热泵空调系统含可燃组元混合物的最危险泄漏工况和泄漏点

各种冷热源的热泵空调系统采用含有可燃组元的替代混合物时,存在最危险泄漏点和泄漏工况,给出了根据混合物的性质判定其所处循环系统最危险泄漏和泄漏工况的准则式。结果有利于设备及替代工质的配套选择以及系统结构改进,以有效地预防燃烧爆炸事故的发生。     

Detailed simulations of the transient hydrogen mixing,leakage and flammability in air in simple geometries

During an accidental release, hydrogen disperses very quickly in air due to a relatively high density difference. A comprehensive understanding of the transient behavior of hydrogen mixing and the associated flammability limits in air is essential to support the fire safety and prevention guidelines. In this study, a buoyancy diffusion computational model is developed to simultaneously solve for the complete set of equations governing the unsteady flow of hydrogen. A simple vertical cylinder is considered to investigate the transient behavior of hydrogen mixing, especially at relatively short times, for different release scenarios: (i) the sudden release of hydrogen at the cylinder bottom into air with open, partially open, and closed tops, and (ii) small hydrogen jet leaks at the bottom into a closed geometry. Other cases involving the hydrogen releases/leaks at the cylinder top are also explored to quantify the relative roles of buoyancy and diffusion in the mixing process. The numerical simulations display the spatial and temporal distributions of hydrogen for all the configurations studied. The complex flow patterns demonstrate the fast formation of flammable zones with implications in the safe and efficient use of hydrogen in various applications.     

低沸点可燃组元混合物贮运过程动态可燃烧性

通过热力学状态及数值分析法对含可燃组分混合物在贮运过程中动态可燃烧性进行研究。当可燃组元为低沸点的可燃混合物时，得出其在贮运过程中发生泄漏捍的燃烧爆炸性与工质性质，可燃组元浓度，泄漏持续时间，系统及环境温度有关，指出低温汽相泄漏的初始时刻泄漏的可燃浓度有可能达到临界可燃浓度，当其与空气以适当的比例混合，而泄漏的局部温度由于磨擦或碰撞等原因快速升高时，有发生燃烧爆炸事故的可以 ，以R32／134a为例给出泄漏时的动态可燃性规律，研究结论对可燃组分混合物在贮运过程中的了燃防爆有参考价值。     

室内可燃气体泄漏过程中的着火与火焰蔓延

采用小尺度箱体及液化石油气模拟室内燃气泄漏的条件,通过电火花点火,实验研究了在燃气泄漏过程中的着火与火焰蔓延的现象与规律.实验结果表明,火焰首先在燃料燃烧的浓限和稀限之间的可燃区域传播,并且在化学恰当比附近区域的传播速度最大,因此燃气的动态浓度场决定了火焰蔓延的形状.并表明可燃区域是一个逐渐发展的区域,最终其高度与窗沿高度相当,在此区域内点火最容易成功,因此是气体燃料泄漏后发生火灾的最危险的区域.实验结果还揭示了在火焰蔓延过程中,存在着由火焰面推进传播向空间整体燃烧过渡的现象.     

Characteristics of flammable,buoyant hydrogen plumes rising from open vertical containers

The dynamics of the dispersion of a fixed mass of the highly buoyant hydrogen when exposed to overlaying atmosphere with a negligible pressure difference from open vertical cylindrical enclosures are examined. Features of the rapid formation and dispersion of flammable mixtures both inside and immediate outside of the enclosure and their corresponding propagation rates were examined using a 3-D CFD model. For the cases considered, the puffs of the fuel–air mixture appear to produce lean flammable boundaries that move mainly at a near constant rate for much of the time. A similar simulation that used an axis-symmetrical 2-D model tended to under-predict the dynamics of the lean and rich mixture boundaries. Hydrogen plume characteristics were compared with that of the less buoyant methane and helium release. Unlike methane, helium propagation rate was found fairly close to that of hydrogen.     

