Ignition characteristics, conditions of criticality and disappearance of criticality of cumene hydroperoxide reaction by modeling approach

The thermal explosion problem of cumene hydroperoxide exothermic reaction which is used in chemical industries for production of some chemical materials is investigated. The analytical solutions of the problem to determine the margin between ignition and non-ignition systems are presented. The solution offers different analytical expressions which relate between the critical parameters for both steady and unsteady-states in different planes of solutions for different cases. The numerical solutions in different planes offer different trajectories of solution as sub-critical (non-ignition) and supercritical (ignition). Also from the numerical solution the relations between the critical parameters are presented. The critical behaviors from both analytical and numerical solutions are concise and pertained the same results.     

Effects of fuel reactivity and injection timing on diesel engine combustion and emissions

Recent strategies for simultaneously reducing NOx and soot emissions have focused on achieving nearly premixed, low-temperature combustion (LTC) in diesel engines. A promising approach in this regard is to vary fuel reactivity in order to control the ignition delay and optimize the level of premixing and reduce emissions. The present study examines such a strategy by performing 3-D simulations in a single-cylinder of a diesel engine. Simulations employ the state-of-the-art two-phase models and a validated semi-detailed reaction mechanism. The fuel reactivity is varied by using a blend of n-heptane and iso-octane, which represent surrogates for gasoline and diesel fuels, respectively. Results indicate that the fuel reactivity strongly influences ignition delay and combustion phasing, whereas the start of injection (SOI) affects combustion phasing. As fuel reactivity is reduced, the ignition delay is increased and the combustion phasing is retarded. The longer ignition delay provides additional time for mixing, and reduces equivalence ratio stratification. Consequently, the premixed combustion is enhanced relative to diffusion combustion, and thus the soot emission is reduced. NO_x emission is also reduced due to reduced diffusion combustion and lower peak temperatures caused by delayed combustion phasing. An operability range is observed in terms of fuel reactivity and SOI, beyond which the mixture may not be sufficiently well mixed, or compression ignited. The study demonstrates the possibility of finding an optimum range of fuel reactivity, SOI, and EGR for significantly reducing engine out emissions for a given load and speed.     

Flame propagation through dust clouds of nano and micron scale aluminum particles

Temperature measurement on propagating flame and minimum explosible concentration are investigated. The dust explosion experiments of nano-particle dust clouds exhibit higher temperature gradient in preheat zone and lower MEC than those of micron particle dust clouds. A heterogeneous model is proposed to describe the oxidation process under two extreme conditions: whether the alumina film is involved in the reaction or not. The new methodology allows the estimation of oxidation kinetics of growing alumina. For micron particle, the model clarifies that the activation energy which has been wrongly considered to be for aluminum oxidation should be for lattice diffusion, and the initial reaction rate is proved to be dominated by the diffusion rate of oxygen through alumina shell as diffusion controlled reaction. For nano-particle, the model explained that why the reported activation energy shows significantly lower than that for micron particle, due to initially ignorable alumina film or considered as kinetically controlled reaction. However, as reaction occurs and alumina builds up on the surface, the interference of alumina somewhat increases the activation energy.     

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (C_dust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and C_dust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various C_dust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (P_ex) and the rate of explosion pressure rise [(dP/dt)_ex]. When dust particle size was augmented gradually, the P_ex and (dP/dt)_ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.     

变压器油在电弧作用下的点燃特性

变压器的充油设备在长期使用过程中,由于超负荷运行时间长、运行维护不当等原因,劣化的变压器油存在更高被电弧点燃的火灾风险。对电弧作用下变压器油的点燃特性进行了实验研究,考虑了电弧能量、变压器油老化程度等参量的影响,利用 Bruceton法统计和计算了临界点燃温度和本质安全温度。研究表明:电弧可以加速变压器油的分解,促进易燃油气混合物的形成;变压器油最低临界点燃温度的电弧能量极值为13.8 J,对应电弧电压约为8.1 kV,在该能量及以上,平均本质安全温度约为124 ℃。研究结果可为制订变电站火灾爆炸事故防护措施提供依据。     

Incendiary characteristics of electrostatic discharge for dust and gas explosion

Paying attention to the ignition potentiality of static electricity, the relation between the discharge characteristics and the ignition of a dust cloud and the gas produced was studied, applying an electrical power supply of which the electrical circuit is adjustable. The effect of ignition characteristics on dust and gas explosions was investigated. The results of the study indicate that the probability of an explosion is influenced by the minimum ignition energy, spark duration time, feeding rate of ignition energy, circuit capacitance, ignition voltage, etc.     

