Detonation mode and frequency analysis under high loss conditions for stoichiometric propane-oxygen

The propagation characteristics of galloping detonations were quantified with a high-time-resolution velocity diagnostic. Combustion waves were initiated in 30-m lengths of 4.1-mm inner diameter transparent tubing filled with stoichiometric propane–oxygen mixtures. Chemiluminescence from the resulting waves was imaged to determine the luminous wave front position and velocity every 83.3 μ. As the mixture initial pressure was decreased from 20 to 7 kPa, the wave was observed to become increasingly unsteady and transition from steady detonation to a galloping detonation. While wave velocities averaged over the full tube length smoothly decreased with initial pressure down to half of the Chapman–Jouguet detonation velocity (D_CJ) at the quenching limit, the actual propagation mechanism was seen to be a galloping wave with a cycle period of approximately 1.0 ms, corresponding to a cycle length of 1.3–2.0 m or 317–488 tube diameters depending on the average wave speed. The long test section length of 7300 tube diameters allowed observation of up to 20 galloping cycles, allowing for statistical analysis of the wave dynamics. In the galloping regime, a bimodal velocity distribution was observed with peaks centered near 0.4 D_CJ and 0.95 D_CJ. Decreasing initial pressure increasingly favored the low velocity mode. Galloping frequencies ranged from 0.8 to 1.0 kHz and were insensitive to initial mixture pressure. Wave deflagration-to-detonation transition and detonation failure trajectories were found to be repeatable in a given test and also across different initial mixture pressures. The temporal duration of wave dwell at the low and high velocity modes during galloping was also quantified. It was found that the mean wave dwell duration in the low velocity mode was a weak function of initial mixture pressure, while the mean dwell time in the high velocity mode depended exponentially on initial mixture pressure. Analysis of the velocity histories using dynamical systems ideas demonstrated trajectories that varied from stable to limit cycles to aperiodic motion with decreasing initial pressure. The results indicate that galloping detonation is a persistent phenomenon at long tube lengths

Carriage of the V279F Null Allele within the Gene Encoding Lp-PLA2 Is Protective from Coronary Artery Disease in South Korean Males

The Asia-specific PLA2G7 994G-T transversion leads to V279F substitution within the lipoprotein-associated phospholipase-A2 (Lp-PLA₂) and to absence of enzyme activity in plasma. This variant offers a unique natural experiment to assess the role of Lp-PLA₂ in the pathogenesis of coronary artery disease (CAD) in humans. Given conflicting results from mostly small studies, a large two-stage case-control study was warranted.PLA2G7 V279F genotypes were initially compared in 2890 male cases diagnosed with CAD before age 60 with 3128 male controls without CAD at age 50 and above and subsequently in a second independent male dataset of 877 CAD cases and 1230 controls. In the first dataset, the prevalence of the 279F null allele was 11.5% in cases and 12.8% in controls. After adjustment for age, body mass index, diabetes, smoking, glucose and lipid levels, the OR (95% CI) for CAD for this allele was 0.80 (0.66-0.97, p = 0.02). The results were very similar in the second dataset, despite lower power, with an allele frequency of 11.2% in cases and 12.5% in controls, leading to a combined OR of 0.80 (0.69-0.92), p = 0.002. The magnitude and direction of this genetic effect were fully consistent with large epidemiological studies on plasma Lp-PLA₂ activity and CAD risk.Natural deficiency in Lp-PLA₂ activity due to carriage of PLA2G7 279F allele protects from CAD in Korean men. These results provide evidence for a causal relationship between Lp-PLA₂ and CAD, and support pharmacological inhibition of this enzyme as an innovative way to prevent CAD

Carriage of the V279F Null Allele within the Gene Encoding Lp-PLA2 Is Protective from Coronary Artery Disease in South Korean Males

The Asia-specific PLA2G7 994G-T transversion leads to V279F substitution within the lipoprotein-associated phospholipase-A2 (Lp-PLA₂) and to absence of enzyme activity in plasma. This variant offers a unique natural experiment to assess the role of Lp-PLA₂ in the pathogenesis of coronary artery disease (CAD) in humans. Given conflicting results from mostly small studies, a large two-stage case-control study was warranted.PLA2G7 V279F genotypes were initially compared in 2890 male cases diagnosed with CAD before age 60 with 3128 male controls without CAD at age 50 and above and subsequently in a second independent male dataset of 877 CAD cases and 1230 controls. In the first dataset, the prevalence of the 279F null allele was 11.5% in cases and 12.8% in controls. After adjustment for age, body mass index, diabetes, smoking, glucose and lipid levels, the OR (95% CI) for CAD for this allele was 0.80 (0.66-0.97, p = 0.02). The results were very similar in the second dataset, despite lower power, with an allele frequency of 11.2% in cases and 12.5% in controls, leading to a combined OR of 0.80 (0.69-0.92), p = 0.002. The magnitude and direction of this genetic effect were fully consistent with large epidemiological studies on plasma Lp-PLA₂ activity and CAD risk.Natural deficiency in Lp-PLA₂ activity due to carriage of PLA2G7 279F allele protects from CAD in Korean men. These results provide evidence for a causal relationship between Lp-PLA₂ and CAD, and support pharmacological inhibition of this enzyme as an innovative way to prevent CAD

