Ejector Mode Research Articles

Numerical study on throttling characteristics of RBCC engines in ejector mode

Investigating the throttling characteristics of Rocket-Based Combined Cycle (RBCC) engines in ejector mode is crucial for adapting to varied flight mission demands. This study employs validated RANS numerical methods to investigate throttling effects on an experimental kerosene-fueled RBCC engine under two representative flight conditions: ground take-off (Mach 0) and supersonic flight (Mach 2.0). Results indicate that, at Mach 0, throttling reduces the entrained secondary stream and increases mixing efficiency, but exacerbates mixing losses, which cannot be offset by the thrust produced by the secondary stream due to its weak working capacity. Therefore, specific impulse performance deteriorates during throttling. Moreover, the optimum mixture ratio remains at 2.4, consistent with conventional rockets. Conversely, at Mach 2.0, throttling mitigates the primary stream’s squeezing effect to augment the entrained secondary stream. Simultaneously, throttling enhances mixing efficiency to improve the utilization of residual fuel. Consequently, the optimum mixture ratio shifts from 2.4 to below 1.4, which contributes to harnessing the work capacity of the secondary stream generated by ram effects, and significantly improves specific impulse performance. This study emphasizes the distinct throttling characteristics at different Mach numbers and provides pivotal insights for developing more efficient RBCC engines in ejector mode.

Numerical Investigation on the Effect of Fuel-Rich Degree in the RBCC Engine under the Ejector Mode

The ejector mode of the Rocket-Based Combined-Cycle (RBCC) engine is characterized by high fuel consumption. This study is aimed at investigating the influence of the rocket fuel-rich degree on the RBCC engine’s performance under the ejector mode combined with simultaneous mixing and combustion (SMC). Numerical simulations were conducted for various rocket mixing ratios (Φ=1.6~3.2) under subsonic (Maf=0.9) and supersonic (Maf=1.8) flight conditions. It was observed that a high fuel-rich degree in the rocket plume negatively impacts the eject performance under all conditions. However, it improves the overall performance (Isp) at high flight Mach numbers (Maf). For supersonic conditions, increasing the fuel-rich degree promotes greater fuel participation in combustion, thereby enhancing RBCC engine performance. Nevertheless, the subsonic-supersonic mixing layer exhibits low evolution, resulting in a decrease in reaction efficiency from 29.2% to 12.0% as the Φ decreases from 3.2 to 1.6. Consequently, there is an inefficient utilization of fuel. To optimize RBCC engine performance, the rocket fuel-rich degree can be appropriately increased. However, this increase should be limited to prevent fuel wastage arising from low reaction efficiency. Under subsonic conditions (Maf=0.9), the low kinetic energy of captured air leads to the occurrence of “negative thrust surface” and “wall impact” phenomena, which hinder the efficient and stable operation of the RBCC engine. Consequently, adjusting the fuel-rich degree alone cannot promote specific impulse (Isp), and a low fuel-rich degree is considered an ideal strategy when combined with adjustable nozzle technology.

Numerical and experimental study of choking phenomenon on RBCC engines in ejector mode

This study investigates the impact of the choking phenomenon on the mixing, combustion, and thrust performance of rocket-based combined-cycle (RBCC) engines operating in ejector mode. Free-suction experiments were conducted on an axisymmetric kerosene-fueled RBCC engine equipped with two different mixer types (an expansion mixer and a constant-section mixer) under low backpressure conditions, and data on wall pressure distribution and net thrust were obtained. To supplement the experimental findings, numerical methods that incorporate compressibility effects and chemical non-equilibrium processes in the rocket nozzle were also utilized. The simulated results showed a high level of agreement with the experimental data, with relative errors of 3.6 % and 1.3 % for pressure and 3.42 % and 0.05 % for thrust, respectively. The simulations revealed two distinct combustion zones in the mixer, namely the chemical non-equilibrium zone within the rocket exhaust and the mixing zone between primary and secondary streams. The former zone was found to be highly susceptible to static pressure mismatch, which could result in significant oscillations. In contrast, the latter zone was almost uniformly distributed in the axial direction. Moreover, the study highlights the negative impact of inlet choking, which can result in low mixing intensity and significant total pressure loss due to the shock structure. Therefore, avoiding inlet choking is recommended for RBCC engines. Finally, the study also shows that exit choking in the constant section mixer, although reducing the entrained secondary air, could achieve higher mixing and combustion intensity than the expansion mixer, resulting in 18.87 % more specific impulse than a conventional rocket at the simulated condition of H5.1 km / Mach 1.0. In conclusion, this research provides critical insights into the choking phenomena on RBCC engines through rigorous numerical and experimental methods.

