Gas Penetration Research Articles

ENVIRONMENTAL SAFETY OF POWER PLANTS: DUAL-FUEL AND GAS ENGINES

The subject of this study is internal combustion engines capable of using gaseous fuels-gas and dual-fuel engines. The main technologies of using natural gas in piston engines from the standpoint of ensuring the highest energy efficiency and environmental safety are considered in the paper. It is noted that the use of natural gas as a fuel, regardless of the technology, reduces carbon dioxide emissions from combustion products by 28 % compared to liquid fuels. All known designs of gas and dual-fuel engines fundamentally implement two technologies, namely, the use of homogeneous or heterogeneous gas-air mixtures. The most common is the technology of homogeneous mixtures, which provides high fuel efficiency and the highest environmental safety indicators. This technology provides for the supply of low-pressure gas to the intake ducts of the engine through automatic gas valves. There are practically no sulfur oxides, solid particles in the exhaust gases of the engine, and the emission of nitrogen oxides can be provided at the level of the requirements of the Tier III standard. At the same time, the main disadvantage of the homogeneous mixture technology is the potential penetration of natural gas with a high level of greenhouse effect (which is up to 28 times higher than the effect of carbon-acid gas) into the exhaust tract of the engine - “methane slippage”. This negative aspect can be largely minimized through the use of heterogeneous technology, when fuel gas is fed directly into the cylinder of a dual-fuel engine under high pressure, up to 50 bar, at the end of the compression process. This technology is becoming widespread in the structures of two-stroke engines due to the specifics of their gas exchange process. However, the high dynamics of the heat release process in the implementation of heterogeneous technology of gas-air mixtures is associated with a high emission of nitrogen oxides, commensurate with the emission of NOx by a diesel engine. Based on the analysis of the parameters and characteristics of the products of leading manufacturers implementing various technologies for using natural gas as fuel, conclusions about the potential advantages and prospects of using gas and dual-fuel engines are made.

Performance intensification and anti-fouling of the two-phase flow enhanced direct contact membrane distillation for seawater desalination

Membrane distillation is a promising approach to the fresh water crisis due to the low operating temperature and high rejection of non-volatile components. However, it suffers from the polarization effect and membrane fouling. This paper aims at developing a novel two-phase flow enhanced direct contact membrane distillation (TP-DCMD) system and investigating the effect of two-phase flow behaviors on the heat and mass transfer and membrane fouling comprehensively. First, a novel experimental system of TP-DCMD is developed and well validated using the mass and energy conservations. Then, the effect of flow parameters on the transmembrane permeate flux is discussed. The introduction of gas could improve the membrane distillation performance by as high as 27 %. The relationship between the performance enhancement and two-phase flow regimes is discussed. Both permeate flux and multiplier clouds are proposed based on two-phase flow regime map. The slug flow demonstrates superior enhancing effect to the other flow regimes due to the low possibility of gas penetration. Finally, the effect of two-phase flows on the membrane fouling is investigated. After 420 min of accelerating fouling test, the transmembrane permeate flux drops by 38.2 % in the conventional DCMD system, while it just decreases by 6.6 % in TP-DCMD system.

A Novel Permeability Model of Coal Considering Gas Slippage and Gas Sorption-Induced Strain

As an unconventional natural gas, coalbed methane (CBM) has been recognized as a significant fuel and chemical feedstock that should be recovered. Permeability is a key factor that controls CBM transport in coal. The slippage effect is an influential phenomenon that occurs during gas penetration processes, especially in low-permeable media. Apparent permeability may differ greatly from intrinsic permeability due to gas slippage. However, the gas slippage effect has not been considered in most analytical permeability models. Based on the cubic law, a new analytical model suited for the permeability analysis of coal under different stress conditions is derived, taking into consideration gas slippage and matrix shrinkage/swelling due to gas desorption/adsorption. To enhance its application, the model is derived under constant hydrostatic stress and pore pressure. The new analytical model is then compared with the existing models, and its reliability is verified by the comparison between the analytical prediction and the experimental permeability data under different stress conditions.

The impact of city gas on income inequality in China: A regional heterogeneity analysis

City gas plays a significant role in reducing pollution, facilitating life, and promoting economic development. In this study, the fixed effects panel regression model based on data for 30 provinces in China, covering 2005–2018, is employed to investigate the interrelationship between city gas and income inequality. Primarily, the impact of gas penetration, gas supply, gas users, and gas pipelines on income inequality in terms of three types of city gas (i.e., coal gas, liquified petroleum gas (LPG), natural gas) is discussed. Then, a heterogeneity analysis of the influence of city gas consumption on income inequality is conducted based on the spatial distribution, economic development, and proportion of gas access. According to the empirical results, the total gas penetration increases while the gas penetration rate plunges. For different types of city gas, the impact of coal gas and natural gas consumption on income inequality is significantly negative. In other words, expanding natural gas consumption can effectively narrow income inequality. Furthermore, the impact of LPG consumption on income inequality is positive. In addition, improvement in the economic level coming from city gas can narrow income inequality. In conclusion, several policy implications for promoting natural gas development and narrowing income inequality are highlighted.

