modeling of isentropic coefficients used in one dimensional model to predict ejector performance at critical mode

a number of isentropic coefficients are used in the one-dimensional models which predict ejector performance at critical mode. some of these coefficients have considerable effects on accuracy of the model. these coefficients depend on geometry, working fluid and operating conditions; but, they are usually taken constants or are presented as functions of geometry and working condition based on a specific experiment. in this work, the idea of using the flow parameters to determine these coefficients is introduced and has been analyzed. for this purpose, four models with different formulations are employed. the fluid has been considered as a real gas; hence, the models which are based on the ideal gas assumption are modified. the experimental data related to some ejectors with different geometries, working fluids and working conditions have been used. using the empirical data, correlations between some of the isentropic coefficients and the flow parameters are developed for some models. using these correlations, entrainment ratios are calculated with the maximum relative error of 35%, while in most cases the maximum relative error is about 10%. however, errors are acceptable since the empirical data are extracted from a vast range of different geometrical and operational conditions.