Deflagration-to-detonation Transitions Research Articles

Impact of Radiative Losses on Flame Acceleration and Deflagration to Detonation Transition of Lean Hydrogen-Air Mixtures in a Macro-Channel with Obstacles

While there has been some recognition regarding the impact of thermal boundary conditions (adiabatic versus isothermal) on premixed flame propagation mechanisms in micro-channels (hydraulic diameters <10 mm), their impact in macro-channels has often been overlooked due to small surface-area-to-volume ratios of the propagating combustion wave. Further, the impact of radiative losses has also been neglected due to its anticipated insignificance based on scaling analysis and the high computational cost associated with resolving it’s spatial, temporal, directional, and wavelength dependencies. However, when channel conditions promote flame acceleration and deflagration-to-detonation transitions (DDT), large pressures are encountered in the vicinity of the combustion wave, thereby increasing the magnitude of radiative losses which in turn can impact the strength and velocity of the combustion wave. This is demonstrated for the first time through simulations of lean (equivalence ratio: 0.5) hydrogen-air mixtures in a macro-channel (hydraulic diameter: 174 mm) with obstacles (Blockage ratio: 0.51). By employing Planck mean absorption coefficients in conjunction with the P-1 radiation model, radiative losses are shown to affect the run-up distances to DDT in a long channel (length: 11.878 m). As anticipated, the differences in run-up distances resulting from radiative losses only increased with system pressure.

Deflagration to detonation transition in JP-10 mist/air mixtures in a large-scale tube

Deflagration to detonation transitions (DDTs) in JP-10 mist/air mixtures have been studied in a horizontal multiphase combustion and explosion tube with inner diameter of 199mm and length of 32.8m. The mist/air mixtures were generated by injecting liquid samples into the experimental tube. Experiments were performed at 298k and 101kPa with equivalence ratio ranged from 0.51 to 2.09. The coupling process of deflagration wave with leading shock wave and low-velocity self-sustained detonation were observed in JP-10 mist/air mixture with a concentration of 142.86g/m3, and the average velocity of the self-sustained detonation wave is 510m/s, which is as low as 26% of C-J value. The low-velocity detonation in JP-10 mist/air mixture can be explained by the low-volatile property of JP-10 liquid and boundary condition. The leanest and richest critical detonable concentrations were studied. The detonation structure was studied by using pressure sensors array mounted in the wave structure test section. A single-head spin detonation wave front was observed and the cellular structure resulting from the spinning movement of the triple point was analyzed.

Simulation of shock–turbulence interaction in non-reactive flow and in turbulent deflagration and detonation regimes using one-dimensional turbulence

The one-dimensional turbulence (ODT) methodology is extended to include an efficient compressible implementation and a model for capturing shock-induced turbulence is presented. Lignell et al. recently introduced a Lagrangian ODT implementation using an adaptive mesh. As the code operates in the incompressible regime (apart from constant-pressure dilatation) it cannot handle compressibility effects and their interactions with turbulence and chemistry. The necessary algorithmic changes to include compressibility effects are highlighted and our model for capturing shock–turbulence interaction is presented. To validate our compressible solver, we compare results for the Sod shock tube problem against a finite volume Riemann solver. To validate our model for shock–turbulence interaction, we present comparisons for a non-reactive and a reactive case. First, results of a shock traveling from light (air) to heavy (SF6) with reshock have been simulated to match mixing width growth data of experiments and turbulent kinetic energy results from LES. Then, for one-step chemistry calibrated to represent an acetylene/air mixture we simulate the interaction of a shock wave with an expanding flame front, and compare results with 2D simulation (2D-sim) data for flame brush formation and ensuing deflagration-to-detonation transitions (DDT). Results for the Sod shock tube comparison show that the shock speed and profile are captured accurately. Results for the non-reactive shock–reshock problem show that interface growth at all simulated Mach numbers is captured accurately and that the turbulent kinetic energy agrees in order of magnitude with LES data. The reactive shock tube results show that the flame brush thickness compares well to 2D-sim data and that the approximate location and timing of the DDT can be captured. The known sensitivity of DDT characteristics to details of individual flow realizations, seen also in ODT, implies that model agreement can be quantified only by comparing flow ensembles, which are presently unavailable other than in an ODT run-to-run sensitivity study that is reported herein.

