Multi-evaporator Refrigeration System Research Articles

Performance of a two-stage ejector with varying liquid fraction at its inlet

Liquid volume fraction (LVF) in a two-phase flow has a certain effect on the flow performance. In this paper, a two-stage ejector applied to a multi-evaporator refrigeration system was proposed by the authors, in which, the refrigerant of R134a is used and then the effects of different LVF in the two-stage ejector inlet on the two-stage performance were studied by using CFD simulation, which is the novelty of this study. One of the main contributions is that the reason for the changed performance of each stage was explained by analyzing the volume flow rates, density and mass flow rates of the two-phase mixture when the LVF of one, two or three inlets varied. The results show that: (1) when the LVF of one inlet increases, the two-phase mass flow rate can be increased evidently due to the significant increase of the density of the mixture; (2) if one inlet fluid is in two-phase, the improvement of the LVF in the first-stage secondary flow, the performance of the first-stage as well as the second-stage can be improved; (3) if two inlet flows are in two-phase, the increase of the LVF of the two inlet flows of the first-stage with the same proportion can offer the best performance to both stages; and (4) when the LVF of three inlet flows increase from 0.04 to 0.12 with the same proportion, the ER of both stages can be increased evidently.

Experimental Study on a Multi-Evaporator Refrigeration System Equipped with EEV-Based Ejector.

This study presents an experimental rig of a multi-evaporator refrigeration system, in which the pressure difference between two evaporators can be maintained by using both the pressure-regulating valve (PRV) and electronic expansion valve (EEV)-based ejector. The proposed EEV-based ejector that is used to partially recover the throttling losses of the PRV consists of an EEV and the main body of an ejector. The established experimental system can work in both PRV-based mode and ejector-based mode by switching the valves. Via experimental means, the performances of both modes were evaluated by varying the cooling loads. Moreover, the effects of the spindle-blocking area percentage of the EEV-based ejector and the condensing temperature on the system performance were identified. The results showed that: (1) the system performance of the ejector-based mode was 3.6% higher than the PRV-based mode; (2) both entrainment ratio and coefficient of performance dropped along with the increase in ejector spindle-blocking area percentage; (3) compared to ejector spindle-blocking area percentage, the condensing temperature had a more evident influence on the system performance.

Performance analysis of auxiliary entrainment ejector used in multi-evaporator refrigeration system

• Effect of diffuser angle on auxuiliary ejector performance is identified. • Effect of diffuser angle on auxiliary inlet position, angle and, ER is investigated. • An optimal parameters (θ = 4°, LR = 1.9, and γ = 45°) of the new-type ejector exists. • An optimal operating scheme exists under different evaporating temperatures. In this paper, a numerical method is used to maximize the performance of an auxiliary entrainment ejector used in multi-evaporator refrigeration system developed for refrigerated trucks in tropical and humid climates. The Computational Fluid Dynamics model based on the previous study is developed to discuss the effects of key design parameters of an auxiliary entrainment ejector. The simulation model is validated against experimental data of an R134a test rig. The numerical models agree fairly well with the experiments. In order to maximize the efficiency of the auxiliary inlet, the diffuser angle is considered critically. The influence of diffuser angle on the optimal position and angle of the auxiliary inlet is then discussed. As well, the effect of secondary flow and auxiliary flow at varying temperatures on the efficiency of auxiliary entrainment ejector is analyzed. The main results are given as follows: (1) optimal auxiliary inlet position as well as angle and entrainment ratio depend on diffuser angle, and optimum parameters combination (length ratio of 1.9, inlet angle of 45°, and diffuser angle of 4°) of the new-type ejector exists; (2) with the secondary flow working temperature of −10 °C, the entrainment ratio reaches its maximum value of 0.658, and the corresponding entrainment ratio at secondary inlet is 2.60; (3) the auxiliary entrainment ejector can only function if the auxiliary flow working temperature is between −30 °C to −22 °C; (4) it is best to connect the secondary inlet to the medium temperature evaporator, with the auxiliary inlet to the low temperature evaporator, when using the secondary and auxiliary inlet connections to separate evaporators with varied operating temperatures.

