Supersonic Ejector Research Articles

Development and testing of a pressure probe for centerline static pressure measurement in supersonic nozzles and ejectors

ABSTRACTThis work describes the development and testing of a system for measuring static pressure along the axis of supersonic flow devices such as convergent–divergent nozzles or ejectors. The pressure probe consists of a capillary tube positioned at the ejector centerline. A hole is perforated perpendicularly on the wall of the thin tube to capture and deport the static pressure measurement to a piezoelectric sensor located at one extremity of the tube outside the ejector. The measurements obtained are compared with numerical results from computational fluid dynamics simulations and wall pressure measurements. The first tests demonstrate the good capability of the pressure probe to detect shocks occurring in the flow and to provide continuous axial distributions of the static pressure.

A Numerical and Experimental Study of Ejector Internal Flow Structure and Geometry Modification for Maximized Performance

A wide range of industrial refrigeration systems are good candidates to benefit from the cooling and refrigeration potential of supersonic ejectors. These are thermally activated and can use waste heat recovery from industrial processes where it is abundantly generated and rejected to the environment. In other circumstances low cost heat from biomass or solar energy may also be used in order to produce a cooling effect. Ejector performance is however typically modest and needs to be maximized in order to take full advantage of the simplicity and low cost of the technology. In the present work, the behavior of ejectors with different nozzle exit positions has been investigated using a prototype as well as a CFD model. The prototype was used in order to measure the performance advantages of refrigerant (R-134a) flowing inside the ejector. For the CFD model, it is assumed that the ejectors are axi-symmetric along x-axis, thus the generated model is in 2D. The preliminary CFD results are validated with experimental data over a wide range of conditions and are in good accordance in terms of entrainment and compression ratios. Next, the flow patterns of four different topologies are studied in order to discuss the optimum geometry in term of ejector entrainment improvement. Finally, The numerical simulations were used to find an optimum value corresponding to maximized entrainment ratio for fixed operating conditions.

Thermodynamic Modelling of Supersonic Gas Ejector with Droplets

This study presents a thermodynamic model for determining the entrainment ratio and double choke limiting pressure of supersonic ejectors within the context of heat driven refrigeration cycles, with and without droplet injection, at the constant area section of the device. Input data include the inlet operating conditions and key geometry parameters (primary throat, mixing section and diffuser outlet diameter), whereas output information includes the ejector entrainment ratio, maximum double choke compression ratio, ejector efficiency, exergy efficiency and exergy destruction index. In single-phase operation, the ejector entrainment ratio and double choke limiting pressure are determined with a mean accuracy of 18 % and 2.5 % , respectively. In two-phase operation, the choked mass flow rate across convergent-divergent nozzles is estimated with a deviation of 10 % . An analysis on the effect of droplet injection confirms the hypothesis that droplet injection reduces by 8 % the pressure and Mach number jumps associated with shock waves occuring at the end of the constant area section. Nonetheless, other factors such as the mixing of the droplets with the main flow are introduced, resulting in an overall reduction by 11 % of the ejector efficiency and by 15 % of the exergy efficiency.

Assessment and prediction of component efficiencies in supersonic ejector with friction losses

In this paper, the influences of friction losses on ejector efficiencies are investigated by the computational fluid dynamics (CFD) technique. Roughness values factor is introduced to analyze ejector performance by considering the components efficiencies. Efficiencies of each component are assessed with different levels of roughness. Validation is given through comparisons between the calculated and experimental values obtained from the ejector refrigerant platform with diverse levels of surface roughness. Results indicate that the efficiency of the ejector decreases with the increase of roughness values and friction losses in constant-area section and diffuser have the most significant impact on ejector performance. By analyzing the relationship between efficiencies and roughness values, an efficiency correlation is built with a coefficient of determination over 0.9654. It is shown that proposed correlations for ejector component efficiencies can be utilized more accurately in system design of ejector based refrigerant system considering the friction losses.