Effects of excess oxidizer coefficient on RBCC engine performance in ejector mode: A theoretical investigation

This study investigates the effects of the primary rocket’s excess oxidizer coefficient (EOC) on the performance of a kerosene-fueled rocket-based combined cycle (RBCC) engine operating in ejector mode using the diffuser and afterburning (DAB) cycle. Employing complete mixing and chemical equilibrium assumptions, a quasi-one-dimensional model is developed to simulate combustion processes in the primary rocket chamber, mixer, and afterburner. Experimental data and full three-dimensional simulation validate the model’s accuracy. Results indicate that increasing EOC mitigates thermal choking effects at the mixer exit, decreasing mixing and Rayleigh losses, enhancing the entrainment ratio, and providing more oxygen for secondary combustion. However, the gas mixture’s compression ratio and total temperature decrease, rendering secondary combustion more susceptible to thermal choking in a constant cross-sectional afterburner. As the flight Mach number increases from 0.8 to 2.5, the optimum EOC for maximum specific impulse shifts from 0.725 to 2.375. While the optimum EOC for maximum thrust remains consistently around 2.0. The findings underscore the necessity of a higher EOC to fully unlock the DAB cycle’s thrust augmentation potential.

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.

A simplified chemical model for RBCC engines operating in ejector mode

The ejector mode in rocket-based combined-cycle (RBCC) engines involves complex mixing and combustion processes between the primary and secondary streams and second fuel, which poses a challenge to numerical simulation. This paper introduces an efficient model to predict RBCC engine performance in ejector mode. Given the slow growth rate of compressible mixing layers with strong compressibility effects and high chemical reactivity of immediate species within the primary stream, the combustion intensity is primarily governed by the mixing process. The model converts residual chemical energy within the primary stream into suitable mass flow rates of H2 and CO and simplifies the intricate chemical kinetics into single-step models. The model was tested on hydrogen- and kerosene-fueled RBCC engine experiments involving diffusion and afterburning (DAB) and simultaneous mixing and combustion cycles. Simulation results align well with experimental data regarding wall pressure distributions, entrainment ratio, and axial thrust. The model suggests intense combustion occurs in both the mixer and afterburner, even in the DAB cycle, which notably reduces the entrainment ratio. Additionally, early combustion within the mixing process incurs extra Rayleigh loss and impairs pressurization performance. The secondary fuel exhibits a jet-wake stabilized combustion mode under a high-temperature and low Mach number environment. In conclusion, the proposed model provides an efficient way of predicting RBCC engine performance in the ejector mode.

Experimental analysis of the air-to-water ejector-based R290 heat pump system for domestic application

The aim of this study was to analyze and compare the performance of an 8 kW ejector-based R290 heat pump system for hot water heating applications that operates with two different expansion devices: a throttling valve and a two-phase ejector. The vapor compression system was designed to compare the unit operating in direct expansion and ejector-based working modes under similar operating conditions. The test results were assessed for overall system performance measured by COP and heating capacity and for single-component operation. The ejector mode allowed operation at the same range of ambient temperatures as direct expansion mode with the potential to completely replace the expansion valve. The COP of ejector system was up to 2.6, which was 38% higher than direct expansion when working under similar conditions for an ambient temperature of −12.8°C. Moreover, the COP in direct expansion mode decreased significantly along the decrease of ambient temperature, whereas for the ejector mode this decrease was considerably smaller.

