Numerical Approach for Real Gas Simulations: Part II — Flow Simulation for Supercritical CO2 Centrifugal Compressor

Abstract

This paper is presented in two parts. Part I (Tabular fluid properties for real gas analysis) describes an approach to creating a tabular representation of the equation of state that is applicable to any fluid. This approach is applied to generating an accurate and robust tabular representation of the RefProp CO2 properties. Part II (this paper) presents numerical simulations of a low flow coefficient supercritical CO2 centrifugal compressor developed for a closed loop power cycle. The real gas tables presented in part I are used in these simulations. Three operating conditions are simulated near the CO2 critical point: normal day (85 bar, 35C), hot day (105 bar, 50 C) and cold day (70 bar, 20C) conditions. The compressor is a single stage overhung design with shrouded impeller, 155 mm impeller tip diameter and a vaneless diffuser. An axial variable inlet guide vane (IGV) is used to control the incoming swirl into the impeller. An in-house three-dimensional computational fluid dynamics (CFD) solver named TACOMA is used with real gas tables for the steady flow simulations. The equilibrium thermodynamic modeling is used in this study. The real gas effects are important in the desired impeller operating range. It is observed that both the operating range (minimum and maximum volumetric flow rate) and the pressure ratio across the impeller are dependent on the inlet conditions. The compressor has nearly 25% higher operating range on a hot day as compared to the normal day conditions. A condensation region is observed near the impeller leading edge which grows as the compressor operating point moves towards choke. The impeller chokes near the mid-chord due to lower speed of sound in the liquid-vapor region resulting in a sharp drop near the choke side of the speedline. This behavior is explained by analyzing the 3D flow field within the impeller and thermodynamic quantities along the streamline. The 3D flow analysis for the flow near the critical point provides useful insight for the designers to modify the current compressor design for higher efficiency.

Theoretical Analysis of Isentropic Exponent Effects on the Internal Flow of Supercritical CO2 Centrifugal Compressors

“…e thermodynamic property table ranges were 250-600 K with 301 nodes for temperature and 1-30 MPa with 301 nodes for pressure. e resolution of the physical property table is enough to meet the table independence requirement according to the investigations of Li et al [28] and Saxena et al [29].…”

A New Method for Impeller Inlet Design of Supercritical CO2 Centrifugal Compressors in Brayton Cycles

“…Concretely, substituting the impeller inlet parameters into Equations (10) and (14), and replacing the absolute Mach number by the relative one using Equation (8) yields…”

“…Kim et al [13] carried out a three-dimensional numerical study for a SCO 2 compressor stage to obtain fundamental data for the design optimization near the critical point and in the supercritical state. More related studies can be found in the references [14,15].…”

Unsteady two-phase region evolution of SCO2 centrifugal compressor

“…15 Nikola 16 pointed out that at high flow rates, the acceleration of flow near the blade leading edge (LE) induces the thermodynamic parameters of SCO 2 to drop below the critical point, thus promoting two-phase region formation. Saxena 17 noticed that the two-phase zone in the impeller passage extended toward the streamwise direction as the mass flow rate increased. Rinaldi 18 found that the two-phase zone at the LE of the main blades extended toward the hub side as the compressor speed increased.…”