Position Of Front Research Articles

Study on the Imbibition Damage Mechanisms of Fracturing Fluid for the Whole Fracturing Process in a Tight Sandstone Gas Reservoir

Tight sandstone gas is a significant unconventional natural gas resource, and has been exploited economically mostly through the application of hydraulic fracturing technology in recent decades. However, formation damage occurs when fracturing fluid percolates into the pores inside sandstones through imbibition driven by capillary pressure during fracturing operations. In this work, the formation damage resulting from the whole operation process composed of fracturing, well shut-in and flowback, and the degree of damage at different moments were investigated through core flow experiments and the low-field Nuclear Magnetic Resonance (NMR) technique. The results show that imbibition damage occurs starting from the contact surface between the formation and the fracturing fluid, which penetrates into an increasingly deep position with time down to a certain depth. The T2 spectra of NMR at different moments indicates that fracturing fluid initially enters the small pores, followed by the large pores due to the larger capillary pressure in the former. Thus, the sandstone cores with low permeability incur a higher degree of damage due to their stronger capability of retaining fracturing fluid compared to high-permeability cores. The front position of the fracturing fluid imbibition at different moments, along with the degree of damage, were characterized through the one-dimensional encoding processing of the NMR signal. These results underlie the effective strategy to relieve formation damage resulting from imbibition during hydraulic fracturing operations.

Study on the Influence of Vent Shape and Blockage Ratio on the Premixed Gas Explosion in the Chamber with a Small Aspect Ratio.

Given the current situation of deflagration caused by gas leakage for domestic use and the study of the single shape of the vent, the differential change law of vent parameters on the small aspect ratio of the chamber explosion flame evolution and explosion overpressure and other effects were experimentally investigated. Based on the theory of geometric similarity, the experimental platform of a small length–diameter ratio explosion chamber was designed and built, and the premixed gas explosion experiment was carried out by changing the shape (square and rectangle) and blockage ratio (0.1, 0.3, 0.5, 0.7, and 0.9). The explosion flame structure, flame front position, flame propagation speed, explosion pressure waveform, overpressure peak value, and so on were tested and analyzed. The results showed that the blockage ratio had the most significant effect on flame propagation and explosion flame evolution. With the increase of blockage ratio, the stretching degree of the flame became more and more obvious, the flame front became sharper and sharper, and the sharp flame changed from the lower part to the upper part. The position of the flame front increased rapidly, and the steepness and peak value of explosion overpressure became larger. The oscillation of the flame propagation velocity was more violent after the turn, and the speed of propagation to the outside of the chamber gradually accelerated. In the same blockage ratio, there was a difference in the time point at which the flame propagation velocity turned under the rectangular and square shape of the vent. In blockage ratios of 0.1, 0.3, 0.5, and 0.9, the peak overpressure reduction under the shape of the blast square relative to the rectangle was 41.3, 47.9, 1.03, and 27.6%. This indicated that the explosion relief effect of the square was better than that of a rectangle. The research results can provide a reference and basis for the reasonable deployment of explosion venting and explosion decompression work.

Approximate solution to the generalized radial Boussinesq equation

This paper presents a method for constructing approximate semi-analytical solutions for the radially symmetric generalized Boussinesq equation, also known as the porous medium equation (PME). This work is a further extension of analysis done by Telyakovskiy et al. (2016), that studied water injection governed by the Boussinesq equation into an unconfined initially dry aquifer. The newly derived approximate solution also has two parts; one to approximate singular behavior near the well and a polynomial part to model far-field behavior. With the prescribed initial and boundary conditions, the analyzed problem can be rewritten in terms of similarity variables resulting in a boundary value problem for a nonlinear ordinary differential equation. Using higher-order physically significant moments of the solution, a closed-form expression for the position of the wetting front and the shape of the phreatic surface is obtained. The introduced solution is valid over a wide range of injection regimes: the time-independent, the power-law injection function with positive exponents, and exponential-law of injection. A highly accurate numerical solver similar to Telyakovskiy et al. (2016) is used to validate the presented approximate solutions. Plus, comparison is made with the experimental data on prediction of the wetting front position for the special case of the Boussinesq equation.