Developing characteristics of subsonic-supersonic shear layers induced by different lobed mixers in the RBCC

The rapid mixing of air and rocket gas is pivotal for enhancing combustion performance in the ejector mode of rocket-based combined-cycle engines. This study experimentally investigated developing characteristics of subsonic-supersonic shear layers induced by sinusoidal- and rectangular-lobed mixers using a subsonic-supersonic shear layer experimental system and an ice-cluster-based planar laser scattering technique. Both sinusoidal- and rectangular-lobed mixers evidently bolstered mixing through the promotion of transverse flow and vortex interactions. Under equivalent conditions, the sinusoidal-lobed mixer offered superior mixing effects. The mixing enhancement of a sinusoidal-lobed mixer exhibited a positive correlation with the wavelength, with excessively small wavelengths potentially impeding mixing. Based on the dimensionless wavelength ratio, a criterion was identified for evaluating the mixing enhancement of the sinusoidal-lobed mixer: a positive mixing enhancement occurred when the dimensionless wavelength ratio was at least 1. Notably, in this study, the shear layer thickness growth rate increased by 12 % when the dimensionless wavelength ratio was 1.905. These findings can offer valuable insights for improving combustion performance via enhanced mixing.

Thermodynamic performance modeling, optimization and numerical simulation of RBCC ejector mode

Ejector mode is a unique and critical phase of wide-range rocket-based combined cycle (RBCC) engine. In this paper, a quasi-one-dimensional thermodynamic performance modeling method, with more detailed model treatments for the inlet-diffuser system, primary/secondary flow interaction, and pressure feedback matching, was developed for operating characteristics studies and multi-objective optimization analysis of the ejector mode of an actual RBCC engine. A series of three-dimensional simulations of separate inlet and full flowpath was completed to validate the modeling study and provide further insight into the operating characteristics. The primary/secondary equilibrium pressure ratio functions a significant effect on ejector mode performance, a higher performance augmentation can be obtained by lower rocket pressure ratio, larger mixing section area ratio, smaller throttling throat and higher equivalence ratio, within an appropriate range. The positive performance augmentation can be realized at low flight Mach conditions, the coordination and trade-off relationships between specific impulse, performance augmentation ratio and thrust-to-area ratio during ejector mode are present by the Pareto-front from MOP analysis. It is further verified by CFD simulation that, the operating back-pressure at the exit of inlet-diffuser system functions a decisive influence on the airbreathing characteristics, the pressure feedback and matching should be well-controlled for secondary flowrate and performance augmentation. The thermodynamic modeling analysis results are basically consistent with those of numerical simulation, to validate the rationality and effectiveness of the modeling method.

Thrust performance of the rocket-based combined-cycle engine under ejector mode

The Rocket-Based Combined-Cycle (RBCC) engine integrates the preponderance of ramjet and rocket engines. It can perform excellently at a lower Mach number than a ramjet while consuming less fuel than the rocket. The higher specific impulse under lower flight Mach conditions guarantees the competitiveness and application prospect of an RBCC engine for reusable space transportation and hypersonic cruise vehicles. With a wide range of Mach numbers, the flow choking between primary and secondary streams in the inner flow passage of the engine becomes complicated. The flow choking not only affects the mass flow ratio between the air stream and rocket plume but also determines the thrust performance of the engine. However, the relationship between flow-choking states and thrust performance has not been revealed. This investigation aimed to provide a better design reference for the RBCC trajectory design and a basis for the RBCC engine geometry design so that the thrust performance under different flow choking states was studied. The findings indicate that the engine is more favorable in the supersonic regime status than others. At the premise of the lower total pressure air stream condition, the thrust of the RBCC engine is attributed to the rocket pressurization effect. On the contrary, its thrust is ascribed to the air stream when the air stream total pressure is higher. Besides, the thrust augmentation of the RBCC engine is essentially due to the larger entrainment ratio. There is a rocket mass flow rate range in different mixer diameters, named region B in this paper, in which the air mass flow rate can be boosted with the rocket mass flow rate and the specific impulse ratio is higher than the rocket specific impulse. Significantly, the RBCC design state should be maintained in region B to meet the operational requirements of engines under the precondition of an insufficient flight Mach number.

