Native File Format Specification¶

Complete reference for FLINT native file formats (non-Cantera builds).

Overview¶

When FLINT is compiled without Cantera support, input data is loaded from a set of pre-generated native files. This path is designed for high-performance production use, where pre-computed tables eliminate runtime polynomial evaluation and mechanism parsing.

FLINT native files support two distinct thermodynamic models:

Ideal gas (multi-species) — 1D tables indexed by temperature
Real fluid (single-species) — 2D tables indexed by pressure and enthalpy

Ideal Gas Native File Structure¶

The complete input for an ideal-gas mechanism consists of up to eight files in the same directory:

INPUT/
  ├── phase.txt               # Species list
  ├── chemistry-info.txt      # Chemistry - Metadata & stoichiometry
  ├── chemistry-Arrhenius.dat # Chemistry - Reaction rates
  ├── chemistry-Troe.dat      # Chemistry - Fall-off parameters (optional)
  ├── chemistry-Lindemann.dat # Chemistry - Fall-off parameters (optional)
  ├── transport.dat           # Transport properties (optional)
  ├── diffusion.dat           # Binary diffusion coefficients (optional)
  └── thermo.dat              # Thermodynamic properties

File Sizes (CORIA 17-species / 44-reaction example): - phase.txt: ~250 bytes - chemistry-info.txt: ~15 KB - chemistry-Arrhenius.dat: ~43 MB (15000 temperature points × 44 reactions) - chemistry-Troe.dat: ~8 MB (fall-off reactions only) - thermo.dat: ~15 MB (15000 temperature points × 17 species × 5 properties)

Total: ~81 MB for a medium-sized mechanism. Pre-tabulation trades memory for speed.

1. `phase.txt` – Species Definitions¶

Purpose: Define species names and molecular weights.

Format: ASCII text, one species per line.

ideal-gas phase
<species_name> <molecular_weight_g_mol>
<species_name> <molecular_weight_g_mol>
...

Example (CORIA mechanism):

ideal-gas phase
H2      2.016000
H       1.008000
O      15.999000
O2     31.998000
OH     17.007000
H2O    18.015000
HO2    33.006000
CH3    15.035000
CH4    16.043000
CO     28.010000
CO2    44.009000
H2O2   34.014000
HCO    29.018000
CH2O   30.026000
CH3O   31.034000
C2H6   30.070000
CH3O2  47.033000

Parsing Notes: - Line 1 is always ideal-gas phase (header) - Subsequent lines: space-separated <name> and <MW> - MW values are floats in g/mol - Species order must match chemistry-info.txt and .dat files

2. `chemistry-info.txt` – Mechanism Metadata¶

Purpose: Provide resume of the chemical mechanism: species/reaction counts, reaction types, and stoichiometry.

Format: ASCII text with three sections.

<mechanism_name>
N.ro species = <count>
N.ro reactions = <count>

General loop info. They are not used if the mechanism is exlplicitly defined

Reaction type
<index> <type>
<index> <type>
...

Reaction definition
<rxn_id> <species_name> <coeff_reactant> <coeff_product> <coeff_thirdbody>
<rxn_id> <species_name> <coeff_reactant> <coeff_product> <coeff_thirdbody>
...

Example (CORIA, header section):

CORIA-CNRS
N.ro species = 17
N.ro reactions = 44

General loop info. They are not used if the mechanism is exlplicitly defined

Reaction type
1 Arrhenius
2 Arrhenius
3 Arrhenius
4 Arrhenius
5 three-body-Arrhenius
6 Arrhenius
7 falloff-Troe
...
44 Arrhenius

Reaction definition
1 H2 1.0 0.0 0.0
1 H 0.0 1.0 0.0
1 O 1.0 0.0 0.0
1 O2 0.0 0.0 0.0
...

Section 1: Mechanism Header

<mechanism_name>
N.ro species = <n_species>
N.ro reactions = <n_reactions>

<mechanism_name>: Arbitrary string (e.g., "CORIA-CNRS", "GRI-MECH3")
n_species: Total number of distinct species
n_reactions: Total number of chemical reactions

Section 2: Reaction Type List

Reaction type
<reaction_index> <reaction_type>
...

