Nozzle Throat Research Articles

Simulation and Experimental Investigation of Performance and Flow Behavior for Steam Ejector Refrigeration System

The ejector refrigeration system is a desirable choice to reduce energy consumption. A Computational Fluid Dynamics CFD simulation using the ANSYS package was performed to investigate the flow inside the ejector and determine the performance of a small-scale steam ejector. The experimental results showed that at the nozzle throat diameter of 2.6 mm and the evaporator temperature of 10oC, increasing boiler temperature from 110oC to 140oC decreases the entrainment ratio by 66.25%. At the boiler temperature of 120oC, increasing the evaporator temperature from 7.5 to 15 oC increases the entrainment ratio by 65.57%. While at the boiler temperature of 120oC and the evaporator temperature of 10oC, increasing the nozzle throat diameter from 2.4 to 2.8 mm decreases the entrainment ratio by 40%. The numerical results showed that reducing the condenser back pressure or increasing the primary fluid temperature, secondary fluid temperature, and nozzle throat diameter moves the second shock waves in the downstream direction. It could be concluded that the second shock series position detects the ejector operation mode. The ejector runs in critical mode if the second shock series position is close to the diffuser. In contrast, if the second shock series position moves toward the upstream, the ejector runs in subcritical mode.

Effect of spindle position on the performance and two-phase characteristics of controllable ejector for transcritical CO[formula omitted] refrigeration system

Spindle-controlled variable-geometry ejectors can adjust the system capacity to meet varying load requirements in transcritical-CO2 refrigeration systems. The present study numerically explores the effect of spindle position on the performance and flow characteristics of a controllable two-phase CO2 ejector. Spindle-controlled ejectors achieved entrainment ratios higher than that of fixed geometry ejector. Motive and suction flows exhibited high sensitivity to the spindle position near the critical operating range because of the transition between operating modes. The two-phase and shock characteristics associated with ejector flow and the variations observed for different spindle positions are discussed. Critical discharge pressures and entrainment ratios are estimated for various spindle positions. The critical compression and entrainment ratios correlate with the respective normalized spindle position in the ejector. The impact of the effective nozzle throat area on the flow entrainment range and achievable pressure lift is investigated. The results showed that the critical entrainment ratio bears a common linear relationship with the critical compression ratio for all positions of the spindle. The study relates the most significant performance and geometric parameters of the spindle-controlled two-phase CO2 ejector.

A novel carbon dioxide capture technology (CCT) based on non-equilibrium condensation characteristics: Numerical modelling, nozzle design and structure optimization

The carbon capture technology (CCT) is widely studied to decrease carbon emissions and improve the global climate. Supersonic separation is becoming a new strategy for decarbonization. In order to simulate the non-equilibrium condensing flow of CO2 in the supersonic nozzle more accurately, the mathematical model is modified and verified, and non-equilibrium condensation of carbon dioxide in different states and different inlet superheat levels are analysed numerically. In order to improve the separation efficiency of the supersonic decarbonization technology and reduce the energy loss, the nozzle structure is appropriately designed and optimized. Firstly, the model is verified by gas properties, different droplet growth models and surface tension models. The result shows that the real gas model can predict the non-equilibrium condensing flow in the nozzle more accurately than the ideal gas model. Among the four droplet growth models, the most suitable one was selected in order to predict the non-equilibrium condensation properties of CO2. The droplet surface tension is corrected by introducing temperature and the droplet radius, and the droplet condensation process is enhanced. Secondly, the condensing flow through the nozzle is predicted in different states (subcritical, near-critical and supercritical). The result shows that the prediction based on the modified model is basically consistent with experimental data. Analysing the predicted Wilson line, it is found that the non-equilibrium effect of the flow is quite different at different inlet conditions. The condensation locations gradually approach the nozzle throat, the condensation interval narrows with an increase in pressure, and the peak of the nucleation rate increases gradually. Furthermore, the effect of inlet superheat on the non-equilibrium condensing flow is investigated. As the temperature rises, the flow loss decreases and the thermal efficiency increases from 61.15% to 67.62%. Finally, four nozzles with different convergence shapes are calculated. One of them demonstrates stronger stability, lower losses and the highest thermal efficiency which is about 67.64%. Additionally, when the expansion angle is 7°, the average of the liquid mass fraction is the highest, about 21.7%.

