Furnace Temperature Curve Optimization Model Based on Differential Evolution Algorithm

When soldering electronic components onto circuit boards,the temperature curves of the reflow ovens across different zones and the conveyor belt speed significantly influence the product quality.This study focuses on optimizing the furnace temperature curve under varying settings of reflow oven zone temperatures and conveyor belt speeds.To address this,the research sequentially develops a heat transfer model for reflow soldering,an optimization model for reflow furnace conditions using the differential evolution algorithm,and an evaluation and decision model combining the differential evolution algorithm with the Technique for Order Preference by Similarity to Ideal Solution(TOPSIS)method.This approach aims to determine the optimal furnace temperature curve,zone temperatures of the reflow oven,and the conveyor belt speed.

ISOCHRONOUS STRESS-STRAIN CURVES OF LOW ALLOY STEEL CROSS-WELD-SPECIMEN AT HIGH TEMPERATURE

In this work, a parametric approach is presented and utilized to determine the creep properties of weldments; then the model of creep strain for cross weld specimen is given. On the basis of the experimental results, attempt has been made to establish equations of the isochronous stress-strain for weld joint that can predict the function of loading and service time in use of the creep data of base metal and weld metal.

Influence of Firing Temperature on Properties of Gun Propellants

Initial firing temperatures play an important role on the combustion rate of propellant. In gun propellants, initial temperature is a key factor for both accuracy and safety. Ideally, the initial temperature of the propellant should not influence the ballistic properties of the round. Nevertheless, constant initial temperature coefficients can not be achieved easily. This work focuses on the influence of the firing temperature on the ballistic properties, the mechanical integrity and the sensitivity to impact of nitrocellulose based propellants. Combustion rates have been determined by closed vessel tests. Ballistic properties have been investigated by firing 5.56 cartridges. The propellants have been conditioned at temperatures ranging from -54 ℃ to ＋71 ℃ before firing. The largest temperature coefficient is observed at high temperatures. The temperature sensitivity of the peak pressure in the combustion chamber can not be fully explained by the results from the closed vessel test. The authors speculated that the mechanical behaviour of the propellant grains at low temperatures influences also the overall ballistic properties of the round. Impact tests with propellants conditioned at low and high temperatures permit to investigate their mechanical strength under extreme temperatures and to better understand the propellant performance during firing. Tests on aged propellants have been conducted as well.

The Effects of Firing Temperatures on the Performance of Insulating Firebricks Containing Different Proportions of Alumina and Sawdust

This work studied the effects of firing temperatures on the refractory properties of insulating firebricks produced from a blend of hydrometallurgically purified clay, high alumina cement and sawdust. Twenty grams out of a bulk (1000 Kg) of clay obtained from Ipetumodu in Nigeria was analyzed for size range, consequent upon which the remaining bulk was sieved to 100 μm, being the average size. The bulk was there after leached under a predetermined condition (1.6 mol/dm3 of oxalic acid at 70oC for 150 min and 200 rev/min agitation speed) and cylindrical samples (5 cm diameter by 5 cm high) containing different quantities of high alumina cement (5% - 20%) and sawdust (1% - 5%) were prepared, dried at 110oC and subsequently fired at 900oC, 1100oC, 1300oC and 1500oC, at the rate of 4oC/min and soaked for 2 hrs. These samples were subjected to different refractory tests (permanent linear change, modulus of rupture, bulk density, cold crushing strength and apparent porosity). Even though samples containing more than 20% alumina crumbled at elevated temperatures, it was still observed that the bricks performed to expectations at lower alumina contents, even at 1500oC. The sample containing 3% sawdust and 10% alumina cement however, gave the desired requirement for preparing good insulating firebricks with reliable phase integrity, as revealed by scanning electron microscopy (SEM).