Experimental study of rocket plume expansion in the rocket-based combined-cycle engine under the ejector mode

In this paper, we utilized high-speed schlieren technology to investigate the impact of expansion degrees on the evolution of subsonic-supersonic mixing layers. Specifically, we explored characteristics of supersonic and subsonic flows with Mach numbers of 1.27 and 0.75. Our findings revealed that over-expansion of the supersonic flow facilitated transition, causing the generation position of Kelvin-Helmholtz vortexes to shift forward by 35.3%∼45.1%. Additionally, the growth rate of visual thickness was observed to increase to 2.7∼3.5 times in the incipient stage of the mixing layer under these conditions. We employed root-mean-square analysis to observe the rapid increase of turbulence intensity and the expansion of the strong turbulent region in the incipient stage (within x/H=0∼1.5), from under-expansion to over-expansion conditions. Our results indicate that the turbulent region and intensity play a crucial role in the evolution of mixing layers. Spatial correlation analysis revealed larger turbulent structures under over-expansion effects, with structures at x/H=1.0 being over 8.2% larger than those under weak under-expansion conditions. This turbulent mechanism explains why the over-expansion of rocket plumes can enhance mixing layers. Therefore, designing the rocket in the over-expansion state is an effective strategy for rocket-based combined-cycle engines under the ejector mode. Moreover, we noted that mixing layers and Kelvin-Helmholtz vortexes deflected to the low-pressure flow side, and pressure-unmatched conditions would cause flatten turbulent structures in subsonic-supersonic mixing layers.

Ejector mode performance improvement of rocket-based combined cycle engine designed for dual-ramjet mode

Rocket-based combined cycle engine integrating multi-mode operation accomplishes an excellent performance under various flight speed conditions. Generally researchers selected dual-ramjet mode as the cruise and design point with adjusting heat addition method to improve the performance under other modes. To analyze the difficulties during the multi-mode design, a unilateral expansive flowpath through central rocket ejection was investigated to analyze the effects of the mixing ratio of oxygen and fuel and the mass flow rate of the primary rocket plume. The results indicate that the mixing ratio increase can significantly strengthen the entrainment amount. There is an optimal operating rate to obtain the highest entrainment ratio due to Fabri choking limit induced by a confined path width. However, the excessively divergent afterbody based on dual-ramjet results in the under-expansion issue of the exhaust gas and negative thrust of the nozzle. The multistage ejection method can effectively overcome this shortage by compressing the exhaust gas. But with the flight speed increasing, the existence of upstream bow shock induces high inlet resistance. Adding extra duct can effectively solve the problem by decreasing inlet convergent ratio and sucking more secondary flow for combustion.

Experimental and numerical study on reinforcement mechanism of embedded rocket on back pressure resistance of RBCC inlet at starting stage

Favorable back pressure resistance of an RBCC inlet is vital for its smooth start and stable operation in ejector-to-ramjet mode transition. As a core component of the RBCC engine, the embedded rocket can be fully used to enhance the back pressure resistance of the inlet at the starting stage. The reinforcement mechanism of an embedded rocket on the back pressure resistance of the RBCC inlet is studied through experimental and numerical methods. An RBCC inlet model fully concerning the combustion pressure in the RBCC combustor, and the expansion and aerodynamic compression of the embedded rocket jet is established, whilst a facility nozzle is deployed to replace the compression section of the inlet in the ground direct-connect tests. The results show that three typical rocket operation stages may exist impacted by both the rocket jet and the back pressure, in terms of ‘Subcritical Rocket Operation’, ‘Critical Rocket Operation’, and ‘Supercritical Rocket Operation’. Meanwhile, two different patterns of flow separation caused by the synergy of the rocket jet and back pressure are observed. In the ‘Subcritical Rocket Operation’ stage, the two separation regions are strongly influenced by the back pressure. Nevertheless, the locations of the separations and the related shock train move downstream synchronously as the rocket pressure increases. In the ‘Supercritical Rocket Operation’ stage, the increasing rocket pressure drives the cowl wall separation downstream gradually, and however, it pushes the shock train moving upstream in the isolator resulting from the aerodynamic compression of the rocket jet in the mainstream. And in the ‘Critical Rocket Operation’ stage, an optimum status is found to exist such that the shock train stands most downstream from the physical throat, at which the RBCC inlet performs at the best back pressure resistance. Therefore, an appropriate embedded rocket operation can effectively reinforce the back pressure resistance of the RBCC inlet. Since the embedded rocket is operated at relatively high pressure, the under-expanded rocket jet will compress the mainstream and form a new aerodynamic throat in the flow passage, which can: 1) increase the real internal contraction ratio of the RBCC inlet to improve the compression efficiency of the incoming flow, and 2) enlarge the aerodynamic expansion ratio of the downstream flow passage. Further, the appropriate embedded rocket operation can also effectively assist the RBCC inlet to start at a certain Mach number, but with higher combustion pressure in the ejector mode.

