Critical diameter and transverse waves of powder combustion

A critical review is given of the problem of powder extinction with a decrease in the sample diameter due to heat release. The results of our experimental studies of the critical combustion diameter are presented. A comparison of the experimental data on the critical diameter as a function of burning rate is shown to be the most informative. These functions follow a power law with an exponent of −1.15 to −1.17. The relations between the sizes of the cells (hot spots) formed by the set of transverse waves on the burning surface and the burning rate follow the same law. The sizes of the cells are 2.1–2.3 times smaller than the critical combustion diameter. It is also found that the transverse wave decays if its curvature exceeds a critical value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 72–78, March–April, 2007.     

Burning and dripping behaviors of polymers under the UL94 vertical burning test conditions

An experimental setup was constructed to record the real‐time mass data of eight pure polymers under the UL94 vertical burning test conditions. The experiments showed that the flame rises up to the clamp or the dripping occurs soon for the pure polymers. The mass burned before the flame reaching the clamp and the dripping occurrence only accounts for a small fraction of the original mass of the specimen, which differentiates the UL94 test from the cone calorimeter test. The mass loss rate of polymer specimens is in the magnitude order of 0.001–0.01 g/s. It was also found that the flame of thin specimens usually reaches the clamp sooner than that of thick specimens. Apart from the dripping behaviors found in large‐scale fires, it is found that the diameter of the first drop for the tested polymers is in the range of 2.0–10.0 mm. The mass of the first drop increases with the first dripping time. The first dripping time and the mass of the first drop increase with the thickness of the specimen, especially for polymers of large‐size dripping type. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of liquid film vaporization on natural convection heat and mass transfer in a vertical tube

A numerical analysis was carried out to investigate the effects of film vaporization on natural convection heat and mass transfer in a vertical tube. Results for interfacial Nusselt and Sherwood numbers are presented for air-ethanol and air-water systems for various conditions. Predicted results show that heat transfer along the gas-liquid interface is dominated by the transport of latent heat in association with the vaporization of the liquid film. Additionally, the predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Chang et al. (1986) and Yan and Lin (1990) is only valid for systems with small liquid mass flow rates. For systems with a high liquid film Reynolds number, Re₁₀, the assumption of an extremely thin film is seriously in error.     

垂直圆管内掺氢甲烷燃烧不稳定性研究

为了研究垂直圆管内掺氢甲烷燃烧的不稳定性,自行搭建了上端为开口下端为闭口的透明圆形燃烧管道（半径r=\mathrm{30}\mathrm{m}\mathrm{m},管长L=\mathrm{600}\mathrm{m}\mathrm{m}）,火焰在开口端被点燃,向闭口端传播,在化学当量比条件下,通过改变氢气体积分数进行实验。结果表明,当氢气体积分数\gamma \ge50%后,火焰在传播过程中出现了数量众多的小尺寸胞状结构,并逐渐演变为平滑的弯曲火焰锋面结构,而在50%的工况中没有出现此现象;对初级不稳定性振荡以及次级不稳定性振荡出现的原因进行了分析,火焰表面积的快速变化是形成初级不稳定性振荡的主要原因,燃烧过程中产生的有限振幅的声学振荡是次级不稳定性振荡产生的原因;对不同工况反应过程组分流量的敏感性分析可以得出,链式反应R1（H+O₂\stackrel{}{̿}O+OH）为促进燃烧反应速率的主导链式反应;火焰传播过程中的最大压力出现在次级不稳定性振荡阶段,由于开口声压损失以及开口预混未燃气体损失,最大压力的数值随氢气体积分数的增加而减小。     

垂直圆管内掺氢甲烷燃烧不稳定性研究

为了研究垂直圆管内掺氢甲烷燃烧的不稳定性,自行搭建了上端为开口下端为闭口的透明圆形燃烧管道（半径r=\mathrm{30}\mathrm{m}\mathrm{m},管长L=\mathrm{600}\mathrm{m}\mathrm{m}）,火焰在开口端被点燃,向闭口端传播,在化学当量比条件下,通过改变氢气体积分数进行实验。结果表明,当氢气体积分数\gamma \ge50%后,火焰在传播过程中出现了数量众多的小尺寸胞状结构,并逐渐演变为平滑的弯曲火焰锋面结构,而在50%的工况中没有出现此现象;对初级不稳定性振荡以及次级不稳定性振荡出现的原因进行了分析,火焰表面积的快速变化是形成初级不稳定性振荡的主要原因,燃烧过程中产生的有限振幅的声学振荡是次级不稳定性振荡产生的原因;对不同工况反应过程组分流量的敏感性分析可以得出,链式反应R1（H+O₂\stackrel{}{̿}O+OH）为促进燃烧反应速率的主导链式反应;火焰传播过程中的最大压力出现在次级不稳定性振荡阶段,由于开口声压损失以及开口预混未燃气体损失,最大压力的数值随氢气体积分数的增加而减小。     

电极直径对EHD强化垂直管内凝结换热的影响

基于EHD强化管内凝结换热的理论分析,对5根直径不同的电极进行了垂直管内EHD强化凝结换热实验研究.结果表明,相同工况条件下,电极直径越大液膜相对减薄率越大,液膜减薄的起始电压越小.液膜相对减薄率随着电压的增加而增加.同时发现,液膜相对减薄率大并不意味着强化系数大.     