Combustion and flame spread on fuel-soaked porous solids

Fires caused by accidental spillage of flammable liquids have been a major safety concern in industries and urban areas. There has been a recent surge of interest in the research concerning the combustion and flame spread over an inert porous media soaked with flammable liquid. This interest has been driven by the need to better understand fire and its behaviour under these conditions and improve the relevant fire safety and prevention technologies. A review of key studies in this subject area has been conducted and summarised, focussing mainly on the theory plus a notable experimental findings about combustion and the flame spread phenomena of fuel-soaked porous media. The review covers topics such as flame spread behaviour, physical flame propagation aspects, heat transfer, temperature distribution; and fuel consumption over inert porous media. The review concludes with some practical safety and environmental considerations for decontamination of land soaked with flammable liquid.     

Numerical modelling of release of subsonic and sonic hydrogen jets

For the general public to use hydrogen as a vehicle fuel, they must be able to handle hydrogen with the same degree of confidence as conventional liquid and gaseous fuels. For refuelling hydrogen cars, hydrogen is stored at high pressures up to 700 bar. The hazards associated with jet releases from accidental leaks of such highly pressurized storage must be considered since a jet release and dispersion can result in a fire or explosion. Therefore, it is essential to understand the dispersion characteristics of hydrogen to determine the extent of the flammable cloud when released from a high-pressure source. These parameters are very important in the establishment of the safety distances and sizes of hazardous zones. This paper describes the work done by us in modelling of dispersion of accidental releases of hydrogen, using the FRED (Fire Explosion Release Dispersion) software. The dispersion module in FRED is validated against experimental data available in the open literature for steady release and dispersion of cold and ambient hydrogen gas. The validation is performed for a wide range of hole sizes (0.5–4 mm), pressure (1.7–400 bar) and temperature (50–298 K).The model predictions of hydrogen gas jet velocity, concentration decay as a function of distance as well as radial concentration distribution are in good agreement with experiments. Overall, it is concluded that FRED can accurately model accidental release and dispersion of hydrogen in unconfined and open configurations.     

Modeling of laminar flame propagation through organic dust cloud with thermal radiation effect

In this research, a mathematical model is performed to analyze the structure of flame propagation through a two-phase mixture consisting of organic fuel particles and air. In contrast to previous analytical studies, thermal radiation effect is taken into consideration, which has not been attempted before. In order to simulate of the dust combustion phenomenon, it is assumed that the flame structure consists of four zones: preheat, vaporization, reaction and post flame (burned). Furthermore, radiative heat transfer equation is employed to describe the thermal radiation exchanged between these zones. The obtained results show that the induced thermal radiation from flame interface into the preheat and vaporization zones plays a significant role in the improvement of vaporization process and burning velocity of organic dust mixture, compared with the case in which the thermal radiation factor is neglected. According to present results, flame structure variables such as the burning velocity, mixture temperature, mass fraction of volatile fuel particles and gaseous fuel mass fraction strongly depend on radiative heat transfer. These predictions have reasonable agreement with published experimental data.     

燃料电池船舶舱内氢气泄漏爆炸的数值模拟研究

以燃料电池客船“Water-Go-Round”号为对象,利用FLUENT软件模拟燃料电池客船舱内管道发生氢气泄漏并引发爆炸的情况,研究不同舱室氢气点火爆炸事故的影响规律。结果表明：可燃氢气云被点燃后,爆炸超压波自点火位置向四周迅速传播,点火位置对超压波的分布影响较大;控制舱爆炸时,超压强度最大,对船体超压危害最大;乘客舱爆炸强度最小,但超压中心分布在乘客舱,超压对乘客造成的危害最大;船舶舱室燃烧火焰温度主要由可燃氢气云的分布决定,燃料电池舱的火焰衰减趋势基本相同;乘客舱受到的高温危害较低,船艏舱无燃烧火焰的高温危害。     

Generalization of the radiative fraction correlation for hydrogen and hydrocarbon jet fires in subsonic and chocked flow regimes