Optimization on key geometries of a highly coupled two-stage ejector

In this paper, an ejector-based multi-evaporator refrigeration system is proposed, in which, the refrigerant mixture coming from the exit of the first-stage enters into the second-stage primary nozzle and entrains its secondary flow refrigerant, thus, the two stages of the ejector are highly coupled. In this case, the primary nozzle throat diameter of the second-stage (PNTD2) plays a key role in affecting and improving both-stage performance. By using Computational Fluid Dynamics (CFD) simulation, key geometries of both stages such as: area ratio (AR), nozzle exit position (NXP), and the angle of constant-pressure mixing chamber (θ) are optimized with varied PNTD2. When PNTD2 increases from 4.1 mm to 5.3 mm, the simulation results show that: (1) Optimum AR of first-stage increases from 2.9 to 5.9, optimum NXP of first-stage rises from 8 mm to 14 mm, and optimum θ of first-stage improves from 7.5° to 15.5°, however, optimal AR of second-stage decreases from 10.5 to 6.9, optimal θ of second-stage decreases from 15.7° to 13.7°, and optimal NXP of second-stage keeps a constant value of 28 mm; (2) AR is the most sensitive one to influence each stage performance, which is followed by NXP, and the least one is θ; and (3) AR and NXP give an evident performance influence to the local stage but a negligible influence to the other stage. The novelty of this work is to investigate the effect of key geometries on a highly coupled two-stage ejector.

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.

Influence of liquid volume fraction on ejector performance: A numerical study

In ejector-based refrigeration systems, the fluid phase state plays a key role in improving the ejector performance. In this study, the effects of liquid volume fraction on ejector performance were investigated by computational fluid dynamics simulations. The ejector was applied in a multi-evaporator refrigeration system used for refrigerated trucks with refrigerant R134a superheated gas and gas-liquid two-phase mixture. Under three operating modes (air-conditioning – freezing, refrigerating – freezing, and air-conditioning – refrigerating modes) of the ejector, by varying liquid volume fraction of the two inlets of the ejector in the range of 0–0.1, the variation trends of primary mass flow rate, secondary mass flow rate and entrainment ratio were obtained. The results indicated that: (1) when secondary fluid is superheated gas, with the increase of primary flow liquid volume fraction, the ejector performance degrades greatly, even reflux occurs; (2) primary mass flow rate is not affected by secondary flow liquid volume fraction; (3) when secondary flow liquid volume fraction reaches a certain value, secondary mass flow rate increases with the increasing of primary flow liquid volume fraction; and (4) for three modes, the entrainment ratio change is the greatest when liquid volume fraction increases from 0 to 0.02. The novelty of this study is that the effects of different liquid volume fraction in the primary inlet and secondary inlet on the ejector performance are identified.

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.

Optimization on crucial ejector geometries in a multi-evaporator refrigeration system for tropical region refrigerated trucks

In this paper, a novel ejector-based multi-evaporator refrigeration system using R134a is proposed to meet different working modes of refrigerated trucks running in tropical regions. The Computational Fluid Dynamics (CFD) simulation is implemented to research the performance of ejectors utilized in both air conditioning - refrigerating mode and air conditioning - freezing mode. Consequently, crucial ejector geometrical parameters such as area ratio (AR), nozzle exit position (NXP), lengths of primary nozzle and diffuser are optimized for each mode. Moreover, a novel approach by taking the deviation of the entrainment ratio or secondary mass flow rate over the deviation of the geometrical parameter into account to evaluate the ejector performance sensitivity to each geometrical parameter is first time presented to the best of the authors’ knowledge. The results show that: 1) Area ratio, NXP, lengths of primary nozzle and diffuser of the ejector in the air conditioning - freezing mode are almost twice as those in the air conditioning - refrigerating mode; 2) The performance of both ejectors is primarily influenced by area ratio and NXP; 3) The proposed sensitivity indicator can better evaluate the sensitivity of the ejector performance to geometrical parameters.

Effects of operating conditions and cooling loads on two-stage ejector performances

This paper presents a two-stage ejector-based multi-evaporator refrigeration system using R134a as the refrigerant and the effects of variable operating conditions such as back pressure and evaporating temperature and cooling loads on each stage are investigated by using CFD simulations. The simulation results indicate that: (1) three operational modes: critical mode, subcritical mode and back flow mode are available for each stage when two stages combine with each other, but the range of pressure lifting ratio generating subcritical and back flow modes is not the same with each other; (2) the entrainment ratio of the first-stage is mainly influenced by the variation of the evaporating temperature of refrigerating and freezing chambers, while the entrainment ratio of the second-stage of is more influenced by the change of the evaporating temperature of the air-conditioning chamber; (3) the effect of the mass flow rate of the first-stage primary flow on the entrainment ratio of the first-stage is more significant, while the effect of the mass flow rate of the second-stage primary flow on the entrainment ratio of the second-stage is more evident.

