China 
Africa 
Explore chapters and articles related to this topic
Solid–Liquid Systems
Published in Wioletta Podgórska, Multiphase Particulate Systems in Turbulent Flows, 2019
Precipitation methods that apply supercritical fluids (SCFs) can be divided into methods in which SCF is used as a solvent and methods in which SCF is used as an antisolvent. The formation of particles requires supersaturation creation. Using supercritical fluids allows one to create very high supersaturation in a very short time and also reduce it in a short time. Supersaturation is created as a result of the rapid expansion of a supercritical solution or fast mixing with antisolvent. Let’s consider the antisolvent method used to produce nicotinic acid (Kubicki, 2006; Bałdyga et al., 2010). The process was performed in a SEDS apparatus (York and Hanna, 1996). The important part of the system is a concentric nozzle. The unsaturated solution of acid in ethanol is introduced through the inner channel, while the supercritical CO2 (scCO2) flows in the outer tube. The mixing chamber is located just before the nozzle exit. Turbulent mixing in the chamber improves mass transfer and the dispersion of liquid solution in supercritical fluid. The presence of antisolvent decreases the solubility of acid in ethanol drops, which results in supersaturation creation, nucleation, and growth of crystals. Local CO2-ethanol density strongly depends on local pressure and flow parameters and is calculated using the Peng-Robinson equation of state. Density variation is taken into account by using Favre averaging for momentum, mass, species, energy equations, as well as transport equations for turbulent kinetic energy and energy dissipation rate.
Compressible Multiphase Flow
Published in Efstathios E. Michaelides, Clayton T. Crowe, John D. Schwarzkopf, Multiphase Flow Handbook, 2016
John D. Schwarzkopf, S. Balachandar, William T. Buttler
Favre averaging is a method of simplifying the equations of uid motion when variations in density are present, such as found in compressible ows. e Favre average is based on a mass-weighted ensemble average and for property B is de ned as B= rB r or rB , r (5.6)
TimeScale Analysis, Numerical Simulation and Validation of Flame Acceleration, and DDT in Hydrogen–Air Mixtures
Published in Combustion Science and Technology, 2021
Aditya Karanam, Sunil Ganju, Jayanta Chattopadhyay
The present problem of hydrogen flame propagation and its transition between multiple regimes is highly turbulent with a wide range of time and length scales. The three main modeling approaches used to handle turbulence are the Reynolds-Averaged Navier–Stokes Equations (RANS), Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). In RANS, the balance equations for averaged quantities are obtained by averaging the instantaneous balance equations. Then, closure rules are applied to model unsteady features such as turbulence and turbulence-chemistry interaction. LES performs a filtering operation to obtain a set of equations which resolve the larger scales of turbulence. The smaller scales are modeled using sub-grid scale models. DNS solves the full instantaneous set of balance equations without requiring any turbulence or combustion models. In the present work, the RANS modeling philosophy has been chosen as this is computationally less expensive and relatively coarse mesh can be used. RANS modeling provides reasonably good estimates on the macroscopic parameters of interest. Due to variation in the density field, the averaging procedure is based on Favre averaging. The Favre-averaged species transport equation for species k is presented in Equation (4).