Constant-area Mixing Chamber Research Articles

Optimization of the two-stage ejector with blocking area ratio of primary nozzle throat under certain liquid volume fraction in two-phase inlet fluids

This paper proposes an innovative two-stage ejector with a needle blocking the main nozzle area within 0%-60% to control the mass flow rate of the inlet. Based on this, under certain liquid volume fraction in two-phase inlet fluids, the nozzle exit positions (NXP1 and NXP2) and constant-area mixing chamber lengths (L1 and L2) of the first and second stages were optimized with blocking area ratio (BAR) of primary nozzle throat by using CFD simulation, the working fluid is R134a. The main results are as follows: (1) The maximum increase rate of the entrainment ratio (ER) of the first and the second-stage with optimization of four geometries without needle blockage are relatively slight, in comparison, both of them with optimization of L1 can reach 23.47% and 20.38%, which obtain the best improvement; (2) Under the condition of a needle blockage at the primary flow inlet of the first-stage ejector, the increase rate of ER1 with optimization of NXP1 and L1 are close to that without needle blockage; (3) Under the condition of a needle blockage at the primary flow inlet of the second-stage ejector, the first-stage ejector can achieve relatively large improvement of ER, especially, the optimization of NXP2 can increase the maximum ER1 by 258.73% of initial value, and the optimization of L2 can increase the maximum ER1 by 239.38% of initial value. The study in this work is helpful to understand the two-stage ejector performance improvement by adjusting the blocking area ratio of primary nozzle throat under certain liquid volume fraction in two-phase inlet fluids.

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

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

Investigation on key geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector

There was no literature that involved the combined investigation on the key geometries optimization and the effect of variable operating conditions (temperature, pressure and dryness fraction of primary and secondary flows) for a R744 two-phase ejector. In this paper, CFD simulation on geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector was carried out for the first time. Firstly, each of three geometries of area ratio (AR), nozzle exit position (NXP) and constant-area mixing chamber length (L2) was optimized to determine their relative sensitivity on ejector performance. Then, two or three geometries under different optimization orders were optimized. Finally, the effects of temperature, pressure and dryness fraction of primary and secondary flows as well as back pressure on ejector performance were analyzed based on the optimal ejector geometries. The results showed that: (1) the relative sensitivity of entrainment ratio (ER) to AR is the highest with value of 44.56%, followed by NXP and L2; (2) optimizing two or three of these parameters under different optimization sequences can achieve certain optimization effect with ER improvement within 5.96%–33.91%, but the result depends on the optimization order; (3) the effect of dryness fraction of the primary flow on the ejector performance is greater than that of temperature; (4) the change of secondary flow pressure has a greater impact on the ejector ER as compared to the dryness fraction of the secondary flow; (5) not only increasing the primary and secondary flow pressure but also reducing the back pressure is conducive to improving the ejector performance.

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

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

Effects of operating conditions and geometries on the performance of nitrogen ejectors for Joule–Thomson cooling

Ejectors in Joule–Thomson (JT) cooling cycles can reduce the evaporator pressure and the compressor power consumption, achieving lower temperatures and higher efficiencies. The ejector, being a major component of a JT cooling cycle with an ejector, is critical to overall cooling performance. In this study, the effects of inlet pressures, outlet pressures, inlet temperatures, nozzle, and mixing geometries on the performance of nitrogen ejectors are investigated experimentally and numerically. The critical back pressure of the ejector increases as the primary and secondary inlet pressures increase. Depending on the working mode, the entrainment ratio varies with the ejector inlet temperature. The clogging due to the deposition of water molecules as impurities occurs when the ejector inlet temperature is below 222 K, resulting in the rapid reduction of mass-flow rates. For nitrogen ejector designs, the following ratios are recommended: constant-area mixing chamber diameter to nozzle outlet diameter, constant-area mixing chamber diameter to nozzle throat diameter, nozzle exit position to constant-area mixing chamber diameter, constant-area mixing chamber length to constant-area mixing chamber diameter. The results of this study will be beneficial in promoting the use of ejectors in JT cooling cycles.