The radiative fraction is one key parameter to characterize the jet flame combustion dynamics and to calculate the thermal radiant heat emitted from jet fire. A theoretical analysis is conducted to clarify the key parameters that dominate the radiative fraction of jet fires, with discussion of the limitation of previous radiative fraction correlations. A completely new dimensionless group, consisting of the mass fraction of fuel at stoichiometric conditions, the density ratio of fuel gas to ambient air and the flame Froude number, is proposed to correlate the radiative fraction of jet fires. The current up-to-date experimental data are used to build the radiative fraction correlation that covers orifice exit diameters from one to hundreds of millimeter, hydrogen, methane and propane fuels, vertical and horizontal jets, buoyance- and momentum-controlled releases, subsonic, sonic and supersonic jets. It is found that the source Froude number can fit the radiative fraction of a particular fuel jet fire. However, the new dimensionless group can correlate the radiative fractions of fuel-different jet fires. The predictive capability of the new correlation exceeds that of previously published work based on the source Froude number only or the global residence time with/without correction factors.     

Risk assessment and ventilation modeling for hydrogen releases in vehicle repair garages

The availability of repair garage infrastructure for hydrogen fuel cell vehicles is becoming increasingly important for future industry growth. Ventilation requirements for hydrogen fuel cell vehicles can affect both retrofitted and purpose-built repair garages and the costs associated with these requirements can be significant. A hazard and operability (HAZOP) study was performed to identify risk-significant scenarios related to light-duty hydrogen vehicles in a repair garage. Detailed simulations and modeling were performed using appropriate computational tools to estimate the location, behavior, and severity of hydrogen release based on key HAZOP scenarios. This work compares current fire code requirements to an alternate ventilation strategy to further reduce potential hazardous conditions. Modeling shows that position, direction, and velocity of ventilation have a significant impact on the amount of instantaneous flammable mass in the domain.     

An experimental investigation on temperature profile of buoyant spill plume from under-ventilated compartment fires in a reduced pressure atmosphere at high altitude

Buoyant fire plume over a building façade spilled from the window of an under-ventilated compartment fire poses a serious fire hazard of flame spread to upper floors, a process in which the high plume temperature is the key parameter. In order to specify its counteracting fire safety design and regulations, the façade plume temperature profiles have been studied and correlated in the past. However, those correlations are all specified at normal atmospheric pressure conditions at sea level, which is needed however to be extended for conditions at low pressure such as in high altitudes. To investigate the effect of low atmospheric pressure on temperature profile of buoyant spill plume, a knowledge which has never been revealed, scale model experiments were carried out correspondingly at two different altitudes (Hefei city: 50 m, 1 atm; Lhasa city: 3650 m, 0.64 atm). Both the lateral (in the direction normal to façade) and vertical (along facade) temperature profile of the spill plume are measured. It is found that the lateral decay of temperature in the reduced pressure atmosphere is much faster than that in the normal pressure condition, but they can be converged and correlated by a proposed non-dimensional equation. For a given total heat release rate, the temperature of the spill plume near the façade wall is much higher in the reduced pressure atmosphere than that in the normal pressure condition at the same height, suggesting that the fire safety regulations to counteract the vertical fire spread to upper floors need to be specified more rigorous in high altitude. Based on the correlation of vertical temperature profile, it is found that the air entrainment of the buoyant spill plume is weaker in the reduced pressure atmosphere, being about 0.8 times of that in the normal pressure condition. Finally, the vertical temperature profile is collapsed non-dimensionally with this entrainment change accounted for. These results and findings at low pressure provide a significant supplement over previous results in the literatures, as well as application of current fire protection measure settings to high altitude locations with considerably reduced atmospheric pressure.     

