Theoretical insights into thermal transport and structural stability mechanisms of triaxial compressed methane hydrate

The heat transfer and stability of methane hydrate in reservoirs have a direct impact on the drilling and production efficiency of hydrate resources,especially in complex stress environments caused by formation subsidence.In this study,we investigated the thermal transport and structural stability of methane hydrate under triaxial compression using molecular dynamics simulations.The results suggest that the thermal conductivity of methane hydrate increases with increasing compression strain.Two phonon transport mechanisms were identified as factors enhancing thermal conductivity.At low compressive strains,a low-frequency phonon transport channel was established due to the overlap of phonon vibration peaks between methane and water molecules.At high compressive strains,the filling of larger phonon bandgaps facilitated the opening of more phonon transport channels.Additionally,we found that a strain of0.04 is a watershed point,where methane hydrate transitions from stable to unstable.Furthermore,a strain of0.06 marks the threshold at which the diffusion capacities of methane and water molecules are at their peaks.At a higher strain of0.08,the increased volume compression reduces the available space,limiting the diffusion ability of water and methane molecules within the hydrate.The synergistic effect of the strong diffusion ability and high probability of collision between atoms increases the thermal conductivity of hydrates during the unstable period compared to the stable period.Our findings offer valuable theoretical insights into the thermal conductivity and stability of methane hydrates in reservoir stress environments.

GLOBAL STABILITY OF WAVE PATTERNS FOR COMPRESSIBLE NAVIER-STOKES SYSTEM WITH FREE BOUNDARY

In this article, we investigate the global stability of the wave patterns with the superposition of viscous contact wave and rarefaction wave for the one-dimensional compressible Navier-Stokes equations with a free boundary. It is shown that for the ideal polytropic gas, the superposition of the viscous contact wave with rarefaction wave is nonlinearly stable for the free boundary problem under the large initial perturbations for any γ 〉 1 with V being the adiabatic exponent provided that the wave strength is suitably small.

ANALYSIS AND APPLICATION OF ELLIPTICITY OF STABILITY EQUATIONS ON FLUID MECHANICS

By using characteristic analysis of the linear and nonlinear parabolic stability equations ( PSE), PSE of primitive disturbance variables are proved to be parabolic intotal. By using sub- characteristic analysis of PSE, the linear PSE are proved to be elliptical and hyperbolic-parabolic for velocity U, in subsonic and supersonic, respectively; the nonlinear PSE are proved to be elliptical and hyperbolic-parabolic for relocity U + u in subsonic and supersonic., respectively . The methods are gained that the remained ellipticity is removed from the PSE by characteristic and sub-characteristic theories , the results for the linear PSE are consistent with the known results, and the influence of the Mach number is also given out. At the same time , the methods of removing the remained ellipticity are further obtained from the nonlinear PSE .

Microstructure and thermal stability of sintered pure tungsten processed by multiple direction compression

Multiple direction compression(MDC)was conducted on sintered pure tungsten(99.9%,mass fraction)with different reductions at 1423 K.The microstructure,microhardness and thermal stability of the MDC-processed samples were studied by X-ray diffraction(XRD),electron backscattered diffraction(EBSD)and differential scanning calorimetry(DSC)compared with those of the initial sintered tungsten.The results show that the dislocation density increases significantly with the reduction of MDC,ranging from 3.08×1014 m-2 for the initial sintered tungsten to 8.08×1014 m-2 for the tungsten after MDC with the reduction of 50%.The average grain size decreases from 83.8 to 14.7μm and the microhardness value increases from HV0.2 417 to HV0.2 521.The recrystallization temperature for the tungsten samples processed by MDC is approximately constant at around 1600 K.The MDC of sintered tungsten results in a decrease of grain size concurrent with an increase of uniformly distributed nucleation sites,which leads to the improvement of the thermal stability.

NONLINEAR STABILITY OF VISCOUS SHOCK WAVES FOR ONE-DIMENSIONAL NONISENTROPIC COMPRESSIBLE NAVIER–STOKES EQUATIONS WITH A CLASS OF LARGE INITIAL PERTURBATION

We study the nonlinear stability of viscous shock waves for the Cauchy problem of one-dimensional nonisentropic compressible Navier–Stokes equations for a viscous and heat conducting ideal polytropic gas. The viscous shock waves are shown to be time asymptotically stable under large initial perturbation with no restriction on the range of the adiabatic exponent provided that the strengths of the viscous shock waves are assumed to be sufficiently small.The proofs are based on the nonlinear energy estimates and the crucial step is to obtain the positive lower and upper bounds of the density and the temperature which are uniformly in time and space.