Analysis of the internal flow features of a CO2 transonic nozzle and optimization of the nozzle shape profile

Two-phase flow Laval nozzles are simple and have promising applications, but the internal flow is intricate and encompasses physical phenomena like phase transition, transonic and Mach waves. Therefore, a 1D mathematical model for validating and designing a CO2 two-phase flow transonic nozzle was developed and validated based on experiments. The findings demonstrated that a grid density of 0.292 mm was appropriate and that the maximum error of the model was less than 6.0 %, with an average error of less than 2.5 %. Additionally, it was discovered that when a phase transition occurred at the nozzle throat, whether through condensation or evaporation, there was a minimum critical pressure ratio in the nozzle; when the phase transition took place in the nozzle diffusion chamber, there was a minimum total pressure ratio. Finally, the Laval nozzle was designed to utilize the isostatic pressure gradient method (IPG), which gave a more significant pressure drop than a conventional nozzle while eliminating the shock wave strings at the nozzle throat. Under the typical operating conditions of a transcritical CO2 heat pump system, the ejector performance was boosted by an average of 5.44 %.

Self-optimized efficient operation of trans-critical CO2 ejector-expansion heat pump water heater system with two-stage evaporation

In application scenarios with lower ambient temperatures and higher sink temperatures, the trans-critical CO2 heat pump water heater system using an ejector instead of a conventional expansion device offers better performance, and the higher pressure differential allows for partial pressure loss recovery through the ejector. However, the complexity of discharge pressure and ejector matching affects the further application of the system. Therefore an optimization method called Extremum seeking control method is proposed to achieve the lowest power consumption implementation under off-design conditions. The ESC controller applied to the system uses the compressor power consumption as the feedback signal and the discharge pressure is used as operating input set-point. The dynamical model of a trans-critical CO2 heat pump water heater built on Modelica/Dymola is used as the study object and optimized for three scenarios: fixed, sloped step profile, and actual ambient temperature. The simulation results show that the ESC applicable to the proposed system can optimize the matching of discharge pressure and ejector driving nozzle throat area to obtain the best performance. Compared with the benchmark system, the extremum seeking control for actual ambient temperature conditions results in a 5.7% increase in system efficiency.

Performance evaluation of supersonic flow for variable geometry radial ejector through CFD models based on DES-turbulence models, GPR machine learning, and MPA optimization

This study aims to derive valuable insights for utilizing computational fluid dynamics (CFD) based on reynolds-averaged navier–stokes (RANS) and detached eddy simulation (DES) turbulence models (TMs) to analyze a specific radial ejector configuration known as the variable geometry radial ejector (VGRE). The VGRE features the primary nozzle and ejector duct plates with adjustable disk-like surfaces, allowing for changes in the nozzle and ejector throat areas within a single ejector. Extensive numerical investigations of the VGRE are conducted by systematically validating CFD models with experimental datasets and subsequently using the most appropriate TM to design a new radial ejector. The study reveals that the DES SST k-ω turbulence model achieves the closest agreement with experimental data, with an average entrainment ratio (ω) discrepancy of only 5 %. However, there are challenges in accurately predicting the critical compression ratio (rc*), especially under varying conditions. Based on the CFD results, the original VGRE exhibited ω values ranging from 0.16 to 0.61, rc* values between 1.5 and 3.1, and ejector efficiency (η) values between 7 % and 17 % at expansion ratio (re) values ranging from 89 to 150 for different nozzle throat separations (d) and different duct throat separations (D). Furthermore, this study presents a comprehensive investigation into predicting and optimizing ω, rc*, and η parameters using a multi-output gaussian process regression (GPR) model and a marine predators algorithm (MPA) approach. The multi-output GPR model was constructed to predict the relationships between boundary conditions (primary pressure (Pprimary) and secondary pressure (Psecondary)), geometric parameters (d and D), and the response variables (ω, rc*, and η). The model evaluation employed a 5-fold cross-validation technique to assess predictive performance, demonstrating strong predictive accuracy with low root mean square error (RMSE) and high coefficient of determination (R2) values. The optimization results revealed that the highest achieved values were ω = 0.303, rc* = 2.678, and η = 0.156, corresponding to specific parameter settings (Pprimary = 160 kPa, Psecondary = 1.8 kPa, d = 0.6 mm, and D = 2.3 mm). This study demonstrates the effectiveness of the multi-output GPR model for accurate prediction and the multi-objective MPA optimization approach for identifying optimal input parameters to maximize entrainment ratio, critical compression ratio, and ejector efficiency in fluid dynamics systems.