Reaction types: - Arrhenius – Standard elementary reaction (2-body, 3-body forms handled by coefficients) - three-body-Arrhenius – Three-body reaction (includes third-body collision partner) - falloff-Troe – Pressure-dependent reaction with Troe fall-off correction

Section 3: Reaction Definition (Stoichiometry Matrix)

<reaction_id> <species_name> <n_reactant> <n_product> <n_thirdbody>

For each reaction <reaction_id>, there are n_species lines (one per species):

Column	Type	Description
`<reaction_id>`	int	Reaction index (1–44 in CORIA)
`<species_name>`	string	Species name (must match `phase.txt`)
`<n_reactant>`	float	Stoichiometric coefficient as reactant (0 if product/inert)
`<n_product>`	float	Stoichiometric coefficient as product (0 if reactant/inert)
`<n_thirdbody>`	float	Third-body efficiency (0 if not involved, 1.0 if collision partner)

Example: Reaction 5 (three-body-Arrhenius):

5 H2 0.0 0.0 0.0    # H2 is neither reactant nor third body
5 H 0.0 1.0 0.0     # H is product
5 O 1.0 0.0 0.0     # O is reactant
5 O2 0.0 0.0 1.0    # O2 is third body (efficiency 1.0)
5 OH 0.0 1.0 0.0    # OH is product
5 H2O 0.0 0.0 1.0   # H2O is third body
...
5 M 0.0 0.0 1.0     # Generic third body (efficiency 1.0)

Parsing Notes: - 846 lines total in CORIA (lines 1–4 header + 44 reactions × 18 species ≈ 844 lines) - Species appear in same order as in phase.txt - Third-body coefficients: 0.0 (not involved), 1.0 (default), other floats (enhanced efficiency)

3. `chemistry-Arrhenius.dat` – Reaction Rate Tables¶

Purpose: Pre-computed forward and backward Arrhenius rates for all reactions across temperature range.

Format: Tecplot ASCII (binary-like structure, not human-readable).

TITLE = "Arrhenius data"
VARIABLES = "Temperature", "kf", "kb"
ZONE T=Reaction1
I=15000, F=POINT
<T_1>  <kf_1>  <kb_1>
<T_2>  <kf_2>  <kb_2>
...
<T_15000>  <kf_15000>  <kb_15000>

ZONE T=Reaction2
I=15000, F=POINT
<T_1>  <kf_1>  <kb_1>
...

Structure:

Component	Description
TITLE	File description (fixed: "Arrhenius data")
VARIABLES	Column headers (fixed: "Temperature", "kf", "kb")
ZONE	Zone header (one per reaction; `T=Reaction<N>`)
I=N	Number of temperature points (e.g., 15000)
F=POINT	Data format (always POINT = single values per line)
Data rows	`Temperature kf kb` (space-separated floats)

Example (first reaction, first 6 points):

ZONE T=Reaction1
I=15000, F=POINT
1.0           0.00000000000000000000E+00    0.00000000000000000000E+00
2.0           0.00000000000000000000E+00    0.00000000000000000000E+00
3.0           0.00000000000000000000E+00    0.00000000000000000000E+00
4.0           0.00000000000000000000E+00    0.00000000000000000000E+00
5.0           4.31541883433268038949E-272    4.26390547350824394621E-194
6.0           4.65732315911476184290E-226    6.07657342283880940567E-161
...

Data Format:

Temperature: 1.0 to 15000.0 K (in 1 K increments)
kf: Forward rate constant (scientific notation, units depend on reaction order)
For 2-body Arrhenius: cm³/mol/s
For 3-body: cm⁶/mol²/s
kb: Backward rate constant (same units as kf, from reverse reaction)
Precision: 20 significant decimal places (double precision)

Temperature Indexing:

The file contains one zone per reaction, with all reactions using the same temperature grid: - Zone 1 (Reaction 1): T = 1 to 15000 K - Zone 2 (Reaction 2): T = 1 to 15000 K - ... - Zone 44 (Reaction 44): T = 1 to 15000 K

To access rate for reaction N at temperature T(K):

line_index = (T - 1)  # Since T starts at 1.0
value = ZONE[N].data[line_index]

File Size: ~43 MB (CORIA: 44 reactions × 15000 points × ~65 bytes/line)

4. `chemistry-Troe.dat` – Fall-off Correction Tables¶

Purpose: Pre-computed Troe fall-off correction factors for pressure-dependent reactions.

Format: Tecplot ASCII (same as chemistry-Arrhenius.dat).

