This table shows the fluid types that are modeled in GUNNS, and their pressure & temperature limits.

Fluid Type	Compound	Phase	min T (K)	max T (K)	min P (kPa)	max P (kPa)	Notes
GUNNS_CO	Carbon Monoxide	Gas	68.16	2000	1e-64	1e6	1
GUNNS_CO2	Carbon Dioxide	Gas	100	2000	1e-64	1e6	1
GUNNS_H2O	Water	Gas	100	2000	1e-64	1e6	1
GUNNS_N2	Diatomic Nitrogen	Gas	64	2000	1e-64	1e6	1
GUNNS_O2	Diatomic Oxygen	Gas	54.5	2000	1e-64	1e6	1
GUNNS_NH3	Ammonia	Gas	100	2000	1e-64	1e6	1
GUNNS_H2	Diatomic Hydrogen	Gas	13.957	2000	1e-64	1e6	1
GUNNS_CH4	Methane	Gas	91	2000	1e-64	1e6	1
GUNNS_HCL	Hydrogen Chloride	Gas	100	2000	1e-64	1e6	1
GUNNS_HCN	Hydrogen Cyanide	Gas	100	2000	1e-64	1e6	1
GUNNS_HE	Monatomic Helium	Gas	2.1768	1500	1e-64	1e6	1
GUNNS_HE_REAL_GAS	Monatomic Helium	Gas	2.1768	1000	0	60000	2
GUNNS_XE_REAL_GAS	Monatomic Xenon	Gas	170	750	0	34473	2
GUNNS_N2_REAL_GAS	Diatomic Nitrogen	Gas	160	750	0	59090.9	2, 5
GUNNS_O2_REAL_GAS	Diatomic Oxygen	Gas	160	750	0	59090.9	2, 5
GUNNS_H2_REAL_GAS	Diatomic Hydrogen	Gas	64	1000	0	80000	2
GUNNS_WATER	Water	Liquid	200	470	1e-64	1e6
GUNNS_HFE7000	HFE-7000	Liquid	150.65	470	1e-64	1e6	3
GUNNS_HFE7100	HFE-7100	Liquid	138.15	468.45	1e-64	1e6	6
GUNNS_PG30	Propylene Glycol 30%	Liquid	260.45	640.5122	1e-64	1e6	4
GUNNS_PG40	Propylene Glycol 40%	Liquid	251.56	638.3176	1e-64	1e6	4
GUNNS_PG50	Propylene Glycol 50%	Liquid	239.65	636.123	1e-64	1e6	4
GUNNS_AMMONIA	Ammonia	Liquid	200	320	1e-64	1e6
GUNNS_OXYGEN	Diatomic Oxygen	Liquid	54.5	160	1e-64	1e6
GUNNS_HYDROGEN	Diatomic Hydrogen	Liquid	13.957	33.145	1e-64	1e5
GUNNS_METHANE	Methane	Liquid	91	190	1e-64	1e6
GUNNS_NAK78	Eutectic NaK 78%	Liquid	260.55	1058.15	1e-64	1e6	7
GUNNS_GALDEN170	Galden HT-170	Liquid	176.15	443.15	1e-64	1e6	8
GUNNS_WATER_PVT	Water	Liquid	273.16	373.5065	1e-10	1075.427	9
GUNNS_NTO	Nitrogen Tetroxide	Liquid	261.95	431.35	1e-64	1e6	10
GUNNS_MMH	Monomethyl Hydrazine	Liquid	220.0	585.0	1e-64	1e6	11
GUNNS_HYDRAZINE	Hydrazine	Liquid	274.68	387.4	1e-64	1e6	11

Notes:

1. Ideal gas.
2. Real-gas compressibility via table-lookup, all other properties identical to the ideal gas type.
3. 3M™ Novec™ Engineered Fluid HFE-7000
4. Prior to release 14.2, PG50 was called GUNNS_PROPGLYCOL. The number in the name represents the percentage of propylene glycol (by mass) in aqueous solution.
5. Max P was 50000 kPa prior to release 14.2.
6. 3M™ Novec™ Engineered Fluid HFE-7100
7. Liquid metal potassium/sodium in the eutectic 78/22% mixture by mass. Most source data comes from: The Thermophysical and Transport Properties of Eutectic NaK Near Room Temperature, O’Donnell, et.al., Feb. 1989 . This fluid is primarily used as a coolant in nuclear fission reactors. The Specific Heat and Thermal Conductivity properties are optimized in the temperature range of 790-960°K. Other properties are optimized near room temperature and values above 80°C held constant. Note: the phase-change properties: Critical Temperature, Saturation Pressure & Temperature, and Heat of Vaporization should not be used. We haven’t found data for these for NaK-78 and we used the properties for water as a placeholder instead.
8. Galden PFPE Perfluoropolyether Fluorinated Fluids . Note: The phase-change properties: saturation curve & Heat of Vaporization are only accurate at 25°C, as data has not been found at other points. We assumed a Critical Temperature equal to water’s and the saturation & heat of vaporization curves will be less accurate away from 25°C.
9. Liquid water with realistic compressibility and thermal expansion via table lookup, all other properties are identical to the GUNNS_WATER type.
10. “USAF Propellant Handbooks, Nitric Acid/Nitrogen Tetroxide Oxidizers”, Vol. II, Martin Marietta Corp., Feb. 1977. This is also good for the MON-1 and MON-3 mixtures of NTO and nitric acid. MON-3 is common, used in the Space Shuttle and modern vehicles.
11. “USAF Propellant Handbooks, Hydrazine Fuels”, Vol. I, Bell Aerospace Corp., Mar. 1970.

