Experimental study on the vapor-liquid two-phase explosion of n-heptane under high-temperature source ignition

Abstract

The combustion and explosion characteristics of vapor-liquid two-phase (aerosol) under high-temperature sources ignition are an essential subject in fuel utilization and prevention of chain explosion accidents. In this study, a series of experiments were carried out in a 20 L vessel to observe instantaneous concentration and droplet size distribution of n-heptane aerosol and analyze the energy characteristics of n-heptane aerosol explosion under high-temperature source ignition. Tests were conducted for n-hexane/air mixtures of mass concentrations from 50.19 g/m3 to 317.87 g/m3, the ignition temperature of 1373.15 K, and the initial temperature and pressure of 22 ℃ and 0.10 MPa, respectively. The influence of the instantaneous concentration and droplet size of n-heptane aerosol on the explosive characteristics under high-temperature source ignition was examined. The results show that the explosion pressure-time history of the n-heptane aerosol at the high-temperature ignition was found to have a “double peak” structure produced in the first and second explosion respectively. The second peak of the pressure in an explosion time history of the n-heptane aerosol ignited at the high-temperature source is 2–5 times the first. The liquid n-heptane was driven by pressure to form aerosol in the vessel. The aerosol flows from the nozzle on the vessel wall to the center of the vessel (high-temperature source). The first explosion formed when the mist contacts a high-temperature source will generate outward radiation pressure waves, which are opposite to the initial converging flow direction of the aerosol, forming a reverse flow, and then causing a transient negative pressure. When the high-temperature product of the first explosion swells and meets the droplets of unburned n-heptane, a second explosion occurs, forming the second explosion peak. Compared with premixed aerosol explosion under electric spark ignition, the explosion process under high-temperature source ignition is more complicated. The explosion is coupled with the flow, and the explosion energy output is significantly increased. The latter is about 1.6 times the former. At the moment when the first explosion was initiated by the high-temperature source, the concentration of 301.14 g/m3 at which the first explosion pressure reached maximum was greater than the stoichiometric concentration of 83.65 g/m3 due to energy consumption caused by heat absorption of the droplets.