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.

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.

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.

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.

Experimental study on temperature stress calculation and temperature control optimization of concrete based on early age parameters

Temperature control curve is the key to achieving temperature control and crack prevention of high concrete dam during construction,and its rationality depends on the accurate measurement of temperature stress.With the simulation testing machine for the temperature stress,in the present study,we carried out the deformation process tests of concrete under three temperature curves:convex,straight and concave.Besides,we not only measured the early-age elastic modulus,creep parameters and stress process,but also proposed the preferred type.The results show that at early age,higher temperature always leads to greater elastic modulus and smaller creep.However,the traditional indoor experiments have underestimated the elastic modulus and creep development at early age,which makes the calculated value of temperature stress too small,thus increasing the cracking risk.In this study,the stress values of the three curves calculated based on the strain and early-age parameters are in good agreement with the temperature stress measured by the temperature stress testing machine,which verifies the method accuracy.When the temperature changes along the concave curve,the law of stress development is in consistent with that of strength.Under this condition,the stress fluctuation is small and the crack prevention safety of the concave type is higher,so the concave type is better.The test results provide a reliable basis and support for temperature control curve design and optimization of concrete dams.

倾斜隧道贴壁火灾顶棚烟气最大温升模拟研究

目的 研究在火灾浮力效应、壁面辐射热反馈及空气卷吸限制效应的协同作用下倾斜隧道贴壁火灾发展的特性。方法 利用FDS软件对倾斜隧道贴壁火进行数值模拟,分析顶棚烟气最大温升及其位置;考虑火源中心与侧壁的距离、坡度,建立了无量纲预测模型,进行拟合分析。结果 当贴壁火热释放速率分别为2.7、4.0和5.0 MW时,隧道贴壁火的顶棚最大温升分别在坡度为2%、1%、0时达到最大值,之后随着坡度的增大呈指数衰减;与轴线火相比,当坡度小于10%时,贴壁火的最大温升点与火源中心更近,坡度大于10%时,贴壁火的最大温升点位置更远;贴壁火和轴线火的拟合结果很好,相关系数分别为0.98和0.95。结论 侧壁限制对最大温升的影响随着火源中心与侧壁距离的缩小而更加显著;随着隧道坡度增大,倾斜隧道贴壁火的火焰同时向火灾上游和侧壁倾斜,空气卷吸和对流换热作用不断增强,从而影响火焰的受力机制和火源表面接受到的辐射热反馈,顶棚最大温升随坡度增加呈指数衰减。

某核电厂电气仪控房间火灾排烟温度分析

对某核电厂电气厂房的2个电气仪控房间进行了火灾排烟温度模拟分析。模拟计算了排烟系统不同时刻启动时,这2个防火空间室内空气温度的变化情况。结果表明:排烟系统启动后房间空气温度迅速降低,然后逐渐升高,最后随着火灾热释放速率的降低而逐渐降低;对于手动启动的排烟系统,需及时启动才能有效排除火灾中的烟气;需合理设置排烟防火阀,以保护排烟风机等。

隧道单侧集中排烟模式下烟气蔓延距离研究

为研究隧道火灾单侧集中排烟模式的烟气流动特性,采用缩尺寸模型试验研究了隧道温度分布和烟气蔓延距离。定量分析不同火源热释放速率及排烟量作用下单侧集中排烟模式隧道纵向温度的变化规律,并通过温度场判断不同火灾工况下的烟气蔓延距离,探究火源热释放速率、排烟量与烟气蔓延距离的数学模型。结果表明:在隧道单侧集中排烟模式下,温度场呈非对称分布,排烟风机侧烟气蔓延距离大于封闭侧;火源上、下游的烟气蔓延距离随火源热释放速率的增大而增大,随排烟量的增大而减小;通过理论分析,得到单侧集中排烟模式下火源上、下游烟气蔓延距离的预测模型。

