• Energy Research

Abstract 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%.

Abstract 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 (Lpm), the constant-area mixing chamber length (Lam) and diameter (Dam), 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 (LVF1) and secondary flow liquid volume fraction (LVF2) (LVF1 = 0 plus LVF2 = 0, LVF1 = 0 plus LVF2 = 0.06, LVF1 = 0.06 plus LVF2 = 0, LVF1 = 0.06 plus LVF2 = 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 LVF1 = 0 plus LVF2 = 0 or LVF1 = 0.06 plus LVF2 = 0.06, ultimate optimal geometrical parameters and maximum ER are the same with each other for six optimization sequences, however, for LVF1 = 0 plus LVF2 = 0.06 or LVF1 = 0.06 plus LVF2 = 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 Dam → Lam → Lpm (S6), Lpm → Lam → Dam (S1) or Lpm → Dam → Lam (S2), and Lam → Lpm → Dam (S3) and Dam → Lam → Lpm, 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.

Abstract 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.

Abstract This paper describes the development of a combined ejector-vapor compression cycle (EVCC) that uses working fluid R134a and air-cooled condensers for both sub-cycles. A large amount of experiments have been conducted to test the influence of the evaporating, generating and condensing temperatures of the ejector sub-cycle (ESC) on the performances of EVCC. The test results show that (1) the performance of the combined cycle is very sensitive to the three temperatures of ESC; (2) the variation of the degree of sub-cooling at the vapor compression sub-cycle (VCSC) is similar to that of COP improvement at EVCC over VCSC; and (3) this system with certain operating conditions results in relatively high COP improvements (15.9–21.0%).

Abstract 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.

In this paper, a hybrid modeling approach is proposed to describe the dynamic behavior of the two phase flow condensers used in air-conditioning and refrigeration systems. The model is formulated based on fundamental energy and mass balance governing equations, and thermodynamic principles, while some constants and less important variables that change very little during normal operation, such as cross-sectional areas, mean void fraction, the derivative of the saturation enthalpy with respect to pressure, etc., are lumped into several unknown parameters. These parameters are then obtained by experimental data using least squares identification method. The proposed modeling method takes advantages of both physical and empirical modeling approaches, can accurately predict the transient behaviors in real-time and significantly reduce the computational burden. Other merits of the proposed approach are that the order of the model is very low and all the state variables can be easily measured. These advantages make it easy to be applied to model based control system design. The model validation studies on an experimental system show that the model predicts the system dynamic well.

Abstract In this paper, a CFD model calibrated by the experimental results from initial designed ejector is used to evaluate the influence of 6 key geometry parameters on the performance (entrainment ratio) of an air-cooled ejector cooling system and, consequently, to find the best design parameters. A new ejector according to the findings from the CFD simulation is then designed and used at the same air-cooled ejector system to verify the simulation results. From both simulation and testing results, we find that: 1) the optimal area ratio, the ratio of primary nozzle exist position and length of constant-area mixing section to primary nozzle diameter are lower than those of water-cooled ejector systems; 2) the optimal converging angle of constant-pressure mixing section and the ratio of primary nozzle exit position and length of constant-area mixing section to the diameter of constant-area mixing section are very close to those of water-cooled ejector systems; 3) substantial performance improvement can be achieved by using the new parameters in the ejector design.

Abstract 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.

Abstract In this paper, a hybrid modeling approach is proposed to describe the dynamic behavior of the two phase flow condensers used in air-conditioning and refrigeration systems. The model is formulated based on fundamental energy and mass balance governing equations, and thermodynamic principles, while some constants and less important variables that change very little during normal operation, such as cross-sectional areas, mean void fraction, the derivative of the saturation enthalpy with respect to pressure, etc., are lumped into several unknown parameters. These parameters are then obtained by experimental data using least squares identification method. The proposed modeling method takes advantages of both physical and empirical modeling approaches, can accurately predict the transient behaviors in real-time and significantly reduce the computational burden. Other merits of the proposed approach are that the order of the model is very low and all the state variables can be easily measured. These advantages make it easy to be applied to model based control system design. The model validation studies on an experimental system show that the model predicts the system dynamic well.