Microstructure studies of air-plasma-spray-deposited CoNiCrAlY coatings before and after thermal cyclic loading for high-temperature application

In the present study, bond-coats for thermal barrier coatings were deposited via air plasma spraying（APS） techniques onto Inconel 800 and Hastelloy C-276 alloy substrates. Scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, X-ray diffraction（XRD）, and atomic force microscopy（AFM） were used to investigate the phases and microstructure of the as-sprayed, APS-deposited Co Ni Cr Al Y bond-coatings. The aim of this work was to study the suitability of the bond-coat materials for high temperature applications. Confirmation of nanoscale grains of the γ/γ′-phase was obtained by TEM, high-resolution TEM, and AFM. We concluded that these changes result from the plastic deformation of the bond-coat during the deposition, resulting in Co Ni Cr Al Y bond-coatings with excellent thermal cyclic resistance suitable for use in high-temperature applications. Cyclic oxidative stability was observed to also depend on the underlying metallic alloy substrate.

Response Surface Modeling of Fuel Rich and Fuel Lean Catalytic Combustion of the Stabilized Confined Turbulent Gaseous Diffusion Flames

The Response Surface Methodology (RSM) has been applied to explore the thermal structure of the experimentally studied catalytic combustion of stabilized confined turbulent gaseous diffusion flames. The Pt/γAl2O3 and Pd/γAl2O3 disc burners were situated in the combustion domain and the experiments were performed under both fuel-rich and fuel-lean conditions at a modified equivalence (fuel/air) ratio (ø) of 0.75 and 0.25 respectively. The thermal structure of these catalytic flames developed over the Pt and Pd disc burners were inspected via measuring the mean temperature profiles in the radial direction at different discrete axial locations along the flames. The RSM considers the effect of the two operating parameters explicitly (r), the radial distance from the center line of the flame, and (x), axial distance along the flame over the disc, on the measured temperature of the flames and finds the predicted maximum temperature and the corresponding process variables. Also the RSM has been employed to elucidate such effects in the three and two dimensions and displays the location of the predicted maximum temperature.

Response Surface Methodology and Artificial Neural Network Methods Comparative Assessment for Fuel Rich and Fuel Lean Catalytic Combustion

Modeling, predictive and generalization capabilities of response surface methodology (RSM) and artificial neural network (ANN) have been performed to assess the thermal structure of the experimentally studied catalytic combustion of stabilized confined turbulent gaseous diffusion flames. The Pt/γAl2O3 and Pd/γAl2O3 disc burners were located in the combustion domain and the experiments were accomplished under both fuel-rich and fuel-lean conditions at a modified equivalence (fuel/air) ratio (ø) of 0.75 and 0.25, respectively. The thermal structure of these catalytic flames developed over the Pt and Pd disc burners was scrutinized via measuring the mean temperature profiles in the radial direction at different discrete axial locations along with the flames. The RSM and ANN methods investigated the effect of the two operating parameters namely (r), the radial distance from the center line of the flame, and (x), axial distance along with the flame over the disc, on the measured temperature of the flames and predicted the corresponding temperatures beside predicting the maximum temperature and the corresponding input process variables. A three-layered Feed Forward Neural Network was developed in conjugation with the hyperbolic tangent sigmoid (tansig) transfer function and an optimized topology of 2:10:1 (input neurons:hidden neurons:output neurons). Also the ANN method has been exploited to illustrate the effects of coded R and X input variables on the response in the three and two dimensions and to locate the predicted maximum temperature. The results indicated the superiority of ANN in the prediction capability as the ranges of & F_Ratio are 0.9181 - 0.9809 & 634.5 - 3528.8 for RSM method compared to 0.9857 - 0.9951 & 7636.4 - 24,028.4 for ANN method beside lower values for error analysis terms.

