Orthogonal analysis method for multi – Objective optimization of CO2 nozzle thermodynamic performance

Abstract

In this study, the structural parameters of the converging diverging (CD) nozzle in a subcritical or supercritical CO2 cycle system were optimized by L16 (44) orthogonal design. The established model was used to analyse the effect of divergence length Ld, divergence angle θ, convergence angle α and throat diameter Dth on the nozzle thermodynamic performance, and further optimization of CD nozzle structural parameters was carried out using comprehensive evaluation coefficients and comprehensive scores. The results indicate that Ld is the primary factor, and while the effects of θ and Dth were inconsistent in different application scenarios, attention should be paid to the size parameters of the nozzle divergence part during design. For CD nozzles with supercritical inlet pressure, there is a linear relationship between Ld and exergy destruction, while for subcritical CD nozzles, the effect of Ld is reduced. For a CD nozzle used in subcritical or transcritical cycles, the outlet exergy and exergy efficiency of the modified nozzle could be increased by 16.4% and 16.2%. For supercritical nozzles used in the Brayton cycle, the optimized exergy destruction was relatively reduced by 37%. The mass flow rate of the nozzle before and after optimization was reduced, with a maximum decrease of 22.7%, which is beneficial for improving the entrainment ratio of the ejector. In addition, it was found that considering this in the modelling process would lead to conservative optimization results. These research results could provide a reference for the design of the CO2 nozzle or motive nozzle of ejector.