Optimization on ejector key geometries of a two-stage ejector-based multi-evaporator refrigeration system

A three-evaporator and dual-ejector refrigeration system was presented in this paper. By combining two single ejectors as a whole, the authors attempted to discuss the effects of key geometric dimensions of each stage on both stage performances with two-dimensional CFD simulation method. First, the CFD models were validated by experimental data. Then, the primary nozzle diameters of both stages were determined by the cooling load demands of the refrigerating and air-conditioning chambers. Next, the detailed effects of key geometric parameters such as the length of constant-pressure mixing section, the length of constant-area mixing section and area ratio of each stage on two-stage performances were identified with simulations. Finally, the optimum values of the key geometric parameters were obtained, and the results showed that the area ratio had the most significant influences on the entrainment ratio of both stages as compared to other parameters.

Type-1 and Type-2 effective Takagi-Sugeno fuzzy models for decentralized control of multi-input-multi-output processes

Effective model is a novel tool for decentralized controller design to handle the interconnected interactions in a multi-input-multi-output (MIMO) process. In this paper, Type-1 and Type-2 effective Takagi-Sugeno fuzzy models (ETSM) are investigated. By means of the loop pairing criterion, simple calculations are given to build Type-1/Type-2 ETSMs which are used to describe a group of non-interacting equivalent single-input-single-output (SISO) systems to represent an MIMO process, consequently the decentralized controller design can be converted to multiple independent single-loop controller designs, and enjoy the well-developed linear control algorithms. The main contributions of this paper are: i) Compared to the existing T-S fuzzy model based decentralized control methods using extra terms to characterize interactions, ETSM is a simple feasible alternative; ii) Compared to the existing effective model methods using linear transfer functions, ETSM can be carried out without requiring exact mathematical process functions, and lays a basis to develop robust controllers since fuzzy system is powerful to handle uncertainties; iii) Type-1 and Type-2 ETSMs are presented under a unified framework to provide objective comparisons. A nonlinear MIMO process is used to demonstrate the ETSMs’ superiority over the effective transfer function (ETF) counterparts as well as the evident advantage of Type-2 ETSMs in terms of robustness. A multi-evaporator refrigeration system is employed to validate the practicability of the proposed methods.

Experimental study on a multi-evaporator refrigeration system with variable area ratio ejector

This paper describes an experimental study on a multi-evaporator refrigeration system (MERS) with conventional pressure regulating valve (PRV) and variable area ratio ejector. Some key performance indicators such as cooling capacity, power consumption and ejector entrainment ratio were evaluated by switching operating modes, adjusting superheat and tuning ejector spindle and so on. The results indicated that: (1) the energy efficiency of the MERS can be improved by up to 12% by replacing the conventional pressure regulating valve with the variable area ratio ejector, (2) sufficient superheat or superheat degree greater than 2°C is conducive to high entrainment performance of the ejector, large cooling capacity of the low-temperature evaporator and low power consumption of the system, and (3) the effect of nozzle spindle position on the performance of the system is evident.

Analysis on performance characteristics of ejector with variable area-ratio for multi-evaporator refrigeration system based on experimental data

This paper presents a study on experiment and analysis of variable area-ratio ejector used in a multi-evaporator refrigeration system (MERS). The experimental rig and method are described, and the entrainment and pressure recovery performances of the variable ejector are measured at various operating and geometric conditions. The critical area ratio is proposed as an indicator of pressure recovery status; area ratios smaller than the critical ones are required to make sure that the system operates at energy saving mode. The investigation results indicate that the entrainment ratio, pressure recovery ratio and critical area ratio are strongly affected by the primary pressure and secondary pressure. Greater entrainment ratio can be obtained by increasing the secondary pressure or decreasing primary pressure. The opposite trends are found for pressure recovery ratio. Moreover, the critical area ratio varies greatly with the operation condition, and can be predicted by a linear fitting function of primary pressure or a power function in terms of secondary pressure.

