Front-capturing method

The purpose of this section is to give an overview of the pertinent high resolution methods often referred in the literature on multiphase reactor modeling. These are The Maker and Cell (MAC) method [96], the Simplified MAC method [6], the volume of fluid (VOF) method[108], the level set (LS) front capturing method [214, 20, 186], and finally the front tracking method [227, 221[. 

It is noticed that after some re-evaluation Tryggvason and co-workers [228, 222] classified their front tracking method [227] as an embedded interface method, since it is best described as a hybrid between a front tracking and a front capturing method. 

Ferreira, J.A. Cimiinato, S. McKee, A front-trackiiig/front-capturing method for the simulation of 3D multi-fluid flows with Ifee surfaces. Journal cf Computational Physics, 2004, 198, 469-499. 

Kwakkel M, Breugem WP, Boersma BJ An efficient multiple marker front-capturing method for two-phase flow, Comput Fluids 63 47—56, 2012. 

This paper is intended to describe recent progress on the development of the level-set method and IBM in the context of the advanced front-capturing and front-tracking methods. The paper is also intended to discuss the application of them for the 3-D DNS of two complex three-phase flow systems as described earlier. 

An alternative and complementary method is to follow the deformation of the material by following the boundaries of a fluid domain, by either the front capturing or front tracking technique. In the former, marker particles are distributed over the fluid domain volume and tracked over time, with the surface of the domain being restored by interpolation techniques in the latter, a separate moving mesh is used to describe the interface between the subdomain and the rest of the volume (86-89). 

The scalar x is defined differently in different interface capturing techniques. For example, x is volume fraction in the VOF volume tracking method, a distance function in the LS method, or a Heaviside (step) function in the front tracking method. 

A hierarchy of computational models is available to simulate dispersed gas-liquid-solid flows in three-phase slurry and fluidized bed reactors [84] continuum (Euler-Euler) method, discrete particle/bubble (Euler-Lagrange) method, or front tracking/capturing methods. While every method has its own... 

Volume of fluid (VOF) method Level set method Phase fleld method Diffuse interface model Interfacial flow Free surface flow Interface tracking Front tracking Front capturing... 

Numerical simulation of hood performance is complex, and results depend on hood design, flow restriction by surrounding surfaces, source strength, and other boundary conditions. Thus, most currently used method.s of hood design are based on experimental studies and analytical models. According to these models, the exhaust airflow rate is calculated based on the desired capture velocity at a particular location in front of the hood. It is easier... 

Another design method uses capture efficiency. There are fewer models for capture efficiency available and none that have been validated over a wide range of conditions. Conroy and Ellenbecker - developed a semi-empirical capture efficiency for flanged slot hoods and point and area sources of contaminant. The point source model uses potential flow theory to describe the flow field in front of a flanged elliptical opening and an empirical factor to describe the turbulent diffusion of contaminant around streamlines. 

Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. Fhe exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. lliis method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved. 

For the analysis of the chemical structure of flames, laser methods will typically provide temperature measurement and concentration profiles of some readily detectable radicals. The following two examples compare selected LIF and CRDS results. Figure 2.1 presents the temperature profile in a fuel-rich (C/O = 0.6) propene-oxygen-argon flame at 50 mbar [42]. For the LIF measurements, 1% NO was added. OH-LIF thermometry would also be possible, but regarding the rather low OH concentrations in fuel-rich flames, especially at low temperatures, this approach does not capture the temperature rise in the flame front [43]. The sensitivity of the CRDS technique, however, is superior, and the OH mole fraction is sufficient to follow the entire temperature profile. Both measurements are in excellent agreement. For all flames studied here, the temperature profile has been measured by LIF and/or CRDS. 

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