Carbon Shell‐Encapsulated Metal Alloy Catalysts with Pt‐Rich Surfaces for Selective Hydrogen Oxidation Reaction

AbstractInvited for this issue's Front Cover are the groups of Kwang Bok Yi and Namgee Jung from Chungnam National University (South Korea). The Front Cover shows the catalyst structure that blocks O2 gas (red circles) penetration and selectively permeates H2 gas (blue circles) via the molecular sieve effect by the carbon shell coated on the metal surface to suppress reverse current flow under shut‐down/start‐up conditions during fuel cell operation. Read the full text of the Research Article at 10.1002/celc.202200342.

Mechanical, rheological, and carbon dioxide barrier properties of polyvinylidene fluoride/TiO2composites for flexible riser applications

AbstractNanoparticles can act as a barrier to gas penetration in the polymer matrix. However, the agglomeration of nanoparticles has a negative effect on the comprehensive properties of nanocomposites. We envisioned surface modification of 25 nm TiO2nanoparticles with KH550 to enable better dispersion in the polyvinylidene fluoride (PVDF) matrix and prepared PVDF/TiO2composites by the melt blending method. The effects of adding modified nanoparticles into the matrix on the thermodynamic, mechanical, rheological, and gas barrier properties were studied. We find that the dispersion of TiO2in the matrix significantly improved after modification, as observed in the SEM images. As a result, the mechanical properties of composites decreased with an increase in the content of nanoparticles, and PVDF/N‐TiO2composites showed better mechanical and CO2barrier properties. The permeability coefficient of PVDF/0.5% N‐TiO2has the lowest permeability coefficient, which is 18.30% lower than primary PVDF. At a high shear rate, the viscosity of PVDF/N‐TiO2with 0.5% and 1.0% content decreased, and the viscosity of 1.5% ratio was higher than that of primary PVDF. PVDF/0.5% N‐TiO2has the best comprehensive performance and also possesses the advantages of simple preparation, low cost, and ease of industrial production.

Drying Kinetics of Macroalgae as a Function of Drying Gas Velocity and Material Bulk Density, Including Shrinkage

Macroalgae have many potential applications and can make important contributions to sustainability and circular economy objectives. Macroalgae are degradable high-moisture biomaterials and drying is a necessary step, but drying is an energy and capital-intensive part of their production process. This study presents convective drying curves for commercially promising fresh and saltwater species (U. ohnoi and O. intermedium), obtained over a range of industry-relevant drying gas velocities (0.3–2 m/s) and material bulk densities (33–100 kg/m3). Pragmatic diffusion-based drying models that account for the influence of drying gas velocity, material bulk density, and material shrinkage are presented. Results provide critical insights into the validity of diffusion model assumptions for compressible biomaterials and new mechanisms describing gas penetration into such materials are proposed. The drying models provided in this work demonstrate a high degree of accuracy for both species.

Catalyst Layers Prepared By Mixing Ionomer Dispersions with Different Equivalent Weights in Proton Exchange Membrane Fuel Cells

Recently, hydrogen-based energy conversion devices are stand out in the world for decarbonization. Fuel cells are common devices for hydrogen utilization, membrane electrode assembly (MEA) has the most important role in all components for fuel cell performance. MEA is consist of membrane and both side of catalyst layer, the catalyst layers were made of electrocatalyst and ionomer by evaporating all solvents in the catalyst inks. The ionomer increases the catalyst utilization by covering the electrocatalyst in the catalyst layer and increases the gas penetration by forming pore in the catalyst layer. Each ionomer dispersion has a different EW, and there are two types of side chains: Long side chain (LSC) and short side chain (SSC). Each ionomer has advantages and disadvantages. The structure of catalyst layer is formed differently according to type of ionomer, which could be make difference performance of catalyst layer. By mixing the ionomer, the advantages can be optimized to obtain a beneficial effect. Many properties of the components to consist of catalyst layers substantially influence their performance and durability due to the different interaction of electrocatalysts and ionomer. In this study, EW conducted an experiment to find out the difference in the properties of the catalyst layer using the same commercial ionomer and the ionomer made by mixing different dispersions. Three types of ionomers were used: i) long-side chain (LSC) ionomer, ii) short-side chain (SSC) and iii) Mixture of dispersions with different EW ionomer. Use an IEC automatic titrator to verify that ionomer dispersions made by mixing different ionomer dispersions have target EW values. Prepared catalyst layers prepared by the ionomers with different side chains were characterized by electrochemical evaluation such as I-V polarization, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and by physical and microscopic characterization such as porosimetry and water uptake. Acknowledgments This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT ＆ Future Planning (NRF-2019M3E6A1063677) and by 2021 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