Deflagration to detonation transitions (DDTs): Predicting DDTs in hydrocarbon explosions

This paper discusses our efforts to expand CFD calculations to include the prediction of deflagration to detonation transitions (DDTs) for various fuels including hydrogen, ethylene, propane and natural gas. The work includes validation against experiments conducted in a variety of configurations including: (1) closed pipes with obstacles; (2) other congested lab-scale geometries; (3) medium to large-scale 3-D obstacle configurations; and (4) large-scale geometries where DDT occurred upon flame propagation from confined to more open configurations. Validation of CFD models against experimental data is a critical step in verifying their accuracy in predicting DDT phenomena. In addition, an extensive set of experiments, which were performed under a variety of conditions that influence the transition to detonation, should be utilized in the validation to demonstrate the robustness of a CFD model and ensure its use for actual large-scale scenarios, i.e., petrochemical facilities where the likelihood of DDTs need to be evaluated for siting studies.The CFD model FLACS has been recently extended to identify whether DDT is likely in a given scenario and indicate the regions where it might occur. The likelihood of DDT has been expressed in terms of spatial pressure gradients across the flame front. This parameter is able to visualize when the flame front captures the pressure front, which is one of the main mechanisms that may initiate a DDT. Reasonable agreement was obtained with experimental observations in terms of explosion pressures, transition times, and flame speeds. The DDT model is currently being extended to include a criterion for estimating the likelihood of DDT by comparison of the geometric dimensions with the detonation cell size of the fuel-air mixture. This paper will provide a summary of some of the results.

Deflagration-to-detonation transition process for spherical aluminum dust/epoxypropane mist/air mixtures in a large-scale experimental tube

The deflagration-to-detonation transitions (DDTs) for clouds of spherical aluminum dust (SAD) mixed with air or epoxypropane mist (EPM) and air were investigated in a 29.6-m-long experimental tube of 199 mm in diameter. The clouds formed through the injection of SAD and SAD/liquid epoxypropane samples into the experimental tube. Explosions of the SAD/air mixture were initiated using a 7-m-long EPM/air cloud explosion ignited by a 40-J electric spark. Explosions in SAD/EPM/air clouds were initiated using a 1.2-m EPM/air cloud explosion ignited by a 40-J electric spark initiated using a 40-J electric spark. Self-sustained detonation waves formed in SAD/EPM/air mixtures instead of in SAD/air mixtures. The stages and characteristics of the DDT process in SAD/air and SAD/EPM/air mixtures were studied and analyzed. Self-sustained detonation was evident from the existence of a transverse wave and a cellular structure. Moreover, a retonation wave formed during the DDT process in SAD/EPM/air clouds.

Simulations of flame acceleration and deflagration-to-detonation transitions in methane–air systems

Flame acceleration and deflagration-to-detonation transitions (DDT) in large obstructed channels filled with a stoichiometric methane–air mixture are simulated using a single-step reaction mechanism. The reaction parameters are calibrated using known velocities and length scales of laminar flames and detonations. Calculations of the flame dynamics and DDT in channels with obstacles are compared to previously reported experimental data. The results obtained using the simple reaction model qualitatively, and in many cases, quantitatively match the experiments and are found to be largely insensitive to small variations in model parameters.

Deflagration-to-detonation transition in nitromethane mist/aluminum dust/air mixtures

Deflagration-to-detonation transitions (DDTs) in nitromethane mist/air mixture clouds, flaked aluminum dust/air mixture clouds, and nitromethane mist/flaked aluminum dust/air mixture clouds have been studied in an experimental tube of inner diameter 199 mm and length 29.6 m. The mist/dust/air multiphase mixtures were generated by injecting liquid/dust samples into the experimental tube. The droplet dimensions of the nitromethane mist cloud were analyzed, calculated, and measured. The multiphase fuel/air mixture clouds were ignited by means of an electric spark. The characteristics of different stages during the DDT process in the clouds of the various mixtures in the experimental tube have been studied and analyzed. A self-sustained quasi-detonation wave formed in the nitromethane mist/air mixture clouds, while a self-sustained detonation wave formed in the flaked aluminum dust/air mixture clouds and in the nitromethane mist/flaked aluminum dust/air mixture clouds. The detonation parameters of the clouds of the different mixtures have been studied and compared.

