Direct numerical simulations and reduced order models for supercritical fluids at rest and in motion

• Grant number: 24/08730-2
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: August 01, 2024
• End date: July 31, 2026
• Field of knowledge: Engineering - Mechanical Engineering - Transport Phenomena
• Principal Investigator: Luben Cabezas Gómez
• Grantee: Davi Bernhard de Souza
• Host Institution: Escola de Engenharia de São Carlos (EESC). Universidade de São Paulo (USP). São Carlos , SP, Brazil
• Associated research grant: 22/15765-1 - Study of transport phenomena applied to emerging technologies of energy conversion and storage, AP.TEM

Abstract

The critical point represents a thermodynamic state beyond which the gaseous and liquidphases of a fluid can no longer coexist. This mechanical instability creates a phenomenonknown as critical anomaly, i.e. all thermodynamic properties diverge algebraically asthe critical point is approached along the reduced temperature. Furthermore, the respectivecritical exponents are essentially fluid universal and correlated in such a way that only twodistinct ones exist. These anomalies often result in unusual fluid behavior. For instance,the fluid compressibility exhibits a sharp increase even for low Mach number flows, whichallows minute temperature differences to generate thermo-acoustic waves. These wavescarry energy from the thermal boundary-layer into the bulk fluid at acoustic speeds. Whenoccurring in a confined medium, this leads to a rapid increase of the bulk temperature.Known as piston effect, this phenomenon was first observed in low gravity experiments and explained soon after using a simplified thermodynamic model as well as simulations of the compressible Navier-Stokes equations. Since these pioneering studies, the influence of near critical thermodynamics states on fluid flow and heat transfer has been extensively explored. Recent work on supercritical heat transfer has i) shown that variable fluidproperties have little effect on the bulk temperate increase, ii) described the necessaryconsistency requirements for the results obtained from simulations of the thermodynamic model and Navier-Stokes equations to agree, iii) derived an analytical expression for the bulk temperature relaxation time, iv) demonstrated the importance of modeling wall impedance when attempting to validate Navier-Stokes simulations and v) shown that wall impedance can essentially eliminate the piston effect. Finally, it is worth pointingout a quite recent attempt to better understand how fluid flow instabilities are affected bythe proximity to the thermodynamic critical point.