Impact of impurities on liquid methane properties under typical rocket operation conditions

•The influence of the LNG impurities N2/C2H6/C3H8 on the thermodynamic properties is calculated and analysed.•The two-phase diagrams, critical temperature Tcr and pressure Pcr, pseudo condensation/evaporation regions for mixtures of CH4 + N2/C2H6/C3H8 are calculated.•The pressure-temperature intervals of non-monotone behavior of non-monotone behavior of specific volume, v, thermal expansion, αp, isobaric and isochoric heat capacities, Cp, Cv, compressibility, βT, speed of sound, w for CH4 + N2/C2H6/C3H8 around the critical point are identified.•The pressure-temperature intervals of change of thermo-physical properties of mixtures CH4 + N2/C2H6/C3H8 in the vicinity of critical points, that should be carefully accounted for CFD simulations of the combustion chamber and cooling channel flow for liquid propellants, are defined. The paper reports the results of the investigation of the influence of small quantities of impurities (nitrogen, ethane, propane, carbon dioxide, and a small number of higher hydrocarbons) on the thermophysical properties of liquified natural gas (LNG). For the first time the two-phase diagrams, critical temperature, pressure, and non-monotone behavior of thermodynamic properties around critical point were calculated, systematically described, and analyzed for selected LNG mixtures (CH4/N2/C2H6/C3H8) with impurities content not higher than 5%. It was shown that qualitative and quantitative composition of impurities impact the thermodynamic properties differently and can change Tcr and Pcr significantly. Areas of retrograde condensation have been identified for mixtures with a C3H8 concentration greater than 2%. The Widom Area (WA) was introduced to characterize a supercritical region with not negligible non-monotonous variations of thermophysical properties. The calculated WAs for mixtures can be larger or smaller for different thermophysical properties relative to purer methane. The obtained results can be useful for rocket engine design in simulations of trans-critical and supercritical reacting flows, mixing, evaporation, combustion, flame stability, thermo-acoustic instabilities, and heat transfer at typical operating conditions. It is also essential for the design of the cooling system of a rocket engine.