Failure of reinforced concrete and tuff stone masonry buildings as consequence of hydrogen pipeline explosions

Pipelines are the most efficient method of transporting large quantities of hydrogen, and the low volumetric energy density of gaseous hydrogen requires that the gas must be compressed to extremely high pressure to be used as a transport fuel. The failure of high pressure hydrogen gas pipelines and subsequent explosion may induce heavy damage to buildings. In this paper, such an issue is addressed for existing reinforced concrete framed buildings and tuff stone masonry buildings. Physical features such as the gas jet release process, flammable cloud size, blast generation and propagation, and explosion effects on structural components of buildings are considered and evaluated through the SLAB integral model, Multi-Energy Method and pressure‒impulse diagrams. Damage to both types of structural components was evaluated and the maximum distance of blast damage was derived in several environmental conditions, contributing to land-use planning and performance-based design/assessment of pipelines and buildings.     

Core-shell microstructured nanocomposites optimized based on Box–Behnken design for enhanced suppression of hydrogen co-flow flames

Combustion characteristics of hydrogen-containing syngas are distinct from those of traditional hydrocarbon fuels. However, the efficiency for any fire extinguishant on syngas/air flame suppression has not been validated up to now. In this study, we developed novel core-shell microstructured nanocomposites (CSMNs) with inner seawater for syngas/air fire extinguishing. To optimize the preparation conditions, we adopted a three-factor three-level Box-Behnken design (BBD). The corrosion rate and minimum extinguishing concentration of CSMNs were determined by corrosion tests and Cup-burner experiments, respectively. Results showed that the BBD method exhibits good agreement between experimental data and fitted models. By applying numerical optimization, we determined that solid mass fraction, rotation speed, and rotation time in a range of 2–10%, 4000–8000 rpm, and 60–180 s, respectively, are the optimal preparation conditions for the CSMN preparation. The degree of corrosion rate and mass concentration in co-flow gaseous fuel/air fire suppression were reduced by more than 50% compared to an aqueous seawater solution. The updated methods can be used to study the fire inhibition efficiency of the emerging class of composite materials with the core-shell structure. This does not only provide solutions for syngas/air flame inhibition, but it also gives new insight into the application of seawater for fire extinguishment.     

Fuzzy-inference-based failure mode and effects analysis of the hydrogen production process using Thermococcus onnurineus NA1

Hydrogen energy can be effectively converted from various energy sources, such as renewable energy or hydrocarbon fuel, and therefore, it is a promising energy source. Various hydrogen production processes have been proposed worldwide because conventional energy conversion systems, after minor design modifications, can be used for such conversions. However, hydrogen gas can be more dangerous than other flammable gases if released into the atmosphere, where it can rapidly reach its explosion limit because of its high diffusion velocity. Therefore, a failure mode and effects analysis (FMEA) for hydrogen production processes is required. In this study, FMEA is performed for hydrogen produced from coal syngas using Thermococcus onnurineus NA1, which was discovered in the Indonesian deep sea. A fuzzy inference methodology is introduced for a quantitative analysis of the linguistic ambiguity of risk priority number estimated from a conventional FMEA. To estimate the objective severity ranking, we introduced the potential asset loss combined with the proportionate cost of each piece of equipment under total capital investment and a weight value influenced by environmental and mankind. Moreover, we proposed a fuzzified risk matrix to effectively represent the fuzzy risk priority number (f-RPN) under the risk matrix; variation with and without fuzzification of risk priority number is then expressed to ascertain why this variation has occurred through a vector diagram. Based on the f-RPN vector diagram, we have performed a design revision of the hydrogen production process using Thermococcus onnurineus NA1 to adjust the risk priority number downward from a broadly unacceptable region.     

