1D Modelling of some Paradigmatic Non-ideal Compressible Flows

orc2019 Tracking Number 103

Systems based on the organic Rankine cycle (ORC) concept for energy harvesting, refrigeration and power generation make use of a variety of working fluids. For instance, complex working fluids and supercritical carbon dioxide are utilized in organic Rankine cycle and supercritical CO2 Brayton cycle power systems. Thermo-fluid characteristics of these vapor flows in expanders or compressors deviate significantly from those of flows of perfect gases and this deviation increases if the processes occur in proximity of saturation or the vapor-liquid critical point. The term Non-Ideal Compressible Fluid Dynamics (NICFD) is utilized to characterize the gas dynamics of dense vapors: a key parameter to study these flows is the fundamental derivative of gasdynamics Γ, which accounts for variations in speed of sound with density over isentropic processes. Although rather extensive theoretical knowledge has been developed on the fundamental aspects of NICFD, these findings have not been thoroughly studied in the context of applications and particularly of energy conversion applications, i.e., flows in turbomachinery and heat exchangers. This study is aimed at evaluating the impact of NICFD effects by analyzing simple 1D flow configurations reproducing some of the main entropy loss mechanisms of turbomachinery. A new general theoretical framework based on quasi one-dimensional control volume analysis is developed: a new matrix of influence coefficients accounting for shaft work, heat addition, wall friction, variation in cross section area and mass injection in the control volume is derived. The approach is based on works by Shapiro (1953). Experimental observations of the expansion of a siloxane vapor in a de Laval nozzle provide a mean for preliminary validation. Simple Rayleigh and Fanno flow configuration are studied to quantify the impact of these phenomena on the performance of turbomachinery (and heat exchangers).