Numerical Analysis on Effect of Structural Parameters on Flow Field and Internal Trajectory in Gas–Steam Ejection Systems

This paper aims to study the influence of the structure of spray holes and throats on the flow field and the internal trajectory of the gas–steam ejection device. The compressible Navier–Stokes equations, discrete ordinate methods and RNG k-ε turbulence model are utilized to simulate the two-phase flow of the rocket gas with multispecies and the water sprays. The comparison between numerical results and experimental data confirms the accuracy and effectiveness of this model. The simulation analysis on the cases of the ejection process with multiple spray hole diameters, number of spray holes, total spray area, and throat diameter are conducted. The shock wave structure inside the gas–steam ejection device is examined. The simulation results show that, instead of the spray hole diameter and number, the total spray area and secondary nozzle throat diameter are the key factors that affect the flow field and internal trajectory of the gas–steam ejection device. Under the existing spray structure, the maximum number of spray holes is 300 to achieve the stability of the flow field and internal ballistic trajectory of gas–steam ejection devices. By comparing the throat diameters of multiple secondary nozzles, it was found that the minimum throat diameter of the secondary nozzles should be no less than 100 mm. The results could be valuable for the design of gas–steam ejection devices.

Flow and acoustic fields investigation of noise reduction by micro vortex generators in supersonic nozzles

The role of micro-vortex generators (MVGs) in supersonic jet noise reduction is investigated. Studies are performed to understand the noise reduction mechanisms of these devices. MVGs are implemented on a scale model representative of GE-F404 nozzles. Configurations consisting of single and dual arrays of MVGs were tested for pressure ratios relevant to takeoff operating conditions. A combination of laboratory measurements and large-eddy simulations are used. Particle image velocimetry reveals that MVGs placed inside the nozzle form oblique shocks that interact with the jet plume's shock cell structure and greatly modify the shock-cell structure issued from the nozzle throat. This modification can weaken the jet plume shock-cell structure and even reduce the jet core velocities. Convective velocities estimated from the high-speed Schlieren imaging showed a significant reduction in magnitude caused by MVGs. When compared to the baseline design, the MVG nozzles showed noise reduction up to 10 dB in the upstream direction and near 5 dB in the peak downstream radiation angle at cold jet conditions. Near-field acoustic measurements showed a significant reduction of low-frequency turbulent mixing noise. Scaling analysis showed that MVGs are capable of delivering the same noise reduction performance across different model scales.

A Simple Design Approach of Two-Phase Ejectors for CO2 Transcritical Heat Pumps

Integrating a two-phase ejector in mechanical vapor compression heat pumps is a practical and low-cost solution for improving performance and reducing energy consumption. Typically, using an ejector to recover part of the important pressure expansion losses in CO2 systems may improve the operating conditions of the compressor. One of the prerequisites for the success of such an application is the proper design of the ejector. This study is mainly dedicated to developing a simple approach for CO2 ejector design. The advantage of using the ejector as an expander in a transcritical CO2 heat pump is first introduced. Compressor operation is particularly improved. The development of an ejector design model for CO2 expanding from transcritical to two-phase conditions is presented. Validation of the thermodynamic model with experimental results from the literature shows the predictions to be within an acceptable range of discrepancy. The primary nozzle throat diameter calculations do not exceed ±8% of error for transcritical conditions. The error of the predicted pressure at the outlet of the ejector is in the limit of -15% to +3%. A practical design example for estimating the transcritical CO2 ejectors geometry integrated in a heat pump is presented. The results show the important decrease of primary nozzle diameters with the drop of Tevap, especially for the throat. A decrease of Dmix also occurs with Tevap and an optimal diameter is obtained for each condition considered. The design of the diffuser is based on a compromise between the outlet velocity and the length of the diffuser. The detailed design procedure with the proposed model, complemented with data from the literature, is a valuable tool for rapidly generating useful results and obtaining preliminary designs transcritical CO2 ejector.