木材点燃温度的测定

本文叙述了在15-32KW/㎡辐射条件下对四种木材点燃性的研究，测定了木材（64×64×18mm）水平方向着火点（点燃状态形成）时的木材表面温度及点燃时间。结果表明：辐射强度等于或高于24KW/㎡时，可以较准确地测量木材的点燃温度，且此温度基本保持一致（±20℃）。辐射强度较低时，木材表观点燃温度大大高于辐射强度较高时的点燃温度，本文对这一现象进行了讨论。     

Development of a method for predicting the ignition of explosive atmospheres by mechanical friction and impacts (MECHEX)

Mechanical friction and impacts is still today a main cause of ignition of explosive atmospheres (ATEX) in the industry and this trend seems to be stable in time. This situation certainly results from a significant gap of knowledge in the underlying mechanisms so that the parameters to play on are not precisely identified. In this programme of European dimensions, the process of degradation of the mechanical energy into heat during friction and impacts have been studied.

An extensive experimental programme is presented to this end. The mechanisms of dissipation of the mechanical energy into heat during friction has been studied with rubbing machines in which a slider equipped with temperature sensors rubs against a rotating wheel. For impacts, a new device has been developed using a special “air driven cannon” to propel a projectile accurately up to 50 m/s onto an inclined target. A very significant effort has been reserved to the investigation of the ignition mechanisms, not only for ATEX but also for dust accumulations.

Some “simple” modelling is proposed on purpose of practical applications. For frictional situations, a critical rubbing power is calculated without any limitations about any lower boundary concerning the rubbing velocity. For “impacts”, the relevant parameter for ignition is not the kinetic energy of the projectile but its velocity and the nature of the materials.     

Friction spark generation and incendivity of several metal alloys

When metal alloys are used as mechanical equipment or tools in explosive atmospheres, the occurrence and incendivity of mechanically generated sparks as ignition sources should be taken into consideration. The formation of mechanically generated sparks was investigated for seven metals, including Q235 steel, 304 stainless steel, TC4 titanium alloy, 6061 aluminum alloy, H62 bronze alloy, AMAK3 zinc alloy, and AZ31B magnesium alloy. The relationship between the physical-chemical properties and generation and incendivity of friction sparks was evaluated. For 6061 aluminum alloy, H62 bronze alloy, AMAK3 zinc alloy, and AZ31B magnesium alloy, no bright friction sparks were observed in the maximum friction velocity of 12 m/s and maximum surface pressure of 3.75 N/mm², because of low hardness, high thermal conductivity, low melting point, and the absence of carbon content. Ignition testing indicated that nano titanium dust layers with MIEL (minimum ignition energy of dust layer) of 1 mJ were not ignited by friction particles from the four metal alloys. However, bright particles were clearly observed for 304 stainless steel, Q235 steel, and TC4 titanium alloy. Friction sparks at the maximum power densities showed incendivity with micro titanium layers having an MIEL of 17.5–25 mJ but not with PMMA, corn starch, and wood dust having MIELs greater than 1 J. Two different particle burning behaviors with different fragmentation mechanisms during the friction process were determined, namely the micro explosion phenomenon for TC4 titanium alloy and particle burst for Q235 steel. Results indicate that the physical-chemical properties of friction metal rods are useful for preliminary evaluation of spark generation. Powder layers with known MIEL can be considered as indicator testing materials to evaluate spark incendivity.     

Comparative study on standardized ignition sources used for explosion testing

For the determination of safety characteristics of gases, vapors and dusts different types of ignition sources are used in international standards and guidelines. The paper presents test results of a comparative calorimetric and visual study between four different types of ignition sources. The ignition procedures were analyzed visually with a high-speed camera and electric recordings. In addition to that, the influence of the electrode-orientation, -distance as well as ignition energy on the reproducibility of the exploding wire igniter was tested.The exploding wire is already in use for standardized determination of safety characteristics of gases, first tests on the suitability of the exploding wire igniter for dust testing have been carried out but are not standardized yet. Using the exploding wire, the ignition energy can be varied from 2 J to 10 000 J (2 x 5000 J) and thus it could be used for gases, vapors, dusts and hybrid mixtures. Moreover it can be used at high initial pressures and it is the only ignition source with an easily measurable ignition energy release. Furthermore, it does not introduce another chemical reaction into the system.Finally, a proposal for a standard ignition source for explosion tests on hybrid mixtures is derived from the test results.