Kriging-based reliability analysis of the long-term stability of a deep driftconstructed in the Callovo-Oxfordian claystone

Callovo-Oxfordian(COx)claystone has been considered as a potential host rock for geological radioactive waste disposal in France(Cigéo project).During the exploitation phase(100 years),the stability of drifts(e.g.galleries/alveoli)within the disposal is assured by the liner,which includes two layers:concrete arch segment and compressible material.The latter exhibits a significant deformation capacity(about 50%)under low stress(<3 MPa).Although the response of these underground structures can be governed by complex thermo-hydro-mechanical coupling,the creep behavior of COx claystone has been considered as the main factor controlling the increase of stress state in the concrete liner and hence the long-term stability of drifts.Therefore,by focusing only on the purely mechanical behavior,this study aims at investigating the uncertainty effect of the COx claystone time-dependent properties on the stability of an alveolus of Cigéo during the exploitation period.To describe the creep behavior of COx claystone,we use Lemaitre’s viscoplastic model with three parameters whose uncertainties are identified from laboratory creep tests.For the reliability analysis,an extension of a well-known Kriging metamodeling technique is proposed to assess the exceedance probability of acceptable stress in the concrete liner of the alveolus.The open-source code Code_Aster is chosen for the direct numerical evaluations of the performance function.The Kriging-based reliability analysis elucidates the effect of the uncertainty of COx claystone on the long-term stability of the concrete liner.Moreover,the role of the compressible material layer between the concrete liner and the host rock is also highlighted.

Preventing Pressure Oscillations Does Not Fix Local Linear Stability Issues of Entropy-Based Split-Form High-Order Schemes

Recently,it was discovered that the entropy-conserving/dissipative high-order split-form discontinuous Galerkin discretizations have robustness issues when trying to solve the sim-ple density wave propagation example for the compressible Euler equations.The issue is related to missing local linear stability,i.e.,the stability of the discretization towards per-turbations added to a stable base flow.This is strongly related to an anti-diffusion mech-anism,that is inherent in entropy-conserving two-point fluxes,which are a key ingredi-ent for the high-order discontinuous Galerkin extension.In this paper,we investigate if pressure equilibrium preservation is a remedy to these recently found local linear stability issues of entropy-conservative/dissipative high-order split-form discontinuous Galerkin methods for the compressible Euler equations.Pressure equilibrium preservation describes the property of a discretization to keep pressure and velocity constant for pure density wave propagation.We present the full theoretical derivation,analysis,and show corresponding numerical results to underline our findings.In addition,we characterize numerical fluxes for the Euler equations that are entropy-conservative,kinetic-energy-preserving,pressure-equilibrium-preserving,and have a density flux that does not depend on the pressure.The source code to reproduce all numerical experiments presented in this article is available online(https://doi.org/10.5281/zenodo.4054366).

An Experimental Investigation on Low Load Combustion Stability and Cold-Firing Capacity of a Gasoline Compression Ignition Engine

Gasoline compression ignition (GCI) is one of the most promising combustion concepts to maintain low pollutant emissions and high efficiency. However, low load combustion stability and firing in cold-start operations are two major challenges for GCI combustion. Strategies including negative valve overlap (NVO), advanced injection strategies, fuel reforming, and intake preheating have been proposed in order to solve these difficulties;however, the cold start is still an obstacle. The objective of this work is to study effective methods to achieve GCI engine cold start-up. This work combines NVO, in-cylinder fuel reforming, and intake preheating to achieve quick firing under cold-start conditions and the subsequent warmup conditions. The results show that start of injection (SOI) during the intake stroke yields the best fuel economy, and injection during the compression stroke has the potential to extend the low load limit. Furthermore, SOI during the NVO period grants the ability to operate under engine conditions with cold intake air and coolant. With highly reactive products made by in-cylinder fuel reforming and fast heat accumulation in the combustion chamber, the NVO injection strategy is highly appropriate for GCI firing. An additional assisted technical method, such as intake preheating, is required to ignite the first firing cycle for a cold-start process. With the combination of NVO, in-cylinder fuel reforming, and intake preheating, the GCI engine successfully started within five combustion cycles in the experiment. After the firing process, the engine could stably operate without further intake preheating;thus, this method is appropriate for engine cold-start and warm-up.