Influence investigation of friction on supersonic ejector performance

The effects of internal surface roughness on ejector performance were investigated by dividing the ejector into five categories: the nozzle, suction chamber, mixing chamber, constant-area section and diffuser with the computational fluid dynamics (CFD) technique. Each section was analyzed with the experimental values obtained from the ejector refrigerant platform by using replaceable nozzles with different levels of roughness. It was ascertained that friction has a noticeable influence on the performance of the ejector; therefore, an increase in the roughness level will essentially lead to a decline in the ejector's working performance. Furthermore, the constant-area section and diffuser have the most significant impact on ejector performance. The correlations regarding the friction influence for numerical calculations and the polishing process of ejector manufacturing are also essentially discussed.

Experimental and Numerical Analysis on the Internal Flow of Supersonic Ejector Under Different Working Modes

ABSTRACTSupersonic ejectors involve very complex phenomena such as interaction between supersonic and subsonic flows, shock trains, instabilities, which strongly influences the performance of supersonic ejector. In this study, the static pressure distribution along the ejector wall and Mach number distribution along the axis are used to investigate the internal flow field of supersonic ejector. Results indicate that when the back pressure is much less than the critical back pressure, there are two series of shock trains, and the change of the back pressure will not affect the flow field before the effective area section, so the entrainment ratio would remain constant. The second shock train moves further upstream and is combined with the first shock train to form a single shock train as the back pressure rises. When the back pressure is greater than the critical back pressure, the position of the shock train, the static pressure at its upstream and the entrainment ratio, will be affected. The “effective area section” in the mixing tube is obtained. The effective area section position moves downstream with the increase of the primary flow pressure, while it moves upstream with the increase of the secondary flow pressure. The entrainment ratio shows inversely proportional relationship with the effective section position. Besides, the first shock train length increases with the increase of primary flow pressure or secondary flow pressure. The critical back pressure represents direct proportional relationship to the first shock train length.

The surface roughness effect on the performance of supersonic ejectors

The paper presents the numerical simulation results of the surface roughness influence on gas-dynamic processes inside flow parts of a supersonic ejector. These simulations are performed using two commercial CFD solvers (Star- CCM+ and Fluent). The results are compared to each other and verified by a full-scale experiment in terms of global flow parameters (the entrainment ratio: the ratio between secondary to primary mass flow rate - ER hereafter) and local flow parameters distribution (the static pressure distribution along the mixing chamber and diffuser walls). A detailed comparative study of the employed methods and approaches in both CFD packages is carried out in order to estimate the roughness effect on the logarithmic law velocity distribution inside the boundary layer. Influence of the surface roughness is compared with the influence of the backpressure (static pressure at the ejector outlet). It has been found out that increasing either the ejector backpressure or the surface roughness height, the shock position displaces upstream. Moreover, the numerical simulation results of an ejector with rough walls in the both CFD solvers are well quantitatively agreed with each other in terms of the mean ER and well qualitatively agree in terms of the local flow parameters distribution. It is found out that in the case of exceeding the “critical roughness height” for the given boundary conditions and ejector’s geometry, the ejector switches to the “off-design” mode and its performance decreases considerably.

Numerical Investigation on the Performance of Different Primary Nozzle Structures in the Supersonic Ejector

Computational fluid dynamics (CFD) technique is employed to investigate the effect of primary nozzle structures on the supersonic ejector performance. The performance of the supersonic ejectors with three different nozzle structures, including conical nozzle, petalage nozzle and crenation nozzle, have been compared under the same conditions. Compared with the ejector equipped with the conical nozzle, the entrainment ratio of petalage nozzle is slightly smaller, while the critical back pressure increases by 5.2%. Just the opposite, the entrainment ratio of crenation nozzle is slightly higher, and the critical back pressure would decrease by 2.1%. In addition, the effects of primary/induced fluid pressure on the flow field (pressure and Mach number along the ejector centerline) are observed and analyzed to explain the mixing process occurring inside the ejector. It finds out that the shape of the mixing layer in the supersonic ejector with different nozzles is different.

