圆柱罐体内氢气-空气混合气体燃烧特性的CFD分析

点火器广泛应用于核电站以实现氢气低浓度下的控制性燃烧,缓解核电站严重事故下的氢气风险。本文应用三维计算流体力学程序GASFLOW对一圆柱罐体内不同浓度的氢气-空混合气体的燃烧特性进行分析。低浓度氢气混合气体只有极少部分燃烧,大于8%的情况下出现明显的氢气燃烧和温度、压力的上升。根据火焰加速σ准则和燃爆转换D/7λ准则,大于11%的情况下可能会出现火焰加速,大于12%的情况下有燃爆转换的可能。不同浓度混合气体燃烧的火焰传播路径也不同,较低浓度气体燃烧火焰先向上再向下传播,较高浓度气体燃烧火焰向四周传播。     

水蒸气对氢气燃烧影响的数值研究

核电站严重事故下,氢气的燃烧风险是影响安全壳完整性的重要因素,而水蒸气的存在对氢气、空气混合气体的燃烧会产生重要的影响。本文采用CAST3M软件,对局部小空间内氢气的燃烧特性以及水蒸气的影响进行研究。首先对THAI装置的典型实验工况进行模拟,表明了相关燃烧模型的可用性。然后将高度为6 m、直径为2.2 m的圆柱空间作为燃烧域,对其分别计算了8%、10%、12%氢气浓度下的燃烧,并与添加25%水蒸气的相应工况进行了对比。通过对燃烧域的温度、压力以及火焰传播速度的分析,表明添加水蒸气后燃烧产生的最大压力下降,火焰的最大温度下降,火焰传播的速度下降。研究表明,水蒸气的存在对氢气的燃烧具有抑制作用,能有效降低氢气燃烧产生的后果。     

严重事故条件下水蒸气对氢气燃烧影响试验研究

通过试验分析严重事故条件下水蒸气对氢气燃烧行为的影响,分别改变初始水蒸气浓度,比较分析氢气燃烧的温度、压力、火焰传播速度和燃尽率,并且对试验结果进行对比分析,可得出以下结论:水蒸气降低了氢气燃烧峰值温度、峰值压力和火焰传播速度,并且水蒸气浓度越高,对氢气燃烧影响越大,但是对氢气燃尽率无影响。     

骤冷网格对层流火焰影响的数值分析

利用计算流体力学(CFD)方法,研究二维简化情形下骤冷网格对层流火焰传播的影响。数值分析结果表明:随着火焰的传播,层流火焰前沿在接近骤冷网格的过程中,向金属网格的传热逐渐超过氢气燃烧产生的化学热,最终火焰的传播被骤冷网格终止。     

安全壳内氢气燃烧特性数值研究

本文基于计算流体力学(CFD)方法,采用涡耗散概念(EDC)模型耦合P1辐射模型,对德国开展的ThAI-HD12氢气燃烧实验进行了数值模拟验证,与实验符合良好.同时通过修正反应机理,获得了更符合实验的结果.通过改变点火位置、氢气浓度,计算得到安全壳内压力、温度等的变化,结果表明:在安全壳空间内,浮力对氢气燃烧火焰传播影...     

核电厂严重事故下氢气燃烧单步模型研究

通过改变指前因子和活化能系数，构建氢气燃烧单步反应机理，利用构建的单步机理开展严重事故下氢气燃烧计算分析，将计算结果与试验数据进行对比分析，同时利用机理开展不同氢气浓度条件下氢气燃烧数值计算。结果表明:单步机理在氢气火焰传播速度方面计算值与试验值符合很好，修正后的氢气燃烧单步机理可用于核电厂氢气燃烧计算分析。      