INFLUENCE OF COMPRESSION-BENDING COUPLING ON THE STABILITY BEHAVIOR OF ANISOTROPIC LAMINATED PANELS

Dynamic-Relaxation Method (DRM) is applied to studying the influence of compression-bending coupling on nonlinear behavior of cylindrically slightly curved panels of unsymmetric laminated composite materials subjected to uniform uniaxial Compression during loading and unloading. Numerical results are given for cross-ply plates and panels under S4S4 and S4S2 boundary conditions. The results show that the effects of absolute value and the sign of the coupling coefficient on the stability behavior of the panles are significant.

STABILITY OF THE RAREFACTION WAVE IN THE SINGULAR LIMIT OF A SHARP INTERFACE PROBLEM FOR THE COMPRESSIBLE NAVIER-STOKES/ALLEN-CAHN SYSTEM

This paper is concerned with the global well-posedness of the solution to the compressible Navier-Stokes/Allen-Cahn system and its sharp interface limit in one-dimensional space.For the perturbations with small energy but possibly large oscillations of rarefaction wave solutions near phase separation,and where the strength of the initial phase field could be arbitrarily large,we prove that the solution of the Cauchy problem exists for all time,and converges to the centered rarefaction wave solution of the corresponding standard two-phase Euler equation as the viscosity and the thickness of the interface tend to zero.The proof is mainly based on a scaling argument and a basic energy method.

偏高岭土基地聚合物对水泥固化红黏土的改善机制

为了研究偏高岭土对水泥固化红黏土的改善效果,开展了三种组合(纯水泥、水泥+偏高岭土以及水泥+偏高岭土+水玻璃)的红黏土固化试验。基于固化剂化学组分和固化土的干密度、pH值以及物相成分等,研究了新型复合碱激发体系作用下偏高岭土对水泥固化红黏土的作用机理。研究表明:当水泥、偏高岭土和水玻璃掺入比分别为12%、5%和3%时,红黏土固化效果最佳,相比于纯水泥固化土其强度提高了2.82倍。在n(SiO_(2))/n(Al_(2)O_(3))从2.53增加至4.05过程中,固化土强度发展较快,随后逐渐趋于稳定。由于水泥水化生成的Ca^(2+)能够平衡固化体系中的部分负电荷,在n(Na_(2)O)/n(Al_(2)O_(3))较小的情况下固化土强度得到了显著提升。最后通过固化土微观形貌及主要物相组成发现,新型复合碱激发体系的试样中含有无定形地聚物凝胶且主要物相特征峰峰值有所降低,说明产生了更多的地聚合凝胶产物。

激振搅拌对水泥稳定碎石与煤矸石抗压强度的影响

为研究激振搅拌工艺对水泥稳定煤矸石混合料性能的影响规律,以水泥稳定碎石为基准,对比分析普通搅拌与激振搅拌对不同水泥剂量的水泥稳定基层材料无侧限抗压强度的影响。结果表明:在干拌状态下增加激振工艺对基层混合料抗压强度的提升效果显著,在激振气压为0.4 MPa、激振干拌30 s后普通湿拌30 s条件下,混合料的7 d无侧限抗压强度达到最佳;激振搅拌对水泥稳定碎石、水泥稳定煤矸石混合料强度的平均提升幅度约为20%,对煤矸石混合料的强度提升更加显著,水泥剂量越少,提升幅度越大。SEM结果表明激振搅拌后混合料中的水化产物分布更加均匀,结构更加致密;经线性拟合计算,当水泥剂量为2.0%~7.0%(质量分数,下同)时,激振搅拌对于碎石混合料可节约8.9%~26.3%的水泥,对于煤矸石混合料可节约12.9%~30.8%水泥,且节约率与水泥剂量成反比。

