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

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 T_cr and P_cr 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.