Explosion behavior of methane-air mixtures and Rayleigh-Taylor instability in the explosion process near the flammability limits

Methane is the main component of natural gas and biogas, and it has been applied in various fields in daily life and industrial production. A better understanding of the explosion behavior of methane mixture can contribute to the safe usage of this fuel and developing advanced explosion protection technology and devices. The main purpose of this study is to investigate the explosion behavior and the Rayleigh-Taylor (RT) instability in the explosion process of methane-air mixtures using a 20 L bomb. Overpressure trajectories and flame images using high-speed schlieren technique are recorded for each experiment. Explosion behavior of methane-air mixtures is investigated at 280 K covering wide equivalence ratios (0.61–1.42) and initial pressures (50 kPa ∼ 250 kPa). By analyzing the explosion image data, flame speed and laminar burning velocity are then calculated. Results indicate that as initial pressure rises, the maximum explosion pressure increases, while the flame speed and the laminar burning velocity decrease. The effect of initial pressure and equivalence ratio (Ф) on the spherical flame is also presented and analyzed through the combination of image data and pressure data. It is suggested that mass of reactants per unit volume and fuel composition have a strong relationship to ignition feasibility and ignition delay time. When Ф is 1.42 and the initial pressure is less than 150 kPa, the dynamic ignition process is recorded. Fores (pressure gradient, density gradient) are generated at the position of the flame front due to combustion. The density of spherical flame zone (burned zone) is lower than the unburned zone, therefore the spherical flame moves upward. The moving density stratification of the flame surface changes the structure of the flame front, which induces the RT instability. Near the flammability limits (Ф = 0.61 or 1.42), the explosion pressure is weak and flame speed is slow (∼0.4 m/s). Due to the longer duration of forces, the effect of forces on the spherical flame becomes apparent. It is found that the RT instability leads to structural instability of the flame.

AN ANALYSIS OF WH-QUESTIONS IN MONGONDOW LANGUAGE

This research aimed to determine the form and function of WH-Questions in the Mongondow language. Mongondow speakers were the subjects of this research. This research was the descriptive qualitative method. The procedure of this research consisted of selecting informants based on certain criteria, collecting through a through interviews and observations, rechecking, the data, and writing the report. The data were analyzed through unitization, categorization, explanation, and interpretation according to Bogdan & Biklen. The results of this research showed that WH-Question ns in Mongondowwasguage were formed from question words, there are 10 forms of question words commonly used in WH-Questhe tions in Mongondow language, there are Onu, Ki'ine, Onda, To'onu, Doda'anda, Nongonu, Mongonu, Pongonu, Na'anda, and Tongonu. For the function of WH-questions in Mongondow language, the question words that form WH-questions sewasas bweren the front position of the sentence in the middle of the sentence when it has a time marker verb. Onu and Onda question words that used verbs of time-marking might use the prefix conn". The word "ki'ine" could be changed to "ine" when asking for an object description because the position of "ki" would be replaced with the words "koi" (to) and "takin" (with). So, the function of WH-questions in Mongondow language has different depending on the structure of the WH-questions sentences and the structure of the question words

Molecular dynamics study of the influence of free volume and orientation of the crystallization front on its velocity in nickel

The influence of free volume and orientation of the crystallization front relative to the growing crystal on the velocity of the front movement in nickel was studied by the method of molecular dynamics. The computational cells had the shape of elongated parallelepipeds, at the ends of which the crystal structure was fixed, that imitated the starting position of the heterogeneous crystallization front. Three different orientations of the front relative to the growing crystal were considered: (100), (110), and (111). The concentration of the excess free volume and the formation of vacancy clusters occurred mainly in last turn, at the final stage of crystallization. The introduction of excess free volume led to a decrease in the crystallization velocity. At high concentrations of several percent, the formation of some clusters occurred already at the early stages of crystal growth, but most of them, nevertheless, were located at the meeting point of two fronts from different ends of the computational cell. Crystallization proceeded faster at the (100) front orientation, slower at the (110) and (111) orientations. The anisotropy of the crystallization velocity is related to the difference between the free energies of an atom near the boundary in the liquid phase and a growing crystal «imbedded» into the boundary, which depends on the orientation of the boundary and, in particular, correlates with the energy of the adatom on the corresponding free surface of the crystal.

