Power density performances and multi-objective optimizations for an irreversible Otto cycle with five specific heat models of working fluid

Abstract

Power density (Pd) performance for an irreversible Otto cycle is analyzed with constant specific heat (SH), linear and nonlinear variable SH, linear and nonlinear variable SH ratio of working fluid. Relationships among dimensionless Pd (P‾d), thermal efficiency (η) and compression ratio (γ) of cycle are obtained taking three loss items account into. Performances are compared when dimensionless power output (P‾) and P‾d are maximized with five SH models. Choosing γ as design parameter, and P‾, η, dimensionless ecological function and P‾d as performance parameters, multi-objective optimizations are carried out for five SH models by using NSGA-Ⅱ. Evaluating deviation indices (D s) under three decision-making methods, the optimal parameter combination with the smallest D is selected as the better one. Results show that SH model has no qualitative influence on relationship curve of P‾d−γ and P‾d−η, but it has quantitative influence. With linearly variable SH and constant SH, γP‾d reaches the maximum and minimum respectively. With linearly variable SH ratio and nonlinearly variable SH, ηP‾d reaches the maximum and minimum respectively. The D of LINMAP method with constant SH model is the smallest, which is 0.1236, and the D of Shannon entropy method with the nonlinear variable SH ratio model is the largest, which is 0.3664.