikasnog nadzvučnog ejektora

This paper presents a CFD analysis of a supersonic ejector model for refrigeration systems. The model is validated by comparing it with previous studies and used to design a high-efficiency ejector. The effects of geometrical parameters such as convergence angle, throat length, divergence angle, and diffuser length on the entrainment ratio are studied. The optimal values of these parameters are found and used to modify the base model. The performance of the optimized model is evaluated under different operating conditions. The paper aims to provide design guidelines for supersonic ejectors in refrigeration applications.

Experimental and numerical study on RBCC engines performance in simultaneous and combustion cycle

The simultaneous mixing and combustion (SMC) cycle is a simple yet effective approach to augment the specific impulse of rocket-based combined cycle (RBCC) engines in ejector mode. However, the thrust performance of RBCC engines in the SMC cycle has not been adequately explored. This study investigates the effects of the flight Mach number, mixer shapes, and mixture ratio (MR) on the thrust performance of an axisymmetric kerosene-fueled RBCC engine. Ground free-suction experiments were conducted, employing a supersonic air ejector to simulate varying flight backpressures. Wall pressure distributions and axial thrusts were obtained. To eliminate undesirable interactions between MRs and backpressures inherent in experiments, numerical simulations were performed. The experimental results show that as flight Mach number increases, specific impulse improves rapidly, and specific impulse augmentation is achieved in the transonic regime. Simulations conducted at H4.0km/Mach0.88 with the expansion mixer reveal that as MR decreases, both mixing efficiency and combustion intensity increase. The balance between specific impulse gain from the secondary stream and loss from the primary stream is achieved at MR = 2.2, with the highest specific impulse being achieved at MR = 2.0. These insights provide valuable guidance for advancing the development of more efficient RBCC engines using the SMC cycle.

Artificial neural network-based predictive model for supersonic ejector in refrigeration system

In this paper, a novel predictive model is proposed for the supersonic ejector in the refrigeration system based on the artificial neural network technique. A composite performance prediction framework for the supersonic ejector is developed by employing a back propagation neural network with particle swarm optimization (PSO-BPNN) algorithm and a dynamic error compensation method. Firstly, we study the model construction problem for the supersonic ejector based on the traditional BPNN modeling method and the PSO algorithm. And then, a dynamic compensation technique is given to improve the predictive accuracy of the ejector model based on the adaptive neural network method. Finally, both the simulation and experimental results are given to verify our method. The experimental validation results show that most prediction errors of our model are less than 4%. Therefore, compared with the existing traditional models, our model has a higher accuracy.

An active mixing enhancement technique in supersonic ejectors using pulsed streamwise injections

The current study numerically investigates an active mixing enhancement technique for a supersonic ejector with a constant area mixing chamber operating under the critical regime. Streamwise control jet injections are alternatively pulsed at the top and bottom sides of the mixing chamber entrance. The induced oscillation of the primary jet enhances the bulk mixing between the primary and secondary streams. The secondary stream penetrates the primary core flow much upstream with the control strategy compared to the no-injection case, improving the onset of mixing by 65.36%. A higher spread of the primary jet along the mixing chamber height is observed with the control strategy indicating an enhanced mixing between the two streams. Dynamic mode decomposition analysis of the fluctuations of the velocity magnitude revealed that the dominant dynamic structures are determined by the pulsation frequency and a dominant flapping mode can be observed. The frequency spectrum of the primary jet oscillation revealed that the dominant frequency of oscillation is dictated by the pulsation frequency of the injection. With an increase in the control jet pulsation frequency, the amplitude of primary jet oscillation reduces near the entrance region of the mixing chamber, whereas the amplitude of oscillation far downstream reaches nearly the same value for all the cases. The power spectral analysis of the unsteady pressure fluctuations along the mixing chamber wall revealed that the wall pressure oscillations are contributed by the control jet pulsation frequency as well as the shock wave reflections produced by the supersonic jet–jet interaction within the mixing chamber.