A Song of Wind and Ice: Increased Frequency of Marine Cold‐Spells in Southwestern Patagonia and Their Possible Effects on Giant Kelp Forests

AbstractIn contrast to other coastal regions of the world, the giant kelp (Macrocystis pyrifera) ecosystem in southwestern Patagonia has been persistent in area and associated biodiversity in the last decades. In this ecoregion, sea surface temperature (SST) records have consistently remained below the upper thermal threshold for kelp survival, however, no studies have analyzed the spatiotemporal variability of SSTs and their anomalies across the geographical diversity of the southwestern Patagonian coastline. We explored the geographical distribution of extreme warm and cold events in this region from latitudes 47°–56°S in a range of ∼1,000 km, identifying the dates and spatial distribution of marine heatwaves (MHWs) and marine cold‐spells (MCSs) from 1982 to 2020. Results show that a peak in the number of MHWs occurred in the great El Niño year of 1998. Additionally, the 2014–2019 period has had more severe and extreme MCSs than the previous decades. We discuss the origin of these events with a focus on three main processes: (a) geographically constrained cold events caused by glacier melting, (b) regional cold events caused by extreme winds linked to the position of the polar front, and (c) extensive SST anomalies linked to planetary‐scale events such as El Niño and La Niña. Overall, those processes were conductive to counteract global warming trends locally/regionally, highlighting southwestern Patagonia as a possible climatic refugium for the giant kelp ecosystem. Despite this, the effects of freshwater inputs and storm turbulence on the exposed coasts facing the Southern Ocean may cause new kinds of stress on this ecosystem.

The role of flat slab subduction, ridge subduction, and tectonic inheritance in Andean deformation

AbstractConvergent plate boundaries show sharp variations in orogenic width and extent of intraplate deformation. Analysis of late Cenozoic contractile deformation along the Andean mountain front and adjacent foreland highlights the contrasting degrees of deformation advance toward the plate interior. The retroarc positions of the Andean topographic front (marked by frontal thrust-belt structures) and foreland deformation front (defined by isolated basement block uplifts) range from 300 to 900 km inboard of the trench axis. Over the ~8000 km arcuate length of the Andes (10°N to 55°S), four discrete maxima of inboard deformation advance are spatially co-located with the Peruvian (5°S–14°S) and Pampean (27°S–33°S) zones of flat slab subduction, the subducted Chile Ridge (45°S–48°S), and the anomalously thick Paleozoic stratigraphic wedge of Bolivia (17°S –23°S). The spatial correspondence of retroarc shortening with specific geodynamic configurations demonstrates the mechanical role of flat slab subduction, slab window development, and combined structural and stratigraphic geometries in shaping the orogenic architecture of Cordilleran margins, largely through lithospheric strengthening, weakening, and/or tectonic inheritance.

Research on Electric Vehicle Electromagnetic Protection Considering Radiation of Two Wireless Chargers

To evaluate the electromagnetic exposure safety of multiple wireless charging vehicles charging simultaneously and explore simple, effective electromagnetic protection methods, in this paper, car model, wireless charger model, and human body models were established by using COMSOL Multiphysics software. The influence of different relative positions of two wireless chargers on magnetic induction strength (B) in simulation space when two cars are charging simultaneously and the influence of different car body materials on a driver’s body surface B and internal induced electric field strength (E) were analyzed. The simulation results showed that the maximum value of B in the car when two wireless chargers work simultaneously was 1.30 times that when a single wireless charger works. The maximum value of simulation space B could be reduced by 24.23% by staggering the front and rear positions of the two wireless chargers. The use of aluminum alloy car body could reduce the maximum value of human body surface B of the driver in the car by 99.76%. The B of the driver’s body surface in the aluminum alloy car body combined with the staggering method of two chargers was the smallest, which was 42.86% of that when the two chargers were not staggered, and its electromagnetic exposure level was 1.33% of the International Commission on Non-Ionizing Radiation Protection public exposure limit. The results showed that the protection method is effective.

