Supersonic Steam Ejector Research Articles

Performance and mixing process investigation of a novel mixing-enhanced ejector

Ejector refrigeration systems rarely require electrical energy, satisfying the requirements of sustainable development and low carbon emissions. However, owing to the limitations of the operating conditions, the ejector performance is not sufficient, and there is an urgent need to improve the entrainment performance of the ejector. In this study, experiments and numerical simulations were performed on a conventional ejector, and the numerical simulation results were fitted with the experimental data. A numerical simulation was used to investigate the mixing process and entrainment performance of the novel mixing-enhanced supersonic steam ejector. The optimal values of lm = 120 mm and dc = 57.5 mm achieved the best entrainment ratio (0.778) for the novel ejector under 5%-tab blockage ratio. The results showed that the entrainment performance of the novel ejector improved by 51.1–54.7% and 10.5–12.3% compared to those of the conventional ejector and the conventional ejector with an adjusted mixing chamber, respectively. This was because the generated streamwise vortices entrained more secondary flow into the mixing chamber, and the mixing process between the primary and secondary flow avoided unnecessary kinetic energy loss, increasing the mixing efficiency and accelerating the secondary flow crashing out of the diffuser.

The effect of variable motive pressures on the performance and shock waves in a supersonic steam ejector with non-equilibrium condensing

Complex flow phenomena and performance are not fully understood in supersonic steam ejectors as the neglect of the non-equilibrium condensation in many simulation studies. In this paper, a non-equilibrium condensation model including real physical properties was combined with the SST k-ω turbulence model to investigate the effects of variable motive pressures on the shock waves, non-equilibrium condensation, and ejector performance. The essential connection between the performance and shock waves was revealed from the mechanism perspective. The results show that when the shock wave intensity is the lowest and the shock wave number is the least whereas the second choking forms, the motive pressure is optimal. The closer the shock wave is to the second choking starting position near the diffuser entrance, the closer the discharge pressure is to the critical one. When the motive pressure increases, a dramatic increase in velocity dominates the increase in the mass flow rate of suction steam in the single choking mode; the over-expanded primary shock train region significantly squeezes the suction steam flow region and dominates the decrease in the mass flow rate of suction steam in the double choking mode; the intensity of the non-equilibrium condensing and shock waves increases, the pseudo-shock region expands, and the condensation shock wave position moves upstream by 1.2% while aerodynamic shock waves move downstream by 25.9%.

Influence of Geometrical Parameters on Supersonic Steam-Ejector Performance

Geometrical parameters such as the nozzle exit position (NXP) and the diameter ratio (DR) considerably influence the performance of steam ejectors. An axisymmetric two-dimensional model for a supersonic steam ejector is developed to investigate the steam-ejector performance with the aim of simultaneously optimizing the NXP (-10 < NXP < 10) and DR (6 < DR < 10), and the simulation results are in good agreement (relative error was merely 7%) with the experimental results. At the optimum value of the NXP, the supersonic steam ejector acquires the optimal entrainment ratio (ER) under the given conditions. Deviation from the optimal NXP value may result in a 12.4% decrease in ER. With optimized NXP, the optimization of DR is more significant; however, a deviation from the optimal DR weakens the ER and the supersonic steam ejector eventually breaks down. Therefore, the NXP should be approximately close to its optimum value, and the DR must be designed at its optimum value to ensure effective operation of the supersonic steam ejector.

Double choking characteristics of three-dimensional steam ejector with non-equilibrium condensing

The phenomenology of double choking in a supersonic steam ejector which plays an influential role in the performance is not fully understood because the non-equilibrium condensing and three-dimensional effects are neglected in most numerical simulations. In this paper, a non-equilibrium condensation model was combined with the k-ω SST turbulence model to simulate three-dimensional supersonic steam ejectors with different working conditions and different structures. The internal relationship between the ejector performance and the double choking characteristics was revealed. The influence of the characteristics of double choking on the shock waves in the ejector was also investigated. Results show that when the minimum distance between the sonic velocity line in the diffuser and the wall is equal to the critical minimum distance which is 0.21 mm, the steam ejector is in a double choking mode. 4 mm downstream of the diffuser entrance is the critical second choking position. The deviations between discharge pressure and critical discharge pressure are getting greater with the increase of the second choking length. The intensity and number of shock waves in the diffuser are positively related to the second choking length with certain working conditions. Three-dimensional effects bring non-negligible differences to both the flow field and the ejector performance.