Hydrogen gas dispersion studies for hydrogen fuel cell vessels II: Fuel cell room releases and the influence of ventilation

Results are presented for computational fluid dynamics (CFD) modeling for varying hydrogen leaks within a hydrogen vessel's Fuel Cell Rack inside a Fuel Cell Room. In the limiting case of no room ventilation, modeling shows that the flammable region produced by the hydrogen leak is initially limited by self-induced entrainment and recirculation of air caused by the buoyant rising of hydrogen. Locally and at shorter times (minutes), this effect can be even more influential in limiting the size of the flammable envelope than Fuel Cell Room ventilation. Interestingly, the more diffuse detectable (but sub-flammable) region is not self-limited. This indicates the recirculation pattern required for the self-limiting effect requires a sufficient concentration of hydrogen to establish and differentiate the rising hydrogen mass from the surrounding air, thereby establishing the recirculation pattern that self-limits the flammable region at short times. Modeling results with the Fuel Cell Room ventilation activated shows that several seconds after a hydrogen leak is initiated, the flammable region reaches a steady state, with only minor fluctuations due to the air currents created by ventilation. The expected trends with ventilation rate are found: for a given leak size, a decreasing flammable envelope is found as ventilation is increased and for a given level of ventilation, an increasing hydrogen leak rate produces a larger flammable region. For the cases and ventilation rates examined, flammable H₂/air mixtures greater than 4% clear the Fuel Cell Room within 1.5 s after the hydrogen leak is turned off. The CFD modeling results for the detectable level of hydrogen that would trigger an alarm showed that higher ventilation rates might have the unintended consequence of making a hydrogen leak harder to detect, depending on the location of the gas detector in the Fuel Cell Room For the hydrogen leak rates considered in this study, we find that a ventilation rate of 15 ACH provides timely hydrogen evacuation while allowing the leak to be detected by the ceiling-mounted hydrogen monitor (for most monitor locations).     

Effects of mesh aluminium alloy and aluminium velvet on the explosion of H2/air,CH4/air and C2H2/air mixtures

MAA (mesh aluminium alloy) is one of the most widely used explosion suppression materials in military and civilian applications. To systematically research the effect of MAA on the explosion reactions of flammable gases, we investigated the effect of MAA and AV (aluminium velvet) on the explosion of hydrogen-air, methane-air and acetylene-air mixtures. The results indicated that MAA and AV suppress the methane-air mixture explosion but significantly promote the explosions of the hydrogen-air mixture and the acetylene-air mixture. MAA and AV have the dual effect of promoting and suppressing an explosion. In addition, with an increase in filling density, the promotion of MAA and AV first strengthens and then weakens. The results of this study show that the properties of flammable gas, not the shapes of the explosion suppression materials, determine whether the dominant effect of explosion suppression material is promotion or suppression. Use of explosion suppression materials is not suitable for all flammable gases, especially highly reactive chemical fuels. Applying explosion suppression materials blindly may greatly increase safety risk.     

气体燃料与气体发动机工作性能

气体燃料组分的变化,不但改变气体燃料与空气混合气热值,影响气体发动机输出功率和燃料经济性,而且可能会引起气体发动机爆震、空燃比改变等一系列液体燃料发动机所遭遇不到的问题。本文探讨如何处理这些问题。     

Is hydrogen a safe fuel?

The safety aspects of hydrogen are systematically examined and compared with those of methane and gasoline. Physical and chemical property data for all three fuels are compiled and used to provide a basis for comparing their various safety features. Each fuel is examined to evaluate its fire hazard, fire damage, explosive hazard and explosive damage characteristics. The fire characteristics of hydrogen, methane and gasoline, while different, do not largely favor the preferred use of any one of the three fuels; however, the threat of fuel-air explosions in confined spaces is greatest for hydrogen. Safety criteria for the storage of liquid hydrogen, liquefied natural gas (LNG) and gasoline are compiled and presented. Gasoline is believed to be the easiest and perhaps the safest fuel to store because of its lower volatility and narrower flammable and detonable limits. It is concluded that all three fuels can be safely stored and used; however, the comparative safety and level of risk for each fuel will vary from one application to another. Generalized safety comparisons are made herein but detailed safety analyses will be required to establish the relative safety of different fuels for each specific fuel application and stipulated accident. The technical data supplied in this paper will provide much of the framework for such analyses.