Start Limits and Positive Control of RBCC Inlet

AbstractA rocket-based combined-cycle (RBCC) inlet is always required to start at a relatively low Mach number in consideration of the fuel consumption in the ejector mode and the comprehensive eng...

Experimental analysis of an ejector for anode recirculation in a 10 kW polymer electrolyte membrane fuel cell system

For analyzing ejector's performance in the system, an ejector for a 10 kW polymer electrolyte membrane fuel cell (PEMFC) system was first designed, manufactured, and a 10 kW PEMFC system bench was built up. A proportional valve and PI pressure feedback control method were adopted to control the hydrogen supply and anode inlet pressure. During the test, performances between dead-ended anode (DEA) mode and ejector mode were compared. Ejector's performances in the system, i.e., volume flow recirculated ratio, difference pressure, dynamic responses of primary pressure, anode inlet pressure, and recirculated gas flow rate during the purge process and current variation condition, were investigated. The results show that pressure adjustment is accurate, continuous, and fast using the proportional valve and PI pressure feedback control method. The hydrogen consumption rate in the ejector mode can reduce from 5% to 10% compared with the rate in the DEA mode except for the stack current 5 A and 10 A conditions. For better water removal out of the anode channel in ejector mode, the maximum stack power increases from 5.11 kW (DEA mode) to 9.56 kW (ejector mode). Anode pressure surge caused by the purge valve switching enhances the ejector's recirculated performance significantly.

A novel experimental method to the internal thrust of rocket-based combined-cycle engine

• The method can be used to study high altitude performance of the RBCC engine. • There is a great moment to the engine when the rocket turns on. • The air mass flow rate is the most important factor to the RBCC engine. The rocket-based combined-cycle engine is considered as a promising scheme to achieve affordable and reusable space transportation. Under the ejector mode, the air mass flow rate is limited by aerodynamic chocking in the mixer of the engine. In other words, the air mass flow rate is determined by the downstream mixing. However for the requirement of critical air mass flow rate, the air stream supplying and the internal thrust measurement are difficult to conduct in the conventional ground experimental facilities. And more importantly, the critical thrust performance is also hard to obtain. A novel experimental facility is introduced in which the air stream is entrained from the ambient atmosphere. The inlet throat is large enough to make sure the inflow air is throttled by aerodynamic chocking in the mixer. In addition, a supersonic ejector is used to simulate the pressure environment at a high altitude. The internal thrust under different flight conditions can be calculated through the measured parameters in the experiments. The results show that the novel experimental facility can be used to study the thrust performance of rocket-based combined-cycle engine in ejector mode. The test precision is acceptable and the thrust difference between the test and the simulation is less than 8%. With the increase of the inflow Mach number, the engine internal drag under the cold flow condition rises as well as the specific impulse augmentation. The specific impulse augmentation could be 16.5% when the entrainment ratio is 1.88 at a simulated condition of H6.8 km / Mach 1.2.

Experimental study on the rocket-ejector system with a throat in the secondary stream

This study concentrated on hot-fire tests to demonstrate the flow characteristics that influenced by the throttle of the secondary stream in the rocket-ejector system. The experiment was conducted under the sea level static (SLS) condition. A small rocket used gaseous oxygen / liquid kerosene propellant as a primary stream supply system, and the high-pressure air bottles were employed as the secondary stream supply system. The equivalence ratio of the rocket, the air mass flow rate and the mixing length were varied to study the performance. The results show that there is a mass flow rate mismatch between the air flow entering through the inlet and the mass flow permitted due to aerodynamic choking. If the air mass flow rate through the throat is more, the inlet may unstart. If the air mass flow rate through the throat is less, the total pressure loss of the air due to the pseudo-shock would be caused under the effect of the suction performance. The unsuitable air mass flow rate is detrimental to RBCC engine operation in the ejector mode. With the increasing equivalence ratio of the rocket, the pressurization performance is improved accordingly, but the suction performance is obviously decreased. When the equivalence ratio is more than 1.1, the reacting shear layer would affect the suction performance.

