Numerical simulation of flame acceleration and deflagration-to-detonation transition in hydrogen-air mixtures with concentration gradients

The present study aims to test the capability of our newly developed density-based solver, ExplosionFoam, for flame acceleration (FA) and deflagration-to-detonation transition (DDT) in mixtures with concentration gradients which is of important safety concern. The solver is based on the open source computational fluid dynamics (CFD) platform OpenFOAM® and uses the hydrogen-air single-step chemistry and the corresponding transport coefficients developed by the authors. Numerical simulations have been conducted for the experimental set up of Ettner et al. [7], which involves flame acceleration and DDT in both homogeneous hydrogen-air mixture as well as an inhomogeneous mixture with concentration gradients in an obstucted channel. The predictions demonstrate good quantitative agreement with the experimental measurements in flame tip position, speed and pressure profiles. Qualitatively, the numerical simulations have reproduced well the flame acceleration and DDT phenomena observed in the experiment. The results have revealed that in the computed cases, DDT is induced by the interaction of the precursor inert shock wave with the wall close to high hydrogen concentration rather than with the obstacle. Some vortex pairs appear ahead of the flame due to the interaction between the obstacles and the gas flow caused by combustion-induced expansion, but they soon disappear after the flame passes through them. Hydrogen cannot be completely consumed especially in the fuel rich region. This is of additional safety concern as the unburned hydrogen can be potentially re-ignited once more fresh air is available in an accidental scenario, resulting in subsequent explosions

Large eddy simulation of upward flame spread on PMMA walls with a fully coupled fluid–solid approach

A fully coupled fluid–solid approach has been developed within FireFOAM 2.2.x, a large eddy simulation (LES) based fire simulation solver within the OpenFOAM® toolbox. Due consideration has been given to couple the radiative heat transfer and soot treatment with pyrolysis calculations. Combustion is modeled using the newly extended eddy dissipation concept (EDC) for the LES published by the authors’ group. Soot formation and oxidation are handled by the published extension of the laminar smoke point concept to turbulent fires using the partially stirred reactor (PaSR) concept also from the authors’ group. The gases radiation properties are evaluated using the established weighted sum of grey gas model while soot absorption coefficient is calculated using a single Planck-mean absorption coefficient. The effect of in-depth radiation is treated with the relatively simple Beer's law and the solid surface regression length is calculated from the local pyrolysis rate. Systematic validation studies have been conducted with several published experiments including simple pyrolysis test without the gaseous region, small scale wall fires and large scale flame spread. The predictions are in very good agreement with the relevant experimental data, demonstrating that the present modeling approach can be used to predict upward flame spread over PMMA with reasonable accuracy. Further parametric studies have also been conducted to demonstrate the effectiveness of the present modifications to capture the underlying physics. The detailed field predictions for vortex structures and flame volume including laminar–turbulent transition have also been analysed to uncover further insight of the unsteady flame spread phenomena. Potentially, the model can be used to aid further fundamental studies of the flame spread phenomena such as investigating the effects of width, inclination angles and side walls on flame spread as well as the predictions of flame spread in practical applications

An improved PaSR-based soot model for turbulent fires

The extension of the laminar smoke point based approach to turbulent combustion using the partially stirred reactor (PaSR) concept proposed by Chen et al. ; has been further improved to overcome the limitation in the formulations of Chen et al. ; which assumed infinitely fast soot oxidation chemistry and constant soot formation characteristic time. In the PaSR approach, each computational cell is split into two zones: the reacting zone and the non-reacting zone. Soot formation and oxidation are assumed to take place at finite rates in the reacting zone and computed from the corresponding laminar rates and the mass fractions for soot formation and oxidation, which are evaluated in each computational cell from the characteristic time scales for turbulent mixing, soot formation and oxidation. Since soot would be produced in not only the fine structures but also surrounding fluids in the Eddy-Dissipation-Concept (EDC) model, the average field parameters between the fine structure and surrounding fluid are employed instead of those Favre-averaged values in Chen et al.’s soot formation model. The newly extended model has been implemented in FireFOAM, a large eddy simulation (LES) based solver for fire simulation based on the open source CFD code OpenFOAM®. Numerical simulations of a 30 cm diameter heptane and toluene pool fires tested by Klassen and Gore [29] were performed for validation. The predicted soot volume fraction and temperature have achieved improved agreement with the experimental measurements in comparison with that of Chen et al. ; , demonstrating the potential of the improved PaSR-based soot model for fire applications

Investigation of thermal breakage and heat transfer in single, insulated and laminated glazing under fire conditions

To make constructions more artistic, various new kinds of glazing are increasingly employed in building envelopes. However, when subjected to a fire, these glass façades may easily break and fall out, significantly accelerating the development of enclosure fire. Thus, it is necessary to investigate and compare their different fire performance and breakage mechanisms. In this work, a total of ten tests, including single coated, insulated and laminated glazing, were heated by a 500 × 500 mm2 pool fire. Breakage time, glass surface and air temperature, incident heat flux and crack initiation and propagation were obtained. The critical conditions of three different kinds of glazing were determined. It was established that the insulated and laminated glass can survive longer than the single glass. The air gap and fire side glass pane was found to play a key role for the thermal resistance of ambient side pane in the insulated glazing. Although both panes of the laminated glazing broke, it could be held together by the layer of gel, effectively avoiding the formation of a new vent. Numerical simulations were performed to investigate the heat transfer process through the glazing panels and the temperatures in the glazing were predicted well. Suggestions for glass fire resistance design are proposed

