Abstract

This article discusses the use of quasi-steady and transient modeling approaches for thermal systems subjected to periodic operating conditions. Specifically, the study quantifies the outlet bulk temperature deviation, between approaches, of an internal flow subjected to time-variable inlet temperature and/or external heat flux - which are common conditions in subsystems of solar-powered cycles. The analysis uses an in-house numerical routine based on the Fourier series and the superposition principle, also considers specific sets of boundary conditions (BCs) as well as laminar and turbulent flow regimes. The results show the contribution to the total deviation of each frequency composing the time-dependent BCs. Moreover, this study discusses how such contributions depend on the flow characteristic time-scale, which is affected by the fluid properties. Through regression analysis, the investigation develops generalized correlations for directly determining the maximal deviation between both modeling approaches without solving temperature fields, i.e., with the appropriate expressions, the method only requires the BCs and the fluid properties. Finally, the method is applied to a simplified solar absorber using real irradiation data, results compared with the fully transient ones, and a cutoff-based adaptation proposed for balancing frequency interferences.