Comparative Appraisal of Response Surface Methodology and Artificial Neural Network Method for Stabilized Turbulent Confined Jet Diffusion Flames Using Bluff-Body Burners

The present study was conducted to present the comparative modeling, predictive and generalization abilities of response surface methodology (RSM) and artificial neural network (ANN) for the thermal structure of stabilized confined jet diffusion flames in the presence of different geometries of bluff-body burners. Two stabilizer disc burners tapered at 30° and 60° and another frustum cone of 60°/30° inclination angle were employed all having the same diameter of 80 (mm) acting as flame holders. The measured radial mean temperature profiles of the developed stabilized flames at different normalized axial distances (x/dj) were considered as the model example of the physical process. The RSM and ANN methods analyze the effect of the two operating parameters namely (r), the radial distance from the center line of the flame, and (x/dj) on the measured temperature of the flames, to find the predicted maximum temperature and the corresponding process variables. A three-layered Feed Forward Neural Network in conjugation with the hyperbolic tangent sigmoid (tansig) as transfer function and the optimized topology of 2:10:1 (input neurons: hidden neurons: output neurons) was developed. Also the ANN method has been employed to illustrate such effects in the three and two dimensions and shows the location of the predicted maximum temperature. The results indicated the superiority of ANN in the prediction capability as the ranges of R2 and F Ratio are 0.868 - 0.947 and 231.7 - 864.1 for RSM method compared to 0.964 - 0.987 and 2878.8 7580.7 for ANN method beside lower values for error analysis terms.

Effect of film types on thermal response,cellular structure,forming defects and mechanical properties of combined in-mold decoration and microcellular injection molding parts

Different types of polymer films were used in the combined in-mold decoration and microcellular injection molding(IMD/MIM)process.The multiphase fluid-solid coupled heat transfer model was established to study the thermal response at the melt filling stage in the IMD/MIM process.It was found that the temperature distributed asymmetrically along the thickness direction due to the changed heat transfer coefficient of the melt on the film side.When polyethylene terephthalate(PET)films were applied,the temperature of the melt-film interface increased faster and to be higher at the end of melt filling stage in comparison with the application of polycarbonate(PC)and thermoplastic polyurethane(TPU)films.And the effects of film types on the cellular structure,forming defects and mechanical properties of IMD/MIM parts were also studied experimentally.The results showed that the film types had no obvious effect on the cells size in the transition layer and the mechanical properties of the parts.Under certain film thickness,the offset distance of core layer was the largest with PET film used,while the offset distance was the smallest with TPU film used.And similar results were found for the warpage of the parts.However,an exactly opposite change occurred for the thickness of film-side transition layer and the bubble marks on the surface of the parts.

Crystal Structure,Thermal Expansivity and High-Temperature Vibrational Spectra on Natural Hydrous Rutile

A natural rutle sample was measured by in situ high-temperature X-ray diffraction(XRD)patterns,as well as Raman and Fourier transform infrared(FTIR).Crystal structure is refined on the sample with 1.4 mol.%Fe and 510±120 ppmw.H2 O.The unit-cell and Ti O6 octahedral volumes are expanded by 0.7%–0.8%for Fe3+incorporation,as compared with the reported Ti-pure samples.The volumetric thermal expansion coefficient(α,K-1)could be approximated as a linear function of T(K):4.95(3)×10-9×T+21.54(5)×10-6,with the averaged valueα0=30.48(5)×10-6 K-1,in the temperature range of 300–1500 K.The internal Ti-O stretching(A1 g and B2 g)and O-Ti-O bending(Eg)modes show‘red shift',whereas the multi-phonon process exhibits‘blue shift'at elevated temperature.The rotational mode(B1 g)for Ti O6 octahedra is nearly insensitive to temperature variations.The OH-stretching bands at 3279 and 3297 cm-1 are measured by high-temperature spectroscopy experiments.Both the IR-active and Raman-active OH-stretching modes shift to lower frequencies at higher temperature,with the signal intensities decreasing.And after quenching,we expect about 43%dehydration around 873 K,and 85%dehydration at 1273 K for this hydrous sample.