严重事故下安全壳内氢气爆燃风险数值模拟研究

采用计算流体力学(CFD)方法对典型核电厂失水事故下的氢气分布和燃烧过程进行安全分析研究。首先基于火焰加速准则对安全壳内燃爆风险进行评估,采用大规模氢气燃烧实验确定了保守燃烧模型(CREBCOM)中的燃烧速率常数。对安全壳内的氢气燃烧过程的数值模拟显示:氢气燃烧过程产生的峰值压力接近7.0×10~5 Pa,将对安全壳完整性产生威胁。     

氢氧预混气体燃烧过程的数值研究

建立了氢氧预混气体燃烧过程的数学模型,分析点火位置、反应机理和基元反应动力学参数对氢气燃烧过程的影响。研究发现:浮升力作用对火焰的传播有明显影响,采用顶部点火更有利于安全可控地消除氢气;不同点火位置下,火焰锋面移动相同距离时氢气消耗量几乎相等;Marinov氢氧反应机理和Warnatz氢氧反应机理预测的氢气燃烧过程较为一致;HO_2形成/消耗反应模块是Marinov机理和Warnatz机理中的制约反应模块,其反应速率对反应机理的总反应速率影响较大。     

秦山二期核电站氢气风险的CFD研究

利用计算流体力学(CFD)程序GASFLOW模拟了波动管大破口事故发生后7 000 s内装有22台氢气复合器的秦山二期核电站安全壳内的水蒸汽及氢气行为,得到了不同阶段的特征性流场及氢气浓度的分层情况,给出了所采用的复合器布置方案的稳定消氢速率为20 g/s,并指出了破口所在蒸汽发生器隔间内发生氢气燃烧火焰加速的可能性.同时,计算结果表明,安全壳内构筑物吸热带走了大部分从一回路释放的热量;压力变化同时受气体总质量(主要是水蒸汽质量)与温度的控制.     

GASFLOW程序以及COM3D程序在反应堆氢气行为分析上的应用

反应堆在事故情况下的氢气风险一直是反应堆安全研究中非常重要的内容。利用氢气风险管理程序GASFLOW计算了反应堆一回路破口事故后安全壳内的氢气分布,对计算结果进行分析。在GASFLOW计算结果的基础上,应用COM3D程序模拟氢气燃烧和爆炸,研究了氢气浓度以及点火位置对火焰扩散的影响。     

Combustion of stratified hydrogen-air mixtures in the 10.7 m combustion test facility cylinder

This paper presents preliminary results from hydrogen concentration gradient combustion experiments in a 10.7 m³ cylinder. These gradients, also referred to as stratified mixtures, were formed from dry mixtures of hydrogen and air at atmospheric temperature. Combustion pressures, burn fractions and flame speeds in concentration gradients were compared with combustion of well-mixed gases containing equivalent amounts of hydrogen. The studied variables included the quantity of hydrogen in the vessel, the steepness of the concentration gradient, the igniter location, and the initial concentration of hydrogen at the bottom of the vessel.Gradients of hydrogen and air with average concentrations of hydrogen below the downward propagation limit produced significantly greater combustion pressures when ignited at the top of the vessel than well-mixed gases with the same quantity of hydrogen. This was the result of considerably higher burn fractions in the gradients than in the well-mixed gas tests. Above the downward propagation limit, gradients of hydrogen ignited at the top of the vessel produced nearly the same combustion pressures as under well-mixed conditions; both gradients and well-mixed gases had high burn fractions. Much higher flame speeds were observed in the gradients than the well-mixed gases.Gradients and well-mixed gases containing up to 14% hydrogen ignited at the bottom of the vessel produced nearly the same combustion pressures. Above 14%, hydrogen, gradients produced lower combustion pressures than well-mixed gases having the same quantity of hydrogen. This can be attributed to lower burn fractions of fuel from the gradients compared with well-mixed gases with similar quantities of hydrogen. When ignited at the bottom of the vessel, 90%, of a gradient's gases remained unburned until several seconds after ignition. The remaining gases were then consumed at a very fast rate.     