The Effect of Different Pressure Conditions on Shock Waves in a Supersonic Steam Ejector

The complex flow phenomena in a three-dimensional supersonic steam ejector were simulated with a non-equilibrium condensation model including real physical properties in different pressure conditions. The different working conditions include discharge pressure, motive pressure, and suction pressure. The influence of different pressures on shock waves in the steam ejector was investigated comprehensively. The intrinsic causes of shock wave variation with pressure conditions were explained in detail. The results show that the width of the primary shock train region expand with an increase in the motive pressure or a decrease in suction pressure. The diamond shock waves move downstream with an increase in motive pressure or a decrease in suction pressure. The shocking position in the diffuser moves upstream until it reaches the diffuser entrance with an increase in discharge pressure or a decrease in motive pressure or suction pressure. The intensity and number of oblique shock waves in the diffuser increase with an increase in motive pressure and suction pressure or a decrease in discharge pressure. The existence of only one shock wave in the diffuser is a necessary and insufficient condition for the ejector to enter a double choking mode.

Assessment of Different CFD Modeling and Solving Approaches for a Supersonic Steam Ejector Simulation

The effects of different modeling and solving approaches on the simulation of a steam ejector have been investigated with the computational fluid dynamics (CFD) technique. The four most frequently used and recommended turbulence models (standard k-ε, RNG k-ε, realizable k-ε and SST k-ω), two near-wall treatments (standard wall function and enhanced wall treatment), two solvers (pressure- and density-based solvers) and two spatial discretization schemes ( the second-order upwind scheme and the quadratic upstream interpolation for convective kinematics (QUICK) of the convection term have been tested and compared for a supersonic steam ejector under the same conditions as experimental data. In total, more than 185 cases of 17 different modeling and solving approaches have been carried out in this work. The simulation results from the pressure-based solver (PBS) are slightly closer to the experimental data than those from the density-based solver (DBS) and are thus utilized in the subsequent simulations. When a high-density mesh with y+ &lt; 1 is used, the SST k-ω model can obtain the best predictions of the maximum entrainment ratio (ER) and an adequate prediction of the critical back pressure (CBP), while the realizable k-ε model with the enhanced wall treatment can obtain the best prediction of the CBP and an adequate prediction of the ER. When the standard wall function is used with the three k-ε models, the realizable k-ε model can obtain the best predictions of the maximum ER, and the three k-ε models can gain the same CBP value. For a steam ejector with recirculation inside the diffuser, the realizable k-ε model or the enhanced wall treatment is recommended for adoption in the modeling approach. When the spatial discretization scheme of the convection term changes from a second-order upwind scheme to a QUICK scheme, the effect can be ignored for the maximum ER calculation, while only the CBP value from the standard k-ε model with the standard wall function is reduced by 2.13%. The calculation deviation of the ER between the two schemes increases with the back pressure at the unchoked flow region, especially when the standard k-ε model is adopted. The realizable k-ε model with the two wall treatments and the SST k-ω model is recommended, while the standard k-ε is more sensitive to the near-wall treatment and the spatial discretization scheme and is not recommended for an ejector simulation.

Energy efficiency and exergy destruction of supersonic steam ejector based on nonequilibrium condensation model

With the increasing importance of the environment, energy issues have become the focus of attention. Reducing exergy destruction of the ejector during mixing can increase the entrainment ratio and improve the refrigerating system working efficiency. In this study, a wet steam model integrating a droplet nucleation and growth formulas with four categories of entropy transport equations was established to further analyze thermodynamics properties of ejector. The simulation analyzes of flow pattern, entrainment ratio and coefficient of performance COP, entropy generation and exergy destruction of dry and wet steam flows were carried out under different inlet conditions and roughness. For smooth wall, in the case of primary and secondary temperatures of 120 °C and 14 °C, the COP reaches 0.5324 with the cooling capacity of 3.103 kW. The entropy generation of wet steam flow reaches the maximum value of 1616.37 J·kg−1·K−1, and the exergy destruction also reaches the maximum of 481.68 kJ/kg. Considering the analysis of the surface roughness, the maximum of exergy destruction near critical condition point increases by 45.14 kJ/kg for the 500 μm rough wall, and exergy destruction ratio reaches the maximum value of 0.740.

Computational fluid dynamics simulation of the supersonic steam ejector using different condensation model

This paper addresses the non-equilibrium condensation (NEC) in supersonic steam ejector under the assumptions of no slip velocity between the droplets and vapor phase and homogenous nucleation. The experimental data carried out by Moore has been used to verify the numerical results. It is illustrated that the maximum value of the flow mach number of the NEC model is lower than that of the equilibrium condensation model, and NEC model increases the ejector?s entrainment ratio in comparison equilibrium condesation model. When using the NEC model, the nucleation characteristics such as subcooling degree, nucleation rate could be obtained in ejector flow field.

