Insight into Heterogeneity Effects in Methane Hydrate Dissociation via Pore-Scale Modeling

A large number (1253) of high-quality streaming potential coefficient (\(C_\mathrm{sp})\) measurements have been carried out on Berea, Boise, Fontainebleau, and Lochaline sandstones (the latter two including both detrital and authigenic overgrowth forms), as a function of pore fluid salinity (\(C_\mathrm{f})\) and rock microstructure. All samples were saturated with fully equilibrated aqueous solutions of NaCl (10\(^{-5}\) and 4.5 mol/dm\(^{3})\) upon which accurate measurements of their electrical conductivity and pH were taken. These \(C_\mathrm{sp}\) measurements represent about a fivefold increase in streaming potential data available in the literature, are consistent with the pre-existing 266 measurements, and have lower experimental uncertainties. The \(C_\mathrm{sp}\) measurements follow a pH-sensitive power law behaviour with respect to \(C_\mathrm{f}\) at medium salinities (\(C_\mathrm{sp} =-\,1.44\times 10^{-9} C_\mathrm{f}^{-\,1.127} \), units: V/Pa and mol/dm\(^{3})\) and show the effect of rock microstructure on the low salinity \(C_\mathrm{sp}\) clearly, producing a smaller decrease in \(C_\mathrm{sp}\) per decade reduction in \(C_\mathrm{f}\) for samples with (i) lower porosity, (ii) larger cementation exponents, (iii) smaller grain sizes (and hence pore and pore throat sizes), and (iv) larger surface conduction. The \(C_\mathrm{sp}\) measurements include 313 made at \(C_\mathrm{f} > 1\) mol/dm\(^{3}\), which confirm the limiting high salinity \(C_\mathrm{sp}\) behaviour noted by Vinogradov et al., which has been ascribed to the attainment of maximum charge density in the electrical double layer occurring when the Debye length approximates to the size of the hydrated metal ion. The zeta potential (\(\zeta \)) was calculated from each \(C_\mathrm{sp}\) measurement. It was found that \(\zeta \) is highly sensitive to pH but not sensitive to rock microstructure. It exhibits a pH-dependent logarithmic behaviour with respect to \(C_\mathrm{f}\) at low to medium salinities (\(\zeta =0.01133 \log _{10} \left( {C_\mathrm{f} } \right) +0.003505\), units: V and mol/dm\(^{3})\) and a limiting zeta potential (zeta potential offset) at high salinities of \({\zeta }_\mathrm{o} = -\,17.36\pm 5.11\) mV in the pH range 6–8, which is also pH dependent. The sensitivity of both \(C_\mathrm{sp}\) and \(\zeta \) to pH and of \(C_\mathrm{sp}\) to rock microstructure indicates that \(C_\mathrm{sp}\) and \(\zeta \) measurements can only be interpreted together with accurate and equilibrated measurements of pore fluid conductivity and pH and supporting microstructural and surface conduction measurements for each sample.     

Contribution to the Modelling of Coupled Heat and Mass Transfers on 3D Real Structure of Heterogeneous Building Materials: Application to Hemp Concrete

