微重力条件下粉尘燃烧机理研究

由落塔仓产生的微重力提供了一种独特的环境 ,可克服粉尘云在地面实验时因受重力影响而产生的沉降现象 ,使封闭容器内的整个燃烧过程能维持粉尘云的稳定悬浮 ,保证粉尘燃烧机理的实验研究中粉尘的实际浓度与初始名义浓度相符 ,令粉尘浓度名符其实地成为一个独立参变量。在 12 m落塔提供的 10 - 2 g,1.2 s微重力环境下对球形铝粉 (d50 =7μm)与球形玉米粉 (d50 =2 0μm)两种粉尘的等容燃烧特性进行了系统的实验研究。实验结果经与地面实验结果进行比较与分析后定量给出了粉尘浓度保持不变时 ,粉尘爆炸特性随扬尘湍流强度衰减而变化的规律以及在地面上重力沉降令粉尘浓度变小后对粉尘爆炸特性的影响。最后还讨论了实验容器过小而引起的部分实验数据失真的原因。     

碳化钛的自蔓延燃烧合成

本文报道了有关碳化钛自蔓延燃烧合成的若干实验结果，重点放在其燃烧方面。此外，在１维非定常条件下对自蔓延燃烧现象进行了数值分析，得到与实测值相近的自蔓延燃烧速度；根据数值分析结果，建立了自蔓延燃烧的简化模型，并导出了估计燃烧速度及参数影响的表达式。     

壁面边界条件下氢气-空气预混燃烧的压力特性

通过纹影测试的实验方法,在顶置点火定容燃烧弹中对壁面边界条件下的氢气-空气预混燃烧压力特性进行了实验研究,得出了不同燃烧当量比(0.6～2.0)、初始压力(0.1～0.3,MPa)、初始温度(300～450,K)下的氢气-空气预混燃烧过程中燃烧压力、最大燃烧压力以及最大燃烧压力升高率等参数的变化规律.结果表明,壁面边界条件下氢气-空气预混燃烧过程中的最大燃烧压力以及最大燃烧压力升高率均随着当量比的增大先增大再减小,均随初始压力的增大呈现出线性增长,随着初始温度的增大而呈现出线性减少.     

点燃式发动机富氧燃烧的基础研究

为了探清混合气的氧浓度对紊流燃烧速度的影响，进行了定容器内预混合气紊流燃烧实验。研究结果表明，在较大的氧浓度下，氢混合气和甲烷混合气具有较高的紊流燃烧速度，而丙烷混合气则具有相反的倾向。     

生物质燃气预混层流燃烧特性的实验研究

在密闭燃烧容器中对常温、常压环境下的生物质燃气预混层流燃烧特性进行了实验研究,研究了不同燃气组分、不同当量比对生物质燃气预混层流火焰传播速度、火焰表面拉伸率和层流燃烧速度的影响规律。研究结果表明:发酵法制取的生物质燃气中甲烷含量越高,其层流火焰传播速度就越快;相同尺寸的火焰锋面上拉伸率越大,层流燃烧速度则越快;随着当量比的增大,层流火焰传播速度、层流燃烧速度呈现出先增大后减小的趋势。     

半焦粒子着火与燃烧过程实验研究

实验研究了半焦粒子着火与燃烧过程,测定了几种尺寸的半焦粒子在不同环境温度下的着火温度、着火滞燃期、燃尽时间和燃烧过程中的粒子温度等与燃烧过程相关的参数变化,对影响半焦粒子燃烧的因素进行分析讨论,并将实验结果与理论计算结果进行了对比分析,两者在一定范围内有较好的一致。     

二甲基醚燃烧过程及其优化的数学模拟（Ⅰ）——燃烧模型

在定容燃烧弹实验和发动机台架实验基础上建立了二甲基醚DME燃烧过程的准维多区的现象学燃烧模型,模型针对DME燃烧过程的特点,建立了新的喷雾混合和着火滞燃期子模型,修正了燃烧放热率子模型,研究了DME燃烧过程中氮氧化物（NOx)生成机理。模型的计算结果和实验结果相当吻合,模型对变工况、变参数有较好的适应能力。NOx生成历程计算分析表明,DME燃烧过程中NOx主要在扩散燃烧阶段生成,燃烧温度低是NOx排放低的主要原因。     

