网络窄缝通道气液逆向渗透传输特性研究

利用网络窄缝通道模拟实验研究DMFC多孔扩散层气液两相逆流传输特性,结果表明,网络结构喉道内水侵渗机制呈"活塞状",孔内侵渗机制包含三种:"弯月形"、"球冠形"和几何截面突变处"夹断"气柱方式;网络结构内空气驱替水的机制包含两种:喉道内呈"活塞状"驱替,孔内呈"球冠形"驱替;气体流量小,气驱液在网络结构内呈现稳定驱替现象,随着气体流量的增加,逐渐向"树冠形"分布转变;网络结构内气相驱动压力随气相流量和水流量的增加而增加;气相饱和度略有减小。     

基于分形几何的非饱和多孔介质热流映射关系

基于分形几何建立了非饱和多孔介质热传导和气液两相渗流的孔隙尺度模型,研究并提出了传热与渗流的映射关系。结果表明非饱和多孔介质的热流特性取决于介质孔隙率、孔隙尺度限、孔隙分形维数、迂曲度分形维数和液相饱和度。孔隙结构固定时,液相相对渗透率与有效热导率均随着液相饱和度的增加而提高,气相相对渗透率则随着饱和度的增加而降低,因此液相和气相相对渗透率与有效热导率分别呈正相关和负相关;液相饱和度不变且孔隙结构发生变化时,液相和气相相对渗透率与有效热导率分别呈负相关和正相关。非饱和多孔介质的相对渗透率与有效热导率满足二次函数关系,热流映射关系对于理解其传热传质机理和预测输运特性具有重要的意义。     

多孔介质内泡沫渗流过程入口效应的数值研究

本文利用随机泡沫数目守恒模型对均质多孔介质内泡沫液渗流过程的入口效应进行了数值研究。随机泡沫数目守恒模型仅利用两个参数:泡沫生成速率Kg与最大泡沫个数n_∞,来描述多孔介质内非稳态驱替过程中泡沫的生成和发展。采用IMPES方法对二维介质内泡沫两相渗流过程的控制方程组进行求解,基于驱替过程中液相压力、液相饱和度及气泡密度分布等参数的分析,对泡沫驱替过程的入口效应进行了详细研究。结果发现,增大泡沫生成速率Kg,泡沫渗流过程的入口效应减小;而增大参数n_∞,虽可提高驱替压差,但对多孔介质入口段各参数的分布规律影响不大。     

非饱和多孔介质内毛细驱动流动分析

将非饱和含湿多孔介质层内液体饱和度当量为无量纲的液层厚度,在多孔层内发生相变时其分布规律直观地反映相变传热能力从而预示临界热流(CHF)的发生.计算结果显示,多孔层中心处最容易出现干涸而导致CHF;残余饱和度和孔隙率越大,越不容易出现CHF;饱和度分布形状(液层形状)主要受输入热流密度和孔隙率的影响,热流越小、孔隙率越大就越平缓.     

PEMFC气体扩散层干燥过程孔隙网络模拟

本文采用三维孔隙网络模型从孔隙尺度上对质子交换膜燃料电池气体扩散层中水分蒸发的干燥过程进行了模拟,并考虑了阴极流道的影响.计算结果表明毛细力在气体扩散层的干燥过程中起主要作用,气相从阴极流道底部开始,以沿气体扩散层厚度方向侵入为先,直到到达气体扩散层底部,随后气相才向流道肩部水平延伸侵入.蒸发速率随气体扩散层中水饱和度...     