Optimization of the ejector parameters for anodic recirculation systems in high-performance dual-stack proton-exchange membrane fuel cells

The anodic recirculation system (ARS) is critical in designing modern proton-exchange membrane fuel cells (PEMFC). Although the flow-through approach can outperform the dead-end approach, it yields excess residual hydrogen during the operation of the fuel-supply system. This residue can reduce the fuel mileage and be explosive when discharged into the environment, particularly in the multi-stack configuration where multiple PEMFCs are connected for high-power purposes. Therefore, it is desirable to develop an appropriate ARS design that facilitates the recycling of this excess hydrogen. This study aimed to develop a one-dimensional ARS model for dual-stack PEMFCs to optimize the recirculation performance, as well as analyze the effect of ARS on fuel cell (FC) stacks. Subsequently, the dual-ejector concept was adopted, having demonstrated higher efficiency than the single-ejector concept in high-performance PEMFCs. In the designing step, ejector geometries were determined based on the sensitivity analysis of the nozzle throat and mixing chamber diameters. The filtering conditions were set for four parameters: hydrogen unrecoverable percentage (4,1% and 5%), stoichiometry (1.5–3), recirculation ratio, and anode inlet pressure (<1.5 bar). Hence, the minimum hydrogen unrecoverable value the system can achieve by optimization was 4.073%. Due to other filtering conditions being close to the upper limit of the requirements, expanding the unrecoverable condition was recommended despite higher hydrogen emissions. Additionally, the performed methodology demonstrated great potential in providing benefits related to simulation speed, accuracy, and the mapping of system-level interrelationships. Through this method, the optimized geometries ensured that filtering parameters were under control during operation. Moreover, this concept could reveal the direct effect of the anode inlet on the working pressure of each FC stack during dynamic simulation with changes in the Nernst voltage and losses.

Optimization Method of Jet Pump Process Parameters and Experimental Study on Optimal Parameter Combinations

The performance of jet pumps depends significantly on their structural and operational parameters. Current research primarily concentrates on theoretical models and laboratory tests, with limited experimental investigations and comprehensive reports concerning jet pump performance under various parameter combinations. In this study, we take a comprehensive approach, integrating both experimental and theoretical methods to assess jet pump performance under optimized process parameters and sand-flushing capabilities. The key findings of this study are as follows: While the characteristic and efficiency equations of jet pumps effectively describe the interaction of critical factors, including the area ratio, pressure ratio, flow rate ratio, and density, they unfortunately do not account for the influence of crucial structural factors, such as the nozzle throat distance, throat length, and diffuser length, on overall performance. Moreover, the parameter optimization technique based on the P-M curve has limitations and requires a tailored design and evaluation to consider the maximum suction capacity in practical engineering contexts. Notably, the nozzle throat distance significantly affects the jet pump suction capacity. Increasing the nozzle throat distance from 6 mm to 12 mm substantially enhances the suction capacity, albeit with a minor reduction in the lifting capacity. Extending the distance from 12 mm to 18 mm initially boosts the suction capacity, followed by a subsequent decline, along with a decrease in the lifting capacity. Interestingly, jet pumps effectively handle sand suction at relatively low pump pressures, creating negative-pressure conditions. As the pump pressure increases, the suction capacity remains relatively stable. However, the lifting capacity increases proportionally with the pump pressure, providing valuable theoretical and technical insights for practical jet pump applications. In summary, our study introduces a comprehensive approach to evaluate jet pumps by integrating experimental and theoretical methods. These insights highlight the intricate relationship between jet pump characteristics and operational parameters, offering essential knowledge for the efficient utilization of jet pumps in various applications.