Performance comparison of supersonic ejectors with different motive gas injection schemes applicable for flowing medium gas laser

A class of flowing medium gas lasers with low generator pressures employ supersonic flows with low cavity pressure and are primarily categorized as high throughput systems capable of being scaled up to MW class. These include; Chemical Oxygen Iodine Laser (COIL) and Hydrogen (Deuterium) Fluoride (HF/DF). The practicability of such laser systems for various applications is enhanced by exhausting the effluents directly to ambient atmosphere. Consequently, ejector based pressure recovery forms a potent configuration for open cycle operation. Conventionally these gas laser systems require at least two ejector stages with low pressure stage being more critical, since it directly entrains the laser media, and the ensuing perturbation of cavity flow, if any, may affect laser operation. Hence, the choice of plausible motive gas injection schemes viz., peripheral or central is a fluid dynamic issue of interest, and a parametric experimental performance comparison would be beneficial. Thus, the focus is to experimentally characterize the effect of variation in motive gas supply pressure, entrainment ratio, back pressure conditions, nozzle injection position operated together with a COIL device and discern the reasons for the behavior.

Ejectors for pressure recovery systems of supersonic chemical lasers

Results of a numerical study of performance characteristics of supersonic ejectors with nozzles of different types are reported. The work was carried out with the aim of developing a high-performance ejector for pressure recovery systems of supersonic chemical lasers. A specific feature of the operation of ejectors in pressure recovery systems consists in that, in this case, the ejecting and ejected gases, as they undergo mixing, have different thermodynamic properties, and the ejection coefficient depends on the ratio between the temperatures of the gases and on the ratio of their molecular masses. Since the operation of an ejector is based on the mixing process, the task consisted in intensification of this process using nozzles of special geometries. The performance of ejectors was judged considering an integral parameter, the product of induction by compression ratio. The calculations of the 3D viscous gas flow in the ejector channel were performed using ANSYS software. In verifying the numerical model, a comparison with experimental data obtained earlier on a model ejector facility and during tests of real pressure recovery systems in operation with supersonic chemical lasers was performed.

Passive scalar mixing studies to identify the mixing length in a supersonic confined jet

Supersonic jet with a co-flow, closely bounded by walls is known as supersonic confined jet. Supersonic confined jet is encountered in practical devices like the supersonic ejector. Mixing of the primary and the secondary fluid inside the confined passage is complex. From a design perspective, it is necessary to have an accurate knowledge of the mixing length (L-MIX). Tracers that do not actively participate in the flow behavior but rather mark the fluids such that they faithfully follow the fluid motion are known as passive scalars. Passive scalars help in the understanding the progression of mixing amidst interacting flows. In this work, we have performed passive scalar mixing studies in a supersonic confined jet for different operating conditions using an existing low area ratio (AR = 3.7) rectangular supersonic gaseous ejector. Air is used as the working fluid in both the primary and the secondary flow. The design Mach number of the primary flow nozzle (M-PD = 1.5-3.0) and the total pressure of the primary flow (P-OP = 4.89-9.89 bar) are varied during the experiments. Using the planar laser-induced fluorescence (PLIF) technique and acetone as the passive scalar, L-MIX is determined. A 266 nm Nd-YAG laser with a repetition rate of 8 Hz is used to excite the acetone molecules in the flow field, and the emitted fluorescence is captured by an ICCD camera. A new method is proposed to study the passive scalar distribution from the acetone PLIF images through digital image processing. Spatial Scalar Fluctuations Intensity (SSFI or psi ) is a parameter defined at every transverse section along the flow direction. Based on the variation of psi along the jet, the location of L-MIX can be identified. L-MIX is defined as the length from the supersonic nozzle exit where psi first attains a value of 0.05. For the first time, L-MIX is quantified in a supersonic confined jet. L-MIX values are observed to be in the range of 3H to 6H for the cases under study, where H is the height of the confined passage. The behavior of L-MIX is closely dependent on the nozzle operating conditions. The values of L-MIX are found to be reduced by 17.67% for the over-expanded flows and increased by 15.76% for the under-expanded flows from the perfectly expanded condition. This study also provides other supersonic confined jet characteristics like the potential core length (L-PC) and the shock cell spacing (S-x) of the primary supersonic jet. Compared to the supersonic free jet, values of L-PC and S-x are found to be different in the supersonic confined jet.