傅里叶变换离子回旋共振质谱技术在稠油高低温氧化评价中的应用

为了认识稠油开发过程中低温氧化与高温氧化阶段原油变化规律,利用静态氧化釜开展稠油的高低温氧化实验,借助傅里叶变换离子回旋共振质谱分析技术对高低温氧化前后的原油分子量及O、N、S杂原子化合物特征开展研究,结果表明:原油低温氧化阶段分子量分布特征与原样相似,相对分子量分布范围在200~750,整体呈平缓状单峰型分布,高温氧化阶段分子量分布范围前移,呈明显的前峰单峰型分布;杂原子化合物中的O元素在低温氧化阶段主要以无环的饱和二元酸形式存在,在高温氧化阶段受环化、芳构化及脱甲基作用的影响,伴随着侧链烷基及杂原子基团的断裂和芳构化过程,造成原油中杂原子化合物向着碳数更小、双键当量(double bond equivalents,DBE)值更低的方向演化。该研究探索了温度与原油结构及化学组成之间的关系,对于指导稠油开发现场具有重要意义。

燃氢燃机高温高湿透平耦合传热和冷却特性

分析了燃氢燃机透平在导热/对流/辐射耦合作用下的流动传热和冷却特性,发展了能够计算高H_(2)O与CO_(2)分压比的灰气体加权和(WSGG)模型。结果表明:燃氢燃机工质中水蒸气含量增大导致金属壁面温度升高;加入辐射的影响后,H_(2)O和CO_(2)含量对传热的影响呈相反的结果,这主要是由于CO_(2)的对流传热能力强于H_(2)O,而H_(2)O的辐射能力强于CO_(2);在对流、导热、辐射3种传热方式都存在的燃氢透平冷却耦合系统中,气膜冷却的速度场几乎不受影响,而温度场则深受耦合及辐射的影响,所定义的耦合条件下气膜冷却效率能够表征燃氢条件下气膜的冷却性能,因此在燃氢燃机透平冷却的设计中,燃氢导致的热负荷恶化需纳入设计变量中。

自然通风条件下隧道火灾动态监测仿真分析

为了降低隧道火灾事故影响,如何动态监测隧道火灾的情况是需要解决的关键性问题。通过FDS场模拟软件建立隧道火灾模型,模拟小型货车车厢位于隧道中部着火的隧道火灾情况。分析火源近场的火源功率及隧道顶部温度变化,探测温度场分布动态变化相关性规律。结果表明:火源功率前200 s内处于稳定阶段,1000 s左右达到最大火源功率。隧道顶部温度整体趋势先升后降,热烟气呈现加速状态,温度变化曲线缓步上升。火源近场隧道顶部温度变化曲线与热释放速率曲线趋于一致,可根据隧道顶部温度变化曲线监测近场火源的动态变化。近场4处测点在12 min内处于临界高程和温度之内,此时火灾扑救人员相对安全,灭火可在此范围内采取内攻方式,为消防救援提供一定参考。

Gleeble高温拉伸样品标距段的选定对拉伸曲线的影响

Gleeble热模拟试验机是目前测定金属材料在高温条件下力学性能的主要设备之一。利用该设备进行高温拉伸试验时,样品属于局部加热,中间温度最高,向两侧随距离增大温度呈现递减状态,因此常选用温差在50~100℃范围内的区域作为标距段长度来获取最终的拉伸曲线。该文选取了两种棒材和两种板材,对其分别进行600、800和1000℃的高温处理,获得不同样品的加热区域温度分布情况,进而给出推荐的标距段长度。随后对4种样品分别进行800℃、应变速率0.1/s下的高温拉伸试验,获得不同标距长度下样品的高温拉伸曲线。通过对曲线结果进行分析,给出利用该设备进行高温拉伸时的样品选用和标距范围的建议。

壳聚糖季铵盐对O/W乳状液的破乳性能评价

以壳聚糖季铵盐作为破乳剂,使用Turbiscan Lab稳定性分析仪实验研究了破乳剂浓度及处理温度对水包油(O/W)型乳状液破乳效果的影响。实验结果表明:在25~55℃的处理温度范围内,破乳剂最佳使用浓度约0.4%(体积分数),破乳剂浓度超过1.0%后,无破乳效果。在操作温度为25℃、破乳剂加药浓度为0.4%时,所测得的背散射光ΔBS曲线显示出良好的油水分层现象,相应的TSI曲线值达到最大,油滴粒径增大7倍,破乳过程在10 min之内基本完成。在25~55℃的处理温度范围内,破乳效果随温度升高而改善,最佳破乳温度为55℃。此研究对油田采出液破乳方法的选择具有指导借鉴意义。