牡蛎壳微粉改性水泥基复合材料性能及微观结构研究

以牡蛎壳粉部分替换传统水泥基材料,在实现牡蛎壳废物利用的同时可有效降低建材成本。利用牡蛎壳粉作为普通硅酸盐水泥掺合料,探讨其不同掺杂量及不同粉磨方式对水泥胶砂强度及安定性的影响,确定了牡蛎壳粉的适宜添加量,并用XRD、SEM及EDS等测试分析手段表征不同掺量下胶凝试块的结构及性能变化。实验结果表明,牡蛎壳的球磨方式对水泥胶砂强度有明显影响,掺加湿法球磨牡蛎壳粉的胶凝材料的强度高于相应的掺加干法球磨的样品。牡蛎壳粉的掺加量为5%时,试样的抗压强度和抗折强度分别达到49.2 MPa和7.55 MPa,显著高于未掺加的空白样品。通过安定性实验制得的试样经过预养、沸煮之后,表面均光滑、无裂纹,体积无明显膨胀或收缩,表明掺杂牡蛎壳粉的胶凝材料安定性良好。微观结构表征结果显示牡蛎壳粉在水泥胶凝材料中独立存在,主要起惰性填充作用。利用牡蛎壳微粉部分替代水泥基材料是可行的,既可以改善水泥基材料综合性能,又能节约材料成本、减少环境污染,具有良好的应用前景。

Investigation on mechanical properties and water stability of porous polyurethane concrete

Porous polyurethane concrete(PPUC)is a novel material for permeable pavements and is considered as an alternative to porous asphalt or porous cement concrete.However,studies of the mechanical properties of PPUC are still insufficient.In this study,the comprehensive mechanical properties and water stability of PPUC with different gradations and polyurethane dosages were investigated,and its water damage mechanism was preliminarily explored.The results show that the flexural strength and Marshall stability of PPUC can more easily reach the index in the standards of porous cement concrete or porous asphalt,while the compressive strength and abrasion resistance are the weak points of its mechanical properties and need to be further optimized.The mechanical properties and water stability of PPUC were effectively improved by increasing the polyurethane dosage and using continuously graded aggregates.PPUC is more susceptible to water damage because water reacts with the residual isocyanate groups within the polyurethane film to generate carbon dioxide gas,which reduces the cohesion and adhesion performance of polyurethane film.This study provides a comprehensive understanding of the mechanical properties of PPUC and an initial insight into the mechanism of water damage.

水泥稳定碎石拉压不同力学特性研究

为解决现行路面结构设计参数及其取值与材料实际力学特性不匹配的问题,选取典型水泥稳定碎石开展了不同水泥剂量与加载速度下的单轴压缩及直接拉伸试验,研究了其应力应变特性,并分析了压拉强度、模量及泊松比的变化规律及其相互关系,揭示了水泥稳定碎石的拉压不同力学特性.结果表明:水泥稳定碎石拉压受力状态下的应力应变性质符合双模量理论的双线性特征,2%~3%水泥剂量范围为水泥稳定碎石力学本质特性发生变化的过渡区;水泥稳定碎石的拉压力学参数与加载速度和水泥剂量均呈良好的非线性变化关系;就影响因素而言,水泥剂量对材料拉压参数的影响程度远大于加载速度;就力学参数而言,模量受水泥剂量和加载速度的影响最大,其次为强度,最后为泊松比,且相较于抗压,抗拉力学参数受这2种因素的影响更为显著;压拉模量比及压拉泊松比的比值受加载速度和水泥剂量的影响较小,压拉强度比随水泥剂量的增大而逐渐减小,当水泥剂量超过3%时,压拉强度比趋于稳定,据此建立了水泥稳定碎石的拉压力学参数量化取值模型.研究结果可为基于双模量理论的路面结构设计参数取值提供依据.

大跨度双层甲板在轴压载荷下的稳定性研究

针对大跨度双层甲板缩尺模型设计并开展了单轴压缩试验,分析了缩尺模型的极限承载力以及结构的崩溃特性.计及初始缺陷的影响建立数值仿真模型,将试验结果与仿真结果进行对比,分析了双层甲板模型在轴压载荷作用下的稳定性及极限承载特性.结果表明:文中研究的双层甲板缩尺模型的失效模式主要是板格屈曲变形,仿真计算得到的模型应力分布情况与试验测量的结果也基本吻合.