Thermodynamic Characterization of Planar Shapes and Curves, and the Query of Temperature in Black Holes

The purpose of this research is to characterize shapes in thermodynamic terms, namely, in terms of total energy, dissipative energy, entropy, and temperature. As case studies, polygons and some well-known curves were taken, and they were characterized using physical terms. The relation between entropy and curvature was elucidated, and the black hole surface gravity and temperature were criticized and reinterpreted from this point of view. Particular energy attributions were evaluated by comparing the position of any edge of a polygon (i.e. its angle with the horizontal axis) with a broken crystal surface. Energies of all edges were added up at all positions between 0˚ - 360˚. In regular polygons, the total energy decreases with the increase of the number of edges. Entropy increases in the reverse order, and the increase of the number of edges increases entropy. It implies that the circle has the lowest energy but the highest surface entropy. In curves (circle, sine-curve, spiral, and exponential curve), the total energy, dissipative energy, and entropy all depend on amplitude and also on specific variables. Black hole entropy expressed in terms of the surface area is a configurational entropy and not thermal entropy;therefore, it does not involve a varying temperature term. The surface gravity of a black hole is connected to acceleration and thus to curvature. To relate it with the temperature needs to be reinterpreted, because, surface gravity behaves like an attractive force not exactly like temperature. Hawking radiation is still possible, but the black hole does not get warmer as it evaporates. Material loss from the black hole gets faster as its radius decreases due to the curvature effect, i.e. by a mechanism similar to the Gibbs-Thomson effect.

Effect of spermine on the melting curve and melting temperature of some tRNAs

Using purified bovine liver tRNA￣(Ile) and yeast tRNA￣(Phe), we studied the effects of spermine on the melting curve and melting temperature(Tm) of the tRNAs. The results showed that the absorbance at 260 nm of tRNAs decreases with the increase of temperature in the presence of 2 mmol/L spermine. We called this phenomenon hypochromism and reverse-Tm of the tRNAs. It is suggested that spermine binds to tRNAs and stabilizes the secondary structure of the tRNAs.

Influence of Firing Temperature on Properties of High Alumina Bricks

To reduce production costs and make full and reasonable use of raw materials,high alumina bricks were prepared using tabular corundum and mullite as aggregates,sillimanite as intermediate particles,and white fused corundum powder,α-alumina micropowder,and Suzhou soil as the matrix,firing at different temperatures(1420,1440,1460,1480,1500 and 1520℃)for 4 h.The apparent porosity(AP),the bulk density(BD),the cold crushing strength(CCS),the thermal shock resistance(TSR),the refractoriness under load(RUL)and the creep rate of the samples were tested.The effects of the firing temperature on the creep rate(1450℃×50 h,under a load of 0.2 MPa)of the samples were studied.The results show that with the sillimanite addition of 22.5 mass%,the sample fired at 1460℃for 4 h performs the best comprehensive properties:the AP of 17.5%,the BD of 2.75 g·cm^(-3),the CCS of 100.5 MPa,the TSR number of 35 cycles,the RUL of 1682℃,and the creep rate of-0.428%,which can prolong the service life of furnaces.

Predicting of Land Surface Temperature Distribution in Freetown City, Sierra Leone by Using Polynomial Curve Fitting Model

Global warming has attracted much concern about the worldwide organization, civil society groups, researchers, and so forth because the worldwide surface temperature has been expanding. This investigation intends to assess and compare the ability of a combination of land cover indices to predict the future distribution of land surface temperatures in Freetown using the Polynomial model analysis. Landsat satellite images of 1988, 1998, 2000, 2010, and 2018 of the Freetown Metropolitan zone were utilized for analysis. The investigation had adopted two land covers indices, Modification of normalized difference water index and Urban Index (UI) (e.g., MNDWI and UI) and applied a multi regression equation for forecasting the future LST. The stimulation results propose that the development will be accompanied by surface temperature increases, especially in Freetown’s western urban area. The temperature prevailing in the west of the metropolitan area may increase in the city somewhere in the range from 1988 to 2018. Additionally, the results of the LST prediction show that the model is perfect. Our discoveries can be represented as a helpful device for policymakers and community awareness by giving a scientific basis for sustainable urban planning and management.

