Appearance of cool flame in flame spread over fuel droplets in microgravity

This research conducted microgravity experiments to investigate phenomena appearing around a droplet existing outside the flame-spread limit. n-Decane droplets are tethered at intersections of SiC fibers. The flame spreads to two- or three-interactive droplets to heat a droplet placed outside the flame-spread limit of the interactive droplets. The cool-flame appearance during the flame spread over droplets was detected using different methods. The droplet diameter was measured with a back illumination to evaluate the vaporization-rate constant and to judge whether the cool flame appears or not. The temperature around the droplet was measured by the thin-filament pyrometry using a near-infrared camera to detect the temperature rise due to cool-flame appearance. The infrared radiation distribution from the combustion products was measured using a mid-wave infrared camera to judge the cool-flame appearance. The results show that a cool flame appears around the droplet existing outside the hot-flame-spread limit and the vaporization completes with the cool flame if the heat input from the hot flame is sufficiently large. This type of flame spread is called hot-to-cool flame spread. The definition of flame spread should be extended considering the cool flame.     

Microgravity experiments on droplet motion during flame spreading along three-fuel-droplet array

Flame spreading along a fuel droplet array at microgravity has been studied as a simple model of spray combustion. A three droplet array with a pendulum suspender was employed to investigate interactions between flame spreading and droplet motion in the array direction. Initial droplet diameter was 0.8 mm and fuel was n-heptane. A silicon carbide pendulum suspender of 15 μm in diameter and 30 mm in length was used for the third droplet. The first fixed droplet was ignited by electric spark. Behavior of the flame and the third droplet was observed using a high-speed video camera. Dimensionless span, which is the averaged droplet span divided by the averaged initial diameter of the three droplets, was varied from 2.7 to 10. Large displacement of the movable droplet was observed after group flame grew around the movable droplet. As the initial dimensionless span increased, the averaged droplet speed after the occurrence of flame spreading to the movable droplet increased steeply, taking the maximum value around 5 in initial dimensionless span, and then decreased gradually. The movable droplet advanced toward the second droplet in small spans and moved away from the second droplet in large spans. The direction of the motion changed around 4.6 in initial dimensionless span. Flame spread induction time from the second to the third droplet increased exponentially as the initial dimensionless span was increased. The induction time of flame spreading to a movable droplet was longer than that of flame spreading to a fixed droplet. From calculations of flame spreading along a 20-droplet array, it was predicted that the droplet speed nearly converged after flame spread to the sixteenth droplet. The maximum speed of the nineteenth droplet appeared around 7.5 in the initial dimensionless span.     

Mechanics and energy analysis on molten pool spreading during laser solid forming

This paper studies the issue that the molten pool width gradually increases under some conditions during laser solid forming (LSF), which can decrease the shape and dimension accuracy of LSFed component to a large extent. By using the statics analysis method and calculating the interfacial tensions at the solid-liquid-gas triple point of molten pool, the proposed two-dimensional (2D) cross-sectional model of single deposition layer illustrates qualitatively that the deposition width would increase with the increasing pool temperature at a certain powder feeding rate, which we called the pool spread behavior here. Meanwhile, by calculating the maximum equilibrium contact angle for keeping solid-liquid-gas triple point balance, it is found that the molten pool is solidified during non-equilibrium state. Furthermore, in order to control the pool temperature and decrease pool spread amount, the optimal match of pool energy and mass inputs is determined for obtaining an optimum balance between the energy input and deposition efficiencies.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment

In this work a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment. The two-dimensional (2D) simulations show that the flame spread rates for the multiple-fuel configuration are higher than those for the flame spreading over a single fuel sheet. This is due to reduced radiation losses from the flame and increased heat feedback to the solid fuel. The flame spread rate exhibits a non-monotonic variation with decrease in the interspace distance between the fuel sheets. Higher radiation heat feedback primarily as gas/flame radiation was found to be responsible for the increase in the flame spread rate with the reduction of the interspace distance. It was noted that as the interspace distance between the fuel sheets was reduced below a certain value, no steady solution could be obtained. However, at very small interspace distances, steady state spread rates were obtained. Here, due to oxygen starvation the flame spread rate decreased and eventually at some interspace distance the flame extinguished. With fuel emittance (equal to absorptance) reduced to ‘0’ the flame spread rate was nearly independent of the interspace distance, except at very small distances where the flame spread rate dropped due to oxygen starvation. A flame extinction plot with the extinction oxygen level was constructed for the multiple-fuel configuration at various interspace distances. The default fuel with an emittance of 0.92 was found to be more flammable in the multiple-fuel configuration than in a single fuel sheet configuration. For a fuel emittance equal to zero, the extinction oxygen limit decreases for both the single and the multiple fuel sheet configurations. However, the two flammability curves cross over at a certain fuel separation distance. The multiple-fuel configurations become less flammable compared to the single fuel sheet configuration below a certain separation distance.     

