Thermodynamic properties of the nitrogen and FK-5-1-12 binary mixed fluids: (p,ρ,T) behavior, solubility of nitrogen and engineering calculation

1,1,1,2,2,4,5,5,5-Nonafluoro-4-(trifluoromethyl)-3-pentanone (commonly called FK-5-1-12) is an environmentally friendly halon alternative widely used in fire suppression systems, often in coexistence with nitrogen (N 2 ) in cylinders or mixed with N 2 in pipelines. However, systematic data on the phase equilibrium and thermodynamic properties of the N 2 -FK-5-1-12 binary mixed fluid-critical for optimizing fire suppression system design-have been lacking in existing research. To fill this gap, this study measured the pressure-density-temperature ( p , ρ , T) behavior of the N 2 -FK-5-1-12 mixed fluid over a temperature range of 273.15–323.15 K and pressures up to 9.8 MPa. Additionally, it determined the solubility of N 2 in FK-5-1-12 within the same temperature range and at pressures up to 5.8 MPa. Based on these experimental data, two key calculation models were developed: a phase equilibrium density formula with a maximum deviation of 0.74 %, and a solubility formula with a maximum deviation of 5.19 %. The solubility was found to exhibit a linear positive correlation with pressure and no dependence on temperature, simplifying engineering calculations. Furthermore, a nitrogen mass fill method and corresponding empirical formula for engineering applications were established. This method achieves accurate superpressurization of cylinder assemblies at 20 °C (with an accuracy of ≤0.87 %) and can predict cylinder pressure at any temperature (with an error of ≤1.45 %). Compared to traditional pressure-based filling methods, the proposed approach improves efficiency by 30 % and eliminates safety hazards from overfilling. These findings provide critical data and theoretical guidance for the design and operation of FK-5-1-12-based fire suppression systems, addressing the long-standing lack of thermodynamic data for this mixed fluid.