Reflooding of degraded cores in ASTEC V2.1: Modelling and validation on PEARL experiments

The injection of water into the reactor vessel at a late stage of a severe accident (core reflooding) is one of the key Severe Accident Management measure to stop the progression of the accident in Light Water Reactors. Therefore, an appropriate modelling of the physical phenomena involved in this process is of paramount importance for the enhancement of safety of these nuclear power plants.This article presents the improvements performed in the ASTEC V2.1 integral severe accident code in order to obtain a comprehensive reflooding model, which is valid regardless the core damage state. The main development consists in replacing, in presence of debris, the single momentum conservation equation (plus the drift flux relation) by two momentum conservation equations (one for each phase) with specific porous friction terms. Moreover, the calculation of the quench front position and the heat transfers downstream of the quench front have been harmonized.The physical models are validated against experimental data from the PEARL facility where a large heated debris bed is quenched. A wide range of thermal–hydraulic (pressure, injection velocity) and geometrical parameters (bed particle diameter and bypass thickness) is investigated. Generally, ASTEC V2.1 provides a good agreement for tests involving either a one or two-dimensional quench front progression. However, conversion ratios of produced steam to injected water seem to be overestimated in two-dimensional tests with fine particle diameters, which raises questions about the modelling of capillarity in those situations.The validated version of the code is then used to gain further insights on degraded core reflooding: radial steam redistribution is mainly driven by the large pressure gradients generated within the bed, whereas water entrainment is driven by the interfacial drag between the redistributed steam and the liquid, both contributing to hinder the coolability of the debris bed.

Simulation based on Galerkin method for solidification of water through energy storage enclosure

In this investigation, to expedite the phenomena of converting water to ice, nano-powders of Al2O3 with various sizes were utilized. For simulation, Galerkin method with involve of implicit technique for unsteady source term was applied. Besides, the grid configuration has been changed during the freezing according the position of ice front. Energy equation has one source term according to freezing process and properties of PCM has been changed with adding nanoparticles which were involved in modeling with considering empirical correlations. Validation test in comparison of published article demonstrates nice agreement. Dispersing additive makes the freezing time to decline around 41.21% because of increment of conduction mode with dispersing nanoparticles. With increase of size up to dp = 40, the freezing time declines about 19.96% but as dp augments up to 50 nm, the needed time augments about 49.21%. Increasing concentration of nano-powders with dp = 40 makes freezing time to decrease around 19.42%.

Experimental study of explosion dynamics of syngas flames in the narrow channel

This article introduced the experimental study of the propagation of a syngas premixed flame in a narrow channel. The structural evolution, flame front position and velocity characteristics of lean and rich premixed flames were investigated at different hydrogen volume fractions as the flame was ignited at the open end of the pipe and propagated to the closed end. The comparative study of the syngas fuel characteristics, flame oscillation frequency and overpressure oscillation frequency prove that the syngas explosion flame oscillation in the narrow passage has a strong coupling relationship with overpressure and fuel heat release rate. The results was shown that the flame structure was strongly influenced by the hydrogen volume fraction of the syngas and the fuel concentration. The distorted tulip flame only appears in lean mixture. At 30% of hydrogen volume fraction, the flame exhibits intense and unstable propagation, manifested as the reciprocating and alternating movement of the flame front. As the volume fraction of hydrogen increased, the velocity of flame propagation and the frequency of oscillation increased. When the hydrogen volume fraction γ ≥ 0.4 at the equivalence ratio of Φ = 0.8, the pressure oscillation amplitude gradually increases and reaching the peak after 200–320 ms. Significantly, when γ = 0.3, the pressure peak increases abnormally. This work can provide support for the safe use of syngas in industry by experimental study of various explosion parameters in the narrow channel.

Late Weichselian and Holocene behavior of the Greenland Ice Sheet in the Kejser Franz Josef Fjord system, NE Greenland

To better understand the past retreat patterns and chronologies of major marine-terminating outlet glaciers, the Late Weichselian and Holocene glacial history of a NE Greenland fjord system is reconstructed using new and previously published swath bathymetry and high-resolution seismic data, supplemented with multi-proxy analysis of sediment cores. The investigated area extends more than 190 km, including Fosters Bugt, Kejser Franz Josef Fjord, Nordfjord and Moskusoksefjord, the drainage routes for Waltershausen Gletscher, one of the largest outlet glaciers in the NE sector of the Greenland Ice Sheet. A complex of moraine ridges identified in Fosters Bugt show former stillstand positions during this period, and, alongside radiocarbon ages, supports the theory of a maximum ice front position in the outer coastal areas during the Younger Dryas-Preboreal period. The retreat moraines distributed within the Kejser Franz Josef Fjord system provide evidence of a stepwise retreat during the last deglaciation, whereby rapid retreat was interrupted by episodes of stillstands and/or readvances of the grounding line. The fjord system was mostly deglaciated prior to 7.8 ka cal BP, with an estimated average retreat rate ranging between 33 and 96 m a−1 since the Younger Dryas. Following this, only minor ice advances in the inner fjords have been identified. The sediment supply from Waltershausen Gletscher dominated the Kejser Franz Josef Fjord, Nordfjord and outer Moskusoksefjord throughout the Holocene period, whereas in the middle and inner Moskusoksefjord the sediment deposits reflect a more local catchment area. The estimated average retreat rate is in the same order of magnitude as the rates from fjords of northern Fennoscandia, indicating the same overall control for the deglaciation, ice melting from increased Northern Hemisphere summer insolation that peaked in Early Holocene.

