Elsevier

Applied Thermal Engineering

Volume 185, 25 February 2021, 116369
Applied Thermal Engineering

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

https://doi.org/10.1016/j.applthermaleng.2020.116369Get rights and content

Highlights

  • Ejector performance is evaluated with auxiliary hole at different chamber.

  • The effect of multiple auxiliary holes on ejector performance is identified.

  • According to optimized auxiliary hole position, auxiliary hole angle is optimized.

  • The effect of area ratio on auxiliary hole position, angle and ER is determined.

Abstract

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.

Introduction

The growth of human civilization has led to an increase in the consumption of energy. Therefore, the increase of energy consumption of refrigeration and thermal comfort has become a major issue to be solved urgently, which has triggered a research upsurge on ejector application technology [2]. An ejector which usually has 5 parts (primary nozzle, suction chamber, constant-pressure mixing chamber, constant-area mixing chamber and diffuser) as shown in Fig. 1 possesses simple structure, little maintenance and long lifespan [3], [4]. These distinctive characteristics makes the ejector suitable for wide applications such as polymer electrolyte membrane fuel cells [5], desalination processing devices [6], [7], and multi-evaporator refrigeration cycles (MERCs) [8].

By applying an ejector to a refrigeration cycle with two or three evaporators, the throttling losses to maintain the pressure difference between evaporators can be partially recovered [9], hence, the energy efficiency of the MERCs can be improved [10]. The schematic and P-h diagram of an ejector-based multi-evaporator refrigeration cycle (EMERC) are presented in Fig. 2 [11]. The working process of the EMERC is as follows [12]: The low pressure and low temperature refrigerant vapor (state 1) is compressed in the compressor to state 2 and then enters the condenser where it condenses to states 3 and 4. The condensed liquid enters the high pressure and low pressure evaporators via expansion valves (3–5 and 4–6). The two-phase or superheated refrigerants from high pressure and low pressure evaporators with states 7 and 8 enter the primary and secondary flow inlets of the ejector. Both of the two flows mix with each other and then enter the suction line of the compressor with lifted pressure (state 1). The suction pressure of the compressor can be increased and thus the compressor compression ratio of the system can be reduced.

The following provides a review of relevant studies quantifying EMERC system performance. Elakdhar et al. [14] proposed such system with two evaporators and evaluated the system performances. The results revealed that the system performances can be improved by 10–32% as compared to a conventional cycle, though it largely depends on the refrigerants employed and working conditions. In the same way, Lin et al. [15] investigated the effects of varying cooling loads on performances of an EMERC equipped with two evaporators. Their results indicated that pressure recovery ratio (PRR) is very sensitive to the varied cooling loads of both primary and secondary flows, and the maximum PRR can reach 60% when the cooling loads vary. Moreover, Wen et al. [16] and Kairouani et al. [10] extended their studies for EMERCs with triple-evaporator and dual-ejector. Their studies showed that the COP of the proposed EMERCs is better than the conventional system.

So far, in order to improve performances of the ejector utilized in an EMERC or other applications, traditional ways have been performed by vast investigators to optimize ejector area ratio (AR, or the ratio of the cross-sectional area of the ejector throat to that of the primary nozzle throat) [17], [18], nozzle exit position (NXP, or the distance between the primary nozzle exit and the constant-area mixing chamber) [19], [20], the angle and length of the constant-pressure mixing chamber [21], and so on. The ejector geometry is the most important factor affecting the performance of the ejector and the system. Under certain working conditions, the optimal performance of the ejector can be obtained by optimizing the geometry of the ejector.

Furthermore, new approaches involved in improvement of ejector performances have been reported. For example, Zhou et al. [22] proposed a dual-nozzle ejector for the dual-evaporator household refrigerator-freezer refrigeration cycle. Two parallel primary nozzles located in the suction chamber, the entrainment effect of the proposed ejector can be improved because the contact area between the primary and secondary flows can be increased. Their study indicated that, as compared to a conventional ejector, the cycle coefficient of performance (COP) can be enhanced by 10.5–30.8% when using the ejector with two nozzles. Meanwhile, the effects of the primary nozzle structures on the ejector performance were mentioned by Rao et al. [23]. By using tip ring supersonic nozzle and elliptic sharp tipped shallow lobed nozzle, 30% higher of entrainment of secondary flow can be obtained. Similarly, the ejector performances with using different nozzle structures (conical, elliptical, square, rectangular and cross-shaped nozzles) were evaluated by Yang et al. [24]. Their study revealed that the cross-shaped nozzle has the best entrainment ratio, which is followed by square, conical, rectangular and elliptical nozzles in terms of the indicator of ER. Detailed analysis on the characteristics of the mixing process under different nozzle shapes with using CFD simulation was also provided by the authors. Omer Genc et al. [25] evaluated ejector performance with mainly optimizing secondary flow tube inclination angle. They disclosed that: (1) for the given conditions, the best performance of the ejector can be obtained when the secondary flow tube inclination angle is of 45°; (2) 4.7% higher entrainment ratio happening to the parallel flow ejector, or the secondary flow is parallel to the primary flow direction compared to the counter flow ejector.