油页岩沸腾炉燃烧效率的数学模型

有机碳燃烧效率是沸腾炉燃烧过程中重要的操作参数之一。本文利用茂名油页岩燃烧的实验结果及工业沸腾炉的现场操作数据，通过油页岩在炉内的停留时间分布函数和燃烧动力学参数，建立了定量评价沸腾炉燃烧效率的数学模型，利用该模型考察了沸腾炉的运转情况。结果表明，理论计算值和实验结果吻合良好。     

炉膛压力与燃烧速度的再探讨

通过对锅炉燃烧的实验，指出了现行各类教科书中，对炉膛压力与燃烧速度问题的论述的不足性，并建议现有教科书应统一公式，明确各参数介定范围，进一步深入研究锅炉燃烧理论。     

玉米秸秆燃烧过程及燃烧动力学分析

采用TG-DTA-DTG热分析联用技术对玉米秸秆在不同升温速率下进行了燃烧实验.考察了着火温度、燃烧速率最大时温度、燃尽温度等燃烧特征参数.根据对不同升温速率下玉米秸秆燃烧过程的分析,用双组分分阶段反应模型能够很好的描述玉米秸秆的燃烧过程.建立了玉米秸秆反应动力学方程,得到了在不同温度区间的燃烧动力学方程和表观活化能、频率因子等燃烧动力学参数,并提出了相应的燃烧机理.     

Effects of hydrogen addition on the explosion characteristics of n-hexane/air mixtures

To study the effects of hydrogen addition on the explosion characteristics (the explosion pressure and maximum rate of pressure rise) of n-hexane/air mixtures, experiments were performed in a cylindrical vessel at 100 kPa, 353 K, with equivalence ratios of 0.8–1.7 and hydrogen addition range from 0% to 80%. Concurrently, flame images were captured by high-speed schlieren photography to study the burning performance. The results indicate that both the explosion pressure and maximum pressure rise rate increase with the increase in hydrogen addition in terms of the lean n-hexane/hydrogen/air mixtures. With respect to the richer mixtures, however, the inverse tendency is observed. With increasing hydrogen fractions, the explosion pressure and maximum pressure rise rate decrease. The peak values of the explosion pressure and maximum pressure rise rate shift to the leaner mixture with increased hydrogen proportion. Moreover, the laminar burning velocities of n-hexane/hydrogen/air mixture were also obtained via the expanding spherical method and the pressure-time histories, respectively. Variation of laminar burning velocity with hydrogen proportion from both methods were studied as well, and the results show that the laminar burning velocity changes significantly under different hydrogen addition.     

A study of numerical hazard prediction method of gas explosion

A 3-dimensional computational fluid dynamics (CFD) simulation of a premixed hydrogen/air explosion in a large-scale domain is performed. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent burning velocity that characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent burning velocity is evaluated by analyzing the experimental data of hydrogen/air premixed explosion. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement. The effect of mesh size on simulated flame propagation velocity is also tested, showing that the numerical result agrees reasonably well with experiment when the mesh size is less than about 20 cm.     

Effect of initial turbulence on vented explosion overpressures from lean hydrogen–air deflagrations