Hydraulic flow, electrical flow and the passage of elastic waves through porous media are all linked by electrokinetic processes. In its simplest form, the passage of elastic waves through the porous medium causes fluid to flow through that medium and that flow gives rise to an electrical streaming potential and electrical counter-current. These processes are frequency-dependent and governed by coupling coefficients which are themselves frequency-dependent. The link between fluid pressure and fluid flow is described by dynamic permeability, which is characterised by the hydraulic coupling coefficient (C_hp). The link between fluid pressure and electrical streaming potential is characterised by the streaming potential coefficient (C_sp). While the steady-state values of such coefficients are well studied and understood, their frequency dependence is not. Previous work has been confined to unconsolidated and disaggregated materials such as sands, gravels and soils. In this work, we present an apparatus for measuring the hydraulic and streaming potential coefficients of high porosity, high permeability consolidated porous media as a function of frequency. The apparatus operates in the range 1 Hz to 2 kHz with a sample of 10 mm diameter and 5–30 mm in length. The full design and validation of the apparatus are described together with the experimental protocol it uses. Initial data are presented for three samples of Boise sandstone, which present as dispersive media with the critical transition frequency of 918.3?±?99.4 Hz. The in-phase and in-quadrature components of the measured hydraulic and streaming potential coefficients have been compared to the Debye-type dispersion model as well as theoretical models based on bundles of capillary tubes and porous media. Initial results indicate that the dynamic permeability data present an extremely good fit to the capillary bundle and Debye-type dispersion models, while the streaming potential coefficient presents an extremely good fit to all of the models up to the critical transition frequency, but diverges at higher frequencies. The streaming potential coefficient data are best fitted by the Pride model and its Walker and Glover simplification. Characteristic pore size values calculated from the measured critical transition frequency fell within 1.73% of independent measures of this parameter, while the values calculated directly from the Packard model showed an underestimation by about 12%.

     

考虑接触界面材料微观结构与势能参数的滑动摩擦计算研究

根据独立振子模型的能量耗散机理,提出了在无磨损界面摩擦中,利用通用界面粘附能量函数计算滑动时接触界面势能的变化从而计算摩擦力和摩擦系数的方法,建立摩擦力和摩擦系数与金属材料表面能与微观结构参数的关系,并利用已有的实验数据进行计算.结果表明,摩擦力与金属材料的表面能基本呈线性关系,与比例参数呈线性负相关关系,与晶格常数基本无关.计算结果与粘附理论公式以及超高真空原子力显微镜试验结果吻合较好.     

Surface Probing with Streaming Potential: The Polymer Flooding Scenario

Transport in Porous Media - The first experimental evidence of polymer adsorption in natural carbonates obtained from streaming potential measurements is presented. The surface probing of the...     

Development of an inverse heat conduction model and its application to determination of heat transfer coefficient during casting solidification

The interfacial heat transfer coefficient (IHTC) is required for the accurate simulation of heat transfer in castings especially for near net-shape processes. The large number of factors influencing heat transfer renders quantification by theoretical means a challenge. Likewise experimental methods applied directly to temperature data collected from castings are also a challenge to interpret because of the transient nature of many casting processes. Inverse methods offer a solution and have been applied successfully to predict the IHTC in many cases. However, most inverse approaches thus far focus on use of in-mold temperature data, which may be a challenge to obtain in cases where the molds are water-cooled. Methods based on temperature data from the casting have the potential to be used however; the latent heat released during the solidification of the molten metal complicates the associated IHTC calculations. Furthermore, there are limits on the maximum distance the thermocouples can be placed from the interface under analysis. An inverse conduction based method have been developed, verified and applied successfully to temperature data collected from within an aluminum casting in proximity to the mold. A modified specific heat method was used to account for latent heat evolution in which the rate of change of fraction solid with temperature was held constant. An analysis conducted with the inverse model suggests that the thermocouples must be placed no more than 2 mm from the interface. The IHTC values calculated for an aluminum alloy casting were shown to vary from 1,200 to 6,200 Wm^?2 K^?1. Additionally, the characteristics of the time-varying IHTC have also been discussed.     

旋翼翼型动态失速流场特性PIV试验研究及L-B模型修正

为测量翼型动态失速的非定常涡流场特性,采用3D-PIV 技术,对典型直升机旋翼翼型SC1095 的动态失速流场特性进行测量,发现涡在不同位置处的输运速度不同:位于翼型表面的涡的无量纲速度为0.39,位于尾迹区的涡的无量纲速度为0.55. 利用前缘涡输运速度变化这一特征,改进了经典的翼型动态失速利什曼-贝多斯（Leishman-Beddoes,L-B）模型,将该模型中固定的涡时间常数修正为可以随涡位置变化的时变函数,修正后的模型计算得到翼型法向力峰值相对原L-B 模型提升5%,力矩系数负峰值相对原L-B 模型提升13%,与试验值相比更加吻合,表明修正后的翼型动态失速模型更好地体现了翼型前缘涡的物理特征.      