Thermal stability of bimodal grain structure in a cobalt-based superalloy subjected to high-temperature exposure

The present work investigates the thermal stability and mechanical properties of a Co-20 Cr-15 W-10 Ni(wt%) alloy with a bimodal grain(BG) structure.The BG structure consisting of fine grains(FGs) and coarse grains(CGs) is thermally stable under high-temperature exposure treatments of 760℃ for 100 h and 870℃ for 100-1000 h.The size of both FGs and CGs remains no significant changes after thermal exposure treatments.The microstructural stability is associated with the slow kinetics of grain growth and the pinning of carbides.The thermal stability enables to maintain the BG structures,leading to the same mechanical properties as the sample without thermal exposure treatment.In particular,the BG alloy samples after thermal exposure treatment exhibit superior mechanical properties of both high strength and high ductility compared to the unimodal grain(UG) structured ones.The BG structure of the alloy samples after thermal exposure is capable of avoiding severe loss of ductility and retaining high strength.More specifically,the ductility of the BG alloy samples after thermal exposure treatments of 870℃ for 500-1000 h is ten times higher(44.6% vs.3.5% and 52.6% vs.5.0%) than that of the UG ones.The finding in the present work may give new insights into high-temperature applications of the Co-20 Cr-15 W-10 Ni alloy and other metallic materials with a BG structure.

Thermal fatigue behaviors of SiC power module by Ag sinter joining under harsh thermal shock test

The excellent properties of SiC bring new challenges for the device packaging.In this study,the bonding strength,fracture behaviors and microstructural evolution of micron-porous Ag joint were elevated during thermal cycling(–50 ℃–250 ℃) in SiC/DBC(direct bonding copper) die attachment structure for different time.During harsh thermal shock test,the strength of sintered joint deceased gradually with the increase of cycling number,and the value just was half of the value of as-sintered after 1 000 cycles.Coarsening of Ag grains was observed in micron-porous joint with the structure inhomogeneity and defects increasing,which were the reasons of the strength decease.In addition,it was also found that the fracture behavior of sintered joints was changed from ductile deformation of Ag grain to brittle fracture of crack propagation after 1 000 cycles.This study will add the understanding in the mechanical properties of Ag sinter joining and its applications at high temperature.

Thermal-structural response and low-cycle fatigue damage of channel wall nozzle

To investigate the thermo-mechanical response of channel wall nozzle under cyclic working loads,the fnite volume fluid-thermal coupling calculation method and the fnite element thermal-structural coupling analysis technique are applied.In combination with the material lowcycle fatigue behavior,the modifed continuous damage model on the basics of local strain approach is adopted to analyze the fatigue damage distribution and accumulation with increasing nozzle work cycles.Simulation results have shown that the variation of the non-uniform temperature distribution of channel wall nozzle during cyclic work plays a signifcant role in the thermal-structural response by altering the material properties;the thermal-mechanical loads interaction results in serious deformation mainly in the front region of slotted liner.In particular,the maximal cyclic strains appear in the intersecting regions of liner gas side wall and symmetric planes of channel and rib,where the fatigue failure takes place initially;with the increase in nozzle work cycles,the residual plastic strain accumulates linearly,and the strain amplitude and increment in each work cycle are separately equal,but the fatigue damage grows up nonlinearly.As a result,a simplifed nonlinear damage accumulation approach has been suggested to estimate the fatigue service life of channel wall nozzle.The predicted node life is obviously conservative to the Miner＇s life.In addition,several workable methods have also been proposed to improve the channel wall nozzle durability.

Nonlinear thermo-structural behavior of sandwich panels with truss cores under through-thickness gradient temperature field

A theoretical analysis is presented to predict the nonlinear thermo-structural response of metallicsandwich panels with truss cores under through-thickness gradient temperature field, which is acommon service condition for metallic thermal protection system （TPS）. The in-planetemperature distribution is assumed to be uniform, and through-thickness temperature field isdetermined by heat conduction. Two typical conditions are analyzed： nonlinear thermal bendingin fixed inside surface temperature, and thermal post-buckling in fixed temperature differencebetween two surfaces. Temperature-dependent mechanical properties are considered, andgradient shear stiffness and bending stiffness due to non-uniform temperature is included. Resultsindicate that the temperature-dependent material properties obviously affect bending resistance;however, the effect is negligible on post-buckling behavior. Influences of geometric parameters onthe thermo-structural behavior of the sandwich panel according to the present theoretical modelare discussed.