To examine the effect of initial turbulence on vented explosions, experiments were performed for lean hydrogen–air mixtures, with hydrogen concentrations ranging from 12 to 15% vol., at elevated initial turbulence. As expected, it was found that an increase in initial turbulence increased the overall flame propagation speed and this increased flame propagation speed translated into higher peak overpressures during the external explosion. The peak pressures generated by flame–acoustic interactions, however, did not vary significantly with initial turbulence. When flame speeds measurements were examined, it was found that the burning velocity increased with flame radius as a power function of radius with a relatively constant exponent over the range of weak initial turbulence studied and did not vary systematically with initial turbulence. Instead, the elevated initial turbulence increased the initial flame propagation velocities of the various mixtures. The initial turbulence thus appears to act primarily by generating higher initial flame wrinkling while having a minimal effect on the growth rate of the wrinkles. For practical purposes of modeling flame propagation and pressure generation in vented explosions, the increase in burning velocity due to turbulence is suggested to be approximated by a single constant factor that increases the effective burning velocity of the mixture. When this approach is applied to a previously developed vent sizing correlation, the correlation performs well for almost all of the peaks. It was found, however, that in certain situations, this approach significantly under predicts the flame–acoustic peak. This suggests that further research may be necessary to better understand the influence of initial turbulence on the development of flame–acoustic peaks in vented explosions.     

Numerical simulation of airflow characteristics during the loss of vacuum accident of CFETR

During a loss of vacuum accident (LOVA) of China Fusion Engineering Test Reactor (CFETR), the high velocity airflow will result in the migration, re-suspension even explosion of the radioactive dust deposited in the vacuum vessel (VV) during plasma burning. In addition, large amounts of hydrogen may be produced from the exothermic reaction between tungsten and water/steam. In order to minimize the risk of explosion and the leakage of radioactivity, the airflow characteristics must be well studied at the first step. In this paper, a break due to a failed component is assumed to take place at the equatorial port. The numerical simulation has been performed by using ANSYS CFX code with a simplified VV model of CFETR. The airflow field during LOVA has been studied in detail under different initial pressure and first wall (FW) temperature. The results show that the FW temperature significantly affects the pressurization process. Besides, the friction velocity is also greatly affected by the FW temperature and initial pressure. From the 3D airflow distribution, the velocity near the lower part of torus can reach up to 87.4 m/s. This study provides indispensable basis for the research about dust resuspension, migration, dust/hydrogen explosion and the radiological safety of CFETR.     

Correlation of turbulent burning velocities of ethanol–air, measured in a fan-stirred bomb up to 1.2 MPa

The turbulent burning velocity is defined by the mass rate of burning and this also requires that the associated flame surface area should be defined. Previous measurements of the radial distribution of the mean reaction progress variable in turbulent explosion flames provide a basis for definitions of such surface areas for turbulent burning velocities. These inter-relationships. in general, are different from those for burner flames. Burning velocities are presented for a spherical flame surface, at which the mass of unburned gas inside it is equal to the mass of burned gas outside it. These can readily be transformed to burning velocities based on other surfaces.The measurements of the turbulent burning velocities presented are the mean from five different explosions, all under the same conditions. These cover a wide range of equivalence ratios, pressures and rms turbulent velocities for ethanol–air mixtures. Two techniques are employed, one based on measurements of high speed schlieren images, the other on pressure transducer measurements. There is good agreement between turbulent burning velocities measured by the two techniques. All the measurement are generalised in plots of burning velocity normalised by the effective unburned gas rms velocity as a function of the Karlovitz stretch factor for different strain rate Markstein numbers. For a given value of this stretch factor a decrease in Markstein number increases the normalised burning velocity. Comparisons are made with the findings of other workers.     

Flame propagation behaviors and influential factors of TiH2 dust explosions at a constant pressure

The flame propagation through TiH₂ dust cloud at near constant pressure condition is studied in a series of experiments using an apparatus with transparent latex balloons. The influential factors for the combustion performance of TiH₂ dust cloud, including dust concentration, particle size, scale of isobaric space and oxygen content are investigated. Results show that the burning velocity increases with dust concentration in the fuel-lean mixtures, and then plateaus after crossing the stoichiometric condition, while the trend of flame speed changing with dust concentration varies for different mean particle sizes (D₅₀) of 48 and 106 μm. The flame propagation speed of dust cloud is positively correlated to the isobaric space scale and oxygen content. The burning mechanism of TiH₂ dust is thought to be mainly controlled by diffusion regime, the appearance of hydrogen gas accelerates the combustion rate of TiH₂ particles and also makes the TiH₂ dust changed from a discrete media to a continuum, which may account for the phenomenon that the flame speed in dust cloud of TiH₂ is larger than that of Ti at the same concentration no matter in air or oxygen atmosphere.     