摩擦磨损的接触界面势垒理论研究

根据系统和能量的观点,从摩擦界面的微观相互作用出发,提出摩擦磨损的接触界面势垒理论,研究接触界面势垒和标准接触界面势垒的计算方法,并推导出摩擦力、摩擦系数以及粘着磨损量的计算公式.研究表明,界面的摩擦磨损性能取决于接触界面势垒的大小和分布,用标准接触界面势垒作为表征摩擦副材料摩擦磨损特性的指标具有较好的独立性和稳定性.利用已有的实验数据进行计算,结果与超高真空原子力显微镜实验结果基本符合,验证了所提出理论和方法的可行性.     

PIV EXPERIMENTS ON FLOWFIELD CHARACTERISTICS OF ROTOR AIRFOIL DYNAMIC STALL AND MODIFICATIONS OF L-B MODEL

为测量翼型动态失速的非定常涡流场特性,采用3D-PIV 技术,对典型直升机旋翼翼型SC1095 的动态失速流场特性进行测量,发现涡在不同位置处的输运速度不同：位于翼型表面的涡的无量纲速度为0.39,位于尾迹区的涡的无量纲速度为0.55. 利用前缘涡输运速度变化这一特征,改进了经典的翼型动态失速利什曼-贝多斯（Leishman-Beddoes,L-B）模型,将该模型中固定的涡时间常数修正为可以随涡位置变化的时变函数,修正后的模型计算得到翼型法向力峰值相对原L-B 模型提升5%,力矩系数负峰值相对原L-B 模型提升13%,与试验值相比更加吻合,表明修正后的翼型动态失速模型更好地体现了翼型前缘涡的物理特征.     

Experimental investigation in pool boiling heat transfer of ammonia/water mixture and heat transfer correlations

The nucleate pool boiling heat transfer coefficient of ammonia/water mixture was investigated on a cylindrical heated surface at low pressure of 4-8 bar and at low mass fraction of 0 < x_NH3 < 0.3 and at different heat flux. The effect of mass fraction, heat flux and pressure on boiling heat transfer coefficient was studied. The results indicate that the heat transfer coefficient in the mixture decreases with increase in ammonia mass fraction, increases with increase in heat flux and pressure in the investigated range. The measured heat transfer coefficient was compared with existing correlations. The experimental data were predicted with an accuracy of ±20% by the correlation of Calus&Rice, correlation of Stephan-Koorner and Inoue-Monde correlation for ammonia/water mixture in the investigated range of low ammonia mass fraction. The empirical constant of the first two correlations is modified by fitting the correlation to the present experimental data. The modified Calus&Rice correlation predicts the present experimental data with an accuracy of ±18% and the modified Stephan-Koorner correlation with an accuracy of ±16%.     

Heat transfer during heat sterilization and cooling processes of canned products

In this paper, an analysis of transient heat transfer during heat sterilization and cooling processes of a cylindrical canned product is presented. In the analysis, most practical case including the boundary condition of third kind (i.e., convection boundary condition, leading to 0.1 ≤ Bi ≤ 100) was employed. A simple analytical model for determining effective heat transfer coefficients for such products is developed. For the heat sterilization process, heating coefficient is incorporated into heat transfer coefficient model. An experimental study was performed to measure the thermal center temperatures of the short-cylindrical canned products (i.e., Tuna fish) during heat sterilization at the retort medium temperatures of 115^∘C and 121^∘C, and during cooling process at 16^∘C. The effective heat transfer coefficient model used the experimental temperature data. Using these effective heat transfer coefficients the center temperature distributions were calculated and compared with the experimental temperature distributions. Agreement was found considerably high. The results of the present study indicate that the heat-transfer analysis technique and heat-transfer coefficient model are reliable, and can provide accurate results for such problems. Received on 12 November 1997     

