Ignition Of Hydrogen Jets Research Articles

Experimental and Simulation Study on Ignition of Hydrogen Jet by Electric Sparks and Hot Surfaces

Experimental and Simulation Study on Ignition of Hydrogen Jet by Electric Sparks and Hot Surfaces

Effects of Lewis numbers and kinetics on spontaneous ignition of hydrogen jets

The transient process following a hydrogen leak into the atmosphere initiates as a contact surface appears separating air heated by the leading shock from hydrogen cooled by expansion. Diffusion of heat and species produces reactive mixture, potentially leading to ignition. Reactions being very temperature-sensitive, their rate peaks close to the hot air-rich side of the interface, where the small fuel concentration depends upon the fuel Lewis number. If the Lewis number is less than unity, diffusion brings in more fuel than temperature-controlled chemistry consumes. If greater, diffusion does not bring in as much fuel as chemistry would burn. Results from the current analysis for multistep kinetics also show that the role of the H Lewis number is crucial. In the short time limit, the evolution of the diffusion layer appears as a perturbation superimposed to the self-similar non-reactive diffusion solution. Initiation is very slow compared with steps consuming one reactant and an intermediate species, while some steps associated with termination are extremely fast. The approximation being made, assuming initiation much smaller than most rates, and termination much faster, is very accurate since the corresponding ratios are of the order of 105. The resulting problem still requires a numerical solution. A time-splitting algorithm combines exact solutions to three subproblems, avoiding issues such as stiffness of the kinetics. The problem is formulated not in physical space but in the similarity variable of the diffusion problem, hence avoiding difficulties associated with the initial singularity when the layer has zero thickness, and the resulting uncertainties. These are distinct advantages over numerical simulations, which however will retain a reasonable accuracy at later times. Results confirm the role of the Lewis number. They also show two distinct regimes: an early one mainly controlled by initiation, and a later one controlled by chain-branching, with a sharp transition.

Ignitability limits for combustion of unintended hydrogen releases: Experimental and theoretical results

The ignition limits of hydrogen/air mixtures in turbulent jets are necessary to establish safety distances based on ignitable hydrogen location for safety codes and standards development. Studies in turbulent natural gas jets have shown that the mean fuel concentration is insufficient to determine the flammable boundaries of the jet. Instead, integration of probability density functions of local fuel concentration within the quiescent flammability limits, termed the flammability factor, was shown to provide a better representation of ignition probability. Recent studies in turbulent hydrogen jets showed that the envelope of ignitable gas composition (based on the mean hydrogen concentration), did not correspond to the known flammability limits for quiescent hydrogen/air mixtures. The objective of this investigation is to validate the flammability factor approach to the prediction of ignition in hydrogen leak scenarios. The ignition probability within a turbulent hydrogen jet was determined using a pulsed Nd:YAG laser as the ignition source. Laser Rayleigh scattering was used to characterize the fuel concentration throughout the jet. Measurements in methane and hydrogen jets exhibit similar trends in the ignition contour, which broadens radially until an axial location is reached after which the contour moves inward to the centerline. Measurements of the mean and fluctuating hydrogen concentration are used to characterize the local composition statistics conditional on whether the laser spark results in a local ignition event or complete light-up of a stable jet flame. The flammability factor is obtained through direct integration of local probability density functions. A model was developed to predict the flammability factor using a presumed probability density function with parameters obtained from experimental data and computer simulations. Intermittency effects that are important in the shear layer are incorporated in a composite probability density function. By comparing the computed flammability factor with the measured ignition probability we have validated the flammability factor approach for application to ignition of hydrogen jets.

The shock-wave mechanism of spontaneous ignition of hydrogen under conditions of sudden efflux from reservoir at high pressure

Analysis is made of the conditions of spontaneous ignition of hydrogen as a result of emergence of a starting shock wave in air before an expanding cold flow of gas. The rise of temperature behind the shock wave causes ignition of the mixture of combustible gas with air, which forms on the contact surface. The condition for spontaneous ignition is the sufficient time of residence of mixture at high temperature for mixing and ignition. The calculations of spontaneous ignition of hydrogen jet are based on a model which takes into account the gasdynamic transport of viscous gas, the kinetics of oxidation of hydrogen, multicomponent diffusion, and thermal conductivity. The range of pressures is determined in a reservoir, during whose depressurization the shock wave forming in air exhibits intensity sufficient for igniting the hydrogen-air mixture behind the front of propagating jet of compressed hydrogen. Results of analysis are given of the dependence of conditions of ignition on the pressure of hydrogen in the reservoir, on the size of the outlet opening, and on the initial temperature of hydrogen and air.

Numerical Simulation of Ignition of Hydrogen Jet in Hot Air

Numerical Simulation of Ignition of Hydrogen Jet in Hot Air