Numerical modelling of deflagration to detonation transition in inhomogeneous hydrogen/air mixtures

Explosions in homogeneous reactive mixtures have been widely studied both experimentally and numerically. However, in practice, combustible mixtures are usually inhomogeneous and subject to both vertical and horizontal concentration gradients. There is still very limited understanding of the explosion characteristics in such situations. The present study aims to investigate deflagration to detonation transition (DDT) in such mixtures. Two cases in a horizontal obstructed channel with 30% and 60% blockage ratios filled with hydrogen/air mixture with vertical concentration gradients are numerically studied. These cases were experimentally investigated by Boeck et al. (2015), and hence some measurements are available for model validation. A density-based solver within the OpenFOAM CFD toolbox is developed and used. To evaluate the convective fluxes contribution, the Harten–Lax–van Leer–Contact (HLLC) scheme is used for shock capturing. The compressible Navier–Stokes equations with a single step Arrhenius reaction are solved. The numerical results are in good qualitative and quantitative agreement with the experiments. The predictions show that the overpressure at the DDT transition stage is higher in the non-uniform mixtures than that in homogeneous mixtures under similar conditions. It is also found that increasing the blockage ratio from 30% to 60% resulted in faster flame propagation and lower propensity to DDT. The Baroclinic torque and the resulting Richtmyer–Meshkov (RM) instability are also analyzed in relation to flame acceleration and DDT

Bifurcation of pulsation instability in one-dimensional H2−O2 detonation with detailed reaction mechanism

Classical modes of one-dimensional (1D) detonation characterized by a simplified reaction model are reproduced by using a real chemical kinetics for the H2−O2 system with argon dilution. As Ar dilution is varied, the bifurcation points of pulsating instability are identified and a formed bifurcation diagram is compared with that obtained by the one-step reaction model. Eventually, the numerical results demonstrate that, for real detonations with detailed chemistry, the criterion of Ng et al. works well on prediction of the 1D detonation instability. Furthermore, the detonability limits are found respectively at low and high Ar dilutions. Above the high Ar dilution limit, detonations decays to the minimum level where long autoignition time and small heat release rate make reestablishment impossible for both 1D and 2D simulations. However, below the low Ar dilution limit, a 1D detonation cannot be sustained due to high instability, while the corresponding cellular detonation can propagate sustainably due to the role of transverse instability

Experimental and modeling analysis of thermal runaway propagation over the large format energy storage battery module with Li4Ti5O12 anode

Insight of the thermal characteristics and potential flame spread over lithium-ion battery (LIB) modules is important for designing battery thermal management system and fire protection measures. Such thermal characteristics and potential flame spread are also dependent on the different anode and cathode materials as well as the electrolyte. In the present study, thermal behavior and flame propagation over seven 50 A h Li(Ni1/3Mn1/3Co1/3)O2/Li4Ti5O12 large format LIBs arranged in rhombus and parallel layouts were investigated by directly heating one of the battery units. Such batteries have already been used commercially for energy storage while relatively little is known about its safety features in connection with potential runaway caused fire and explosion hazards. It was found in the present heating tests that fire-impingement resulted in elevated temperatures in the immediate vicinity of the LIBs that were in the range of between 200 °C and 900 °C. Such temperature aggravated thermal runaway (TR) propagation, resulting in rapid temperature rise within the battery module and even explosions after 20 min of “smoldering period”. The thermal runaway and subsequent fire and explosion observed in the heating test was attributed to the violent reduction of the cathode material which coexisted with the electrolyte when the temperature exceeded 260 °C. Separate laboratory tests, which measured the heat and gases generation from samples of the anode and cathode materials using C80 calorimeter, provided insight of the physical-chemistry processes inside the battery when the temperature reaches between 30 °C and 300 °C. The self-accelerating decomposition temperature of the cell, regarded as the critical temperature to trigger TR propagation, was calculated as 126.1 and 139.2 °C using the classical Semenov and Frank-Kamenetskii models and the measurements of the calorimeter with the samples. These are consistent with the measured values in the heating tests in which TR propagated. The events leading to the explosions in the test for the rhombus layout was further analyzed and two possible explanations were postulated and analyzed based on either internal catalytic reactions or Boiling Liquid Expansion Vapor Explosion (BLEVE)

TRPM8 mechanism of autonomic nerve response to cold in respiratory airway

Breathing cold air without proper temperature exchange can induce strong respiratory autonomic responses including cough, airway constriction and mucosal secretion, and can exacerbate existing asthma conditions and even directly trigger an asthma attack. Vagal afferent fiber is thought to be involved in the cold-induced respiratory responses through autonomic nerve reflex. However, molecular mechanisms by which vagal afferent fibers are excited by cold remain unknown. Using retrograde labeling, immunostaining, calcium imaging, and electrophysiological recordings, here we show that a subpopulation of airway vagal afferent nerves express TRPM8 receptors and that activation of TRPM8 receptors by cold excites these airway autonomic nerves. Thus activation of TRPM8 receptors may provoke autonomic nerve reflex to increase airway resistance. This putative autonomic response may be associated with cold-induced exacerbation of asthma and other pulmonary disorders, making TRPM8 receptors a possible target for prevention of cold-associated respiratory disorders