Thermodynamic Modeling Investigation Of Supersonic Ejectors

Ejectors are widely used in the refrigeration system,fuel cell system and desalination plant due to their simple structure,long service life and low maintenance cost.The supersonic ejector is the key component in systems due to its unique gas pressurization way.The research on the thermodynamic model of the supersonic ejector is significant in the analysis,optimization and control of the system.In the supersonic ejector,there are many complex phenomena,such as transonic gas flow,high intensity turbulence,shock wave and fluid mixing.Meanwhile,working mediums are various in different systems.Great differences in physical properties of different working fluids also increase the difficulty in the thermodynamic modeling of the ejector.The research on the thermodynamic model of the supersonic ejector has been done for nearly eighty years.However,there are still some problems,such as the inadequate research on the component efficiencies,the complex structure of the thermodynamic model,the low computational accuracy of the ejector performance for the subcritical mode and the insufficient research on the condensation phenomenon.In this paper,the working mechanism and model structure of supersonic ejector are studied based on the numerical simulation and theoretical derivation.The specific research contents of this paper are described as follows:An axisymmetric computational fluid dynamics(CFD)model is used to investigate the effect of the area ratio on ejector component efficiencies.The geometrical parameters and operating conditions are designed for thermal vapor compression(TVC)used in multi-effect distillation(MED)desalination system.By changing the primary nozzle throat diameter and ejector throat diameter,two groups of ejectors are tested under different operating conditions.The relations among area ratio,component efficiencies and flow filed are discussed based on the simulation result.Finally,empirical correlations are summarized to estimate ejector component efficiencies.A control oriental model is proposed to predict the performance of the supersonic ejector.A theoretical model is developed based on the thermodynamic principles and ideal gas property firstly.And then the proposed model is simplified to linear equations with four unknown parameters which can be determined by traditional identification methods easily.The accuracy of the model is validated by literature and experimental data.The results show that the model has a strong ability to predict the ejector entrainment ratio and critical back pressure.Furthermore,the proposed method is more accuracy and simpler than the previously published models.This paper focuses on the problem of the thermodynamic modeling of the supersonic ejector under both critical and subcritical mode.Unlike the traditional 1D mathematical model,the proposed model takes into account the non-uniform distribution of the flow field in the radial direction.The key problem is to obtain the mixing pressure.For the critical point,it is assumed that the mixing progress occurs in the constant area mixing chamber under a uniform mixing pressure which is between the stagnation pressure and choking pressure of the secondary flow.By using an exponential relationship to approximate the velocity distribution of the secondary flow in the radial direction,the entrainment ratio,the back pressure and the mixing pressure can be calculated.However,a constant area mixing assumption is made when the ejector operates under the subcritical mode.By adjusting the mixing pressure,the performance of the ejector under the sub-critical mode is obtained.A novel thermodynamic model of the ejector is proposed based on the real gas relations.The influences of the superheat level of inlet flow on the condensation phenomenon and the ejector performance are discussed.The relation between the superheat level of the inlet flow and the condensation phenomenon at different cross section is also obtained.It is found that condensation is a pervasive phenomenon in the ejector chambers.But the degree of condensation varies with the working mediums.Meanwhile,the degree of the condensation phenomenon can be reduced by increasing the superheat level of the inlet flow.