Numerical study on flame spread of an n-decane droplet array in different temperature environment under microgravity

A series of numerical calculations of flame spread of an n-decane droplet array was conducted at different ambient temperatures (T_a = 300 and 573 K) for S/d₀ from 1.5 to 10, where S is the droplet interval and d₀ is the initial droplet diameter. The authors compared these numerical results with experimental results under similar conditions at different ambient temperatures for the first time in this study. Good qualitative agreement in flame spread behavior between numerical results and microgravity experiments is obtained. Flame spread mode changed with an increase in S/d₀. Also, appearance of the flame spread mode in a stepping-stone manner (Mode III in [Jpn. Soc. Mech. Eng. 68 (672) (2002) 2423]) in a normal temperature environment was verified by numerical calculations and microgravity experiments, although it was not predicted in the theoretical analysis. In addition, good qualitative agreement of flame spread rate V_f versus S/d₀ was obtained between numerical and experimental results, although numerical results were at least twice as large as experimental results. V_f had a maximum peak at a specific S/d₀ for a different ambient temperature. Employment of improved reaction model and consideration for thermal radiation heat transfer are expected to produce quantitatively better results. An increase in surface temperature of unburned droplets and the development of a flammable gas layer around the droplets were promoted in a high-temperature environment, due to an increase in heat transfer from ambient air to the droplet. As a result, V_f was increased by the higher ambient temperature, suggesting that ambient temperature plays a significant role both in the flame spread mode and the flame spread rate through promotion of a flammable gas layer around unburned droplets.     

高压Ni77P23非晶合金自由体积变化的同步辐射研究

利用同步辐射高能X光散射的方法,研究了室温下非晶合金Ni77P23的自由体积的变化所引起的压缩行为的变化规律,通过傅里叶变换得到不同压力下的径向分布函数,并由此获得了不同压力下,该非晶合金的配位数、近邻原子间距等原子构型的结构信息. 研究表明,至直30.5GPa压力,Ni77P23合金仍保持稳定的非晶结构,根据Bridgman方程通过拟合数据,得到状态方程为 -ΔV/V0=0.08606P-3.2×10-4P2+5.7×10-6P3.     

高压Ni77P23非晶合金自由体积变化的同步辐射研究

利用同步辐射高能X光散射的方法，研究了室温下非晶合金Ni₇₇P₂₃的自由体积的变化所引起的压缩行为的变化规律，通过傅里叶变换得到不同压力下的径向分布函数，并由此获得了不同压力下，该非晶合金的配位数、近邻原子间距等原子构型的结构信息. 研究表明，至直30.5GPa压力，Ni₇₇P₂₃合金仍保持稳定的非晶结构,根据Bridgman方程通过拟合数据，得到状态方程为-ΔV/V₀=0.08606P-3.2×10^-4P²+5.7×10^-6P³. 关键词： 非晶合金 自由体积 同步辐射     

交联聚乙烯电缆绝缘材料中电树枝的导电特性研究

利用光学显微观察、局部放电测量和共聚焦Raman光谱分析相结合的方法, 研究了交联聚乙烯(XLPE)电缆绝缘材料中两种典型电树枝的导电特性.尽管具有相似的培养条件, 两种电树枝却呈现出完全不同的形态,其中9 kV下典型电树枝为枝-松枝状, 11 kV下为枝状, 而且电树枝生长及局部放电规律呈现出明显的差异.枝-松枝状电树枝主干通道内存在无序石墨碳的沉积, 根据石墨碳G带与D带的相对强度,估算碳层厚度约为8 nm,树枝通道单位长度电阻小于 10 Ω· μm^-1,足以抑制电树枝内局部放电的发展,电树枝呈现出导电型电树枝特征. 枝状电树枝通道内观察到荧光背景,存在材料劣化的产物,但不存在无序石墨碳的聚集, 通道具有明显的非导电特性而不足以抑制电树枝内局部放电的连续作用. 最后提出了XLPE电缆绝缘材料中导电型和非导电型电树枝的单通道生长模型, 利用等效电路理论对XLPE电缆绝缘材料中两种不同导电特性电树枝的生长机理进行了探讨.     