Optimization of three key ejector geometries under fixed and varied operating conditions: A numerical study

In this paper, as for an ejector utilized in a carbon capture system, by using CFD simulation, its three key geometries such as length of constant-pressure mixing chamber (XL3), length and diameter of constant-area mixing chamber (XL5 and D5) were first optimized individually. Then, the relative sensitivity of ejector performance to the three geometries was identified. Next, the effect of six optimization sequences (S1: XL3 → D5 → XL5, S2: XL3 → XL5 → D5, S3: XL5 → D5 → XL3, S4: XL5 → XL3 → D5, S5: D5 → XL3 → XL5, S6: D5 → XL5 → XL3) of the three geometries on entrainment ratio (ER) was evaluated. And then, the ER under varied working conditions was analyzed. Finally, four typical working conditions were selected for doing re-optimization to check whether the ejector performance can be significantly improved and how big difference of the optimized geometries with re-optimization can be generated. The results showed that: (1) the relative sensitivity of ER to D5 is greater than XL3, and that to XL5 is the least; (2) Among six optimization sequences of three geometries, S2 offers the best ejector performance, both S5 and S6 generate the equal and lowest ER; (3) The change of ER with the primary flow pressure is largely related to the back pressure; (4) Re-optimization under four typical operating conditions can improve ejector performance by 4.3% ∼ 128.8%, and optimum D5 and XL3 increases evidently as compared to that without re-optimization. The research results would enhance the application of ejector in carbon capture and relevant chemical fields.

Multi-round optimization of an ejector with different mixing chamber geometries at various liquid volume fractions of inlet fluids

• Two inlet fluids of the ejector have four liquid volume fraction combinations. • Multi-round optimization with six sequences of three geometries is conducted. • Optimal three geometries and ER under six optimization sequences are obtained. • Recommended sequence for each liquid volume fraction combination is presented. • Ultimate optimal three parameters and ER largely depend on inlet fluid states. The performance of the ejector relies on the geometrical parameters and the fluid phase state. Although the optimization of geometries of the ejector has been conducted in previous studies, the influence of different optimization sequence of geometric parameters was ignored in previous literatures. To close the knowledge gap, the goal of this paper is to investigate whether different optimization sequences affect the ejector performance and whether the ejector performance is the same under different optimization sequences after multiple rounds of optimization. Therefore, three geometrical parameters, namely the constant-pressure mixing chamber length (L pm ), the constant-area mixing chamber length (L am ) and diameter (D am ), are selected for the optimization study; besides, multi-round optimization with six optimization sequences of these three parameters is conducted by CFD simulations under four different combinations of primary flow liquid volume fraction (LVF 1 ) and secondary flow liquid volume fraction (LVF 2 ) (LVF 1 = 0 plus LVF 2 = 0, LVF 1 = 0 plus LVF 2 = 0.06, LVF 1 = 0.06 plus LVF 2 = 0, LVF 1 = 0.06 plus LVF 2 = 0.06) for the first time. The results showed that: (1) for each LVF combination, the three optimal parameters and corresponding maximum ER produced by one round optimization of six different optimization sequences are evidently different; (2) after multiple rounds of optimization, for LVF 1 = 0 plus LVF 2 = 0 or LVF 1 = 0.06 plus LVF 2 = 0.06, ultimate optimal geometrical parameters and maximum ER are the same with each other for six optimization sequences, however, for LVF 1 = 0 plus LVF 2 = 0.06 or LVF 1 = 0.06 plus LVF 2 = 0, each of them still has two different ultimate maximum ER; (4) for those four combinations, different sequence takes different optimization rounds, recommended sequences are D am → L am → L pm (S6), L pm → L am → D am (S1) or L pm → D am → L am (S2), and L am → L pm → D am (S3) and D am → L am → L pm , respectively; and (5) the ultimate optimal parameters and maximum ER in four combinations differ significantly because they are largely dependent on the inlet fluid states.

Effect of mixing chamber length on ejector performance with fixed/varied area ratio under three operating conditions in refrigerated trucks

In this paper, an ejector-based multi-evaporator refrigeration system is proposed to provide three switchable operating modes of refrigerated trucks, namely, refrigerating – freezing (R-F), air-conditioning – freezing (A-F) and air-conditioning – refrigerating (A-R) modes. Under these three operating modes, CFD simulations are conducted to evaluate the optimum ejector constant-pressure mixing chamber length (Lpm) and constant-area mixing chamber length (Lam) as well as the sum of these two lengths (Lpam) with fixed and varied ejector area ratio (AR, which is expressed as ratio of the cross-sectional area of constant-area mixing chamber to primary nozzle outlet area). The results show that: (1) optimization of Lpm and Lam has small effect on ejector entrainment ratio (ER, which is defined as the ratio of secondary mass flow rate to primary mass flow rate) with fixed AR of 3 under three operating modes, and the maximum increment of ER are 0.0752, 0.0884 and 0.0205 for R-F, A-F and A-R mode, respectively; (2) under varied AR, when optimizing Lpm and Lam, the increment of ejector ER is 1.615, 2.249 and 0.021 for R-F, A-F and A-R modes, respectively; (3) the optimal Lpm and Lpam of the ejector increase first and then decrease with the increase of AR, however, the optimal Lam is irregular with the increase of AR; (4) compared with Lpm and Lam, the effect of AR on ejector performance is the most prominent when AR gets larger. The novelty of this study is that the relations between the optimal mixing chamber length and varied AR are identified.