隧道内非对称双火源火焰特性及顶棚烟气最高温度研究

针对自然通风下隧道火灾非对称双火源协同作用下火焰特性和顶棚射流烟气最高温度开展研究。在缩尺寸隧道火灾模拟试验平台开展试验,结果表明,随着火源间距的增大,非对称双火源由完全融合到间歇融合,最后完全分离。火焰完全融合且火源热释放速率不同时,火焰呈现非对称形态,存在一个火焰高度,火焰高度主要由较大热释放速率火源决定。随着火源间距的增大,存在两个火焰高度,火焰高度最大值均大于对应工况火焰完全融合时的火焰高度。双火源一侧接收到的辐射热流随离开火源距离的增大而减小,固定水平位置处接收到的热辐射随着火源间距的变化会有波动,但整体变化不大。非对称双火源火羽流顶棚射流烟气最高温度整体随火源间距的增大而减小。当S/D<0.5时,烟气最高温度主要受两个火源热释放速率的影响,火源间距对烟气最高温度的影响较小;而当S/D≥0.5时,烟气最高温度主要受对应侧火源热释放速率的影响,而受临近火源热释放速率的影响较小。在此基础上,建立了非对称双火源顶棚射流烟气最高温度分段预测模型。

煤炭地下气化粗煤气爆炸极限及安全性研究

煤炭地下气化的产物为可燃气体,可能存在着火爆炸风险。因此,研究粗煤气的爆炸极限对确保煤炭地下气化安全有重要意义。但高温高压下可燃气体的爆炸极限实验极其困难,根据热自燃着火理论,通过计算常压下的着火界限并与常压下爆炸极限的实验值进行对比,计算得到修正系数,用修正系数对高温高压下的着火界限进行修正,得到可燃组分的爆炸极限,再将各可燃气体的爆炸极限按组分加权平均得到粗煤气的爆炸极限。结果表明,高温高压下可燃气体的爆炸下限极低,空气中含有微量的可燃气体就可能引发着火爆炸。为防止着火爆炸需在启停阶段采用氮气置换气化通道中的空气,营造安全气化环境。而正常气化时,粗煤气中的氧含量很难达到高温高压下粗煤气的爆炸上限,反而比较安全。研究提供了一种高温高压下计算气体爆炸极限的方法,为煤炭地下气化爆炸安全性分析提供了参考。

基于Master Curve方法的Q345R钢断裂韧性研究

进行了国产Q345R钢在韧脆转变区的拉伸试验、夏比冲击试验和落锤试验,分别采用单温度法和多温度法获得1英寸厚CT试样的Master Curve曲线,并和ASME,API以及BS等规范中断裂韧性估算公式进行对比。结果表明,单温度法和多温度法获得的参考温度T_0基本一致,大约为-105℃。主曲线法能够很好地包络各种估算公式推断出的断裂韧性与温度关系曲线,并且在足够保守的前提下,比ASME的K_(IC)下包络线更具经济性和灵活性。

冷却介质温度对Ti65合金冷却曲线及显微组织的影响

高温热处理后冷却速率对Ti65合金组织性能影响显著。本工作系统研究了冷却介质温度对Ti65合金高温热处理后冷却曲线及显微组织的影响规律。结果表明,在油和空气两种冷却介质条件下,介质温度变化对冷却速率曲线的影响规律相反:室温条件下,油淬最大冷却速率为73.2℃/s,而空冷条件下的最大冷却速率仅为11.2℃/s;随着温度升高,油淬冷却速率曲线右移,最大冷却速率和最低膜沸腾温度提高;室温至60℃范围内,油淬的冷却速率曲线包含蒸气膜、沸腾和对流三个阶段;油温升至80℃时,油淬冷却速率曲线的蒸气膜阶段消失;随油温升高,显微组织呈现α+β两相组织向马氏体组织转变的趋势。与之相反的是,随温度升高,空冷的冷却速率曲线左移,最大冷却速率减小,沸腾阶段最大冷却速率对应的温度升高;相比于不同温度油淬,不同温度空冷条件下的显微组织为典型的双态组织且未见明显差异。油温对冷却曲线的影响主要归因于油黏度/流动性的变化,而空气温度对冷却曲线的影响主要归因于空气的密度和温度梯度等多个复杂因素。

