DPM Wet Combustion and Reactive Particles -- Devolatilization, Char Burning, and Multicomponent Evaporation

#+CATEGORY: cfd

#+TAGS: DPM combustion devolatilization char-burning multicomponent evaporation law wet-combustion coal biomass

#+MATURITY: growing

DPM Wet Combustion and Reactive Particles

Fluent's DPM supports five particle types with increasing physics fidelity. The two most complex are the multicomponent type (evaporating liquid mixture) and the combusting type (devolatilization + char oxidation). These models are essential for coal/biomass combustion, spray drying of suspensions, and waste incineration -- but not for the shisha simulation (which uses inert particles). This page documents the full reactive particle workflow for completeness.

DPM Laws Overview

Fluent numbers the physical laws applied to a particle sequentially. Each law activates during a specific stage of the particle's lifetime:

The particle transitions from one law to the next based on conditions:

Devolatilization Models (Combusting Type)

Single-rate model (Kobayashi et al.)

Parameters: A_1 (pre-exponential), E_1 (activation energy), f_{vol,0} (initial volatile mass fraction).

Two-competing-rates model (Kobayashi et al., 1977)

Two parallel reactions at different activation energies allow the model to capture temperature-dependent volatile yield.

CPD model (Fletcher)

Chemical Percolation Devolatilization -- a structural model based on coal macromolecular network topology. More physics than the single/two-rate models, but requires additional material parameters (cross-linking probability, coordination number). Fluent provides this as an option but it is rarely used outside research settings.

Activation

1. Particle Type = Combusting 2. Materials panel: set Volatile Fraction, Char Fraction 3. Models > DPM > Injections > select injection 4. Set Devolutization Model to single-rate or two-competing-rates 5. Set kinetic parameters A, E, \alpha (from proximate/ultimate analysis)

Char Surface Reaction Models

After devolatilization, the remaining char (carbon + ash) oxidizes at the particle surface.

Kinetics/diffusion-limited model (Field et al., 1967)

where:

• =A_p = \pi d_p^2= (particle surface area)
• p_{ox} = oxidizer partial pressure
• =D_0 = C_1 [(T_p + T_\infty)/2]^{0.75}= (diffusion rate coefficient)
• =R_0 = C_2 \exp(-E/(R T_p))= (kinetic rate coefficient)

Activation

1. After devolatilization law, the particle automatically enters char combustion (Law 5 or 6) 2. Set char reaction parameters:

• C_1 (diffusion rate constant): 5e-12 kg/(m^2 s Pa K^{0.75}) for C + O2
• C_2 (kinetic rate constant): specific to coal type
• E (activation energy): 7.2e7 to 1.5e8 J/mol depending on rank
• Oxidizing species index (typically O2)

Ash inert residue

After all char is consumed, the remaining ash heats up via Law 8 (inert heating). The ash particle then continues tracking until it escapes, is trapped, or reaches maximum steps.

Multicomponent Evaporation (Law 10)

For liquid particles containing multiple evaporating species (e.g., fuel blends, bio-oil), multicomponent evaporation applies a species mass balance:

where B_{M,i} is the Spalding mass transfer number for species i:

The vapour mass fraction at the surface Y_{i,g} comes from Raoult's law or from the Clausius-Clapeyron equation for each species.

Setup

1. Particle Type = Multicomponent 2. Specify the number of species in the particle (up to 3) 3. For each species: set Species Index, Boiling Temperature, Latent Heat, Initial Mass Fraction 4. Fluent evaporates each species in sequence (lowest boiling point first, then the next)

Example: bio-oil droplet (3 species)

The droplet first loses water (at T_p > 373 K), then acetic acid, then guaiacol. This sequence is handled automatically by Law 10.

Swelling Model (Law 9)

Some coals (especially bituminous) swell during devolatilization, increasing their volume. The swelling factor C_{sw} relates the maximum particle diameter to the initial:

During devolatilization, the particle diameter increases linearly from d_{p,0} to d_{p,max} as volatile mass is released.

Typical swelling factors:

Material Setup for Combusting Particles

Required material properties

Creating the material in Fluent

1. Materials > Create/Edit > dpm-particle-material 2. Set all properties above 3. Assign to injection in the injection panel

Complete Combusting Particle Workflow

Shisha Applicability Note

For the shisha simulation:

• Particle type = Inert (not combusting)
• No evaporation, devolatilization, or char reaction
• Smoke particles are solid carbonaceous aerosol (not burning inside the chamber)
• Laws active: only Law 0 (heating/cooling via Ranz-Marshall)
• Laws 1--10 are all inactive

This page documents reactive particle models for completeness and for future cases (e.g., if you model tobacco combustion in the bowl, or biomass pyrolysis).

See Also