Drop-in replacement in a R134 ejector refrigeration cycle by HFO refrigerants

The present work reports a numerical analysis of a single-phase supersonic ejector working with R134a as well as hydrofluoroolefin (HFO) refrigerants R1234yf and R1234ze(E). Comparisons were made regarding the ejector performances under varying operating conditions and refrigerant mixture proportions. The calculations have been then extended to an existing ejector heat driven refrigeration cycle (EHDRC). R1234yf appears to be a good candidate for drop-in replacement of R134a in a real EHDRC, while using R1234ze(E) would induce some modifications due to its thermodynamic properties. Maintaining the same pressure ratio for the ejector would lead on one hand to better entrainment ratio using R1234ze(E) and on the other hand to reduced coefficient of performance (COP) and cooling power by 4.2% and 26.6% in average, respectively. Using R1234yf under the same conditions induces a decrease of 5.2% for the entrainment ratio, 9.6% for the COP and 19.8% for the cooling power in average.

Far-field hot-wire measurements on free jet from complex supersonic nozzles

Complex nozzles use modifications to the exit geometry of supersonic nozzle to enhance the rate of mixing. Complex nozzles have significant beneficial effects when applied in air-breathing engines and supersonic ejectors. The far-field flow structure of supersonic free jets from complex nozzles have not been studied. This is relevant to applications such as thermal signature reduction, noise reduction, and in dynamics of explosive volcanic plumes. In this study, the centerline velocity decay rate is quantified from hot-wire measurements in the far-field of complex supersonic nozzles for the first time. Experiments are conducted on a reference conical nozzle of Mach number 1.8, a six chevron nozzle and an Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle with six lobes, at Nozzle Pressure Ratio NPR=6. All the nozzles have the same throat dimensions and exit area. The hot-wire measurement method is validated by comparison of the behavior of a subsonic turbulent jet with previous results, and the velocity decay rates and velocity profiles are found to agree well. The centerline velocity decay rate is maximum in the ESTS lobed nozzle (by 30%), followed by the chevron nozzle (by 10.5%). Mean velocity profiles are found to be self-similar by 40D. Details of the study are provided in the article.

New methods for analyzing transport phenomena in supersonic ejectors

This work aims at providing novel insights into the quantification and the location of the transfers and the irreversibilities within supersonic ejectors, and their connection with the entrainment. In this study, we propose two different and complementary approaches. First of all, recent analysis tools based on momentum and energy tubes (Meyers and Meneveau (2013)) are extended to the present compressible flow context and applied to the mean-flow structure of turbulent flow within the ejector. Furthermore, the transport equation for the mean-flow total exergy is derived and exergy transport tubes are proposed as a tool for the investigation of transport phenomena within supersonic ejectors. In addition to this topological approach, an analysis based on classical stream tubes is performed in order to quantitatively investigate transfers between the primary and the secondary streams all along the ejector. Finally, the present work identifies the location of exergy losses and their origins. Throughout this analysis, new local and cumulative parameters related to transfers and irreversibilities are introduced. The proposed methodology sheds light on the complex phenomena at play and may serve as a basis for the analysis of transport phenomena within supersonic ejectors. For the ejector under consideration, although global transfers are more important in on-design conditions, it is shown that the net gain in exergy of the secondary stream is maximum for a value of the back pressure that is close to the critical back pressure, as irreversibilities due to viscous dissipation are significantly lowered compared to other operating points.

Computational fluid-dynamics modeling of supersonic ejectors: Screening of turbulence modeling approaches

Abstract Ejector refrigeration has not been able to penetrate the market because of the low coefficient of performance and because of the high influence of ejector performance on the efficiency of the refrigeration system. Improving the performance of ejector refrigeration systems relies on the understanding of the fluid dynamic phenomena, which can be obtained through computational fluid-dynamics approaches. Unfortunately, no agreement has been reached yet on the closures for the turbulence modeling in the Reynolds Averaged Navier-Stokes (RANS) approach. This paper contributes to the existing discussion by presenting a numerical study of the turbulent compressible fluid in a supersonic ejector. Seven RANS turbulence closures have been compared and, in addition, the turbulence models were tested under different near-wall modeling options in order to investigate the wall treatment effect on the numerical results. The numerical results have been validated by literature data consisting in entrainment ratio and wall static pressure measurements, for different ejector geometries and operating conditions. As a result, the k-ω SST model shows better performance in terms of global and local flow phenomena predictions for the different ejector geometries and operating conditions: (a) on the global point of view, the entrainment ratio is well predicted with a maximum relative error equal to about 10%, (b) on the local point of view, the shock wave position, the pressure recovery and the wall static pressure values are well predicted. The results, taking into account previous numerical studies, suggest the use of the k-ω SST model to simulate ejector fluid dynamics.