Numerical investigation on flow and mixing characteristics inside a converging-diverging mixing duct of rocket-based combined-cycle engine in ejector mode

The supersonic mixing layers with complex background waves are formed by the rocket jet and the entrained air in the mixing section of rocket-based combined-cycle (RBCC) engine during ejector mode. To achieve mixing enhancement of primary and secondary flows in confined space, a converging-diverging mixing duct was designed in this study. For a typical flight regime of RBCC ejector mode, the effects of the mixer geometry including the contraction ratio, the throat position of mixer and the converging angle on the flow structures and mixing characteristics in the converging-diverging mixing duct under no backpressure condition were numerically investigated. A detailed discussion on the mechanism of mixing enhancement in this mixer configuration was also presented. The results indicate that appropriately increasing the contraction ratio can significantly strengthen the interactions of background shock waves with supersonic mixing layer in the converging-diverging mixing duct, which effectively promotes the rapid and sufficient mixing between primary flow and secondary flow. Besides, shortening the length from the mixer inlet to the throat position can enhance the growth rate of mixing layer and the mixing efficiency of two flows in the upstream region of converging-diverging mixing duct to some extent. However, in the downstream far-field region, the variations of throat position have little impact on the scalar mixing process in supersonic mixing layer. Moreover, it is found that increasing the converging angle has no obvious effect on intensifying the mixing performance of the converging-diverging mixing duct. On the contrary, it may lead to a dramatic decrease in total pressure recovery. Further analysis reveals that the oblique shocks generated at the compression corner and the separation shocks formed in the divergent section interact with the supersonic mixing layer, which contributes to mixing enhancement in the converging-diverging mixing duct.

Mixing and combustion augmentation of the RBCC with different mixer configurations in ejector mode

Achieving mixing and combustion augmentation of air and rocket exhaust gases become a crucial issue in RBCC ejector mode, in which the mixer is an effective way to improve mixing efficiency. In current study, the three-dimensional coupled implicit Reynolds average Navier-Stokes (RANS) equations, SST k-ω turbulent model and Eddy-Dissipation Concept (EDC) reaction model are employed to investigate the flow fields with different mixer configurations, namely the lobe, rectangle, triangle, and pentagon. The simulated pressure of the prototype has the same trend with the experimental results. The different grid scales bring only a subtle difference for the simulation results, which prove the validity of the calculation results. Simultaneously, the vortex structure, mixing efficiency, combustion efficiency and total pressure loss are investigated. The results show that the lobed mixer and rectangle mixer retain the best mixing combustion augmentation characteristics, and the triangle is the worst. Besides, with the number of lobes decreases, the larger scale vortex structure is induced downstream the mixers. It improves the combustion performance by enhancing the penetration depth. Finally, all mixers only generate the total pressure loss downstream instead of the current location, revealing that mixers have less impact on the friction loss of the inflow.

Effects of component geometries and inflow conditions on ejector operational mode

The ejector system is a useful device for creating high altitude conditions for ground tests of supersonic engines. The ejector performance can be immediately changed by varying the ejector operational mode. The ejector nozzle pressure ratio is well known to have a significant effect of the operational mode of an ejector. However, the effects of the mixing duct length and other geometric design parameters on the ejector mode change have not been clearly determined. In this study, the effects of ejector component geometries and inflow conditions on ejector operational mode are investigated by numerical analysis. By changing the inflow conditions and geometric parameters, twelve test cases are studied. Using the numerical test results, the flow pattern and suction pressure performance of the ejector with a fixed secondary mass flow rate are compared. In the numerical test results, a high primary nozzle stagnation pressure induces a highly underexpanded flow, resulting in the critical operational mode. For the critical operational mode, the mixing duct must be sufficiently long to accommodate the shock train and the choking zone. Primary nozzles with wide angles also induce widely expanded nozzle flows and result in the critical operational mode.