非稳态人口温度下相变材料蓄热性能研究

太阳能辐射强度随时间发生变化,造成蓄热单元入口处传热流体温度呈现非稳态变化。为了分析非稳态的入口温度对相变材料蓄热特性的影响,假设传热流体入口温度1 h内随时间呈线性变化,并且1 h内的平均温度恒定在60℃。讨论了入口温度随时间线性增加及线性降低两种变化形式,对相变材料的熔化速率、熔化分数、蓄热量、固液界面位置等参数的影响。结果表明,当1 h内平均入口温度不变,而初始入口温度在30～90℃的范围内变化时,随初始入口温度增加,尽管熔化速率增加,熔化时间从42.75 min减小到20.58 min;但1 h内的总蓄热量却从72.6 kW减小到45.3 kW。     

低温液体BOG再液化系统压力及蒸发率试验研究

为研究低温液体吸热产生蒸发气(Boil-Off Gas,BOG)的动态过程,寻求合理调控低温液体压力和温度的方法,搭建了一套低温液体BOG再液化试验系统。以液氮为工质对120L高真空变密度多层绝热储罐进行了压力、温度及蒸发率测试试验,分析了以上参数与时间的变化规律,计算了储罐静态蒸发率与漏热量。结果表明:储罐压力随时间增加而逐渐上升,在480min之前压力上升速率较快,为10.9Pa/s,之后上升速率逐渐减小。从液相到气相的温度依次升高,液相内部的温度相差较小,约为1.2℃;随时间的增加,液相和气液分界面的温度逐渐升高,气相的温度逐渐降低,480min后达到相对稳定的状态。初始充装率为0.7时,自然蒸发的BOG流量随时间增加逐渐减小;经计算,储罐静态蒸发率为2.04%/d,漏热量为4.1W。试验结果为后续开展低温液体BOG再液化研究提供了相关依据。     

PEMFC气体扩散层内PTFE含量及分布对气液两相流影响的LBM研究

基于数值重建的碳纸气体扩散层(GDL)三维微结构,采用格子玻尔兹曼方法(LBM)模拟了质子交换膜燃料电池(PEMFC)GDL内的气液两相流,详细研究了聚四氟乙烯(PTFE)含量及分布对GDL内两相输运的影响。计算结果发现:PTFE非均匀分布(靠近气流通道侧含量更多)时GDL内更容易积水;PTFE含量较多时GDL内积累的液态水含量较少。基于计算结果推导了气液两相相对渗透率,分析发现:PTFE含量及分布对气相相对渗透率影响较小;液相饱和度小于0.8时,PTFE含量较低时液相相对渗透率较高,而当液相饱和度大于0.8时,PTFE含量较高时液相相对渗透率较高;与PTFE非均匀分布时相比,PTFE均匀分布时液相相对渗透率较高。     

液相饱和度对多孔介质稳态导热系数的影响

在非饱和含湿多孔介质传热传质的过程中,由于湿分迁移的存在,使得真实导热系数的测量和处理存在争议。为此,运用有限元方法,求解了二维稳态条件下非饱和含湿多孔体特征单元的温度和热流密度,给出了不同液相饱和度时的有效导热系数。结果表明,对于通常的导热能力固体远大于液体、液体远大于气体的情形,随着液相饱和度的增加,多孔介质的导热系数先迅速增加,然后逐渐趋于平稳。     

基于弹性力学参数的天然气识别评价技术研究*

利用测井资料对低孔渗天然气储层进行识别对有效地勘探开发该类型储层具有重要意义。本文研究了天然气储层的岩石力学参数的特征及与含气饱和度的关系,通过理论推导和分析,总结了天然气储层识别方法,建立了低孔渗天然气储层含气饱和度计算模型。采用纵横波速比、泊松比、拉梅系数和体积模量两两交图版来定性识别天然气储层。模量比差比基本不受岩性和孔隙度影响,与含气饱和度呈线性正比关系,可用于定量评价低孔渗天然气储层的含气饱和度。利用上述方法对南堡凹陷深层的测井资料进行了处理分析,有效地识别了低孔渗天然气层段,对该天然气层段的定量评价结果表明,可以有效提高气层解释符合率。     

含气孔裂隙地层的声波测井响应特征

本文以径向分层的孔、裂隙地层多极子井孔声场理论为基础,利用孔、裂隙介质弹性波动统一理论,数值模拟了井壁上有侵入时,单极和偶极声源激发的井孔声场随原状地层含气饱和度变化的特征和规律。结果表明：当原状地层裂隙发育时,相较于饱含水,饱含气时纵波幅度衰减严重,横波波至提前,波幅增大,斯通利波幅度变化不明显,低频时弯曲波到时提前,幅度增大;当原状地层裂隙不发育时,单极子、偶极子波形都对流体性质没有明显响应。可见,原状地层中裂隙的存在加剧了含气对纵波、横波的幅度以及到时的影响,而侵入带的存在使得斯通利波和高频弯曲波对原状地层流体性质变化不敏感。数值计算结果为利用声波响应特征识别致密气层提供了参考。     