基于低温-雨雪-冻雨综合强度指数的湖北省冰冻事件分型及灾损评估

低温雨雪冰冻是一种复合型气候事件,极端低温雨雪冰冻事件发生频率低,但造成的影响往往十分严重,开展低温雨雪冰冻事件演变特征研究对有效防御和减轻灾害影响具有重要意义。利用湖北省1981-2024年逐日气象观测资料,从低温、雨雪、冻雨等3个方面遴选指标,构建大范围持续低温雨雪冰冻事件综合评估模型;依据致灾因子类型、承灾体受灾特征对事件分型并排序;结合历史灾损数据分析了湖北省1981年以来大范围持续低温雨雪冰冻事件的致灾特征。结果表明:(1)近44 a湖北省共出现64次大范围持续低温雨雪冰冻事件,平均每年出现1.5次,1月出现频率最高,约占42%,其次是2月,约占34%。(2)湖北省大范围持续低温雨雪冰冻事件判别为六大分型,其中雨雪为主型占40%,低温雨雪冻雨型占31%,低温雨雪型占16%,其他类型共占13%,最强10次事件均属于低温雨雪冻雨型。(3)湖北省因低温雨雪冰冻灾害造成的直接经济损失总体呈增加趋势,但直接经济损失占GDP比重呈下降趋势。(4)湖北省大范围持续低温雨雪冰冻事件“综合强度指数-直接经济损失”相关关系呈幂指数分布,当事件综合强度指数超过1.85时,直接经济损失迅速增加。

聚对苯二甲酸乙二醇酯的动态黏弹性能

对聚对苯二甲酸乙二醇酯(PET)进行不同频率下的温度扫描试验和不同温度下的频率扫描试验,得到PET材料的玻璃化转变温度及动态黏弹性参数随测试温度、频率的变化特性。根据时间-温度等效原理构建出宽广频域的PET动态黏弹性主曲线,得到相应的移位因子,利用WLF方程对温度移位因子进行拟合,拟合结果与试验结果吻合较好。此外,研究表明,PET损耗模量主曲线和损耗因子主曲线均呈现明显的峰值,且峰值两边具有非对称性,因此,引入改进的分数阶Zener模型(mFDZ)对PET的动态黏弹性试验主曲线进行综合分析,通过建立合适的目标函数,基于Matlab平台对mFDZ模型进行参数识别,得到了统一的模型参数值。根据得到模型参数计算的mFDZ模型曲线与PET黏弹性试验主曲线、非对称的Cole-Cole曲线及Wicket曲线均吻合良好,这表明,mFDZ模型能较好地描述PET材料动态黏弹性的非对称性,可适用于描述PET材料在宽广频域内的动态黏弹性能。

一种基于HTCC的高隔离度开关电路设计

基于开关芯片实现的开关电路是射频前端的基本单元之一,其功能是实现射频信号的导通和关断,在小功率射频信号传输中应用广泛。本文以6 GHz~15 GHz超宽带、60 dB隔离度为设计目标,采用高温共烧陶瓷基板工艺和级联腔体隔离技术,以级联开关芯片为基本电路结构,设计了高隔离度开关电路。该电路包含一只限幅器和两只吸收式砷化镓单刀单掷开关,装配于两个相邻的隔离腔体结构中,通过类同轴垂直传输过渡到基板内对称带状线传输结构,再辅以两侧地孔,实现电磁屏蔽。对开关电路进行电磁场建模仿真,结果表明该结构具有良好的传输特性和隔离特性;在射频前端电路制造完成后对开关电路进行装配和测试,测试结果与建模仿真结果一致性高,验证了本文设计的开关电路具有高隔离度特性。