Instantaneous Squeeze-Film Force Between a Heat Exchanger Tube and a Support Plate for Arbitrary Tube Motion

The instantaneous squeeze-film force between a heat exchanger tube and a support plate is studied. Based on a two-dimensional rectangular plate model, a short-sleeve squeeze-film model for arbitrary tube motion is developed. The instantaneous squeeze-film force is expressed in normal and tangential directions. The normal squeeze-film force consists of four nonlinear terms, the viscous, unsteady inertia, convective inertia and centripetal inertia terms. Three nonlinear terms, the viscous, unsteady inertia and Coriolis inertia terms, make up the tangential squeeze-film force. An experimental apparatus was developed in order to evaluate the theoretical models against measurements of a finite length squeeze film. A modified model based on the experimental data is obtained where the viscous terms for both directions are multiplied by the instantaneous Reynolds number. All the inertia terms are multiplied by constant coefficients. The modified model is in good agreement with most experimental cases for unsymmetrical linear motion, approximate circular motion and elliptical motion. The form of the modified model is suitable for predicting instantaneous squeeze-film forces in the simulation of heat exchanger tube vibration. Further work using different sized components and fluid properties is required in order to finalize coefficient values.     

外加极化电位对双相不锈钢在硫酸介质中腐蚀磨损行为及摩擦性能的影响

利用专门设计制造的腐蚀磨损试验机，对在不同极化电位下双相不锈钢于硫酸介质中的腐蚀磨损行为和摩擦性能进行了试验研究，并且借助于扫描电子显微镜等对试样的磨损表面作了观察与分析．结果表明：采用适当的阴极保护可以有效地降低双相不锈钢的腐蚀磨损率；在钝化电位下的腐蚀磨损率比在自然电位和阴极保护电位下的都高；外加极化电位在阴极保护电位区或与动态腐蚀电位相当时，摩擦系数比自然电位下的低；钝化区的摩擦系数比自然电位下的高，可见此时钝化膜的减摩性能差；在钝化电位下当载荷高于１１．７Ｎ时，双相不锈钢的表面钝化膜遭受严重破坏     

Dislocation-mediated strain hardening in tungsten: Thermo-mechanical plasticity theory and experimental validation

A self-consistent thermo-mechanical model to study the strain-hardening behavior of polycrystalline tungsten was developed and validated by a dedicated experimental route. Dislocation–dislocation multiplication and storage, as well dislocation-grain boundary (GB) pinning were the major mechanisms underlying the evolution of plastic deformation, thus providing a link between the strain hardening behavior and material's microstructure. The microstructure of the polycrystalline tungsten samples has been thoroughly investigated by scanning and electron microscopy. The model was applied to compute stress–strain loading curves of commercial tungsten grades, in the as-received and as-annealed states, in the temperature range of 500–1000 °C. Fitting the model to the independent experimental results obtained using a single crystal and as-received polycrystalline tungsten, the model demonstrated its capability to predict the deformation behavior of as-annealed samples in a wide temperature range and applied strain. The relevance of the dislocation-mediated plasticity mechanisms used in the model have been validated using transmission electron microscopy examination of the samples deformed up to different amounts of strain. On the basis of the experimental validation, the limitations of the model are determined and discussed.     

Computational study of steady streaming from oscillating microbubbles with uniform and wavy wall motions

Steady streaming flow fields of a 5 μm bubble oscillating with uniform radial wall motion and a 500 μm bubble oscillating with wavy wall motion were simulated using a computational fluid dynamics method that incorporated fluid–structure interactions. The steady streaming flow fields for both bubbles were calculated, and they exhibited upward jet flow with two symmetrical counter-rotating vortices. The maximum streaming velocity ranged from a few to tens of millimeters per second. The simulated flow fields were compared with the theory and experimental measurements using particle image velocimetry. The simulation results agreed well with the theoretical and experimental data. Therefore, the proposed computational method would provide a useful tool to predict steady streaming flow fields of oscillating bubbles.     