TITLE = "Fall-off data"
VARIABLES = "Temperature", "k_inf", "k_0", "k_c", "F_cent"
ZONE T=Reaction7
I=15000, F=POINT
<T_1>  <k_inf_1>  <k_0_1>  <k_c_1>  <F_cent_1>
...

Structure:

Component	Description
TITLE	File description (fixed: "Fall-off data")
VARIABLES	Column headers (5 values: T, k_inf, k_0, k_c, F_cent)
ZONE	One zone per fall-off reaction (not all reactions)
I=N	Number of temperature points (e.g., 15000)
Data rows	Pre-computed Troe fall-off parameters

Troe Parameters:

Column	Description	Formula
`Temperature`	Temperature (K)	1.0 to 15000.0
`k_inf`	High-pressure limit rate	Computed from Arrhenius A, b, Ea
`k_0`	Low-pressure limit rate	Third-order rate coefficient
`k_c`	Blending coefficient	Intermediate value for fall-off
`F_cent`	Centering factor	Troe fall-off correction factor

Troe Fall-off Theory:

The Troe correction extends the 2-body rate to handle pressure effects:

\[k(P) = k_{\infty} \left( \frac{k_0 [M]}{1 + k_0 [M]} \right) F_c^x\]

where: - \(k_{\infty}\) = high-pressure limit (provided in chemistry-Arrhenius.dat) - \(k_0\) = low-pressure limit (from Troe parameters) - \(F_c\) = centering factor = \((1-a) \exp(-T/T^*) + a \exp(-T/T^{**}) + \exp(-T^{***}/T)\) - \([M]\) = third-body concentration - \(x\) = fall-off function exponent

This file provides the pre-computed Troe parameters to avoid runtime calculation.

Parsing Notes: - Only 4 zones in CORIA (reactions 7, 16, 19, 37 are fall-off types) - Zones are labeled with original reaction index (e.g., T=Reaction7, T=Reaction16) - Zone order in file may not match reaction order (must parse zone header) - F_cent values ≈ 1.0 at low T, < 1.0 at moderate T (indicates correction strength)

File Size: ~8 MB (CORIA: 4 fall-off reactions × 15000 points × ~130 bytes/line)

5. `thermo.dat` – Thermodynamic Property Tables¶

Purpose: Pre-computed thermodynamic properties (Cp, H, S, dCp) for all species across temperature range.

Format: Tecplot ASCII (human-readable header, dense numeric data).

TITLE = "Mass Thermodynamic Properties"
VARIABLES = "Temperature", "Cp", "Enthalpy", "Entropy", "dCp"
ZONE T=H2
I=15000, F=POINT
<T_1>  <Cp_1>  <H_1>  <S_1>  <dCp_1>
<T_2>  <Cp_2>  <H_2>  <S_2>  <dCp_2>
...

ZONE T=H
I=15000, F=POINT
...

ZONE T=O
I=15000, F=POINT
...

Structure:

Component	Description
TITLE	File description (fixed: "Mass Thermodynamic Properties")
VARIABLES	Column headers (5 values: T, Cp, Enthalpy, Entropy, dCp)
ZONE	One zone per species; `T=<species_name>`
I=N	Number of temperature points (e.g., 15000)
F=POINT	Data format (always POINT)
Data rows	Temperature and properties (space-separated floats)

Properties (per unit mass, not molar):

Column	Unit (SI)	Description
`Temperature`	K	1.0 to 15000.0 K (1 K increments)
`Cp`	J/(kg·K)	Heat capacity at constant pressure
`Enthalpy`	J/kg	Specific enthalpy (H - H_ref)
`Entropy`	J/(kg·K)	Specific entropy
`dCp`	J/(kg·K)	Temperature derivative of Cp (usually ~0 for ideal gas)

Example (H2 species, first 10 points):

ZONE T=H2
I=15000, F=POINT
1.0 13654.381351 -4093931.342254 -13115.572143 0.000000
2.0 13654.381351 -4080276.960903 -3651.076207 0.000000
3.0 13654.381351 -4066622.579552 1885.299003 0.000000
4.0 13654.381351 -4052968.198202 5813.419728 0.000000
5.0 13654.381351 -4039313.816851 8860.306874 0.000000
6.0 13654.381351 -4025659.435500 11349.794939 0.000000
7.0 13654.381351 -4012005.054150 13454.627107 0.000000
8.0 13654.381351 -3998350.672799 15277.915664 0.000000
9.0 13654.381351 -3984696.291448 16886.170149 0.000000
10.0 13654.381351 -3971041.910098 18324.802809 0.000000

