Numerical Simulation And Experimental Study On Dynamic Performance Of Transcritical CO₂ Ejector Expansion Refrigeration System

The use of an ejector can recover the throttling loss and reduce the compressor power,thus the transcritical CO₂ ejector expansion refrigeration system?EERS?is considered as a promising measure to improve the system performance.However,due to the complex flow characteristics in the ejector,as well as the highly nonlinear and the strong coupling in the ejector expansion refrigeration cycle,it is very difficult to study the ejector performance and the dynamical coupling law of system.Therefore,studies on ejector characteristics and dynamic performances of EERS have great theory and application significance.The investigations were carried out for both the CO₂ two-phase ejector and the ejector expansion refrigeration cycle,giving the following conclusions.?1?On the basis of real properties of CO₂,and the flow phenomena such as the transonic flow,chocked flow and shock wave,the ejector model was developed covering the entire operation mode.The ejector was divided into the nozzle,suction chamber,mixing and diffuser sections,and then the lumped parameter method was used to model each section.Meanwhile,the constant pressure mixing theory and the constant area mixing theory were respectively utilized in the mixing section model.The experimental results show that the largest relative errors of entrainment ratio between the model prediction and experimental data are within 10%based on the constant pressure mixing ejector model.While the relative errors of back pressure based on the constant area mixing ejector model are within 11%.Thus,the developed models are accurate and reliable for predicting the performance of CO₂two-phase ejector.?2?The dynamic model of EERS was proposed to analyze the system dynamic characteristics.The time-varying governing equations of the gas cooler,the evaporator and the separator were formulated based on the mass and energy conservation.The compressor and the expansion valve were modeled by the steady state equation.The developed ejector model was used in the system model.Moreover,the two possible states of CO₂ at the evaporator outlet,i.e.superheat and two-phase,were considered in the model,and the smooth transition between the two states was realized.It is helpful to improve the robustness of prediction and rigorously analyze the dynamic evolution laws of EERS under large disturbances.?3?The experimental setup was built to verify the ejector model and the system dynamic model.Furthermore,the effects of structural size of the adjustable ejector on the system performance were tested.The validation results show that the dynamic responses of pressure parameters present similar trends to that of experimental data.The response of entrainment ratio based on the model agrees with the experimental result,while the difference of primary flow rate between the prediction and experiment is slightly larger.The experimental results show the nozzle throat area can effectively change the ejector inlet and outlet pressure,but too small throat area may lead to the instability of system.The change of nozzle exit position?NXP?has obvious effect on the ejector performance,and there is an optimal NXP for the mass flow rate of the primary flow and the suction flow,as well as the entrainment ratio.?4?The dynamic performances of EERS were simulated under the disturbance of inner parameters,including the compressor speed,the expansion valve opening and the nozzle throat area of ejector,as well as the outer parameters,including the cooling water flow rate of gas cooler and the chilled water flow rate of evaporator.The simulation results show that the compressor speed can effectively change the system pressure and the entrainment ratio,while expansion valve opening is able to well control the evaporator pressure and the pressure lift ratio of ejector.The change in nozzle throat area can be used to regulate the gas cooler pressure and entrainment ratio.The adjustment of cooling water flow rate is beneficial to change the system pressure and refrigerant mass flow rate,resulting in large change in the system cooling capacity,heating capacity and COP.By contrast,when the chilled water flow rate is adjusted,the change of system pressure and refrigerant mass flow rate are less obvious than that from cooling water adjustment.?5?Combining the spatial distribution model and experiment data,an evaluation method of component efficiency of ejector was proposed.The ejector was divided into a lot of elements,and the governing equation was formulated for each element.Moreover,the double-flow model was employed to describe the flow loss in the mixing and diffuser section.Based on the variation of the efficiencies of nozzle,suction chamber,mixing section and diffuser section with the working conditions and the structural parameters,the corresponding empirical correlations were developed.Then the correlations were incorporated into the dynamic model of EERS,and the predicted responses adopting variable efficiencies and fixed ones were compared.The variable efficiencies are suggested to be applied in the dynamic simulation of EERS.Even if fixed values are employed,they should be determined according to the working conditions and ejector geometries.?6?A stability analysis method was put forward based on the First Approximation Theory of Lyapunov Stability Theorem and the stability margin calculation.The system stability was analyzed by the eigenvalue forms of coefficient matrix of linear governing equations.The minimum logarithmic decrement was utilized to represent the capability of resisting disturbance.A case study on the gas cooler verifies the consistence between the mathematical stability and the actual stability attribute.Stability analysis of the transcritical CO₂ ejector expansion refrigeration cycle is also performed to forecast the practical range of working conditions and structural parameters,within which the system can operate stability.?7?The modeling method and experimental setup of EERS were extended to the dynamic simulation and experimental study of an improved ejector expansion refrigeration cycle,namely the transcritical CO₂ ejector expansion refrigeration cycle with two-stage evaporation?EERS-TE?.The simulation results show that,under the conditions of combination adjustment,the COP of EERS-TE increases from 2.40 to 3.58 improved by49.17%.While the COP of EERS rises from 2.10 to 2.50 with an increase of 19.05%.The increase of COP for the EERS-TE is 2.58 times larger than that of EERS.The results show the performance of EERS-TE is superior to that of EERS.The experimental results also show the second evaporator can obviously improve the system performance.The system cooling capacity,heating capacity and COP all increase with the increase of the flow rate of the second evaporator chilled water.Moreover,the adjustment of ejector area ratio?the mixing section area to the area of throat area?can also improve system performance.For the system with small area ratio of ejector,the improvement through the second evaporator is particularly prominent.The present study is helpful for a better understanding of the inner complex flow of ejector and the dynamic characteristic of EERS.The proposed theoretical method and the research results are expected to benefit the application research of ejector expansion refrigeration technology.