Investigation on the influence of geometric parameters on two-phase ejector performance with R290

In this paper, the effect of geometric parameters on a two-phase ejector performance was experimentally studied, and the R290 was used as the working fluid. The four geometric parameters of the ejector were discussed in detail, including the diameter of the nozzle throat (Dt), the nozzle exit position (NXP), the diameter of the mixing section (Dm) and the length of the mixing section (Lm). The experimental results indicated that the pressure lift ratio (rp) of the ejector was sensitive to Dt and NXP. Within the increase of Dt and NXP discussed in this paper, the rp of the ejector varied from 1.13 to 1.33 and 1.32 to 1.07, respectively. In addition, a maximum rp of the ejector could be obtained when Dm was 3 mm to 4 mm under different nozzle throat diameter. When the Dt was 0.7 mm, the entrainment ratio of the ejector varied from 0.402 to 1.406 as the Dm varied from 2 mm to 6 mm. The Lm of the ejector diffuser had little effect on the ejector performance. Moreover, the empirical correlations for the efficiencies of R290 two-phase ejector components were summarized based on the experimental result and ejector model.

Numerical investigation of a Coandă-based fluidic thrust vectoring system for subsonic nozzles

The numerical investigation of a novel subsonic thrust concept for fluidic thrust vectoring (FTV) of jet engines is the subject of this publication. FTV possibly offers advantages over conventional mechanical thrust vector control such as decreased complexity, mass and maintenance costs. The operating principle of the FTV nozzles under investigation at the Institute of Jet Propulsion is based on the Coandă effect. The applied concept of thrust vectoring uses dedicated secondary flow channels which are mounted in parallel around the nozzle inner cone or the nozzle wall. If required, bleed air, provided by extraction from the engine compressor or from an external source, is injected at a specific nozzle pressure ratio at the nozzle throat A_8 through these secondary ducts, resulting in the redirection of the secondary jet towards the convex Coandă surface. The interaction between the primary mass flow and the secondary jet leads to a redirection of the primary exhaust mass flow of the engine and thus to a vectoring of the exhaust flow. In this paper, the influence of different nozzle geometric parameters and different operating points are investigated within an extensive parametric study applied to a convergent two-dimensional thrust vectoring nozzle using computational fluid dynamics tools. Thrust vector deflection of up to {20}^{circ } at a maximum secondary to primary mass flow ratio of 10 % is achieved. Reducing this mass flow rate to 5 % still yields vectoring angles of up to {15}^{circ } whereby similar deflection angles compared to conventional mechanical thrust vectoring systems are achievable.

Experiments of ablation characteristics for different nozzle materials and transient simulations on thermochemical erosion in hybrid rocket motors

This paper focuses on investigating ablation characteristics of different nozzle materials in hybrid rocket motors by firing tests and numerical simulations. Hybrid rocket nozzles with different ablation materials are designed and manufactured. Four firing tests with working time of 40 s are conducted, and 90% hydrogen peroxide and polyethylene are adopted. The test results indicate that erosion rates of T705 graphite, whole felt C/C composite, axial rod braided C/C composite, and carbon/ceramic composite are 0.045 mm/s, 0.022 mm/s, 0.03 mm/s, and 0.01 mm/s. Circumferentially uneven ablation is found in the nozzle throat of T705 graphite and C/C composite, and this is mainly due to asymmetric oxidizer injection, asymmetric combustion, and uneven material properties. Transient numerical simulations of 40 s on nozzle thermochemical erosion are carried out, and the regression process of fuel wall and nozzle wall is solved by a dynamic grid technique. The error of average throat erosion rate between simulation results and test results is only 1.82%. The evolution process of nozzle profile and flow field structure are exhibited, and coupling effects between the erosion rate and nozzle profile are obtained. Simulation results show that as the working time increases, protrusions and pits appear on the nozzle inner wall and gradually extend. The temperature of nozzle wall and nozzle erosion rate on the windward side is higher, while they are lower on the leeward side. Nozzle throat erosion rate history is divided into four stages, and time nonlinear characteristics of the throat erosion rate are revealed.

