The ARCHES component was initially designed for predicting the heat-flux from large buoyant pool fires with potential hazard hazards immersed in or near a pool fire of transportation fuel. Since then, this component has been extended to solve many industrially relevant problems such as industrial flares, oxy-coal combustion of natural gas and coal, and coal gasification.
ARCHES solves the conservative, finite volume, compressible, low-Mach formulation of the Navier-Stokes equations with a pressure projection that includes the effect of variable density, reaction, and heat transfer modes in the gas phase including radiation. Given the wide range of length and time scales that are present in many combustion problems of interest, ARCHES utilizes models for bridging the molecular (micro) scales to the full, large (macro) scales. This bridging occurs through the use of subgrid and resolved scale mixing, reaction, and turbulence models. Momentum turbulence closure is accomplished by standard dynamic large eddy simulation (LES) closure models. Fast chemistry scales are modeled by preprocessing the full chemical mechanism, modeled in various idealized configurations (e.g., equilibrium or flamelets), then tracking a set of reduced parameters on the LES mesh that map the full thermochemical state-space. Subgrid turbulence species mixing processes are included by using presumed PDF methods and using models for certain moments of the distribution (e.g., scalar variance and mixture fraction). The chemistry and mixing models are usually completely preprocessed together into one tabular format to give a mixing table.
ARCHES also includes a statistical description of the coal particle number density function for modeling two-phase flows. The coal particulate phase is represented using the direct quadrature method of moments (DQMOM). This Eulerian reference frame approach solves a discrete set of moments of the number density equation using numerical quadrature. The number density function is described by the particle properties including particle number, size, and coal properties (velocity, volatile mass fraction, char mass fraction, energy content, size). Various physical models are implemented to include the effect of coal gas devolatilization, char oxidation, particle drag, and size changes, which in turn effect the coal number density function. The DQMOM description of the coal phase is completely coupled with the gas phase description to produce a completely coupled, gas/solid phase description of the flow with closed mass, momentum, and energy balances.