Optimization of Steam Ejector Performance Using CFD

The present work introduces a numerical investigation of steam ejector optimum performance at constant pressure ratio used in many applications. The present study aims to maximize the ejector efficiency by optimizing the ejector mass ratio. The effect of geometrical parameters on ejector mass ratio and its efficiency is investigated at constant operating conditions. These parameters are the ejector convergent section angle, the constant area mixing chamber length and the angle of the ejector divergent section. The results showed that the ejector wall static pressure distributions were greatly affected by the investigated geometrical parameters. Furthermore, In order to avoid separation, the ejector divergent section angle must be selected carefully together with the operating conditions. The ejector mass ratio and efficiency increased with increasing the previously stated three geometrical parameters to gain their upper limit values, subsequent to that, the efficiency and mass ratio decreased with increasing these geometrical parameters. Moreover, there are certain optimum ejector convergent, divergent angles and the constant area mixing chamber length in order to optimize the ejector mass ratio and consequently its efficiency at given constant operating condition.

Numerical study on the auxiliary entrainment performance of an ejector with different area ratio

In this paper, an ejector-based multi-evaporator refrigeration system is proposed for the tropical region refrigerated trucks. CFD simulations are performed to study the auxiliary entrainment effect of the ejector operated in air-conditioning - freezing mode. First, the performance of the ejector with various auxiliary hole position in different chambers is investigated. Then, based on the optimal auxiliary hole position of each chamber, the influence of double and triple auxiliary holes on the ejector performance is identified. Next, optimal angle with the optimized auxiliary hole position is determined. Last, the effect of the ejector area ratio on optimal auxiliary hole position, angle and entrainment ratio is evaluated. The main results show that: (1) Optimal auxiliary hole position is 59.5 mm or the length ratio of distance between the inlet of constant-area mixing chamber and the auxiliary hole center point to the length of the constant-area mixing chamber is 1.90, and optimal auxiliary hole angle is 45°; (2) The effect of the auxiliary hole position on the ejector performance is stronger than the auxiliary hole angle; (3) the optimum auxiliary hole position, angle as well as entrainment ratio are affected by the ejector area ratio.

Parametric analysis of a combined ejector-vapor compression refrigeration cycle

Abstract From the last few years, the use of efficient ejector in refrigeration systems has been paid a lot of attention. In this article a description of a refrigeration system that combines a basic vapor compression refrigeration cycle with an ejector cooling cycle is presented. A one-dimensional mathematical model is developed using the flow governing thermodynamic equations based on a constant area ejector flow model. The model includes effects of friction at the constant-area mixing chamber. The current model is based on the NIST-REFPROP database for refrigerant property calculations. The model has basically been used to determine the effect of the ejector geometry and operating conditions on the performance of the whole refrigeration system. The results show that the proposed model predicts ejector performance, entrainment ratio and the coefficient of performance of the system and their sensitivity to evaporating and generating temperature of the cascade refrigeration cycle. The simulated performance has been then compared with the available experimental data from the literature for validation.

Experimental studies on a rectangular ejector with air

Experimental studies have been conducted to depict the flow patterns in a small rectangular ejector by using schlieren flow visualisation technique. A transparent ejector is designed, fabricated, and operated using air as working fluid. Geometry of the ejector is in form of two-dimensional extrusion with two suction inlets: one on top and other at bottom of the convergent-divergent nozzle. Ejector performance has been analysed by measuring the mass flow rates at primary and secondary inlets. The results of flow visualisation studies have been compared with those obtained using numerical computations. Variation of entrainment ratio with change in primary stream pressure has been presented. Moreover, the shock wave patterns in constant area mixing chamber and diffuser for set of inlet and outlet conditions have been visualised and presented.