General Notes:

• The T limits range usually encompasses the fluid’s entire saturation curve from triple point to critical point. The max T value is the upper limit applied to the fluid’s density property, and this may be more or less than the critical temperature.
• Some fluid’s minimum T extend below the triple point to support modeling of sublimation/frosting.
• Although the ideal gasses have a wide T range, their properties are optimized for best accuracy around room temperature, and may diverge significantly from truth near the limits.
• For some liquid types, compressibility data is unavailable. We give these liquids the compressibility of water.

Most of our properties are approximations to the NIST Chemistry WebBook. We consider NIST as our “truth” authority on the real fluid properties.

Molecular Weight is a constant for each compound. Multiple fluid types can represent the same compound (diatomic oxygen, for example) so they share the same molecular weight value. We store it in Trick units of (1/mol). We often abbreviate this as MW or MWeight. More on this subject here.

Like molecular weight, the liquid-vapor critical temperature Tc is a constant for each compound. All temperatures in GUNNS are stored in units of Kelvins (K).

In GUNNS, the Pressure-Volume-Temperature (PVT or compressibility) equation of state of all fluids is always exactly reversible between P = f(T, V) and V = f(T, P) at a given temperature (isothermal), where density is the inverse of volume. GUNNS itself is flexible to the type of equation of state used, as long as this isothermal reversibility is always satisfied. Thus the equation of state is actually implemented in the fluid properties code and not in GUNNS “core” code. The compressibility between Pressure (kPa) and Density (kg/m3) are implemented differently depending on the equation of state for the type of fluid. Currently we use these types of compressibility equations of state:

• Ideal Gas, in which the compressibility is linear with temperature, as:

PV/n = RT

• Real Gas, in which the compressibility is implemented in table lookups. The tables are limited to ranges where density & pressure are reversible, i.e. for every (P, T) there is a unique solution for density and vice-versa.
• Liquid, where we model density ρ as the following function of P & T, where a, b, c, d, e and f are constants:

ρ = (a + b·P) + (c + d·P)·T + (e + f·P)·T ²

We curve fit dynamic viscosity in units of (Pa*s) as a function of temperature.

We curve fit specific heat at constant pressure Cp as a function of temperature, with units of (J/kg/K). This assumes a thermally perfect gas — it is only a function of temperature and not of pressure.

Like with density and pressure, GUNNS demands an exactly reversible conversion between temperature and specific enthalpy h. Specific enthalpy is related to temperature and specific heat as:

h = Cp · T

Given either h or T, we can find the other term via Cp. We fit Cp as a linear function of T so that:

Cp = a + b · T

h = a·T + b·T ²

Then we can easily solve h given T, and T given h (via the quadratic equation) for reversibility.

We curve fit fluid thermal conductivity as a function of temperature, in units of (W/m/K).

We curve fit Prandtl Number (unitless) as a function of temperature.

We curve fit adiabatic index, also called gamma γ or the heat capacity ratio, as a function of temperature. It is unitless. Since

γ = Cp/Cv,

given Cp and this curve fit for γ we can find the specific heat at constant volume Cv.

Like density & pressure, GUNNS requires saturation pressure, also called vapor pressure Ps and saturation temperature Ts to be exactly reversible, so Ps = f(Ts = f(Ps)). This defines the saturation curve for the fluid in pressure and temperature, which is useful for modeling vapor-liquid phase changes (condensation, boiling etc). Normally saturation pressure is modeled with the Antoine equation for limited ranges of temperature. However we need a single fit for the entire range of temperatures, so we fit saturation pressure as:

log10(Ps) = a + b·(1/Tr) + c·(1/Tr) ²

where Tr is the reduced temperature, T/Tc, and a, b, c are constants. This quadratic can then be solved for Tr given Ps, and then Ts found from Tr. These curve fits are accurate within 5% for the entire range between the triple & critical points. They also extend below the triple point, but accuracy begins to worsen. NIST does not give saturation curve data below the triple point, but we still attempt to model it for solid-vapor phase changes (sublimation & frosting).

Saturation pressure is given in units of (kPa). The saturation curve has no meaning above the critical point, so Ps or Ts is upper-bounded at the critical point before solving for the other parameter.

Usually denoted as dHvap or L, we approximate the latent heat (enthalpy) of vaporization, with units (kJ/kg), as this function of reduced temperature Tr:

L = A · e ^(-α·Tr) · (1-Tr) ^β,

where A, α and β are constants. L is always zero for T > Tc. For temperatures below the triple point, accuracy begins to worsen but is good enough for modeling sublimation near the triple point. This equation came from the NIST webbook for Methane, and we applied it to all fluid types.