Observations: - Cp is constant for each species (ideal gas approximation) - H varies linearly with T (for ideal gas: \(H = H_{ref} + C_p (T - T_{ref})\)) - S varies logarithmically with T (for ideal gas: \(S = S_{ref} + C_p \ln(T/T_{ref})\)) - dCp ≈ 0 (indicates ideal gas model, no temperature dependence of Cp)

Zone Ordering:

Species zones appear in same order as in phase.txt:

H2
H
O
O2
OH
...
CH3O2 (last)

File Size: ~15 MB (CORIA: 17 species × 15000 points × ~60 bytes/line)

6. `diffusion.dat` – Binary Diffusion Coefficient Tables¶

Purpose: Pre-computed binary (pair) diffusion coefficients \(\mathcal{D}_{ij}(T)\) for every unique species pair, consumed by the mixture-averaged multicomponent diffusion model (see Species Diffusion). Optional — required only when the multicomponent diffusion closure is selected; the constant-Schmidt closure needs no diffusion table.

Format: Tecplot ASCII (same structure as transport.dat), one zone per species pair, a single property variable per zone.

TITLE = "Binary Diffusion Coefficients (Pref=101325.000000 Pa)"
VARIABLES = "Temperature", "Dij"
ZONE T="H2-H"
I=15000, F=POINT
<T_1>  <Dij_1>
<T_2>  <Dij_2>
...

ZONE T="H2-O"
I=15000, F=POINT
...

The TITLE carries the reference pressure Pref (in Pa) at which the coefficients were tabulated, as (Pref=<value> Pa). FLINT parses it and rescales each coefficient to the local pressure by \(p_\text{ref}/p\) (since \(\mathcal{D}\propto 1/p\)). If the tag is absent, FLINT defaults to 1 atm (101325 Pa).

Structure:

Component	Description
TITLE	File description (e.g. "Binary Diffusion Coefficients")
VARIABLES	Column headers ("Temperature", "Dij")
ZONE	One zone per unique unordered species pair
I=N	Number of temperature points (must match `thermo.dat`)
F=POINT	Data format (always POINT)
Data rows	`Temperature Dij` (space-separated floats)

Variable:

Column	Unit (SI)	Description
`Temperature`	K	Same grid as `thermo.dat` (1 K increments)
`Dij`	m²/s	Binary diffusion coefficient of the pair, at the reference pressure `Pref` (see TITLE)

Zone ordering (upper-triangular):

For \(N_s\) species the file holds \(N_s(N_s-1)/2\) zones, ordered with the first pair index running outermost and using the species order from phase.txt:

\[ (1,2),\,(1,3),\,\dots,\,(1,N_s),\,(2,3),\,(2,4),\,\dots,\,(N_s-1,N_s) \]

Only one of \((i,j)\) / \((j,i)\) is stored, since \(\mathcal{D}_{ij} = \mathcal{D}_{ji}\).

Parsing Notes: - The temperature range must match thermo.dat (mismatch returns error code 3). - The number of zones must equal \(N_s(N_s-1)/2\) (mismatch returns error code 4). - Coefficients are tabulated against temperature at the reference pressure Pref from the TITLE; FLINT rescales them to the local pressure by \(p_\text{ref}/p\) (exact, since \(\mathcal{D}\propto 1/p\)), so the model is valid at any pressure. - Single-species phases have no pairs and require no diffusion.dat.

Complete Example: CORIA Mechanism¶

1. phase.txt:

ideal-gas phase
H2      2.016000
H       1.008000
O      15.999000
O2     31.998000
OH     17.007000
H2O    18.015000
HO2    33.006000
CH3    15.035000
CH4    16.043000
CO     28.010000
CO2    44.009000
H2O2   34.014000
HCO    29.018000
CH2O   30.026000
CH3O   31.034000
C2H6   30.070000
CH3O2  47.033000

2. chemistry-info.txt (excerpt):

CORIA-CNRS
N.ro species = 17
N.ro reactions = 44

Reaction type
1 Arrhenius
2 Arrhenius
...
7 falloff-Troe
...
44 Arrhenius

Reaction definition
1 H2 1.0 0.0 0.0
1 H 0.0 1.0 0.0
1 O 1.0 0.0 0.0
1 O2 0.0 0.0 0.0
... (repeated for each species)

3. chemistry-Arrhenius.dat:

44 Tecplot zones with 15000 temperature points each.

4. chemistry-Troe.dat:

4 Tecplot zones for fall-off reactions (Reactions 7, 16, 19, 37).

5. thermo.dat:

17 Tecplot zones for species with properties at 15000 temperature points each.

Real Fluid Native Files¶

When using the real fluid model, the input directory contains a different set of files. Chemistry is not supported for real fluids — only thermodynamic and transport tables are needed.

