Assessment of the Ignition Temperature of Hydrogen-Oxygen Mixtures under High Temperature Microheaters by Finite Element Analysis

This study evaluates the ignition risk of hydrogen-oxygen mixtures exposed to high-temperature microheaters used for hydrogen sensors by means of thermo-fluid simulations incorporating chemical kinetics. To reproduce the known ignition temperature of 833 K for hydrogen-oxygen mixtures, the activation energy in the Arrhenius rate expression was calibrated. Simulations were carried out for (i) a microheater with a 0.2 mm × 0.2 mm heated zone and (ii) a rotationally symmetric ellipsoidal coil heater 1 mm long with a mid-radius of 0.25 mm. Ignition occurred at 1173 K for the microheater, which is higher than the 923 K predicted for the coil heater. This difference is attributed to the microheater's very narrow hot zone: radicals generated there readily diffuse into the surrounding cooler gas and are quenched, thereby suppressing the chain reactions required for ignition.