Performance of steam ejector with nonequilibrium condensation for multi-effect distillation with thermal vapour compression (MED-TVC) seawater desalination system
Introduction
The multi-effect distillation with thermal vapour compression (MED-TVC) seawater desalination system provides a solution for freshwater production, which achieves energy saving by upgrading the low-grade energy [1,2]. The steam ejector, as the main part of the TVC unit, recovers the low-pressure steam from the last effect of evaporators, which accomplish high efficient utilisation of the low-grade energy [3,4]. Thermodynamic and economic analysis has shown that the reduction of the exergy destruction in the TVC was cost-effective for the entire seawater desalination system [5]. The MED-TVC seawater desalination system also contributes to environmental protection by integrating renewable energy [[6], [7], [8]].
The studies on steam ejectors for MED-TVC seawater desalination system including the novel design [9], thermodynamic analysis [10] and optimisation studies [11]. Ghaebi and Abbaspour [12] integrated heat recovery steam generators to MED-TVC seawater desalination systems. The thermodynamic analysis illustrated that the novel design improved the exergy efficiency by 57.5%. Tang et al. [13] designed a new steam ejector with pressure regulation and the optimisation improved entrainment ratios by 11.77%. Tang et al. [14] also reported that the multi-optimisation of the entrainment passage can lead to an improvement of the entrainment ratio of 28.75% for the newly designed steam ejector for MED-TVC seawater desalination systems. Gu et al. [15] proposed a steam ejector with a variable geometry by inserting a spindle inside the primary nozzle and averaged entrainment ratios reached 1.39 compared to the normal ejector of 0.69. Xue et al. [16] reported a novel design of two-stage vacuum ejectors and the experimental test demonstrated that the new design provided the vacuum pressure of about 5.3 kPa compared to normal ejectors of 18.6 kPa. Sadeghi et al. [17] proposed a thermodynamic study and optimised power - ejector - desalination systems, and their study illustrated that the decrease of the total exergy destruction resulted in high temperatures at the exit of the steam ejector to produce distilled water.
Park [18] optimised steam ejectors for the seawater desalination system by introducing the swirling flow to the primary nozzle using computational fluid dynamics (CFD) modelling, which could obtain high entrainment ratios by changing the strength of the swirling flow. Sharif [19] carried out the 2D axisymmetric and three-dimensional (3D) simulation of a steam ejector and the comparison revealed that the axisymmetric simulation obtained similar results to 3D models considering the overall performance of the steam ejector. Liu et al. [20] computationally investigated the influence of area ratios on steam ejectors for MED-TVC seawater desalination systems and they found that the entrainment ratio increased from 0.025 to 0.8. Khalid et al. [21] optimised the location of steam ejectors for MED-TVC seawater desalination systems and best unit performances could be achieved by integrating the steam ejector at the middle effect regardless of the number of effects. Wang et al. [22] performed the optimising study on primary nozzles within steam ejectors for MED-TVC seawater desalination systems using CFD modelling and they showed that the overall efficiency of the steam ejector increased by 14.41%.
The aforementioned CFD and thermodynamic analysis improved the understanding of steam ejectors for MED-TVC seawater desalination systems ignoring nonequilibrium condensation processes in steam ejectors. Fortunately, the importance of the condensation phenomenon has been gradually realized in recent studies. Bonanos [23] proposed a physical model of a steam ejector with assumptions of the one-dimensional and perfect gas model without considering the condensation process, and it was emphasized that the low temperatures due to the acceleration of the fluid may lead to the condensation of the steam, thus invalidating the assumption of perfect gas behaviour. Liu et al. [24] performed a thermodynamic analysis of steam ejectors for MED-TVC seawater desalination units, which investigated the impact of condensation behaviours on steam ejectors based on the homogeneous equilibrium theory assuming that the phase change occurred instantaneously when the steam reached the saturation state. Their study reported that the condensation is a pervasive phenomenon inside steam ejectors. Tang et al. [25] carried out visualization experiments observing condensation behaviours within steam ejectors for MED-TVC seawater desalination systems. The experimental test demonstrated that massive condensing droplets were observed with nonuniform distributions of the droplet size over the cross-area plane within supersonic flows. These studies illustrate that nonequilibrium condensation processes are significant for performance evaluations of steam ejectors for MED-TVC seawater desalination systems.
The present study fills in the scientific gap including the development of the two-phase wet steam model for nonequilibrium condensation in supersonic flows, the comparison of single-phase flow and two-phase flow models for steam ejectors and the performance evaluation of a steam ejector for MED-TVC seawater desalination systems based on the two-phase flow model. Specifically, the nonequilibrium condensation process is integrated to develop a mathematical model to evaluate performances of steam ejectors for MED-TVC seawater desalination systems. The assessments of the single-phase and two-phase flow models are carried out to describe the flow features within steam ejectors. The nonequilibrium condensation processes are discussed inside steam ejectors for MED-TVC seawater desalination systems.
Section snippets
MED-TVC seawater desalination system
The MED-TVC seawater desalination system mainly consists of a seawater supply unit, multi-evaporators, a steam ejector and a condenser, as shown in Fig. 1. Seawater is fed in condensers with heat exchange between seawater and heating steam in the parallel tubes. During this pre-process in the condenser, a small quantity of the feed seawater is used as cooling water, while the main part is sprayed into each effect of the multi-evaporators, where the seawater is heated to evaporate into water
Mathematical model
Fundamental equations governing the flow features in a steam ejector are compressible Navier-Stokes equations [[27], [28], [29]]. Considering the complicated flow structure inside steam ejectors, including the supersonic flow, condensing flow, flow separation and shock waves, the shear stress transport (SST) k-ω turbulence model is employed for performance evaluations of steam ejectors for MED-TVC seawater desalination systems.
CFD validation of nonequilibrium condensations in supersonic flows
The numerical validation of nonequilibrium condensations in supersonic flows is carried out based on the case 252 of the Laval nozzle designed by Moses and Stein [35,36], who performed the experimental measurement on the pressure distribution and the droplet size of the wet steam flow. The supersonic flow condition is assigned to the exit of the Laval nozzle. The computed pressure and droplet radius are compared against experimental data, as shown in Fig. 3. It can be observed that numerical
Conclusions
Performance analyses of a steam ejector are carried out for MED-TVC seawater desalination systems considering nonequilibrium condensation behaviours. The single-phase flow model with ignoring phase change processes predicts an unphysical temperature of the steam in the supersonic flow with the minimum value of approximately 122 K in the primary nozzle. The two-phase flow model predicts that the steam expands further and departs from equilibrium states to induce the occurrence of the
CRediT authorship contribution statement
Chuang Wen:Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing.Hongbing Ding:Formal analysis, Writing - review & editing.Yan Yang:Conceptualization, Methodology, Investigation, Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This project has received funding from the European Union's Horizon 2020 - Research and Innovation Framework Programme under the Marie Sklodowska-Curie grant agreement No 792876 and the National Natural Science Foundation of China under Grant 51876143.
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