Comprehensive analysis of spherical bubble oscillations and shock wave emission in laser-induced cavitation

The dynamics of spherical laser-induced cavitation bubbles in water is investigated by plasma photography, time-resolved shadowgraphs and sensitive single-shot probe beam scattering that portrays the transition from initial nonlinear to late linear oscillations. The frequency of late oscillations yields the bubble's gas content. Numerical simulations with an extended Gilmore model using plasma size as input and oscillation times as fit parameter provide insights into experimentally not accessible bubble parameters and shock wave emission. Model extensions include a term covering the initial shock-driven acceleration of the bubble wall, an automated method determining shock front position and pressure decay and a complete energy balance for the partitioning of absorbed laser energy into vaporization, bubble and shock wave energy and dissipation through viscosity and condensation. These tools are used for analysing a scattering signal covering 102 oscillation cycles from a bubble with 36 μm maximum radius produced by a plasma with 1550 K average temperature. Predicted bubble wall velocities during expansion agree well with experimental data. Upon first collapse, most energy was stored in the compressed liquid around the bubble and radiated away acoustically. The collapsed bubble contained more vapour than gas and had a pressure of 13.5 GPa. The decay of the rebound shock wave pressure with radius r was initially $\mathrm{\ \propto }{r^{ - 1.8}}$ , and energy dissipation at the shock front heated the liquid near the bubble wall to temperatures above the superheat limit. The shock-induced temperature rise reduces damping during late bubble oscillations. Damping in first collapse increases significantly for small bubbles with less than 10 μm radius.

Experimental study of spruce wood reaction to fire in single burning item test

The aim of this work is to study and characterize the fire behavior of vertically oriented spruce wood panels using experiments conducted at the scales of cone calorimeter and single burning item tests. For this purpose, first incombustible panels were exposed to burner powers of 15, 20, 30, and 50 kW in the single burning item tests to obtain a mapping of the total heat fluxes received by the panel. Subsequently, wood panels were exposed to the same burner powers for exposure times of 15, 20, and 30 min. Very thin thermocouples were embedded inside the wood panel to measure accurately the in-depth temperatures while the lateral position of the char front on the exposed surface and the depth of the char layer were also measured for each test. The latter measurement permitted to establish a char depth map according to the burner power and exposure time. Correspondingly, it was observed that for a fixed exposure time, the degraded area on the surface grows linearly with the burner power. Moreover, the in-depth char front position deduced from the 300 °C isotherm was found to comply very well with that obtained from direct measurements. Finally, a comparison is made between the char front depths measured with the single burning item and those measured with the cone calorimeter for similar heat fluxes, showing that the corresponding charring rates from these two tests deviate from one another only at low heat fluxes.

Systematic extraction of a control-oriented model from perturbative experiments and SOLPS-ITER for emission front control in TCV

Systematic extraction of locally valid dynamic models from experiments is necessary for controller design and the validation of high fidelity models. This paper describes the extraction of a dynamic model in the form of a transfer function, giving the dynamic response of the CIII (465.0 nm) emission front position to deuterium gas puffing in the TCV divertor during flattop, relevant for heat exhaust control. The model is extracted using frequency response data from both SOLPS-ITER simulations and perturbative experiments. We use the steady-state solutions of the model SOLPS-ITER to obtain an additional data point at the zero frequency, as the identifiable frequency range by perturbative experiments is lower bounded by discharge time. We specifically approach the problem from a control engineering point of view, aiming to develop control-oriented models for the systematic design of impurity emission front controllers. We find a transfer function structure based on a diffusive process to best describe the obtained frequency response data. The resulting transfer function model accurately reproduces the local dynamic response measured during experiments, so it can be used to assess new controllers offline for similar discharge scenarios.

