Study On The Key Influencing Factors And Performance Of Novel Bi-ejector Refrigeration System

The ejector refrigeration system is one of refrigeration modes which can use low grade energy, and is gainning more and more attention in recent years because of its simplicity in construction, installation and maintenance. Traditionally, the pump is the only moving part of the ejector refrigeration system, which adds additional mechanical energy consumption and usually needs more maintainability work than other equipments. Recently, many researchers have tried to develop a new ejector refrigeration system without the pump. The novel bi-ejector refrigeration system proposed by our research team and its key influencing factors were studied in this paper.Properly selecting refrigerant is an effect method to improve the performance of ejector refrigeration system. In addition, under the same working condition, the generation pressures, the condensation pressures and the evaporation pressures of the refrigerants with similar normal boiling points are also very closer. By introducing refrigerant real property, a thermodynamic model of ejector was established. The pressure and velocity change trends in the ejector and the entrainment ratio of the ejector with the refrigerants having similar normal boiling points were compared, and the main reason for the performance difference of injector with various refrigerants was analyzed according to refrigerant property. The COP values of several pairs of refrigerants with similar normal boiling points were compared. The results indicate that for refrigerants with similar normal boiling points, the higher the specific enthalpy of the driving vapor, the higher the entrainment ratio of ejector is.Taking the momentum equation of ejector mixing chamber as base equation, a thermodynamic off-design model for ejector cooling system was set up with the real properties of working fluid. Using the model, the effect of ejector characteristic area ratio on the performance of ejector and ejector refrigeration system is studied, and the change of the performance of a designed ejector refrigeration system is also studied when R11 is replaced with R123 and R141b. The results indicate that the larger the ejector characteristic area ratio, the larger the maximun entrainment ratio and the COP the ejector and the ejector refrigeration system can reach, but at the same time, the lower the system critical condensation temperature is and the smaller the condensation temperature range in which the system can work is.Considering the important physical phenomenon of "condensation shock" and "the wall resistance", a mathematical model predicting the maximum promoting pressure of injector is established with energy, momentum and mass conservation equations. According to the characteristic of injector in novel bi-ejector refrigeration system, the performance of injector is studied by using the model. The performances of injector with various refrigerants are also studied and compared. The results indicate that the injector performance is affected by the entrainment ratio, the operation parameters, the injector structure and the refrigerant, the entrainment ratio of injector with water is far higher than those with other refrigerants.The performance coefficient of the novel bi-ejector refrigeration system was derived, and the expression was also compared with that of the traditional ejector refrigeration system. The comparison result indicates that no matter how much the entrainment ratio of the added injector is, the performance coefficient of the novel bi-ejector refrigeration system will always be equal to that of the traditional system as long as their operating conditions are the same. Exergy analysis was done on the novel bi-ejector refrigeration system, and the result was also compared with that of the traditional ejector refrigeration system. The comparison results indicate that under the same operating condition, the two systems have equal exergy efficiencies and total exergy losses; Except the generator and the added injector, each component of the two systems has equal exergy loss; The generator exergy loss of the novel system is lower because its refrigerant inlet temperature is higher, but the added injector also generates exergy loss, which makes the total exergy losses of the two systems are still equal.The method to calculate the hourly and average solar radiation on solar collector in various districts was presented. The novel solar bi-ejector refrigeration system was analysed with energy and exergy methods and was also compared with the traditional system. The results indicate that with the increasing generation temperature, the entrainment ratio of injector and the thermal efficiency of heat-collecting subcycle both decrease, the entrainment ratio of ejector and the performance coefficient of the refrigeration subcycle both increase, and the overall energy efficiency and exergy efficiency both first increase and then decrease; with the increasing condensation temperature, the entrainment ratios of ejector and injector and the overall energy efficiency and exergy efficiency all decrease; in the whole system, the solar collector has the largest exergy loss rate of over 90% and the ejector takes the second place, which has the exergy loss rate of about 5%.