Three-dimensional numerical simulation of hybrid rocket motor based on dynamic mesh technology

The three-dimensional dynamic numerical simulation is rare in previous studies of hybrid rocket motors, therefore, a three-dimensional dynamic numerical model is established. A hybrid rocket motor is designed, which adopts 98 wt% hydrogen peroxide and polyethylene as the propellants. Then, a ground experiment is conducted to verify the model. The results show that the method of adopting dynamic mesh technology to simulate the three-dimensional regression of the combustion surface is feasible. The average errors of thrust and pressure in the simulation and experiment are 5.6% and 7.9%, respectively. Moreover, the inner surface of grain is scanned by computed tomography after the experiment. The average deviation of port diameter is 5% between the simulation and experiment, therefore, the accuracy of this model is acceptable. Then, the results also show that the thrust and pressure both gradually decrease with time in the simulation and experiment at 5-10 s. The main reason is the rapid reduction in the fuel mass flow rate. However, the pressure in simulation is stable but decreases with time in experiment at 10-25 s. The main reason is the expansion of the nozzle throat caused by the ablation. The results demonstrate that the numerical method can accurately predict motor performance. In addition, the flow field is approximately uniform along the circumference by the dynamic numerical method, which is difficult to obtain for the conventional steady-state and two-dimensional methods. It is the first time to realize the three-dimensional regression in hybrid rocket motors. Therefore, this method can be used to predict the three-dimensional regression of the complex shapes grain and also can guide the design of grain shape in hybrid rocket motors.

Numerical Simulation of Flashing Flows in a Converging–Diverging Nozzle with Interfacial Area Transport Equation

Flashing flows of initially sub-cooled water in a converging–diverging nozzle is investigated numerically in the framework of the two-fluid model (TFM). The thermal non-equilibrium effect of phase change is considered by an interfacial heat transfer model, while the pressure jump across the interface is ignored. The bubble size distribution induced by nucleation, bubble growth/shrinkage, coalescence, and breakup is described based on the interfacial area transport equation (IATE) and constant bubble number density model (CBND), respectively. The results are compared with the experimental data. Satisfactory prediction of the axial pressure distribution along the nozzle as well as the flashing inception, is achieved by the TFM-IATE coupling method. It was also found that the vapor production in the diverging section was overpredicted, and the radial gas volume fraction distribution deviated from the experiment. The radial diameter profiles exhibit opposite patterns at the nozzle throat and near the outlet, and similar trends can be observed for the superheated degree. A poly-disperse method is suggested to be introduced to describe the evolution of interfacial area concentration.

Experimental investigation of a novel variable geometry radial ejector

This work focuses on improving ejector performance in variable conditions by developing a radial flow variable geometry radial ejector (VGRE). The research objective was to demonstrate the performance of the improved VGRE, which can operate more effectively under changing operating conditions compared to a fixed geometry design. The improved VGRE was developed and experimentally tested to enable adjustment of the nozzle and ejector throat areas to suit different ejector operating conditions in various applications. Radial ejectors have a smaller size than axial ejectors, which is a significant advantage in some applications. Results showed that the improved VGRE achieved higher entrainment ratio and critical compression ratio compared to a fixed geometry design and that optimal performance was obtained with nozzle and duct throat separations of 0.5 mm and 3.0 mm, respectively. The results also indicated that the wall static pressure in the throat region of the improved VGRE was similar to that observed in conventional axial ejectors and showed an average improvement of 107 % in entrainment ratio and 76 % in critical compression ratio relative to previous studies of air ejectors. The findings suggest that the improved VGRE concept could be applied to a wide range of applications in the future.