INPUT/
  ├── phase.txt        # Single species name
  ├── thermo.dat       # 2D (p × h) thermodynamic property grid
  └── transport.dat    # 2D (p × h) transport property grid (optional)

1. `phase.txt` — Real Fluid Species¶

The format is similar to the ideal-gas phase file, but the header line must be real-gas phase and only one species is listed (without a molecular weight, since real fluid tables are pre-computed for a specific substance):

real-gas phase
<species_name>

Example (CO₂):

real-gas phase
CO2

2. `thermo.dat` — 2D Thermodynamic Table¶

Purpose: Tabulate thermodynamic properties as functions of pressure \(p\) and specific enthalpy \(h\) on a uniform structured grid.

Format: Tecplot structured binary or ASCII (szplt/dat), single block, 8 variables.

The mesh coordinate order is:

I-axis → pressure \(p\) (Pa)
J-axis → specific enthalpy \(h\) (J/kg)

Variables (in order):

Index	Symbol	Description	Unit
1	\(\rho\)	Density	kg/m³
2	\(T\)	Temperature	K
3	\((\partial\rho/\partial T)_p\)	Density derivative w.r.t. temperature at const. \(p\)	kg/(m³·K)
4	\(c_p = (\partial h/\partial T)_p\)	Heat capacity at constant pressure	J/(kg·K)
5	\(s\)	Specific entropy	J/(kg·K)
6	\((\partial\rho/\partial p)_h\)	Density derivative w.r.t. pressure at const. \(h\)	kg/(m³·Pa)
7	\(a\)	Speed of sound	m/s

The grid parameters extracted at load time are:

\(p_\text{min}\), \(\Delta p\) — from the I-axis mesh coordinates
\(h_\text{min}\), \(\Delta h\) — from the J-axis mesh coordinates

An optional second table (h_tab2D) indexed by \((p, T)\) is also read from the same file when present, enabling pT2h lookups.

3. `transport.dat` — 2D Transport Table¶

Purpose: Tabulate transport properties on the same \((p, h)\) grid as thermo.dat.

Format: Tecplot structured binary or ASCII, single block, 2 variables.

The mesh must have the same dimensions as thermo.dat (mismatch returns error code 4).

Variables (in order):

Index	Symbol	Description	Unit
1	\(\mu\)	Dynamic viscosity	Pa·s
2	\(k\)	Thermal conductivity	W/(m·K)

Loading Real Fluid Data¶

use FLINT_Lib_Thermodynamic
use FLINT_Load_ThermoTransport

integer :: err
err = read_realfluid_thermo("path/to/INPUT/")
err = read_realfluid_transport("path/to/INPUT/")  ! optional

Native File Format Specification¶

Overview¶

Ideal Gas Native File Structure¶

1. phase.txt – Species Definitions¶

2. chemistry-info.txt – Mechanism Metadata¶

3. chemistry-Arrhenius.dat – Reaction Rate Tables¶

4. chemistry-Troe.dat – Fall-off Correction Tables¶

5. thermo.dat – Thermodynamic Property Tables¶

6. diffusion.dat – Binary Diffusion Coefficient Tables¶

Complete Example: CORIA Mechanism¶

Real Fluid Native Files¶

1. phase.txt — Real Fluid Species¶

2. thermo.dat — 2D Thermodynamic Table¶

3. transport.dat — 2D Transport Table¶

Loading Real Fluid Data¶

1. `phase.txt` – Species Definitions¶

2. `chemistry-info.txt` – Mechanism Metadata¶

3. `chemistry-Arrhenius.dat` – Reaction Rate Tables¶

4. `chemistry-Troe.dat` – Fall-off Correction Tables¶

5. `thermo.dat` – Thermodynamic Property Tables¶

6. `diffusion.dat` – Binary Diffusion Coefficient Tables¶

1. `phase.txt` — Real Fluid Species¶

2. `thermo.dat` — 2D Thermodynamic Table¶

3. `transport.dat` — 2D Transport Table¶