METHOD OF CALCULATING DEFINITION OF BORDERS OF UNSTABLE DETONATION OF NATURAL GAS IN GAS-STANDING INSTALLATIONS
A method has been developed for calculating the values of the compression ratio of a natural gas for the boundaries of three isolated regions: a non-detonation, unstable detonation and guaranteed (100%) detonation. The detonation of gas in the cylinder of a gas piston unit is considered as the simultaneous ignition of the entire compressible gas mixture when the autoignition temperature is reached. It is believed that the autoignition temperature of natural gas with high methane content (96-98% by volume) is completely determined by the autoignition temperature of methane. Based on the analysis of experimental data, the concept of the upper and lower limits of the autoignition temperature of hydrocarbon gases is introduced. The temperature of the gas-air mixture upon its compression was determined from the polytropic equation, taking into account the irreversibility of the real processes. The boundaries of the region of unstable detonation were graphically located as points of intersection of the curve of temperature change upon compression of the gas mixture with the curves of the upper and lower limits of the autoignition temperature of this mixture at different compression ratios. Two cases of determining the dependence of the upper and lower limits of the autoignition temperature of methane on the compression ratio are considered: using the N.N. Semenov's formula and the known experimental curve of the self-ignition temperature of hydrocarbon gases at different pressures. In the first case, a good agreement was obtained between the calculated and experimental values of the compression ratios limiting the region with unstable detonation. In the second case, when using the universal experimental curve of the self-ignition temperature of hydrocarbon gases, it was additionally assumed that the curves of the upper and lower limits of the autoignition temperature are equidistant at all pressures above atmospheric pressure. In this case, the calculations are in good agreement with the experimental values for the no-knock compression ratios and satisfactorily in the size of the region with unstable detonation.