高放废液储罐氢气爆炸事故试验研究

高放废液在贮存过程中会产生氢气,若未能及时排出或被稀释,当混合气体中氢气的浓度达到爆炸临界点时,有可能发生爆炸,导致放射性物质释放。本文利用建立的高放废液储罐氢气爆炸事故试验装置,通过试验研究了氢气在储罐内的爆炸压力及壁面温度。测试结果表明,氢气浓度为30%,点火位置在储罐顶部中心附近时,约在点火后70 ms达到最大爆炸压力,最大爆炸超压值约为0.596 5 MPa,最高壁面温度约为110 ℃。     

Performance Test of the Quenching Meshes for Hydrogen Control

The quenching distance of hydrogen gas was experimentally investigated by considering the effects of the initial pressure and steam addition. The quenching distance decreases with the initial pressure and there is a little increase with the addition of steam. Performance tests have been carried out to check the applicability of quenching mesh for the purpose of arresting hydrogen flame propagation during a severe accident in nuclear power plants. The experimental facility for the performance test of the quenching mesh consisted of a model compartment, a visualization system and an ignition system. Dimensions of the single model compartment were 300x300x300 mm. Three-compartments are connected in parallel. The quenching mesh is located between the first and second compartments. It was observed that the flame from the first compartment where the ignition starts does not propagate to the second compartment. The quenching mesh played a role of preventing flame propagation.     

Air-hydrogen deflagration tests at the University of Pisa

In severe accidents, large amounts of hydrogen may be released in the safety containment of a nuclear plant and the gas mixture may become explosive. The University of Pisa and ENEA have undertaken an experimental program to study the physics of flame propagation in a containment model under accident conditions. Up to now 41 deflagration tests have been performed at the HYDRO-SC facility at ambient pressure and temperature. Concentrations, water spray conditions, ignition source and gas turbulence levels were varied. The vessel volume was 0.5 m³, the ignition sources were an electrical spark discharge and an electrically heated surface (glow-plug), the hydrogen molar fractions were in the range 4–16%, the turbulence was generated by fan or spray and two different spray nozzles were utilized. The experimental data indicate that the peak pressures nearly fit the adiabatic isochoric values at the highest hydrogen concentrations and gas turbulences. Weak pressure waves were observed for H₂ molar fractions greater than 10%. A careful examination of the pressure and temperature transients gave information on the flame path and on the heat transfer process during and after combustion. Scale effects on the peak pressures were not observed by comparison of the HYDRO-SC results with data obtained in other laboratories. The glow plug igniter has proved to be a reliable tool for use in deliberate ignition schemes for hydrogen control in nuclear plants.     

Laser Rayleigh measurement of mixing processes and control of hydrogen combustion using quenching meshes

Two issues concerning hydrogen combustion under a severe accident scenario are addressed: (1) a laser Rayleigh scattering technique to investigate hydrogen mixing processes; and (2) the installation of metallic meshes between compartments to control and isolate hydrogen combustion within a single compartment. The Rayleigh scattering techniques are tested to determine hydrogen/air mixing processes locally and temporally as a non-intrusive probing method. To simulate mixing processes, helium is injected into a chamber filled with n-butane. Results show that helium concentration can be successfully monitored with sufficiently fast responses. Isolation and control of hydrogen burning is simulated by installing metallic meshes between compartments. Hydrogen is injected into one compartment and subsequently transported to the second compartment. Two sets of experiments are conducted with and without installing metallic meshes between the compartments. With the mixture ignited near the second compartment outlet, hydrogen combustion can be successfully contained within the second compartment with meshes, while flame propagates to the first compartment when meshes are not installed. These results demonstrate that hydrogen combustion can be controlled and isolated by installing meshes locally such that unwanted rapid pressure rise in a containment can be prevented. It also suggests the applicability of meshes for equipment survivability and protection from flame propagation by enclosing equipments with properly designed meshes.     