Suppression of hydrogen/oxygen/nitrogen explosions by fine water mist containing sodium hydroxide additive

Complementary sets of experiments, consisting of burning velocity measurements and vented explosion tests, have been undertaken for a wide range of hydrogen–oxygen–air test compositions using fine water mist with NaOH additive (SMD ∼ 4 μm). In contrast to pure water mists, burning velocity measurements identified a critical mist concentration (for a given gas composition) above which a sudden large decrease in burning velocity is observed. The critical concentration was also found to correspond to an inerting concentration during vented explosion testing. Prior to reaching the critical concentration, the NaOH additive had a negligible effect on both the burning velocity measurements and explosion tests. This clearly indicates that the NaOH additive is acting as a chemical inhibitor. The inhibiting effect is generally considered to occur due to homogeneous gas phase mechanisms and it is thought likely that only the fraction of the entire mist (with droplet diameter < 2.5 μm) would evaporate sufficiently quickly to allow vaporised NaOH to take part in the inhibition. The experimental data obtained have enabled the construction of an inerting map to facilitate the design of a practical mist inerting system.     

Combustion behaviors and explosibility of suspended metal hydride TiH2 dust

In the production and storage processes of metal hydride material of TiH₂, there are at least three kinds of explosion hazards, for example, TiH₂ dust explosion, H₂ explosion and hybrid H₂/TiH₂ dust explosion. In this study, combustion behaviors of TiH₂ dust cloud under isobaric and isochoric conditions were studied using a visual dust combustion facility and a standard 20-L spherical explosion vessel bomb, respectively, and Ti dust and hybrid H₂/Ti dust were used as the reference materials. Experimental results showed that at equal dust concentrations, the flame propagation speed S_f, burning velocity S_L, maximum pressure rise P_ex and maximum rate of pressure rise (dP/dt)_ex of TiH₂ dust were all higher than those of Ti dust, while much smaller than those of hybrid H₂/Ti dust except the maximum pressure rise P_ex. The hydrogen state and content were the primary factors for the combustion differences of dust explosions. The values of explosion index K_st showed that the explosion risks of these samples increased as follows: Ti ˂ TiH₂ ˂ hybrid H₂/Ti dust.     

Study of interaction of entrained coal dust particles in lean methane–air premixed flames

This study investigates the interaction of micron-sized coal particles entrained into lean methane–air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0–25 μm, 53–63 μm, and 75–90 μm, is used. The effects of different coal dust concentrations (10–300 g/m³) entrained into the mixture of methane–air at three lean equivalence ratios, ?, of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust–methane–air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53–63 μm and 75–90 μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0–25 μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.     

Turbulent combustion evolution of stoichiometric H2/CH4/air mixtures within a spherical space

Turbulent combustion evolutions of stoichiometric H₂/CH₄/air mixtures were experimentally studied within a spherical constant-volume combustion vessel. A series of initial turbulent ambience (with the range of turbulence intensity from 0 to 1.309 m/s) and a series of hydrogen volumetric fraction (with the range from 0.3 to 0.9) were taken as the variables to studied the influences of turbulence intensity and the fuel composition on the turbulent combustion evolutions. The evolutions of explosion overpressure were studied upon the variations of maximal pressure, the influences of turbulence intensity mainly located at heat loss while the influences of fuel composition mainly located at adiabatic explosion. Subsequently, the evolutions of burnt mass were discussed, the competition between pressure rising and temperature rising induced by the heat release during combustion was considered as major influence mechanism. Then, the nexus between burning velocity and the related burnt mass rate were discussed, the variations regulations of maximal burning velocity brought by turbulence intensity and hydrogen volumetric fraction were analysed. Finally, the nexus between maximum burning velocity and heat loss was discussed.