Analytical considerations of flow boiling heat transfer in metal-foam filled tubes

Flow boiling in metal-foam filled tube was analytically investigated based on a modified microstructure model, an original boiling heat transfer model and fin analysis for metal foams. Microstructure model of metal foams was established, by which fiber diameter and surface area density were precisely predicted. The heat transfer model for flow boiling in metal foams was based on annular pattern, in which two phase fluid was composed by vapor region in the center of the tube and liquid region near the wall. However, it was assumed that nucleate boiling performed only in the liquid region. Fin analysis and heat transfer network for metal foams were integrated to obtain the convective heat transfer coefficient at interface. The analytical solution was verified by its good agreement with experimental data. The parametric study on heat transfer coefficient and boiling mechanism was also carried out.     

强流脉冲电子束表面改性Al-Pb合金的摩擦磨损行为研究

利用强流脉冲电子束技术对Al-Pb合金进行表面改性处理,测试改性试样表面硬度分布及其形貌和组织,采用销-盘式摩擦磨损试验机、扫描电子显微镜和X射线光电子能谱仪对改性Al-Pb合金的摩擦磨损性能及其磨损机理进行分析.结果表明:改性Al-Pb合金的组织结构得到改善,最大硬度出现在距合金表面约40 μm的亚表层;由熔化区、热影响和准静态热应力影响重叠区和过渡区组成,改性层表现出良好的耐磨性;在高载荷下改性合金表现出的低磨损率和摩擦系数与其磨损表面形成的润滑膜密切相关;随着载荷增加,其磨损机制主要表现为氧化磨损和粘着磨损.     

RESISTANCE EFFECT OF ELECTRIC DOUBLE LAYER ON LIQUID FLOW IN MICROCHANNEL

Poisson-Boltzrnann equation for EDL (electric double layer) and Navier-Stokes equation for liquid flows were numerically solved to investigate resistance effect of electric double layer on liquid flow in microchannel. The dimension analysis indicates that the resistance effect of electric double layer can be estimated by an electric resistance number, which is proportional to the square of the liquid dielectric constant and the solid surface zeta potential, and inverse-proportional to the liquid dynamic viscosity, electric conductivity and the square of the channel width. An "electric current density balancing" (ECDB) condition was proposed to evaluate the flow-induced streaming potential, instead of conventional "electric current balancing" (ECB) condition which may induce spurious local backflow in neighborhood of the solid wall of the microchannel. The numerical results of the flow rate loss ratio and velocity profile are also given to demonstrate the resistance effect of electric double layer in microchannel.     

Liquid and gas entrainment to a small break hole from a stratified two-phase region

A theoretical and experimental study has been conducted for liquid and gas entrainment to a small break hole from a stratified two-phase region. Theoretical correlations previously obtained for top, side and bottom entrainment were modified to express the relation between break flow rate, break quality and bulk water level so that they can be used easily for any break geometry. The modified correlations were assessed with experimental data obtained under room temperature and low pressure conditions using air and water. The experiment results were predicted well with the present model without using any adjustment coefficient when the upstream flow was symmetrical around the break. The effects of vortex, crossflow and wavy flow, observed in the experiment but not taken into account in the model, were empirically correlated based on the present correlation. By using the empirically modified correlations, the data in the literature, including high-pressure steam-water conditions, were reasonably predicted.     

A Theoretical Model with Experimental Verification to Predict Hydrodynamics of Foams

An experimentally validated theoretical model, based on hydraulic resistance network and scale analysis at the pore level, is developed to predict the pressure drop for flow through foams. The complex microstructure of the foams is modeled as a matrix of interconnected solid ligaments forming simple cubic arrays of cylinders. New correlations for permeability and form drag (inertia) coefficient are presented as functions of the mean pore and ligament diameter as well as the foam porosity. The present model makes it possible to conduct parametric studies. Results obtained from the proposed model are successfully compared with our experimental data as well those found in the literature to observe good agreement.