Flame-quenching model of the quenching mesh for H2–air mixtures

A deflagration to detonation transition (DDT) occurrence is one of the most important issues concerning safety during severe accidents in nuclear power plants because it can damage the integrity of the containment. It is possible to arrest the acceleration of a flame which can cause DDT by installing quenching meshes between the compartments. To evaluate the applicability of a quenching mesh to nuclear power plants, it requires a means to evaluate a flame arrest of a quenching mesh under a given combustion condition. The flame-quenching models developed by previous researchers were derived to fit the experimental geometry and to consider various thermal boundary conditions from a flame to the mesh wall. Flame-quenching tests were carried out at the 10% hydrogen concentration in a dry air by changing atmospheric pressure to 2.2 bar as the initial pressure. The quenching criterion of a quenching mesh with a 0.3 mm gap distance for hydrogen–air mixtures is established by using the experimental data. The flame-quenching models are also evaluated by using the experimental data. A flame-quenching model that can be used to evaluate a flame arrest for various hydrogen–air mixtures in nuclear power plants is proposed.     

The Advantages of Non-Thermal Plasma for Detonation Initiation Compared with Spark Plug

In this paper, the characteristics of detonation combustion ignited by AC-driven non-thermal plasma and spark plug in air/acetylene mixture have been compared in a double-tube experiment system. The two tubes had the same structure, and their closed ends were installed with a plasma generator and a spark plug, respectively. The propagation characteristics of the flame were measured by pressure sensors and ion probes. The experiment results show that, compared with a spark plug, the non-thermal plasma obviously broadened the range of equivalence ratio when the detonation wave could develop successfully, it also heightened the pressure value of detonation wave. Meanwhile, the detonation wave development time and the entire flame propagation time were reduced by half. All of these advantages benefited from the larger ignition volume when a non-thermal plasma was applied.     

低活化铁素体/马氏体钢CLF-1氢氘的渗透行为

低活化铁素体/马氏体钢(RAFM钢)作为聚变堆结构材料中最有前景的候选材料,其氢同位素渗透行为备受关注。采用氢同位素气相驱动渗透的方法,对中国低活化铁素体/马氏体钢CLF-1的氢同位素渗透行为进行了研究,研究了温度、气体压强、样品表面状态等因素对其渗透行为的影响。结果表明:氢、氘在RAFM CLF-1钢中渗透扩散过程为体扩散控制,渗透率与温度的关系式均遵循Arrhenius关系;在实验测试过程中,由于样品表面发生氧化现象和缺陷捕获造成H₂和D₂渗透实验中渗透通量出现下降的现象。     

Effect of spray on performance of the hydrogen mitigation system during LB-LOCA for CPR1000 NPP

During the course of the hypothetical large break loss-of-coolant accident (LB-LOCA) in a nuclear power plant (NPP), hydrogen is generated by a reaction between steam and the fuel-cladding inside the reactor pressure vessel (RPV). It is then ejected from the break into the containment along with a large amount of steam. Management of hydrogen safety and prevention of over-pressurization could be implemented through a hydrogen mitigation system (HMS) and spray system in CPR1000 NPP. The computational fluid dynamics (CFD) code GASFLOW is utilized in this study to analyze the spray effect on the performance of HMS during LB-LOCA. Results show that as a kind of HMS, deliberate igniter system (DIS) could initiate hydrogen combustion immediately after the flammability limit of the gas mixture has been reached. However, it will increase the temperature and pressure drastically. Operating the DIS under spray condition could result in hydrogen combustion being suppressed by suspended droplets inside the containment. Furthermore, the droplets could also mitigate local the temperature rise. Operation of a PAR system, another kind of HMS, consumes hydrogen steadily with a lower recombination rate which is not affected noticeably by the spray system. Numerical results indicate that the dual concept, namely the integrated application of DIS and PAR systems, is a constructive improvement for hydrogen safety under spray condition during LB-LOCA.