Thermal effects accompanying spontaneous ignitions in gases I. An investigation of the heating effects which accompany the rapid admission of inert gas to an evacuated vessel

Abstract

In experiments on spontaneous ignition of gases, transient temperature changes normally accompany the entry of gases to evacuated vessels. They may invalidate much quantitative experimental work and give rise to spurious observations. In the present investigation, temperature–time histories accompanying gas entry have been mapped with a fine (13 μ m) thermocouple for many positions in a spherical vessel. It is found that although transient (characteristically, 0.1 s), the temperature rises can be large—perhaps 200 °C—and are appreciable even for gases at low pressures. They arise from adiabatic heating of the entering gas. Experiments with 18 different gases show that heating is greatest for monatomic molecules (large γ ; low C v ) and least for complex molecules (small γ ; large C v ). The effects are mitigated by conductive heat losses; such losses are greatest at low pressures, in small vessels, and for gases of high thermal diffusivity. The effects of convection are apparent at higher pressures. The implications of the results are various. Not only do they concern many laboratory measurements on spontaneous ignition, but they are also important for the large-scale handling of potentially reactive gases or gas mixtures, especially involving simple small molecules (as in mixtures with air, halogens or oxygen).