冻融损伤过程中纤维加筋土的抗压性能与裂隙演化

冻融损伤土的微观结构,土中产生微裂隙,宏观上表现为土的力学性能下降。文章完成了石灰固化土和纤维与石灰加筋固化土的冻融试验、无侧限抗压强度试验、CT扫描试验,分析冻融损伤过程中石灰固化土和纤维与石灰加筋固化土的抗压性能与裂隙演化特征,评价纤维对提高土的抗冻融性能的积极作用。结果表明:石灰固化土、纤维与石灰加筋固化土的抗压强度与破坏应变均随冻融次数的增加呈阶段性下降,即降幅较大阶段、降幅较小阶段和强度基本稳定阶段,纤维加筋显著提高了土的抗变形性能;土的压实度越大,纤维加筋提高土的力学性能就越明显;冻融过程中,裂隙主要分布在试样的表面、上部和下部,面裂隙率随扫描层数的增加呈先减小后增大的变化规律;纤维与石灰加筋固化土的体裂隙率、裂隙长度均值、裂隙宽度均值均小于石灰固化土,纤维加筋延缓了裂隙的形成、发展与连通。纤维与石灰加筋固化土的抗压性能与抗冻融性能显著优于石灰固化土,纤维加筋减弱了冻融土的微观结构损伤程度。

金融法视阈下的行政执法和解制度探析

《中华人民共和国金融稳定法(草案)》规定了行政执法当事人承诺制度,正在为金融领域行政执法和解制度的构建提供法律依据。与此同时,虽然行政和解暂未形成理论共识,但金融法视阈下行政执法和解的实践已然先行于理论。针对金融风险处置和证券期货领域中的金融纠纷,行政机关在符合依法行政的要求下通过行使自由裁量权,借助协商民主、公众参与等私法化手段采取的行政执法和解措施,可有效化解纠纷且实现各方获益的经济及社会效果。通过对金融领域行政执法和解措施的实践检视,提出以各方均可获益作为启动行政执法和解制度的前提,通过举证责任的倾斜、“自认”的可适用性与行刑衔接的合规机制相结合,将有助于构建更加符合实践理性的行政执法和解制度。

高温作用后新型高强QN1803不锈钢轴压构件局部稳定性能研究

通过建立新型高强度QN1803不锈钢工字形和方矩形截面短柱的有限元模型对其高温作用后的局部稳定性能进行了数值模拟分析.通过参数分析探究了残余应力、局部初始几何缺陷及温度工况对构件局部稳定性能及极限承载力的影响.研究结果表明:对于方矩形构件及截面正则化长细比小于1.0的厚实工字形构件,残余应力对承载力的影响较小;对于薄柔工字形构件,残余应力会降低其截面承载力,且降低幅度随着温度的升高而下降.此外,随着温度升高,初始几何缺陷对不锈钢工字形构件承载力产生显著影响且正则化宽厚比的影响限值变大,对不锈钢方矩形构件承载力的影响则相反.最后,本文基于参数分析结果,评估了现行欧洲规范EN 1993-1-4、美国规范ASCE/SEI-8-02以及中国《不锈钢结构技术规程》(CECS 410:2015)中不锈钢构件局部稳定性能常温设计方法的适用性,发现了现有设计方法中对高强不锈钢轴心受压构件的高温作用后设计指导尚存在不合理之处并提出了优化建议.

计算长度反推法在钢压杆整体稳定试验中的应用

钢压杆的整体稳定承载力与构件的初始几何缺陷、残余应力、钢材弹塑性以及端部的约束情况密切相关。试验观测表明,实际测得的压杆稳定极限承载力高于按压杆几何长度确定的屈曲荷载。这主要是由于压杆两端支座实际上很难达到理想的铰接状态,较强的支座约束情况引起构件计算长度取值变小,钢压杆的稳定极限承载力随之提高。根据传统的构件稳定理论,利用了压杆整体稳定承载力试验弹性阶段的荷载-位移关系,提出了一种反推计算长度的方法。借助可靠的有限元模型分析验证了该方法的正确性。将此方法应用于实际的钢压杆整体稳定承载力试验数据分析,不仅合理解释了试验现象与结果,还消除了不同支座形式对钢压杆整体稳定承载力大小的影响。相关结论可为钢压杆整体稳定承载力试验研究提供参考。