Multi-phase simulation of ammonium nitrate emulsion detonations

International audienceAmmonium-nitrate-based explosives used by the mining industry exhibit strong non-ideal detonation behaviour. Detonation velocities in rate-sticks with radii close to the failure radius, can be as low as one third of the ideal detonation velocity, which poses a significant challenge for their accurate predictive computational modelling. Given that these emulsions are highly heterogeneous, multi-phase formulations are well suited for their representation in numerical hydrocodes. To this end, a single-pressure, single-velocity multi-phase model is employed for the simulation of an explosive emulsion widely used by the mining industry. The model is modified to rectify a problem related to the calculation of a unique detonation state, and is evaluated using a high-resolution, shock-capturing Riemann problem-based scheme. In order to perform high-resolution numerical simulations at a reduced cost, a shock-following method is implemented and validated against the full-domain solutions. An improved iterative fitting procedure for steady-state detonation kinetics is also presented. Validation against experimental evidence shows that the model can reproduce confined VOD experimental data, solely by adjusting the reaction kinetics to match unconfined experimental VOD data. Furthermore, the model can match experimental front curvature measurement without further adjustments. (c) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Dynamics of lean premixed flames stabilized on a meso-scale bluff-body in an unconfined flow field

Two-dimensional direct numerical simulations were conducted to investigate the dynamics of lean premixed flames stabilized on a meso-scale bluff-body in hydrogen-air and syngas-air mixtures. To eliminate the flow confinement effect due to the narrow channel, a larger domain size at twenty times the bluff-body dimension was used in the new simulations. Flame/flow dynamics were examined as the mean inflow velocity is incrementally raised until blow-off occurs. As the mean inflow velocity is increased, several distinct modes in the flame shape and fluctuation patterns were observed. In contrast to our previous study with a narrow channel, the onset of local extinction was observed during the asymmetric vortex shedding mode. Consequently, the flame stabilization and blow-off behavior was found to be dictated by the combined effects of the hot product gas pocket entrained into the extinction zone and the ability to auto-ignite the mixture within the given residence time corresponding to the lateral flame fluctuations. A proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic theory of Zukoski and Marble

On the effects of fractal geometry on reacting and nonreacting flows in a low-swirl burner: A numerical study with large-eddy simulation

Understanding and optimizing turbulence in combustion systems has a profound effect on combustion efficiency, emission reduction, and safety in applications ranging from industrial burners to propulsion systems in aerospace. This paper contributes to this pivotal field by investigating a real-scale low-swirl combustor utilizing a range of turbulence generation plates characterized by different blockage ratios and numbers of fractal pattern iterations. The work encompasses two states, namely nonreacting and premixed reacting modes, utilizing a large-eddy simulation method equipped with a dynamic Smagorinsky sub-grid model and adaptive mesh refinement for high accuracy. Verifying the robustness of the approach, the simulation aligns closely with experimental data, registering a maximum error of less than 0.9% in the swirl number. The research provides an insightful evaluation of the impact of four different fractal geometries on turbulence intensity, a vital element for attaining more complete combustion. Findings indicate a fractal with a 73% blockage ratio and four iteration levels enhances several key parameters including turbulence intensity, flow residence time, vorticity, and velocity gradient in the nonreacting mode. Conversely, a fractal with a 73% blockage ratio and three iteration levels shows the least progress in the reacting mode. Moreover, the paper delves into a comparative analysis of these two cases in the reacting mode, particularly observing the reaction zone. The appropriate fractal geometry unveiled significantly improves combustion efficiency, evidenced by an increased presence of the OH radical and a decrease in NO emission gas, thus demonstrating potential for wider application in enhancing combustion systems

A numerical study of the auto-ignited laminar lifted methane/hydrogen mixtures in heated co-flow air

In the present study, the liftoff characteristics of a laminar HCNG flame are investigated using 2-D numerical simulations by varying the fuel jet velocity. It is observed that the liftoff height decreases with increasing fuel jet velocity. Gradual transition from the tribrachial flame to the MILD combustion is also observed. From both Da analysis and transport budget analysis, it is verified that auto-ignition is the main stabilization mechanism of the lifted flame. Additional simulations are carried out with modified hydrogen diffusivity, showing that the decrease of liftoff height is mainly attributed to the high diffusivity of hydrogen molecule. Preferential diffusion effect of hydrogen is clarified by revising the ignition delay time term in the liftoff height relation. It is also shown that the revised ignition delay time increases as the fuel jet velocity decreases since hydrogen ratio RH decreases at the upstream of flame front with decreasing fuel jet velocity. The high diffusive nature of hydrogen molecule increases the ignition delay time at lower fuel jet velocity, which, in turn, leads to the increase of the liftoff height

On the effects of NH3 addition to a reacting mixture of H2/CH4 under MILD combustion regime: numerical modeling with a modified EDC combustion model

This paper examines the behavior of reacting NH3/H2/CH4 mixtures in moderate or intense low oxygen dilution (MILD) condition. A series of axisymmetric, turbulent reacting flow simulations are carried out incorporating a modified version of eddy dissipation concept and a few reaction mechanisms. The effects of adding a progressively increasing amount of NH3 to a reacting H2/CH4 mixture in moderate condition are investigated. It is observed that addition of NH3 to MILD combustion leads to markedly different behaviors compared to that in conventional combustion. Most notably, the inherently strong preheating of reactants in MILD combustion causes thermal cracking of NH3 prior to ignition. The resultant production of H2 profoundly affects the reacting flow as such increasing the NH3 mass fraction in the fuel blend decreases the flame lift-off. Further, unlike that in conventional combustion, adding NH3 to MILD combustion increases the process reactivity. In addition, the usual flame thickening typically seen in NH3 flames is not observed here, while in keeping with the thermodynamic predictions, NH3 addition lowers the temperature of combustion products. The results also show that in sharp contrast to that reported for conventional combustion, addition of NH3 in MILD condition does not increase the emission of NO, while the mass fraction of NO2 drops slightly. Overall, it is concluded that MILD combustion could be a promising route to NH3 combustion