多孔介质内受迫对流凝结时两相共存区的非达西模型

本文提出多孔介质内受迫对流凝结时两相共存区的二维非达西流模型。分析了蒸气在多孔介质内沿水平平板和填充国管内受迫对流凝结时两相共存区的厚度。假定局部平衡和暂先假设蒸气为理想气体，化简了能量方程。     

Modelling of candle burning with a self-trimmed wick

As an example of a coupled gas-phase diffusion flame with porous media flow, a candle burning model with a porous wick is offered in this paper. The porous media analysis includes capillarity-induced liquid flow, liquid vaporisation, vapour motion and re-condensation and multi-phase heat transfer. Coupling with the gas phase flame is through the conservation of mass, momentum and energy at the wick surface. The steady state solutions obtained not only yield the flame structure but also the detailed flow pattern and saturation distributions inside the wick. One of the novel features of the present model is the capability to address the self-trimming phenomena of candle burning. The self-trimming wick length and the associated flame characteristics have been computed as a function of gravity level, wick permeability and wick diameter.     

PEM燃料电池毛细压力流道拓展模型

多孔电极与流道内的水淹现象是制约质子交换膜(PEM)燃料电池性能提高的关键难题.传统的宏观两相模型普遍存在液态水饱和度预测值偏低的不足,而流道内气液两相雾状流的假设则是低估水淹程度的一个主要原因.本文提出了反映流道壁面浸润差异性的毛细压力流道拓展模型,并对低气速下燃料电池内的传热传质进行了模拟.计算结果表明,该模型克服...     

Transformation of seismic velocity data to extract porosity and saturation values for rocks

For wave propagation at low frequencies in a porous medium, the Gassmann-Domenico relations are well-established for homogeneous partial saturation by a liquid. They provide the correct relations for seismic velocities in terms of constituent bulk and shear moduli, solid and fluid densities, porosity and saturation. It has not been possible, however, to invert these relations easily to determine porosity and saturation when the seismic velocities are known. Also, the state (or distribution) of saturation, i.e., whether or not liquid and gas are homogeneously mixed in the pore space, is another important variable for reservoir evaluation. A reliable ability to determine the state of saturation from velocity data continues to be problematic. It is shown how transforming compressional and shear wave velocity data to the (rho/lambda, mu/lambda)-plane (where lambda and mu are the Lame parameters and rho is the total density) results in a set of quasi-orthogonal coordinates for porosity and liquid saturation that greatly aids in the interpretation of seismic data for the physical parameters of most interest. A second transformation of the same data then permits isolation of the liquid saturation value, and also provides some direct information about the state of saturation. By thus replotting the data in the (lambda/mu, rho/mu)-plane, inferences can be made concerning the degree of patchy (inhomogeneous) versus homogeneous saturation that is present in the region of the medium sampled by the data. Our examples include igneous and sedimentary rocks, as well as man-made porous materials. These results have potential applications in various areas of interest, including petroleum exploration and reservoir characterization, geothermal resource evaluation, environmental restoration monitoring, and geotechnical site characterization.     

Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems,Part I: H2O2 generation

Cavitational/sonochemical activity can be significantly enhanced or reduced depending on the gases dissolved in the liquid. Although many researchers have suggested the order of importance of dissolved gas conditions that affect the degree of sonoluminescence (SL), sonochemiluminescence (SCL), and compound degradation, the most suitable gas condition for sonochemical oxidation reactions is currently unknown. In this study (Part I), the effects of gas saturation and sparging on the generation of H₂O₂ were investigated in a 28-kHz sonoreactor system. Four gas modes, saturation/closed, saturation/open, sparging/closed, and sparging/open, were applied to Ar, O₂, N₂, and binary gas mixtures. The change in dissolved oxygen (DO) concentration during ultrasonic irradiation was measured and was used as an indicator of whether the gaseous exchange between liquid and air altered the gas content of the liquid. Considerable difference in the DO concentration was observed for the gas saturation/open mode, ranging from –11.5 mg/L (O₂ 100 %) to +4.3 mg/L (N₂ 100 %), while no significant difference was observed in the other gas modes. The change in the gas content significantly reduced the linearity for H₂O₂ generation, which followed pseudo-zero-order kinetics, and either positively or negatively affected H₂O₂ generation. Ar:O₂ (75:25) and Ar:O₂ (50:50) resulted in the highest and second-highest H₂O₂ generation for both gas saturation and sparging, respectively. In addition, gas sparging resulted in much higher H₂O₂ generation for all gas conditions compared to gas saturation; this was because of the significant change in the cavitational active zone and concentrated ultrasonic energy, which formed a bulb-shaped active zone, especially for the Ar/O₂ mixtures adjacent to the transducer at the bottom. The sparging flow rate and position also significantly affected H₂O₂ generation; the highest H₂O₂ generation was obtained when the sparger was placed at the bottom adjacent to the transducer, with a flow rate of 3 L/min.In Part II, the generation of nitrogen oxides, including nitrite (NO₂^–) and nitrate (NO₃^–), was investigated using the same ultrasonic system with three gas modes: saturation/open, saturation/closed, and sparging/closed.     

In-situ observation for natural gas hydrate in porous medium: Water performance and formation characteristic

Extensive efforts have been made regarding gas hydrate sample reconstruction in the laboratory for a better understanding and development of natural gas resources. Magnetic resonance imaging (MRI) is a useful method for directly observing the reconstruction of methane hydrate, yet relevant studies remain limited. In this study, a 9.4-T 400-MHz MRI instrument was employed to investigate CH₄ hydrate formation in porous media involving various initial water saturation levels and sand diameters. Pressure histories and MRI signal variations were monitored to discuss the process of CH₄ hydrate growth, and the three main formation stages of induction, rapid growth, and slow formation were determined. Furthermore, the liquid water performance in MRI micro-images was analyzed to predict the characteristics of CH₄ hydrate formation. The results indicated that CH₄ hydrate formed in a spatially and temporally random manner and that pore plugging occurred owing to the residual water encased in grown hydrate. Additionally, phase saturations, water conversion percentages, and formation rates were defined to evaluate the effect of sand diameter and initial water saturation on CH₄ hydrate formation. With the reduction in the diameter of quartz glass beads from 400 μm to 100 μm, the average hydrate formation rate increased from 0.0010 min⁻¹ to 0.0034 min⁻¹, respectively. When the initial water saturation decreased to the optimized value (0.22 in this study), the water conversion percentage and hydrate saturation increased.     

Discrete models of liquid-vapour phase change phenomena in porous media

This survey presents recent developments in the modelling of liquid-vapor phase change phenomena in porous media within the framework of the discrete approach. The pore network (or discrete) approach is first illustrated through the example of the evaporation at low capillary number of a pure liquid from a capillary porous medium. In particular, it is shown that invasion percolation patterns characterise the phase distribution. Then first results concerning the vaporisation of a liquid within the porous wick of a capillary evaporator are presented. These results show that capillary fingering patterns can be expected within the wick.     

Study of acoustic radiation during air stream filtration through a porous medium

The paper presents results of laboratory experiments on studying the characteristics of acoustic emission generated by a flow of compressed air, which is filtered by porous pumice samples with and without partial fluid saturation. The construction features of the laboratory setup and details of the experiments are described. Porous samples with dry and partially fluid-filled pores are used. The visual patterns of the acoustic emission spectrum, which occurs under stationary filtration of the compressed air, are presented, and its amplitude-frequency distribution characteristic for different sample porosities and different degrees of their fluid saturation is shown. It is demonstrated that the relaxation times of the emission noise level differ. This is revealed during the sharp elimination of the drop in pressure from such samples, i.e., in the nonstationary filtration mode.