Effect of ordering on the volume change and the shape memory effect in a Fe3Pt alloy during martensitic transformation

The effect of heat treatment on the volume effect ΔV/V and the shape recovery during the martensitic transformation in a Fe-24 at % Pt alloy is studied. An increase in the annealing time from 0 to 300 min at 650°C is found to decrease ΔV/V gradually from 1.26% for the quenched state to 0.987% for τ = 300 min. The characteristic temperatures of the martensitic transformation decrease and the transformation hysteresis passes through a minimum. The shape memory effect in heat-treated samples reaches 100% after gT = 40 min.     

Experimental study on downward/opposed flame spread and extinction over electric wires in partial gravity environments

Downward/opposed flame spread over laboratory wire samples under varied gravity conditions were investigated in the range from 0 G to 1 G. Reduced gravity experiments are conducted by parabolic flights of an airplane. Limiting oxygen concentrations (LOCs) and flame spread rates ($V_{\mathrm{f}}$) are obtained as a function of gravity level, with oxygen concentration, forced flow velocity, and wire characteristics such as insulation thickness and core material as experimental variables. The samples used in this study consist of low-density polyethylene (LDPE) insulation over metallic cores. Copper (Cu) and nickel-chrome (NiCr) were selected as core materials. It is found that the effect of gravity on the insulation flammability varies with the thermal conductivity of the wire core; the LOCs of the Cu sample are less affected by gravity, while those of the NiCr sample decrease with decreasing gravity level. On the other hand, $V_{\mathrm{f}}$ increase monotonically with increasing gravity level in the Cu sample, while $V_{\mathrm{f}}$ of the NiCr sample show a peak value under the low gravity conditions. It is suggested that these differences in the response of LOCs and $V_{\mathrm{f}}$ to the gravity level due to the difference in core materials are controlled by the fuel concentration in the reaction zone, which is a function of $V_{\mathrm{f}}$. It is also found that the molten LDPE produced during the flame spread process shows unique behaviors depending on the gravity levels and wire characteristics. Some characteristics of the dynamic motion of the molten LDPE during the flame spread process, such as deformation and dripping, are also summarized in this paper. The experimental data obtained in this study provide useful information on the flammability of materials in a partial gravity environment and will serve as a database for fire safety design in future space exploration.     

矢量场的散度在运动空间区域中的体积分随时间变化率的严密理论

给出运动空间区域中充满介质的情况下,以及运动空间区域中包含有介质和真空的情况下,矢量场的散度在运动空间区域中的体积分随时间变化率的两种计算方法.阐明了若以一种特别的方法来看待空间积分区域的变动,那么两种情况就可以用统一的方式来计算.指出在运动空间区域中包含有介质和真空的情况下,如果保留速度v在积分区域V的边界面S上的值和实际情况一样,而赋予区域V以一个其空间变化率连续或者分块连续的新的速度场v(r,t),那么锁定体元的计算方法仍然适用.     

吸气剂在高真空多层绝热LNG槽车内吸附性能研究

为了利用吸附等温线理论模型模拟高真空多层绝热LNG槽车内吸气剂的吸附过程,对在真空技术中常用的等温式进行了分析研究。通过实验获得吸气剂在室温下的吸氢等温线,同时利用实验数据得到考虑分子间作用力的BET模型中的参数值。将模型结果与实验数据进行对比分析,结果表明:考虑分子间作用力的BET模型不能用作高真空多层绝热LNG槽车真空夹层内吸气剂吸附量的计算。利用Temkin和Freundlich等温式分段对实验数据进行拟合,对于1#吸气剂,拟合方程式计算得出的吸附量与实验数据的相对误差仅为2.6%,表明拟合方程能够对吸气剂在室温下的吸氢量进行准确预测。     

How does brain MRI lesion volume change on serial scans in patients with multiple sclerosis?

Although lesion load changes on conventional T₂-weighted brain magnetic resonance imaging (MRI) scans from patients with multiple sclerosis (MS) are used to monitor the effect of treatment, there is no clear definition of how lesion load changes over years according to the lesion load present at a baseline evaluation. In the present study, we evaluated the relationship between lesion load changes over time and lesion load at a baseline evaluation in a group of untreated patients with MS. We scanned nineteen patients on two separate occasions with a mean interval 16.4 months between the two examinations. In each scanning session, a scan with forty contiguous 3-mm-thick axial slices was acquired. We assessed MRI lesion loads using a semi-automated local thresholding technique. Both a linear (p < 0.0001) and a quadratic component (p = 0.0008) of the baseline volume were significant in describing the follow-up volume. The equation to model this finding was as follows: V_f = β₀ V_b + β₁ (V_b)², where V_f is the lesion volume at follow-up, V_b is the lesion volume at baseline, β₀ = 0.834 (SE = 0.098), and β₁ = 0.014 (SE = 0.003) (mL)⁻¹. Our data indicate that lesion volume changes detectable on serial brain MRI studies from patients with MS are dependent on the extent of lesion burden present on the baseline MRI scans. This finding has to be considered when planning phase III trials.     