In addition to the above-mentioned single secondary flow entrainment of the ejector, some researchers proposed multi-entrainment or auxiliary entrainment to improve ejector performances. For instance, Ding et al. [26] proposed a double entrainment ejector, in which one more primary nozzle flow entrains the outlet flow of the mixed primary and secondary fluids, to keep the ejector working in an extra-low evaporating temperature. Results showed that the entrainment ratio of the ejector can achieve 0.0736─0.0813 when the evaporating temperature is as low as −22 to −18 °C. A new ejector structure with an auxiliary entrainment inlet in the ejector low pressure region was proposed by Chen et al [27]. Results showed that the optimized ejector has higher entrainment rate and lower back pressure compared with the traditional ejector. Based on that, further study results showed that there is always an optimal location where the lowest pressure is corresponding to the maximum entrainment ratio [28]. Besides, Bodys et al. [29] gained the same conclusions by simulating R744 two-phase ejector. Tang et al. [30] proposed a multi-entrainment ejector by opening extra holes in the sections of mixing chamber, throat and diffuser. They evaluated the auxiliary entrainment effect of the ejector with varied primary flow pressure. Under designed conditions, in comparison, best auxiliary entrainment effect can be achieved by opening the auxiliary hole in the section of the throat.

Until now, little literature mentioned auxiliary entrainment ejector as shown in Fig. 3, this type of ejector has three additional auxiliary entrainment inlets at the lower pressure area of the mixing chamber, the throat and diffuser. The auxiliary entrainment inlets are connected in the same way as the secondary entrainment inlet. The auxiliary entrainment ejector was used in MED-TVC desalination system [28]. The results showed that the auxiliary entrainment ejector outlet mass flow rate increases 15.5% compared with the conventional ejector by CFD method.

However, the auxiliary entrainment ejector operated in air-conditioning system was not mentioned to the best of the authors’ knowledge; as for the EMERC, especially in air-conditioning - freezing mode, the influence of different combinations of auxiliary hole in different chambers on ejector performance was not reported by researchers; moreover, no reference mentioned that the optimized auxiliary hole position, angle and entrainment ratio change or not if AR varies.

Limitations of previous work lead to the purpose of this paper. The aim of present study is to: (1) seek the optimal auxiliary hole position of each chamber with fixed AR; (2) find the influence of various auxiliary hole combinations on the ejector performances with fixed AR; (3) optimize auxiliary hole angle based on optimized hole position with fixed AR; (4) search for the effect of varied AR on the optimized auxiliary hole geometric parameters as well as ejector performances. To be specific, the rest of this paper consists of following parts: Section 2, which describes the experimental system and the ejector model; Section 3, which introduces the CFD modeling and validation results; Section 4, which presents results and analysis of structure optimization of the auxiliary entrainment ejector and gives the application of auxiliary entrainment ejector in multi-evaporator refrigeration system.

Section snippets

System description and initial ejector geometries

The proposed system is an ejector-based multi-evaporator refrigeration system, and its schematic is shown in Fig. 4, where it has the following main components: a compressor, a condenser, two electronic expansion valves (EEVs), two electric evaporators, and two ejectors. The photograph of the experimental facility is illustrated in Figs. 5 and 6. The main components such as the compressor, condensing device and ejector are installed in the lower layer of the test facility, and the evaporator

Mathematical model

As for the flow and mixing process in the ejector, assumptions are given as follows: steady and saturated in both flows; isentropic expansion and adiabatic inner wall [36]. Thus, governing equations for conservation of mass, momentum and energy can be derived as below:

Continuity equation(ρui)xi=0

Momentum equation(ρujui)xj=τijxj-Pxi

Energy equation(ui(ρE+P))xi=̇αeffTxi+uiτijwhere stress tensor τij is:τij=μeffuixj+ujxi-23ukxkδijwhere i, j and k are space vector directions,u is

Results and discussion

The influence of auxiliary hole position, the number of auxiliary hole and auxiliary hole angle are investigated in the following sections using proven CFD mode. Especially, the impact of auxiliary hole position and angle on the ejector performance under various ARs is explained.

Limitations

The specific connection way of the EMERC is shown in Fig. 21, in which the secondary flow has two inlets, one is the traditional inlet A, another is the auxiliary inlet B; thus, the Ms is the total mass flow rate of A and B. Since the ejector outlet flow rate can be increased, the cooling efficiency of freezer chamber can be improved. Nevertheless, limitations of this article include: (1) All the results of the study are based on the CFD simulation, and the optimized ejector has not been

Conclusions

In this study, auxiliary entrainment performance is first evaluated with opening single auxiliary hole in four chambers. Then, based on the optimum auxiliary hole position of each chamber, the influence of double and triple holes on the ejector performance is analyzed. Next, the effect of auxiliary hole angle is studied. Finally, the influence of AR on the optimal auxiliary hole position and angle as well as the maximum ER is determined. The main conclusions are made as follows:

  • (1)

    The maximum

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

The authors appreciate the National Natural Science Foundation of China (Grant No.: 51806235).

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