High-irradiance laser ignition of explosives

A current issue important to high explosive safety is deflagration-to-detonation transitions (DDTs) in accident scenarios. In order to better understand the reactive mechanisms involved in DDT and to begin to approach the fast ignition and heating rates seen in DDT, high-irradiance ( h 800W/cm 2 ) CO 2 laser ignition experiments were performed on the common high explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Reported data include time to ignition as a function of laser irradiance, energy, and ignition temperature. A simple dual ignition criteria model (DICM) was used to interpret the HMX results. The DICM requires two basic criteria for ignition: (1) a minimum surface temperature must be reached and (2) a minimum energy concentration must exist within the solid. The DICM sucessfully predicted the slope transition trend and the critical ignition energy for HMX to within 10% of the measured values. TATB had a single dependence on irradiance over the entire range of heating rates.

Anomalous detonation velocities following type II deflagration-to-detonation transitions in pentaerythritol tetranitrate

The detonation velocity following type II deflagration-to-detonation transitions (DDT) has been observed to be markedly different to that which would be expected were the charges directly initiated. In charges of pentaerythritol tetranitrate (PETN) velocities as high as 5.6 mm μs−1 have been observed compared to 3.7 mm μs−1 which is both predicted and measured for the charge were it to be directly initiated. Experiments have also been carried out that attempt to measure the temperature within columns of PETN prior to a type II DDT event. Other experiments and hydrocode simulations have been carried out that attempt to isolate the possible causes of the anomalous detonation velocity and as a result determine what the critical factors that determine the velocity are. These studies have led to a greater understanding not only of the reasons behind the change in detonation velocity, but also of the stages that occur in the build up to type II DDT.

Deflagration to detonation transitions and strong deflagrations in alkane and alkene air mixtures

Deflagration to detonation transitions and strong deflagrations for a large range of stoichiometric alkane (CH 4, C 2H 6, C 3H 8, C 4H 10) and air mixtures and alkene (C 2H 4, C 3H 6, C 4H 8) and air mixtures have been studied systematically by varying surface roughness via the introduction of Shchelkin spirals. The use of such spirals, rather than repeated obstacles such as orifice plates, is preferred, as improved acceleration and higher flame speeds are obtained due to reduced momentum losses. The results obtained indicate that deflagration to detonation transitions (DDT) and quasi-detonations can be obtained for all fuels tested. Using short obstacles it is demonstrated that quasi-stable strong deflagrations of considerable duration, supported only by a smooth-walled tube, can easily be established and typically serve as a basis for further acceleration and transition to detonation. The effects of different obstacle exit velocities on the duration of the strong deflagration phase and on DDT parameters have been investigated. By further increasing the obstacle length, DDT via quasi-detonations has been investigated and it is demonstrated that the relative detonability of the fuels under strongly turbulent conditions differs systematically from that obtained in smooth tube experiments.

Deflagration to detonation transition in mixtures of alkane LNG/LPG constituents with [formula omitted

Deflagration to detonation transitions (DDT) of methane, methane ethane , ethane, propane, and butane in mixtures with increasing dilution of nitrogen have been studied in a smooth 2″ id detonation tube with a length/diameter ratio of 220. The results obtained demonstrate the presence of two regimes of DDT depending on the reactivity of the mixture. The first regime displays rapid acceleration to a stable detonation, while the second regime contains a quasistable, strong deflagration of rapidly increasing duration with decreasing mixture reactivity. It is established that the order of DDT under the conditions employed follows the order of autoignition temperatures for the fuels considered. The conditional use of transition parameters for estimates of relative detonabilities based on a qualitative similarity between induction times and times to transition is demonstrated to hold only for the first transition regime.