电场对甲烷-空气混合气燃烧特性的影响

利用定容燃烧弹研究了外加电场对甲烷-空气混合气燃烧特性的作用。加载电压为0、-5、-10和-12 kV,混合气的过量空气系数λ分别为0.8、1.0和1.6,代表浓燃、当量比和稀燃三种混合气状态。结果表明,电场方向上的火焰发展半径和火焰传播速度明显增加,原先近似圆球形的火焰形状发生变化。特别是稀燃混合气,电场作用下火焰的传播状况增加最为显著,火焰近似呈圆柱形。在加载电压为-12 kV时,对于λ=0.8、1.0和1.6的混合气,火焰拉伸速度最大值分别增加了42.3%、29.7%和111.7%。同时,混合气燃烧压力的发展明显加快,压力峰值出现的时间明显提前。对于浓燃和当量比浓度的混合气,压力峰值变化不大,而稀燃混合气的压力峰值增加较为明显。在-12 kV电压作用下,浓燃、当量比和稀燃三种状态下混合气的压力峰值时间提前了13.4%、7.5%和24.6%,相应的燃烧压力峰值增大了2.2%、1.0%和8.1%。本文应用外加电场对燃烧火焰产生的离子风效应及其对传播火焰形成的拉伸作用对试验结果进行了说明。     

Numerical simulation of transient natural convection induced by electrochemical reactions confined between vertical plane Cu electrodes

A transient natural convection develops along vertical plane Cu electrodes during the galvanostatic electrolysis. The time variations of both the concentration profile of CuSO₄ and the velocity profile of a 0.6 M CuSO₄ aqueous electrolyte were measured. The mass transfer rate of Cu²⁺ ion as well as the natural convective fluid motion was analyzed. The effect of electrode spacing on the transient natural convection was focused. The overshooting phenomenon caused by the transient diffusive and convective mass transfer rate was observed. Numerical simulation may provide as better understanding of the phenomena.     

Investigation of the thermal behavior effect of the surrounding material environment on the swirling flame structure

This paper is focusing on the numerical simulation of a swirling flame, resulting from the interaction of multiple fires, evolving in a free and unlimited environment. A typical system, formed by a central fire source surrounded by four heat sources, is used. Since the thermal characteristic of the surrounding sources is the main engine for the rotation of flame, a detailed study is performed by varying the heating flux of these sources. This study shows that an increase of the heating flux of surrounding sources has as a result an intensification of the penetrating air puffs through the openings between the surrounding four heat sources. These puffs tangentially drive the central flame, thus producing a marked improvement on the angular momentum. Moreover, this study shows that the flame height is strongly affected by the flame rotation. Moreover, two different aspects of the flame height evolution are observed from the flow visualization and the thermal and dynamic fields for the different cases studied.     

Experimental studies on characteristics of fire whirl in a vertical shaft

An internal fire whirl can be generated readily in a tall shaft model with appropriate gap width at one corner. Experimental study was carried out to investigate the relationship between the characteristics of an IFW and the corner gap width in a 9‐m‐tall vertical shaft model. The vertical shaft had a 2.1 m by 2.1 m square section with gasoline pool fire of different diameters burning inside. The gap width was varied to investigate its impact on fire whirl characteristics, such as flame development, swirling intensity, flame height, flame temperature, and heat release rate of the gasoline pool fire. Vigorous flame swirling motions were generated when the ratio of the gap width to the shaft section perimeter was within the range 0.16 to 0.21. From the flame streamline angle, it was observed that the swirling component was much stronger than buoyancy component near the bottom of burning region. The swirling component decreased and became roughly the same as buoyancy near the middle. Finally, it diminished to being much weaker than buoyancy near the top of the fire. These observations suggest that the Froude number Fr decreased from a large number to 1, and then continued to decrease to 0.     

Experimental research on the burning behavior of dragon juniper tree

Tree crown fire is one of the extreme fire behaviors in the wildland‐urban interface. This paper presents an experimental study on the burning behaviors of single and triple dragon juniper trees. The mass loss, flame height, plume temperature, radiation, and fire interaction are measured. It is found that the foliage moisture content and flame mergence dominate the mass consumed percentage (defined as the ratio of the total mass loss to initially total mass), while the tree crown height and flame mergence determine the flame height. The peak mass loss rate is mainly affected by the moisture content and tree species. For triple tree fires, the peak mass loss rate is also affected by the spacing due to the coupled effects of heat feedback enhancement and air entrainment restriction. Results also show that the flame height significantly increases as the spacing decreases. The spacing holds a significant effect on the fire plume temperature distribution and thermal radiation field. Empirical correlations are also developed for the flame height, radial temperature, and radiant heat flux distribution based on physical interpretation of the tree burning behaviors.     