Measurement and calculating of supersonic ejectors

This paper deals with numerical and experimental investigation of the flow in an air to air supersonic ejector with constant area mixing chamber. The mixing chamber of previous ejector was completely repaired since a scratch from previous turning had been found. As a result, a new geometry of the mixing chamber was created. Several measurements were conducted with different nozzle position (NP): 1 mm, 2 mm and 3 mm. Furthermore, for a given NP, two different values of stagnation pressure of 200 kPa and 300 kPa at the primary air inlet were investigated in more detail. All numerical simulations were performed in the ANSYS Fluent software. It was found that the influence of the position of the nozzle influences the ejection factor only to a certain extent. For the other parameters of the ejector is also a need to find the optimum position of the nozzle. Repair of the mixing chamber has contributed to reduce the pressure difference at the wall of the mixing chamber.

Thermodynamic model for all modes performance analysis of supersonic ejector considering non-uniform distribution of flow field

This paper focuses on the problem of the modeling of the supersonic ejector under both critical and subcritical mode. Unlike the traditional 1D mathematical model, the proposed model takes into account the non-uniform distribution of the flow field in the radial direction. For the critical point, it is assumed that the mixing progress occurs in the constant area mixing chamber under a uniform mixing pressure which is between the stagnation pressure and choking pressure of the secondary flow. However, a constant area mixing assumption is made when the ejector operates under the subcritical mode. By using an exponential relationship to approximate the velocity distribution of the secondary flow in the radial direction, both the entrainment ratio and back pressure can be calculated. The influence of component efficiencies on the ejector performance for different working fluids is presented and analyzed. Finally, experimental results are used to validate the effectiveness of the proposed model.

Three-dimensional numerical investigations on rectangular cross-section ejector

The performance and flow structure in a three-dimensional rectangular ejector have been numerically investigated by using air as the working fluid. Commercial software ANSYS Fluent 15.5 has been used for numerical simulation of supersonic, compressible, turbulent flow ejectors. In order to confirm the appropriate turbulence model, the computed results are validated against indigenous experimental data. The study aims to optimize the convergent-divergent (C-D) nozzle position for each operating condition and to bring out the influence of reflected shock waves and boundary layer in the constant area mixing chamber on the performance of the ejector. Also, the variation of entrainment ratio (ER) for a range of primary, secondary and exit pressure has been investigated. Both the on-design and the off-design characteristics and local flow structure of a rectangular ejector have been considered.

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.

PIV and CTA Measurement of Constant Area Mixing in Subsonic Air Ejector

The article deals with experimental study of constant area mixing in subsonic axi-symmetric air ejector. Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA) measurements of four different mixing regimes, each with different ejection ratio were performed. For PIV measuring, the velocity fields inside the constant area mixing chamber were taken through the vitreous wall of the chamber, while the laser beam entered it from the opened outflow of the ejector. For CTA measuring, probe perpendicular to the ejector axis was used. Data obtained from both methods are compared. Basic descriptions of the results are given and it is claimed that results are reliable

Experimental investigation on ejector cooling system performance at low generator temperatures and a preliminary study on solar energy

In this paper, the performance of an ejector cooling system with constant-area mixing chamber was investigated at low generator temperatures. The cooling system was tested by using hot water as driving fluid and R123 as working fluid. The effects of operating temperatures on the cooling capacity and on the coefficient of performance (COP) of the system were experimentally investigated at an ejector area ratio of 7.17, which is suitable for solar cooling. As a result, COP of 0.42 was obtained at a vapor generator temperature of 74°C, evaporator temperature of 10°C, and at a critical condenser temperature of 29°C. A solar application of the system with single-glazed selective-type collectors in a horizontal position was conducted as a preliminary study. In the solar application, a cooling capacity of 1080 W for a 9.2 m2 collection area at an evaporator temperature of 10°C was obtained.

PIV measurement of constant area mixing in subsonic air ejector

** Abstract: The article deals with experimental study of constant area mixing in subsonic axi-symmetric air ejector. The velocity fields inside the constant area mixing chamber were taken through the vitreous wall of the chamber, while the laser beam entered it from the opened outflow of the ejector. PIV measurements of four different mixing regimes, each with different ejection ratio were performed. Basic description of results are given and it is claimed that results are reliable. Obtained data will be later compared with constant temperature anemometry and with numerical calculations.

Simulation of an ejector used in refrigeration systems

The present study describes a computer simulation model for refrigerant ejector. A one-dimensional mathematical model was developed using the equations governing the flow and thermodynamics based on the constant-area ejector flow model. The model includes effects of friction at the constant-area mixing chamber. The current model is based on the NIST-REFPROP database for refrigerants’ properties calculations. The simulated performance is compared with the available experimental data from the literature for validation. The effects of operating parameters and environment friendly refrigerants on critical entrainment ratio are studied.