The hazard of Deflagration to Detonation transition for hydrogen-mathane mixture in un-obstructed pipelines

The introduction of hydrogen into the gas distribution line can represent a great step toward carbon neutrality. Hydrogen's advantages have to be balanced against concerns about the safety of blended gas during transport, which under specific conditions can lead to the phenomenon of detonation. This step responsible to lead the mixture from deflagration to detonation is the main topic of the current study and it is better known as the deflagration-to-detonation transition (DDT). Over the years, many experiments have been performed to study the safety characteristics of hydrogen/methane mixtures along a pipeline and the properties of DDTs, but none of them has ever been conducted in an unobstructed circular steel tube with a diameter of 5 cm. For this reason, this configuration was chosen for the present work, aimed at detecting the exact probability of a DDT transition as a function of initial conditions such as absolute pressure and hydrogen and methane concentrations. The initial pressure was varied from 1.1 to 2 bar, while the fuel fraction was varied from 100% H2 to 100% CH4, always maintaining the equivalence ratio λ to 1. To this end, six photodiodes and six piezoelectric sensors were installed along the tube, and a high-voltage induction spark was used as the ignition source. It has been observed that the transition from deflagration to detonation occurs mainly around values of 80% hydrogen and that an increase in pressure leads to a lower hydrogen/methane ratio capable of detonating in unobstructed pipes. These and many other results are detailed in this work. The investigations in this work can be a starting point for various experiments to obtain experimental data on pipe diameters, initial pressures, runup distances, and hydrogen/methane fractions for safety measures and calculations of possible DDT. In this paper, an initial comparison is presented between experiments carried out in a closed tube and an open one, under the same initial conditions.