Preliminary study of the primary nozzle position of a supersonic air ejector with a constant-area mixing chamber

This work aims at investigating the primary nozzle position in a proposed supersonic air ejector device. The ejector is primarily made up of a supersonic primary nozzle, which is located in the axis of the ejector, a suction chamber or secondary stream inlet, a mixing chamber and a diffuser. The ejector design allows to translate the primary nozzle in the axis direction and fix it in a chosen distance from the beginning of the mixing chamber and hence influence the secondary mass flow rate. In a limit case, it is possible to set the nozzle to such a position where no secondary flow occurs. If we ignore the case where no secondary flow occurs, five different nozzle distances have been investigated in this paper. Some cases seem to be alike and there are no significant dissimilarities between them. Courses of relative back-pressure ratio are carried out against the entrainment ratio and transition between on-design and off-design regimes is determined. Measurements of the mixed flow based on the standard ISO 5167 are performed by means of orifice plate method. In addition, a comparison between experiments and simulations performed by Ansys Fluent software is presented in order to indicate further improvements to the numerical model.

CFD Modeling of the Supersonic Condensation Inside a Steam Ejector

Abstract Supersonic steam ejectors are commonly used in many applications, for example suction of non-condensable gases in steam power plants or heat-powered chillers. In the specific case of ejector chillers, CFD studies of the ejector are necessary for optimization of this device because ejector performances have a direct impact on the COP of the chiller. The complex ejector flow and the high Mach numbers make characterization of the phase change inside this component difficult. Metastability effects also have to be accounted for and some CFD commercial software provide built-in wet-steam models for this purpose. A simpler approach for numerical modeling of multiphase ejector is the Homogeneous Equilibrium Model (HEM) in which the phase change occurs in equilibrium conditions, i.e. metastability is neglected. These second kinds of models are still important tools for preliminary analysis of condensing ejectors. In the present paper a comparison between commercial software wet-steam models and an in-house developed model based on HEM is presented.

CFD Modelling of the Condensation Inside a Supersonic Ejector Working with R134a

Abstract The present work is about CFD modelling of condensing flow inside a supersonic ejector. The geometry used for the simulations reproduces a small-scale prototype ejector chiller built at Georgia Institute of Technology (Atlanta). The working fluid is R134a, whose expansion inside the primary nozzle and mixing chamber can lead to non-equilibrium condensation phenomena. These alter the pressure and Mach profiles along the ejector, thus generating severe thermodynamic losses. The numerical analysis of non-equilibrium condensation requires modelling of the microscopic behaviour of the fluid with a high level of fidelity. In this study, the condensation of R134a is simulated by means of two in-house developed numerical models. The first considers equilibrium conditions between the phases whereas the latter reproduces the non-equilibrium behaviour of the phase transition. Comparisons are made to understand the limitations and advantages of both approaches.

CFD Design Study of a Pressure Probe for Centerline Static Pressure Measurement in Supersonic Ejectors

Abstract The measurement of the static pressure of the flow inside a supersonic ejector can be achieved by using a thin tube with a radially drilled hole to capture the flow pressure, and which is inserted along the ejector axis. This paper presents a numerical study by CFD permitting to predict the disturbances generated by the presence of the probe in the ejector. Also this study allows guiding the design of the probe, in particular of the capillary tube diameter for the least disturbed measurement. A probe prototype has been built and tested on an ejector test bench.

Publisher’s Note: “Parametric experimental studies on mixing characteristics within a low area ratio rectangular supersonic gaseous ejector” [Phys. Fluids 28, 076101 (2016)

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