考虑弹头滚转姿态变化影响的防热层厚度优化

基于气动热/结构温度响应耦合计算方法,研究了弹头固定攻角再入条件下锥身典型子午面结构温度响应特性,以及滚转姿态变化时刻对各子午面结构温度响应的影响,建立了考虑弹头滚转姿态变化影响的防热层厚度优化方法,并基于该方法开展了弹头防热层厚度优化。最后,研究了滚转姿态变化次数对优化效果的影响。结果表明:在再入弹道前段进行滚转姿态变化可使外壁面气动加热量更为合理,有效降低弹体锥身壳体内壁面温升。当迎、背风面壳体内壁面最大温升约束为25 K时,与无滚转姿态变化状态下相比,滚转姿态变化设计可使防热层厚度减小0.725 mm,减小幅度为5.4%。通过与滚转姿态变化时刻的联合设计可有效降低锥身防热层厚度。与单次滚转姿态变化时相比,两次滚转姿态变化时防热层厚度减小量仅增大1.9%,增加滚转姿态变化次数对优化效果影响较小。

空间折展结构热响应研究现状与展望

中国航天正面向世界科技前沿和国家重大战略需求飞速前进,航天事业已成为我国整体发展战略的重要组成部分。近年来,随着航天核心技术的逐渐提升和突破,高精度、大尺度和轻量化的新型空间折展结构逐渐得到广泛应用,然而,其柔性大、刚度小,尤其是受空间极端环境显著影响等问题也备受关注。例如,空间折展结构中的典型应用——空间可展开天线,该新型结构作为航天和国防领域传递与获取信息的核心装备,在轨服役期间受空间热交变环境影响,其形面精度和网面稳定性问题凸显,甚至干扰航天器及其附件在轨工作姿态,从而导致航天器失效。为满足应用需求,文中归纳总结了伸展臂、太阳翼及可展开天线等空间折展结构的热致变形和热致振动等热响应研究进展。基于研究现状,从热-结构耦合情况、非线性因素、新型结构组合、结构连接及热控措施优化等方面展望空间折展结构热响应研究发展趋势,为我国空间折展结构的优化设计和防护方案提供理论依据及应用参考。

基于电磁-流热双向耦合的变压器绕组温升计算及结构参数优化

变压器温升是影响其运行状态和使用寿命的关键因素。为了更准确地得到油浸式变压器绕组区域的温度场分布,采用有限元法对绕组的电磁-流热耦合过程进行了数值研究。根据电磁场及流体-温度场分布的特点,建立了变压器及绕组区域的计算模型。基于异构网格节点数据映射方法,将电磁场分析得到的绕组非平均损耗作为热源边界条件加载到流体-温度场计算网格中,并根据每次迭代时的温度对该热源进行修正,实现电磁-流热的双向耦合。在此基础上,建立热点温度与绕组结构参数之间的响应面,开展了对绕组热点温度及导体材料用量最小化的多目标优化研究。采用非支配排序遗传算法(non-dominated sorting genetic algorithm,NSGA-Ⅱ)获得了Pareto最优解,并对Pareto前沿面上的4种优化方案进行了分析。结果表明,优化后的绕组热点温度及导体用量明显降低。该文研究为变压器结构的优化设计提供了一种可行方法。

基于振动测量的热防护结构脱粘损伤检测方法

针对典型飞行器热防护结构脱粘损伤现场快速无损检测的需求,提出了一种基于敲击测量频响函数的脱粘损伤识别方法。首先,利用有限元方法建立了含脱粘损伤的热防护结构有限元模型并进行了振动特性分析,分析了脱粘损伤引起的结构局部刚度下降规律以及频响函数对脱粘损伤的敏感性;其次,确定了脱粘损伤识别的特征频率范围,给出了一个脱粘损伤指标;最后,采用力锤与激光多普勒测振仪对含脱粘的热防护结构进行了测量。实验结果表明,当脱粘面积达到60%时,脱粘损伤指标比健康件增加了1倍以上,当脱粘面积达到80%时,脱粘损伤指标为健康件的5倍以上,验证了所提出的方法能够有效识别出热防护结构的脱粘损伤。

Solvability and thermal response of cellulose with different crystal configurations

Cellulose is a biodegradable and renewable natural material that it is naturally resistant to breaking and modification. Moreover, the crystalline structure of cellulose is a major factor restricting its industrial utilization. In this study, cellulose polymorphs were prepared from natural cellulose, and their solvability and thermal response were investigated. Using liquid- and solid-state NMR signals, the distinct types and dissolving states of cellulose polymorphs were identified. The thermal behavior of the polymorphic forms of cellulose-d was also evaluated, and cellulose Ⅱ exhibited the poorest thermal stability and a unique exothermic reaction.