Experimental investigation on the influence of annular pool shape characteristics on n-heptane ring fires

The ring fire usually originates from the combustion of the annular-distributed fuel, and it may occur in the low-temperature environments such as the Arctic area and plateau area, which is kind of different from the conventional environment. In this paper, the annular pools with various shape characteristics were designed. The outside diameter ranged from 14.9 to 30.0 cm, the inside diameter increased from 0 to 24.5 cm, and the diameter ratio was from 0 to 0.858. A series of n-heptane ring fire experiments were carried out. The results showed that for the annular pools with the same outside diameter, the mass loss rate (MLR) per unit area presents a piecewise trend with the diameter ratio. When the diameter ratio increases from 0 to 0.7, the MLR per unit area increases linearly because of the domination of air entrainment effect. When the diameter ratio is larger than 0.7, the MLR per unit area is independent of the annular shape due to the strong heat dissipation effect. Finally, a parameter named the outside circumference per unit area is put forward to characterize the annular pool. A corrected pressure modeling is established to normalize the ring fire with various shape characteristics.     

The effect of column diameter and bed height on minimum fluidization velocity

Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semicorrelated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Experimental study on fire behaviors of external thermal insulation composite system under vertical radiation

Using a vertical thermal radiator, we perform a set of experiments on the external thermal insulation composite system (ETICS) and pure extruded polystyrene (XPS). Several important parameters, including time to ignition, mass loss, and sample temperature, were measured. The combustion degree of XPS used during the experiments was B1 and B2. Results show that the whole burning process can be divided into 3 typical stages. Because of the protection effect of the outer layer of ETICS, the burning process of ETICS was noticeably different from that of pure XPS. The experimental results indicated that the protection effect of the outer layer weakened the difference between B1 and B2 flame‐retardant XPS. The time to ignition was increased under the effect of outer layer, while the core material (XPS) was easier to be ignited when the outer layer falls out. The research results are useful to the theoretical and numerical study on the fire characteristics of foamed polymer under vertical thermal radiation condition.     

稳定段长径比对喷嘴雾化性能的影响

利用相位多普勒粒子分析仪,对BW J(Ⅲ)型干气雾化进料喷嘴的2种新型喷头雾化特性进行了冷态实验研究。在不同的喷嘴稳定段长径比下,对喷嘴的雾化粒径及喷射角分别进行了试验测量,并对雾化粒径和喷射角随长径比的变化规律进行了分析。同时,文中介绍了新型雾化进料喷嘴的结构、冷态试验研究装置和测量设备。结果表明,稳定段长径比对于雾化粒径及喷射角有比较明显的影响,试验结论为雾化喷嘴的设计提供了一定依据。     

口模直径对PP/硅藻土复合材料流动性的影响

熔体体积流动速率(MVR)是表征聚合物流动特性的重要参数之一。应用熔体流动速率仪测定了3种粒径硅藻土(体积分数为10％)填充聚丙烯复合材料的MVR,考察口模直径和挤出工艺条件对复合体系流动特性的影响。复合材料的MVR随着口模直径的增加呈二次函数形式提高,随着填料粒径的增加而下降;当载荷及口模直径一定时,复合体系的MVR与温度呈线性函数关系;当载荷及温度固定时,复合体系的MVR与口模直径呈二次函数关系。     

Effects of oil-droplet diameter on the stability of squalene oil in spray-dried powder

ABSTRACT

Effects of the oil-droplet diameter and emulsification with polymerized sodium caseinate (PNC) on the stability of squalene oil (SQ) retention in spray-dried powder were investigated. The SQ droplet diameter significantly affected the stability of the oil in spray-dried powders. The degradation behavior of SQ powders at 105°C was correlated using the Avrami equation. This oxidation mechanism may occur because of the propagative transfer of radical oxidation between oil-droplet particles. SQ emulsified with 5?wt% PNC and small oil droplets had better oxidative stability when compared with 3?wt% sodium caseinate.     

HMX flame structure for combustion in air at a pressure of 1 atm

The chemical structure of HMX flame during combustion in air at a pressure of 1 atm was calculated using molecular beam mass spectrometric sampling. HMX vapor was recorded for the first time near the burning surface. A total of 11 species were identified in the HMX flame (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and HMX vapor), and their concentration profiles were measured. The HMX combustion was unstable. The species concentration profiles exhibit periodic pulsations related to variation in the HMX burning rate. The HMX flame structure at various distances to the burning surface was determined using the average value of the burning rate. Two main zones of chemical reactions in the flame were found. In the first zone ≈0.8 mm wide adjacent to the burning surface, HMX vapor decomposes and NO₂, N₂O, and CH₂O react with each other to form HCN and NO. In the second zone ≈0.8–1.5 mm wide, HCN was oxidized by nitric oxide to form the final combustion products. The composition of the final combustion products was analyzed. The global reaction of HMX gasification at a pressure of 1 atm was established. Heat release values in the condensed phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using literature data from microthermocouple measurements) were analyzed and compared. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 26–43, November–December, 2008.