Native Word Order Processing Is Not Uniform: An ERP Study of Verb-Second Word Order.

Studies of native syntactic processing often target phrase structure violations that do not occur in natural production. In contrast, this study examines how variation in basic word order is processed, looking specifically at structures traditionally labelled as violations but that do occur naturally. We examined Swedish verb-second (V2) and verb-third (V3) word order processing in adult native Swedish speakers, manipulating sentence-initial adverbials (temporal idag ‘today’, spatial hemma ‘at home’ and sentential kanske ‘maybe’) in acceptability judgements, in simultaneously recorded event-related potentials (ERP) to visually presented sentences and in a written sentence completion task. An initial corpus study showed that the adverbials differ in frequency in fronted position (idag > kanske > hemma), and although all occur mainly with V2 word order, kanske occurs more frequently with V3 in natural production than both idag and hemma. The experimental results reflected these patterns such that V2 sentences were overall more frequently produced and were deemed more acceptable than V3 sentences. The ERP results consisted of a biphasic N400/P600 response to V3 word order that indicated effects of word retrieval and sentence reanalysis. We also found consistent effects of adverbials. As predicted, V3 was produced more frequently and judged as more acceptable in Kanske sentences than in sentences with the other two adverbials. The ERP analyses showed stronger effects for idag and hemma with V3, especially regarding the P600. The results suggest that the naturally occurring word order ‘violation’, V3 with kanske, is processed differently than V3 with other adverbials where the V2 norm is stronger. Moreover, these patterns are related to individuals’ own production patterns. Overall, the results suggest a more varied native word order processing than previously reported.

Analysis of nozzle clogging position in a continuous casting mold

Abstract In this paper, the flow field of the submerged entry nozzle (SEN) in the continuous casting process is analyzed in detail by numerical simulation. It is found that the front and rear positions of the SEN outlets and the bottom of the nozzle are prone to the accumulation of inclusions. Compared with the actual SEN, the clogging positions with the simulated results are consistent. The composition of nozzle nodules at two different positions is analyzed by scanning electron microscope (SEM), and the inclusions are alumina. The changes in molten steel level characteristics in the mold without and after clogging are compared and analyzed. It is found that the velocity of molten steel near the surface with nozzle clogging reaches 0.483 m/s, the free level fluctuation value of molten steel is 8.9 mm, which is easy to cause steel defects.

Selection criteria of mesh size and time step in FEM analysis of highly nonlinear unsaturated seepage process

The selection criteria of the mesh size and time step that ensures a convergent result in FEM analysis of highly nonlinear unsaturated seepage process is established. First, the concept of critical mesh size (Lc) and critical time step (Δtc) are theoretically deduced, and their framework is set up. Subsequently, formulas for the prediction of Lc and Δtc are proposed through a series of numerical experiments. Additionally, the reliability of the proposed criteria is verified by analytical solutions, model test results, and quasi-engineering problems, respectively. Finally, the ranges of Lc and Δtc under various conditions in engineering applications are recommended. The results show that the proposed criteria can predict the approximate range of Lc and Δtc, so as to obtain the convergent wetting front position and water content distribution. In addition, the criteria can be applied to the scale and dimensions of laboratory experiments and engineering cases. Moreover, the mesh size and time step were interdependent. Under limiting conditions, the appropriate mesh size should be selected prior to the time step. The research results can provide a reference for improving the accuracy of the FEM analysis of unsaturated seepage.

Digital holographic microscopy implementation for capillary filling measurements in nanoporous materials.

We report the implementation of lensless off-axis digital holographic microscopy as a non-destructive optical analyzer for nano-scale structures. The measurement capacity of the system was validated by analyzing the topography of a metallic grid with ≈150nm thick opaque features. In addition, an experimental configuration of self-reference was included to study the dynamics of the capillary filling phenomena in nanostructured porous silicon. The fluid front position as a function of time was extracted from the holograms, and the typical square root of time kinematics was recovered. The results shown are in agreement with previous works on capillary imbibition in similar structures and confirm a first step towards unifying holographic methods with fluid dynamics theory to develop a spatially resolved capillary tomography system for nanoporous materials characterization.