Hydrogen flammability and explosion concentration limits for a wide temperature range

Abstract

This article presents a theoretical approach for quantifying the flammability and explosion limits of hydrogen-air mixtures over a wide range of temperatures (from 90 K to 850 K) at normal pressure. In a first part of paper, a critical review of the published results is made. We demonstrate that existing empirical datasets and phenomenology-based analytical and computational models critically depend upon features of experimental facility, measurement procedure and criterion for combustion limits. This dependence results into variations of values of concentration limits, obtained in different experimental setups, making it difficult to reconcile data and limiting predictive capabilities and conservatism of the empirical datasets. Further, we describe a non-empirical framework for studying the fundamental concentration limits for the basic combustion models in hydrogen-air mixtures – adiabatic spherical flame balls, plane deflagration flames, plane detonation waves. This framework does not rely on experimental combustion data, but rather uses mathematical models based on “the first physical and chemical principles”. The proposed framework introduces three innovations: 1) the first non-empiric explanation of the ternary "hydrogen-air-water steam" flammability diagram's structure for slow ascending and descending flames, 2) the first theoretical explanation of the quasi-invariance of the adiabatic flame temperature for the near-limits flames, 3) the new kinetic criterion for flame acceleration, which provides conservative predictions for both the lower and the upper concentration limits of the flame acceleration. We conclude this work by discussion of additional features for empowering of the non-empirical models and future experiments that could further improve overall knowledge of nature and quantitative accuracy of the concentration limits.