基于响应面法的单元发动机喷注器优化设计

针对某新研单元发动机喷注器热阻优化的问题,采用仿真计算分析研究支架结构参数对支架热阻与结构强度的影响,并基于响应面法(RSM)对喷注器支架进行结构优化设计。计算结果表明:在计算范围内,对支架隔热性能影响最大的是支架高度,其次是支架厚度,最后是支架孔直径。随着支架高度的增加,支架热阻逐渐增大,而支架热阻则随着支架厚度与孔直径的增加而降低。对支架结构等效应力影响最大的是支架孔直径,其次是支架高度,最后是支架厚度,并且随着支架孔直径的增加,支架结构等效应力逐渐增大,而支架最大等效应力随着支架高度和厚度的增加而减小。通过采用响应面法改进后的支架在结构强度满足发动机力学环境的条件下,喷注器热阻得到明显提高。

Nonlinear response analysis and experimental verification for thin-walled plates to thermal-acoustic loads

For large deflection strongly nonlinear response problem of thin-walled structure to thermal-acoustic load, thermal-acoustic excitation test and corresponding simulation analysis for clamped metallic thin-walled plate have been implemented. Comparing calculated values with experimental values shows the consistency and verifies the effectiveness of calculation method and model for thin-walled plate subjected to thermal-acoustic load. Then this paper further completes dynamic response calculation for the cross reinforcement plate under different thermalacoustic load combinations. Based on the obtained time-domain displacement response, analyses about structure vibration forms are mainly focused on three typical motions of post-buckled plate,indicating that the relative strength between thermal load and acoustic load determines jump forms of plate. The Probability spectrum Density Functions（PDF） of displacement response were drawn and analyzed by employing statistical analysis method, and it clearly shows that the PDF of postbuckled plate exhibits bimodal phenomena. Then the Power Spectral Density（PSD） functions were used to analyze variations of response frequencies and corresponding peaks with the increase of temperatures, as well as how softening and hardening areas of the plate are determined. In the last section, this paper discusses the change laws of tensile stress and compressive stress in pre/post buckling areas, and gives the reasons for N glyph trend of the stress Root Mean Square（RMS）.

类前缘防热层流场与热响应耦合计算研究

本文在以往对类前缘防热层热响应计算分析基础上,进一步研究实现了外流场高超音速NS方程数值计算表面气动加热与防热层结构热响应的耦合计算,这对于常用的非耦合计算方法来说是一进步,也为进一步开展外流场/结构热响应/热应力全耦合一体化计算研究和防热层表面吹气强化传热问题的场协同研究打下了基础。

建筑结构抗火性能研究回顾及展望

火灾产生的高温可使建筑结构严重破坏甚至倒塌 ,为了保证建筑结构具有足够的抗火能力 ,必须进行建筑结构抗火分析和设计。各国学者对建筑结构抗火性能进行了大量研究 ,取得了丰硕的成果。本文简要介绍火灾科学的产生及发展的进程 ,重点回顾建筑结构抗火性能研究的进展 ,进而对今后发展进行了展望。

热声载荷作用下金属薄壁结构的振动响应与试验验证

针对航空航天薄壁结构在热声载荷作用下的非线性振动响应问题,基于声振耦合理论,采用耦合的有限元/边界元法对四边固支高温合金矩形薄壁结构进行了动力学响应计算。重点研究了薄壁结构在行波加载与扩散场加载条件下的振动应力/应变响应规律,讨论了温升对结构振动响应的影响规律,分析了薄壁结构热屈曲(Thermal-buckling)和跳变(Snap-through)响应特性。通过将薄壁结构在不同温度条件下的振动模态以及动态应变响应的仿真结果与热环境下的声激振试验结果进行对比,表明计算的基频量值及随温度的变化关系与试验结果获得较好的一致性,计算的应变响应与试验测试结果量值相当,验证了热声响应计算方法与模型的有效性。该研究提出的金属薄壁结构在热声载荷作用下的非线性振动响应计算方法及分析结论对进一步开展热声疲劳寿命预测及动强度设计提供依据。