Effects of magnetic nozzle strength and orientation on radio-frequency plasma expansion

To improve the efficiency of radio-frequency magnetic nozzle plasma thrusters, it is important to better understand the coupling between plasma expansion and a convergent–divergent magnetic field. This study explores the effects of magnetic field strength and orientation on plasma expansion in a magnetic nozzle. Two-dimensional measurements of the plasma characteristics obtained both in the source and in the expansion region are presented to investigate the influence of magnetic field strength on the formation of high-density conics in a symmetric magnetic nozzle. The measurements are repeated in a deflected magnetic nozzle using a novel magnetic steering system. Measurements of the ion saturation current and floating potential profiles are used respectively to qualitatively assess the plasma density distribution and the presence of high-energy electrons for the magnetic field configurations analysed. In the symmetric magnetic nozzle configuration, it is observed that the ion saturation current peaks on axis in the plasma source, but downstream of the nozzle throat, a double-peaked hollow profile is observed for all cases studied. The location of the high-density conics structure matches the most radial field lines that intersect the antenna and can freely expand downstream outside the source. Negative values of the floating potential are measured in the same peripheral regions, which could be a sign of the presence of high-energy electrons. When the magnetic field is deflected, the ion saturation current profile shows only a single peak centred around the bent field line that reconnects to the antenna. Again, a region of negative floating potential is measured at the location of the maximum ion current. Thus, it is shown how, independent of magnetic field strength and orientation, the magnetic field lines interacting with the antenna dictate the local plasma profiles downstream from the magnetic nozzle.

Numerical Investigation of Transonic Flow-Induced Spontaneous Condensation in Micro-Ejector Nozzles.

Micro-cooling systems are compact refrigeration systems widely applicable in microchemical analysis, biomedicine, and microelectromechanical systems (MEMS). These systems rely on the use of micro-ejectors to achieve precise, fast, and reliable flow and temperature control. However, the efficiency of micro-cooling systems is hindered by spontaneous condensation occurring downstream of the nozzle throat and within the nozzle itself, impacting the performance of the micro-ejector. A micro-scale ejector mathematical model describing wet steam flow was simulated to investigate the steam condensation phenomenon and its influence on flow, incorporating equations for liquid phase mass fraction and droplet number density transfer. The simulation results of wet vapor flow and ideal gas flow were compared and analyzed. The findings revealed that the pressure at the micro-nozzle outlet exceeded predictions based on the ideal gas assumption, while the velocity fell below it. These discrepancies indicated that condensation of the working fluid reduces the pumping capacity and the efficiency of the micro-cooling system. Furthermore, simulations explored the impact of inlet pressure and temperature conditions on spontaneous condensation within the nozzle. The results demonstrated that the properties of the working fluid directly influence transonic flow condensation, underscoring the importance of selecting appropriate working fluid parameters for nozzle design to ensure nozzle stability and optimal micro-ejector operation.

Transient numerical study of CO2 two-phase flow in a needle adjustable ejector

Steady-state numerical modeling is mostly used for predicting the flow and transport characteristics in an ejector, but it may not be able to reflect the actual performances of an ejector when operating condition varies. Moreover effects of needle movement on the performances of an ejector is not very clear. In this study, a transient heterogeneous mixture model was developed for a two-phase CO2 needle adjustable ejector with motive nozzle inlet in supercritical, critical or sub-critical regions. The dynamic mesh method was used to control the needle movement, and the unsteady coupling solution between the needle movement and the flow field is solved for the adjustable two-phase CO2 ejector. Modeling results show that the transient modeling can predict dynamic performances of CO2 ejector more accurately than the steady-state modeling. Based on the transient modeling, it is found that as the needle moves further into the motive nozzle throat, effective nozzle throat area decreases, expansion angle of motive flow increases then decreases, the velocity at the motive nozzle outlet decreases then increases, and entrainment ratio decreases, then increases sharply, then decreases, finally become negative. High pressure at the motive nozzle inlet leads to high entrainment ratio owning to high velocity at the motive nozzle throat outlet. A sudden increase of pressure at the motive nozzle inlet in different thermodynamic state regions (supercritical, sub-critical) leads to different dynamic performance responses and oscillation times of ejector. The mechanisms of boundary layer separation and backflow vortices as well as shock waves were explored to explain the suction flow chocking phenomena. This study is helpful to understand flow mechanisms inside the CO2 ejector, and improve optimal design and control of an adjustable two-phase CO2 ejector in variable operating conditions in practice.