高温环境对激光火工品发火性能的影响

为了研究高温环境对激光火工品发火性能的影响,建立了激光火工品的热力场仿真模型,并搭建光学测试系统,探究了激光火工品自聚焦透镜在100℃高温下的激光光斑尺寸变化。仿真结果表明:高温环境下自聚焦透镜长度产生92.4μm的变形量,导致激光光斑尺寸从94μm增大至150μm;试验结果表明:高温环境下激光火工品输出光斑在243~272μm之间变化,自聚焦透镜变形量的增大导致激光火工品的刺激功率密度降低、作用时间超差。研究表明自聚焦透镜变形降低了激光火工品的发火可靠性。

火灾下部分包覆钢-混凝土组合构件升温数值模拟分析

为研究部分包覆钢-混凝土组合(steel-concrete partially encased composite,PEC)构件火灾下的传热性能,首先采用PEC构件标准火灾试验验证了二维有限元模型,然后对火灾下带有防火保护的PEC构件的升温过程及主要影响参数进行研究分析。研究表明,不同防火保护条件下,PEC构件H型钢腹板最高点温度均位于翼缘与腹板交点处,PEC构件截面温度最高点位置均位于翼缘端部。H型钢截面形状系数、翼缘厚度、腹板厚度以及H型钢高度、宽度等参数对PEC构件升温均有影响,其中影响较大的是翼缘厚度与H型钢宽度。通过将H型钢标准规格截面进行分组模拟,计算PEC构件在标准升温条件下所需防火层厚度,给出不同防火保护措施及不同耐火极限下,PEC构件防火层建议厚度设计值。

Study on the hydrogen absorption properties of a YGdTbDyHo rare-earth high-entropy alloy

This study investigated the microstructure and hydrogen absorption properties of a rare-earth high-entropy alloy(HEA),YGdTbDyHo.Results indicated that the YGdTbDyHo alloy had a microstructure of equiaxed grains,with the alloy elements distributed homogeneously.Upon hydrogen absorption,the phase structure of the HEA changed from a solid solution with an hexagonal-close-packed(HCP)structure to a high-entropy hydride with an faced-centered-cubic(FCC)structure without any secondary phase precipitated.The alloy demonstrated a maximum hydrogen storage capacity of 2.33 H/M(hydrogen atom/metal atom)at 723 K,with an enthalpy change(ΔH)of-141.09 kJ·mol^(-1)and an entropy change(ΔS)of-119.14 J·mol^(-1)·K^(-1).The kinetic mechanism of hydrogen absorption was hydride nucleation and growth,with an apparent activation energy(E_(a))of 20.90 kJ·mol^(-1).Without any activation,the YGdTbDyHo alloy could absorb hydrogen quickly(180 s at 923 K)with nearly no incubation period observed.The reason for the obtained value of 2.33 H/M was that the hydrogen atoms occupied both tetrahedral and octahedral interstices.These results demonstrate the potential application of HEAs as a high-capacity hydrogen storage material with a large H/M ratio,which can be used in the deuterium storage field.

CALCULATIONS OF THE PHASE EQUILIBRIA AND CRITICAL CURVES WITH THE EQUATION OF STATE FOR BINARY MIXTURES

A rational equation of state of the perturbation type with a repulsion and attraction term has been applied to reproduce critical curves of six different binary systems up to high temperatures and pressures. A square well potential for intermolecular interaction is used. With pairwise combination rules for these potentials three adjustable parameters are needed. The experimental critical point and phase equilibrium data are compared with the values predicted using the equation of state. Good agreement is obtained for the analysis of the critical pressure composition data and molar volumes.

