Calculated Evaluation of the Thermal Physical Properties of Nitrogen as a Working Fluid of Cryogenic Piston Engines. Heat Conductivity Calculation

Abstract

Vehicles with internal combustion engines (ICEs) used by many enterprises or high fire hazard facilities (airports, docks, elevators, chemical plants, refineries) can be sources of ignition due to the peculiarity of their technological (work) cycle. Recently, vehicle designers have devoted much attention to power units that are alternatives of ICEs (electric engines, hybrid power units, and cryogenic engines), with the power units being capable of ensuring a higher fire safety in transport. Kreogenic engines as power plants in special vehicles used in the above-mentioned high fire hazard facilities are particularly important. In such engines, the working medium (WM) is liquid nitrogen, and the heat source to implement the work cycle is the warmth of the environment. Nitrogen is the most affordable non-flammable gas, which is why in terms of economic considerations, it is most acceptable for use as WM for a cryogenic piston engine without disturbing nitrogen content balance in the atmosphere. The increased interest in creating cryogenic power units for vehicles determined the relevance of a detailed study of thermodynamic and kinetic characteristics (transfer coefficients) of molecular nitrogen over a wide range of pressures and temperatures. The article presents an original method and the results of calculating the thermal conductivity of nitrogen used as a working fluid for transport piston units. A description of the developed mathematical model of kinetic characteristics in dense molecular media (dense gases and liquids) is presented. The mathematical model and computational procedures are based both on the formal scheme of Enskog and statistical-mechanical approach within the framework of the thermodynamic perturbation theory with no empirical parameters involved. The features of the method are sufficient minimum of initial information, high precision, and applicability for any practically important state ranges. Using gaseous and liquid nitrogen as an example, the calculated values of their thermal conductivity are compared with the available in the literature experimental data at pressures up to 5 MPa in the temperature range from 80 to 300 K. The results of calculations according to the proposed method allow us to predict the kinetic characteristics of nitrogen in experimentally unexplored state ranges up to pressures of 1000 MPa and temperatures up to 5000 K. Errors in the calculation of the thermal conductivity of nitrogen are at the level of ordinary experimental errors.