Study on Ignition Probability of Flammable Materials after Leakage Accidents

AbstractIt is a key step of quantitative risk analysis (QRA) to estimate ignition probability of flammable materials after leakage accidents. This paper reviews the available literature and expert opinion on how to evaluate and determine ignition probability value, and it was detailedly discussed on the main influencing factors of ignition probability, including flammable material properties, mass flow rate of flammable materials spillage, ignition resources and ignition controls. Moreover, the operational and practical ignition probability value could be estimated from the all way of classifications of flammable material, mass flow rate, ignition resources, hazardous areas and ignition prevention and control measure. Furthermore, the more practical ignition probability model was put forward that the ignition probability was the maximum value of the probability decided by material properties (PMP), mass flow rate(PQ) and ignition resources(PIS) with the factor of preventing and controlling ignition (KIC). Finally, the further research was proposed to assign some feasible weigh factors of the ignition probability for flammable materials after leakage accidents

Transition region ignition characteristics of n-heptane fuel sprays

Ignition studies were perferred on monodisperse n-heptane sprays at atmospheric pressure over a range of equivalence ratios and droplet diameters. A capacitive discharge spark ignition system was used as the ignition source, providing independent control of spark energy and duration. Preliminary measurements were made to optimize spark duration and spark gap, optimum conditions being those at which the maximum frequency or probability of ignition was observed. The effect of spark duration on ignition frequency for several spark energies was determined for equivalence ratios of 0.5 and 1.0 and initial droplet diameters of 28 and 68 microns. Spark duration had little effect on ignition frequency over the entire 15 to 170 mu s range examined. Spark durations of 70 to 80 mu s were used for all subsequent work. The spark gap was optimized at equivalence ratios of 0.6, 0.8 and 1.0 and initial droplet diameters of 30, 40, 50, 60 and 70 microns by varying the electrode spacing from 0.5 to 5.0 mm while maintaining a constant spark energy. The optimum gap was determined to be 3.0 mm for nearly all conditions

Technical Reference: Using Lightning Location in the Wildland Fire Assessment System

Location of the ground position of lightning discharges has been possible for several years. A technique for using ignition probability calculations together with lightning location data results in maps that are useful to fire managers in making decisions on timescales from historical to real-time. These maps, part of the Wildland Fire Assessment System, will aid fire managers in assessing the potential that lightning will result in reportable fires. The maps are generated using ignition probability based on duff depth, fuel moisture, and fuel type. To match fire potential classes, ignition probabilities are classified into ignition potential classes of low. medium, high, very high, and extreme. Possible benefits are savings in time, fire suppression costs, and perhaps even lives

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work in non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability P_ign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high P_ign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model’s flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the S_T -versus-u' curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.M.P. Sitte gratefully acknowledges financial support from the Gates Cambridge Trust. The experiments were carried out by E. Bach who was a Masters student from Karlsruhe Institute of Technology visiting the University of Cambridge in 2011.This is the final version of the article. It first appeared from Taylor & Francis via http://dx.doi.org/10.1080/13647830.2016.115575

Spark Ignition Characteristics of a L02/LCH4 Engine at Altitude Conditions

The use of non-toxic propellants in future exploration vehicles would enable safer, more cost effective mission scenarios. One promising "green" alternative to existing hypergols is liquid methane/liquid oxygen. To demonstrate performance and prove feasibility of this propellant combination, a 100lbf LO2/LCH4 engine was developed and tested under the NASA Propulsion and Cryogenic Advanced Development (PCAD) project. Since high ignition energy is a perceived drawback of this propellant combination, a test program was performed to explore ignition performance and reliability versus delivered spark energy. The sensitivity of ignition to spark timing and repetition rate was also examined. Three different exciter units were used with the engine s augmented (torch) igniter. Propellant temperature was also varied within the liquid range. Captured waveforms indicated spark behavior in hot fire conditions was inconsistent compared to the well-behaved dry sparks (in quiescent, room air). The escalating pressure and flow environment increases spark impedance and may at some point compromise an exciter s ability to deliver a spark. Reduced spark energies of these sparks result in more erratic ignitions and adversely affect ignition probability. The timing of the sparks relative to the pressure/flow conditions also impacted the probability of ignition. Sparks occurring early in the flow could trigger ignition with energies as low as 1-6mJ, though multiple, similarly timed sparks of 55-75mJ were required for reliable ignition. An optimum time interval for spark application and ignition coincided with propellant introduction to the igniter and engine. Shifts of ignition timing were manifested by changes in the characteristics of the resulting ignition

Local Ignition in Carbon/Oxygen White Dwarfs -- I: One-zone Ignition and Spherical Shock Ignition of Detonations

The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hotspots by examining the burning of one zone of material, and then investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formula for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf metallicity, we then find that ignition times can decrease by ~20% with addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar increase in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, eg a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we find that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required matchheads of ~4500 detonation thicknesses - tens of centimeters to hundreds of meters - which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.Comment: 29 pages; accepted to ApJ. Comments welcome at http://www.cita.utoronto.ca/~ljdursi/thisweek/ . Updated version addressing referee comment