氧体积分数对氨煤混燃NO生成特性的影响机理

氨为一种理想的无碳燃料,通过氨煤混燃替代和降低燃煤发电中部分煤炭燃料是实现碳减排的有效措施,但氨煤混燃会生成额外的NO,其中氧体积分数变化对燃烧时NO的生成具有显著影响。利用高温管式炉研究了不同温度(1000℃~1500℃)、不同掺氨比(0%~20%)下氧体积分数对氨煤混燃时NO生成特性的影响。结果表明:当过量空气系数(α)从1.1增加至1.2时,纯煤粉燃烧条件下,氧体积分数的增加促进了燃料-N到NO的转化;1100℃与1300℃下,氧体积分数的增加抑制了氨煤混燃过程燃料-N到NO的转化;1500℃下,氧体积分数的增加促进了氨煤混燃过程燃料-N到NO的转化,且掺氨比和温度是氧体积分数增加时燃料-N转化为NO的氮转化率变化的重要影响因素。氧体积分数的增加利于系统氧化性自由基生成,从而促进了NH_(2)→HNO→NO,NH_(2)→H_(2)NO→HNO→NO以及NCO与O,O_(2)自由基等氧化的进行,促进了混燃过程中NO生成。

纵向风下大间距排烟阀集中排烟模式温度及烟气蔓延研究

为研究盾构隧道组合排烟模式下火灾时烟气流动特性,采用数值模拟方法研究了盾构隧道在纵向通风与集中排烟协同作用下的隧道温度分布和烟气蔓延长度。选取火源功率50 MW,分析不同纵向风速、排烟口间距及排烟量作用下隧道内温度分布和烟气蔓延长度。结果表明:大间距排烟阀组合排烟隧道火源下游的顶板下方纵向温度分布符合双指数衰减模型,隧道内温度分布情况主要受纵向风速和火源热释放速率主导,集中排烟量对隧道纵向温度分布影响不大;大间距排烟阀组合排烟模式下,火源下游的烟气蔓延长度随热释放速率的增大而增大,随排烟量的增大而减小,随纵向风速的增大而增大。对于大间距排烟阀集中排烟模式,具体设计应考虑三项原则:(1)纵向风速达到临界风速来抑制烟气向火源上游回流;(2)兼顾人员安全和漏风量的设计标准,在满足漏风量的设计前提下尽量缩短排烟阀间距;(3)排烟量达到一定量来控制烟气在两个排烟阀范围内。通过量纲分析,得到大间距排烟阀组合排烟模式下火源下游烟气蔓延长度预测模型。

预应力混凝土桥梁火灾后损伤分析与技术状况评估

为研究火灾对预应力混凝土连续梁桥的影响,以遭受火灾的某城市快速路桥梁为例,采用火灾动力学模拟软件FDS,构建受火桥梁的空间温度场分析模型,将数值模拟结果与现场检测推定过火温度进行比较,并结合实测混凝土与预应力筋力学性能折减及混凝土截面损失,计算灾后主梁承载能力,进而确定受火桥梁技术状况,为同类桥梁受火后检测评估提供参考。

600 MW四角切圆燃煤锅炉灵活性改造后燃烧优化的数值模拟及验证

某600 MW四角切圆燃煤锅炉在完成灵活性改造后,于高负荷运行时出现炉膛出口烟气温度偏差大以及NO_(x)排放浓度控制难等问题。为此,运用流体计算仿真软件构建炉内燃烧、流动和传热全耦合的三维模型进行数值模拟计算,旨在确定锅炉的最佳运行参数。其中,采用Standard k-ɛ模型描述流体流动,运用双步竞争模型模拟煤中挥发分的析出过程,借助动力/扩散控制模型阐释焦炭燃烧反应,通过拉格朗日随机轨道模型考量煤粉颗粒与烟气的两相流过程,利用P1辐射模型计算炉内辐射传热,采用有限速率/涡耗散模型模拟气相燃烧,并通过后处理方式计算烟气中NO_(x)的质量浓度。重点分析了在100%锅炉最大连续蒸发量(BMCR)工况下,过量空气系数α、一次风率β和二次风角度θ等参数对炉膛切圆燃烧和出口处NO_(x)排放的影响。数值模拟结果表明:在锅炉实现良好切圆燃烧和低NO_(x)排放的条件下,最优运行参数为α=1.1,β=30%和θ=0°。通过锅炉燃烧调整试验对模拟结果的准确性予以了验证,试验结果表明:在最优运行参数下,锅炉省煤器出口的NO_(x)质量浓度降低了9.96%,排烟温度降低了2.66%,烟气温度偏差降低了2.26%,锅炉效率提高了0.28%。