Numerical investigation of geometry parameters for pressure recovery of an adjustable ejector in multi-evaporator refrigeration system

In this paper, Computational Fluid Dynamics (CFD) technique is used to investigate the optimum geometry parameters of the adjustable ejector, which is used in variable cooling loads conditions, for the performance of pressure recovery in a multi-evaporator refrigeration system (EMERS) using R134a as the refrigerant. The performance of pressure recovery reflects the performance of the compression energy saving. The developed CFD model is first validated by actual experimental data of an EMERS. The k–ω–sst model is chosen as the turbulence model, and then used to find the optimum geometry parameters: nozzle diverging angle, length of the constant-pressure mixing section, nozzle exit position and converging angle of the constant-pressure mixing section, for high performance in pressure recovery properties. Through the analysis of physical mechanism, the results indicate that the pressure recovery ratio can be efficiently improved after the geometries optimization which is very sensitive to the nozzle diverging angle and the length of the constant-pressure mixing section.

Experimental investigation of the adjustable ejector in a multi-evaporator refrigeration system

In this paper, the experimental investigation was carried out for the performance of adjustable ejector used in a multi-evaporator refrigeration system. The adjustable ejector with a spindle to adjust the area of nozzle throat was applied to deal with the considerable variation in primary cooling load for air-conditioning in such system. The adaptability of the adjustable ejector for the system was first evaluated by the tests and the results show the adjustable ejector can efficiently deal the problem of variable primary cooling load in the system. The tests for the performance of pressure recovery were subsequently carried out. The experimental results indicate that the Pressure Recovery Ratio (PRR) and Relative Pressure Recovery Ratio (RPRR) can reach 20% and 8%, respectively, in the system with design operating condition, which means the adjustable ejector applied in multi-evaporator refrigeration system may be a promising approach for a practical multi-evaporator refrigeration system with significant energy saving. It also can be found that PRR and RPRR are reduced with decreasing of the primary cooling load.

The characteristics of pressure recovery in an adjustable ejector multi-evaporator refrigeration system

In this paper, Computational Fluid Dynamics (CFD) technique is used to investigate the adaption of adjustable ejector for variable cooling loads and the characteristics of ejector pressure recovery in a multi-evaporator refrigeration system (EMERS) using R134a as the refrigerant. The performance of pressure recovery reflects the performance of the compression energy saving. The developed CFD model is first validated by actual experimental data of an EMERS (ejector-based multi-evaporator refrigeration system). Turbulence model constants are carefully selected in order to minimize the model prediction error. The calibrated model is then solved to find the adaption property of the adjustable ejector and the effects of varying cooling loads on and the pressure recovery ratio. The results indicate that the adjustable ejector using spindle to adjust the throat area of primary nozzle is an efficient solution to control the primary operating pressure in constant for system stability. Pressure recovery ratio is sensitive to the varying of cooling loads and the relationship between them is then presented.

Pressure recovery ratio in a variable cooling loads ejector-based multi-evaporator refrigeration system

In this paper, the Computational Fluid Dynamics (CFD) technique is used to investigate the influences of varying cooling loads on the ejector pressure recovery performance in an ejector-based multi-evaporator refrigeration system (EMERS) using R134a as the refrigerant. The performance of pressure recovery in the EMERS reflects the performance of the compression energy saving. The developed CFD model is first validated by actual experimental data from the EMERS. Turbulence model constants are carefully selected in order to minimize the model prediction error. Over 200 different cases are studied using the model to find the effects of varying cooling loads on pressure recovery ratio. The results indicate that pressure recovery ratio is very sensitive to the varying primary and secondary flow cooling loads. The maximum pressure recovery ratio can reach 60% as the cooling loads vary. It was found that in order to keep the system stable, the primary and secondary cooling loads should be maintained within ±5% and ±10%, respectively, in which case the pressure recovery ratio will have a maximum ratio of 32.8%.

Use of ejectors in a multi-evaporator refrigeration system for performance enhancement

In this study, an improved cooling cycle for a conventional multi-evaporators simple compression system utilizing ejector for vapour precompression is analyzed. The ejector-enhanced refrigeration cycle consists of multi-evaporators that operate at different pressure and temperature levels. A one-dimensional mathematical model of the ejector 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 energy efficiency and the performance characteristics of the novel cycle are theoretically investigated. The comparison between the novel and conventional system was made under the same operating conditions. Also, a comparison of the system performances with environment friendly refrigerants (R290, R600a, R717, R134a, R152a, and R141b) is made. The theoretical results show that the COP of the novel cycle is better than the conventional system.