Role of the rate of change of Total Column Ozone during different seasons on the prediction of Indian summer monsoon rainfall over Gangetic West Bengal, India

A critical analysis of Total Column Ozone (TCO) over Dum Dum, West Bengal, India is done during different seasons for the period of 1997 to 2005. It is confirmed that pre-monsoon rate of change of TCO over Dum Dum is highly correlated with Gangetic West Bengal, India monsoon rainfall of same year and that of TCO during other seasons are independent of monsoon rainfall over Gangetic West Bengal. It is also concluded that pre-monsoon rate of change of TCO may play an important role to forecast monsoon rainfall of a particular year over Gangetic West Bengal. It is also showed that other parameters affecting monsoon rainfall are insignificant w.r.t. TCO variations. A possible explanation based on chemical kinetics of O₃ are also presented.     

Variable fluid properties and variable heat flux effects on the flow and heat transfer in a non-Newtonian Maxwell fluid over an unsteady stretching sheet with slip velocity

The effects of variable fluid properties and variable heat flux on the flow and heat transfer of a non-Newtonian Maxwell fluid over an unsteady stretching sheet in the presence of slip velocity have been studied. The governing differential equations are transformed into a set of coupled non-linear ordinary differential equations and then solved with a numerical technique using appropriate boundary conditions for various physical parameters. The numerical solution for the governing non-linear boundary value problem is based on applying the fourth-order Runge-Kutta method coupled with the shooting technique over the entire range of physical parameters. The effects of various parameters like the viscosity parameter, thermal conductivity parameter, unsteadiness parameter, slip velocity parameter, the Deborah number, and the Prandtl number on the flow and temperature profiles as well as on the local skin-friction coefficient and the local Nusselt number are presented and discussed. Comparison of numerical results is made with the earlier published results under limiting cases.     

我国北方不同类型下垫面地表反照率特征

地表反照率反映了地面对太阳辐射的反射能力,在地表能量平衡中起着非常重要的作用,本文利用"中国北方协同观测"的资料,分析了不同下垫面10个站点7-9月的反照率变化特征和对降水的响应,并讨论了榆中、通榆草地、通榆农田、密云、玛曲和锦州站地表反照率与太阳高度角和土壤湿度的关系,结果表明:反照率的相对关系是东北干旱草甸西北高寒草甸西北绿洲农田黄土高原自然植被东北半干旱草甸东北农田东北果林,并且反照率基本上和降水对应,降水量大的站点反照率小,降水量小的站点反照率大,反照率的日变化曲线分为早晚对称型,早上比较大、中午下午比较小和晚上最大、早上最小的不对称型三种,选取的6个观测站反照率的谷值一般出现在降水日,峰值一般出现在没有降水、太阳辐射比较强的日子,地表反照率随着太阳高度角的增大而减小,当太阳高度角大于40°时,地表反照率基本不变,榆中和通榆农田地表反照率随着土壤湿度增大减小较明显,其次是通榆草地,玛曲、密云和锦州地表反照率随着土壤湿度变化不明显。     

小体积比两相分离早期过程的三维格子气模型研究

采用Monte Carlo模拟方法研究了小体积比三维格子气模型中的相分离过程, 比较了sc, fcc, bcc 等3种晶格类型和T/T_c=0.45–0.85等 5个相对温度的系统演化行为, 计算了系统的结构因子函数, 发现结构因子在相分离早期不满足标度关系, 生长指数等于1/6, 小于经典Lifshitz-Slyozov理论的值. 关键词： 格子气模型 相分离过程 Monte Carlo模拟 生长律     

温度对星敏感器光学系统像面位移的影响研究

光学系统的温度变化会使系统产生像面位移。根据镜面材料特性公式,选用铝合金作为星敏感器的镜筒,透镜组选用ψ值大的材料,并进一步通过I-DEAS软件进行了计算验证。在此基础上根据矩阵光学理论和温度的变化推导出了含有多个光学器件的星敏感器像面位移计算公式,并运用该公式对一个特定的星敏感器光学系统的像面位移进行了计算。结果发现,光学系统从一个温度均匀分布状态变到另一个温度均匀分布状态的温差越大,像面位移量的绝对值越大。同时还发现,对此星敏感器成像精度影响最大的透镜是第4块、第5块和第6块,明显地减少了在热补偿条件下的系统像面位移。