Experimental and numerical study on low-temperature supersonic ejector

Ejectors have emerged as a viable solution for handling Boil-Off-Gas (BOG) generated by cryogenic storage tanks. The efficiency of ejector-based systems plays is highly dependent on the ejector's performance which is influenced by various factors including the geometry of the ejector and the boundary conditions. In this study, an axisymmetric supersonic ejector was designed, optimized, and evaluated for BOG removal. The baseline geometric dimensions design of the ejector was obtained via theoretical analysis. Subsequently, computational fluid dynamics (CFD) simulations were employed to optimize the boundary conditions and geometrical parameters. Experimental investigations were conducted to validate the design and evaluate the ejector's performance. The results highlighted the significant influence of two key parameters, namely, the mixing chamber diameter (Dm) and the nozzle exit position (NXP), on the ejector's performance. Optimized Dm showed an increase of about 12.5% compared with the baseline geometry. By operating the ejector within its on-design geometric and boundary conditions, the maximum entrainment ratio (ER) was achieved. The CFD models successfully identified and validated various flow phenomena, including double choke, single choke, and backflow, which were consistent with the experimental observations. Both the numerical models and experimental findings demonstrated that the modified ejector achieved an entrainment ratio of 1.06 under its design condition showing a 33.66% increase compared with that obtained using the baseline geometry. These results indicate the potential of the supersonic ejector as an alternative solution for BOG applications, emphasizing its efficacy in handling this industrial challenge.

Effect of wall roughness on secondary flow choking in supersonic air ejector with adjustable motive nozzle

Wall roughness is present to a certain extent in all engineering devices and systems. However, it is rarely considered during numerical simulations of supersonic ejectors. In this study, the effect of wall roughness on choking within a supersonic ejector with an adjustable motive nozzle was investigated in detail. Wall roughness was studied through numerical simulations. Characteristic curves showed that roughness had almost no influence on secondary flow choking when choking occured geometrically before the mixing chamber. By contrast, when choking occurred within the mixing chamber, its phenomenology was significantly altered by wall roughness. In addition, the results confirmed the ability of an ejector to switch from the on-design working regime to the off-design one when wall roughness is considered. The numerical results were also compared with available experimental data. Accounting for wall roughness in the numerical analysis significantly improved the agreement between the experimental and numerical data, especially in terms of wall static pressure distributions. This study sheds light on secondary flow choking within a supersonic ejector considering wall roughness.

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.

Working mechanism and characteristics analysis of a novel configuration of a supersonic ejector

The growing awareness of utilizing low-grade energy has encouraged the scientific community to conduct research on supersonic ejectors. Most previous studies have investigated on the geometry parameters or profiles of supersonic ejectors, but few studies have focused on improving the ejector configuration. This paper presents a novel configuration of a supersonic ejector which has the advantage of better performance than the conventional ejector in double-choking mode. First, the novel configuration and its working mechanism are presented, and the entropy generation of the primary fluid is compared theoretically between the novel and conventional configurations. Second, validation of the novel configuration mechanism is performed using CFD. The velocity and pressure distribution, performance characteristics, and entropy generation at different outlet pressures are presented and analyzed in comparison to the conventional ejector. The normal shock intensity in the novel configuration is reduced and the total entropy generation decreased, thereby improving the ejector's performance. Finally, the impact of the geometric parameters of the novel configuration is evaluated, pointing out its limits and the need of further investigation and experimental studies for optimal design.

A theoretical model for performance evaluation of a novel configuration of supersonic ejectors

Few researches have investigated on the improvement of supersonic ejector configuration. This paper presents a theoretical model for evaluating the performance of a novel supersonic ejector configuration that has not been investigated before. Under various geometry parameters and operating conditions, the working performance of ejectors in conventional and novel configurations is investigated. In addition, the performance characteristics of the novel configuration were analyzed, and the Kantrowitz limit was introduced to obtain a reference value for the compression surface radius on a theoretical basis. 24 ejectors with different geometry parameters, including the conventional and novel ejectors, are designed and tested using CFD approach. The results show that the performance of the novel ejectors is better than that of the conventional ejectors within the deterioration point, and the trend of their critical outlet pressure is consistent with the theoretical model. Moreover, the deterioration point is well predicted, and the performance of the novel configuration decreases when the compression surface radius exceeds the deterioration point. Last, the relationship between the flow field and key parameters of the novel configuration was illustrated by analyzing the Mach number contours and pressure distribution of the conventional and novel ejectors.