1-D Mathematical Modeling and CFD Investigation on Supersonic Steam Ejector in MED-TVC

Thermo Vapour Compressor TVC is the heart or prime mover of Multi-Effect Desalination MED system, it is also described as a steam ejector. Steam ejector is an economical device that utilizes high pressure steam to compress low pressure steam without any rotating parts or external power sources. The ejector performance is highly dependent on its geometry and operating conditions. The aim of the present work is to design and obtain steam ejector geometry, based on pre-specified operating conditions, and to investigate the flow behavior inside the ejector by using Computational Fluid Dynamics CFD through using “ANSYS FLUENT 14.5” software. In first section of this paper, 1-D mathematical model is carried out to predict the ejector geometry at designed operating conditions. The second part, describes the flow behavior inside the designed model of the ejector by CFD. CFD is a most reliable tool to reveal the phenomenon of the mixing process at different parts of the supersonic turbulent flow ejector and to study the effect of operating conditions on the effective ejector area. Obtaining such effects, using experimental work, is much costly and difficult. Finally, the results show the effect of the operating conditions on the entrainment ratio and the critical back pressure.

CFD-Exergy analysis of the flow in a supersonic steam ejector

The current study aims to carry out a CFD-exergy based analysis to assess the main areas of loss in a supersonic steam ejector encountered in ejector refrigeration systems. The governing equations for a compressible flow are solved using finite volume approach based on SST k-ω model to handle turbulence effects. Flow rates and the computed mean temperatures and pressures have been used to calculate the exergy losses within the different regions of the ejector as well as its overall exergy efficiency. The primary mass flow rate, the secondary mass flow rate and the entrainment ratio predicted by the model have been compared with the experimental data from the literature.

A Numerical Study on the Supersonic Steam Ejector Use in Steam Turbine System

Supersonic steam ejector is widely used in steam energy systems such as refrigeration, wood drying equipment, papermaking machine, and steam turbine. In this paper the Computational Fluids Dynamics (CFD) method was employed to simulate a supersonic steam ejector, SST k-w turbulence model was adopted, and both real gas model and ideal gas model for fluid property were considered and compared. The mixing chamber angle, throat length, and nozzle exit position (NXP) primary pressure and temperature effects on entrainment ratio were investigated. The results show that performance of the ejector is underestimated using ideal gas model, and the entrainment ratio is 20%–40% lower than that when using real gas model. There is an optimum mixing chamber angel and NXP makes the entrainment ratio achieve its maximum; as throat length is decreased within a range, the entrainment ratio remains unchanged. Primary fluid pressure has a critical value, and the entrainment ratio reaches its peak at working critical pressure; when working steam superheat degree increases, the entrainment ratio is increased.

Computational fluid dynamics simulation of the supersonic steam ejector. Part 2. Optimal design of geometry and the effect of operating conditions on the ejector

The aim of this study is to investigate the use of computational fluid dynamics in predicting the performance and optimal design of the geometry of a steam ejector used in a steam turbine. In the current part, the real gas model was considered using IAPWS IF97 model, and the influences of working fluid pressure and backpressure were investigated. The results illustrate that working critical pressure and backflow critical pressure exist in the flow. Moreover, the entrainment ratio reaches its peak at the working critical pressure. The performance of the ejector was nearly the same when the outlet pressure was lower than the critical backpressure. Effects of ejector geometries were also investigated. The distance between the primary nozzle and the mixing chamber was at optimum, the length of the mixing chamber and the diameter of the throat had an optimal value according to the entrainment ratio. When the length of the diffuser or throat was decreased within a range, the entrainment ratio did not change significantly.

Computational fluid dynamics simulation of the supersonic steam ejector. Part 1: Comparative study of different equations of state

The aim of this study is to investigate the use of computational fluid dynamics in predicting the performance and geometry of the optimal design of a steam ejector used in a steam turbine. Many scholars have analysed the steam ejector using the ideal gas model, which lacks accuracy in terms of calculating the flow field of the ejector. This study is reported in a series of two papers. The first part covers the validation of CFX 11.0 results using different equations of state (EOS) on the converging–diverging nozzle flow field carried out with the experimental value. The IAPWS IF97 real gas model works well with the experimental value. The flow field of the ejector was analysed using different EOS after grid-dependent learning. The results show that the performance of the ejector was underestimated under the ideal gas model; the entrainment ratio was 20–40 per cent lower than when using the real gas model. The effect of the optimal geometrical design and operating conditions will be discussed in Part 2.