Design, fabrication, and investigations of prototype supersonic micro-ejector for innovative cooling system of electronic components

Innovative approach for electronics cooling was proposed, i.e. heat generated by a part of electronic equipment, e.g. CPU, is absorbed by heat driven micro-ejector refrigeration system that can provide cooling of other electronic components, e.g. GPU. It was demonstrated the procedure of design, fabrication, and experimentation on the supersonic micro-ejectors for the case of isobutane as a working fluid for such system. It was demonstrated that it possible to design and fabricate the micro-ejector of cooling capacity in the range 3 W suitable for cooling of electronic equipment. For the discussed micro-ejector entrainment ratio obtained was app. 0.20 under conditions of the motive heat source temperature app. 60 °C and evaporation temperature app. 22 °C. There were demonstrated difficulties in fabrication of the supersonic ejector due to small dimensions of the motive nozzle as well as mixing chamber. Required throat diameter of the motive nozzle was less than 200 μm and the diameter of the mixing chamber was 260 μm. For reported tests conditions the boundary temperature of the on-design operation condition was between 21 – 25 °C. However, the operation of the micro-ejector under off-design operation regime was stable and repeatable. Micro-ejector tests required an indirect method of measurement of critical motive mass flow rate to be applied. The performance line of tested ejector fits to ejectors theory and meet the basic assumptions in terms of thermal capacity and operating temperature. CFD technique used for numerical predictions of the ejector performance fits to experimental measured values with acceptable accuracy.

Studies on Shock Wave Structures in Supersonic Ejector and Its Influence on Ejector Performance

Supersonic ejectors are an economically feasible and environmentally friendly technology. Shock wave structures significantly affect the ejector performance. In the present work, the first shock wave structures in a supersonic ejector were analyzed with experimental and numerical methods. The experimental results by the Schlieren method were used to verify the accuracy of the numerical simulation results. Then the influence of the inlet pressure on the first shock wave structures inside the ejector was analyzed under critical operating conditions. The numerical simulation results showed that the dimensionless length and height of the first shock wave inside the ejector increase with the increase of the primary pressure and decrease with the increase of the secondary pressure. Meanwhile, the relationship between the performance of the ejector and the shock structures was established, the entrainment ratio is positively correlated with the first shock wave aspect ratio and has a good linear relationship. The results may be beneficial to reveal the ejector’s working mechanism and design high performance ejectors in the future.

Low-frequency unsteadiness of recompression shock structures in the diffuser of supersonic ejectors

Supersonic ejectors are passive gasdynamic devices that compress a low-pressure fluid by utilizing the kinetic energy of a high-pressure fluid in a variable area duct. The ejector consists of a primary supersonic nozzle in a mixing duct where the secondary flow is entrained and mixed. The mixed flow can undergo a series of recompression shocks resulting in a subsonic flow in the diverging portion to aid pressure recovery. Recompression shocks usually lead to unsteady shock boundary layer interactions. The performance of the ejector is influenced by shear layers, shock and expansion waves, and their mutual interactions. While existing literature has extensively dealt with mixing of the primary and secondary flows, the unsteadiness in flow resulting from recompression shocks has been seldom investigated. Fluctuations in pressure due to the unsteadiness of the shock often lead structural fatigue issues. This paper reports a detailed investigation on low-frequency unsteadiness of recompression shock using high-speed schlieren images and dynamic pressure measurements. Modal analyses using proper orthogonal decomposition and dynamic mode decomposition techniques are used to determine the dominant spatial modes and associated frequencies. Multimodal frequencies ranging between 80 and 300 Hz are observed. These findings are further corroborated by Fourier and wavelet transformations of the experimentally measured wall static pressure signals. Subsequently, scaling parameter is established for the dominant frequencies based on flow velocities upstream of the shock and the distance between two consecutive shocks. This results in a unique scaling frequency of 4.58% ± 18%, for the recompression shock independent of operating conditions.