Chemical Sulphate Corrosion on Cement Composites in Various Model Environments

Background: Cementitious composites, which are subject to increasing demands, are often exposed to various external attacks, such as aggressive groundwater and surface water, chemicals in the soil, gas penetration, or phenomena related to water freezing and melting. One of the most common reasons for the deterioration of cement composites is the corrosion process. Corrosion results in irreversible damage that occurs during the chemical reaction of the material with the components of the environment. Methods: The paper deals with experimental study of chemical sulphate corrosion of cement composites prepared from three types of cement: ordinary Portland cement; sulphate-resistant cement; and special hybrid cement, and industrial by-products and wastes: silica fume, zeolite and a special mineral admixture based on blast furnace granular slag as cement partial substitutes. Samples of cement composites were subjected to corrosion experiments in a sulphate environment, which took place in the laboratory under model conditions for 180 resp. 270 days. Results: The deterioration parameters: changes in the weight and thickness of the samples, surface and mineralogical changes, leachability of the basic components of the cement matrix as well as changes in the liquid phase proved the degradation process due to chemical sulphate corrosion, model solutions of H2SO4 with pH 3 and 4, and solution of MgSO4 with c (SO4 2 -) = 3 and 10 g /L. Conclusion: By comparing the leachability of the alkali components from cement composites, it can be concluded that for the most aggressive model solution (H2SO4 with pH 3), both slagcontaining formulations are the most stable in terms of the total ratio of leached calcium and silicon. This finding is also supported by the results of water absorbency tests, which confirmed that despite the increase in absorbency after chemical corrosion, cement composites with slag content reach the lowest values.

Analysis on the supersonic gas jet submerged in liquid cross flow

Experimental studies of the influence of the liquid cross flow on the submerged gas jet with constant injection pressure have been presented in our former research (Dong et al., 2021). In the present study, the development of submerged gas jets with different initial injection pressures (0.7 MPa, 1.3 MPa and 2.0 MPa) subjected to liquid cross flow with varying velocities (0.35 m/s, 0.7 m/s, 1.0 m/s, 1.5 m/s, 2.0 m/s) were further experimentally and theoretically investigated to evaluate the effects of the cross flow velocity and initial injection pressure on the gas jet evolution and flow characteristics. Experimentally, the evolution and morphology of the gas jet with different initial injection pressures were captured by the full-scale experimental system designed in our former research. Theoretically, an analytical correlation was proposed to predict the penetration of gas jet in liquid cross flow, and the experimental results of the gas jet tip evolutions were compared with a modified vortex ball model. It turns out that the proposed correlations were able to predict the gas jet development accurately, including expansion angle, gas jet penetration length and gas jet tip evolution, which could provide convincing parameters assessment for the submerged gas jet in liquid cross flow.

Uncertainty effect of CO2 molecular diffusion on oil recovery and gas storage in underground formations

CO2 molecular diffusion in underground reservoir fluids, quantified by diffusion (D) coefficient, is of great importance for CO2 storage and enhanced oil recovery (EOR) projects. The reported CO2 D coefficients values are uncertain due to difficulty of accurately measuring this parameter specifically at high pressure-high temperature conditions. Hence, this study aims to investigate the impact of the uncertainty in the value of CO2 D coefficient on both oil recovery and CO2 storage during intermittent CO2 assisted gravity drainage (CO2-GAGD) injection in oil reservoirs. The impact of the CO2 diffusion on oil compositional changes and fluid distribution in porous medium was also investigated here. To do so, using data from a Malaysian oil field, a tuned equation of state along with a composition simulation model were used to emphasis on the effect of diffusion during the intermittent CO2-GAGD process. The effects of reservoir permeability, reservoir porosity, injection rate, and soaking time on oil recovery and gas storage were tested across different values of CO2 D coefficient. Including the gas diffusion in the simulation resulted in enhancement of the gas penetration though the porous medium leading to the enhanced oil recovery especially at the final soaking-injection/production cycles of the intermitted CO2 injection. The lowest impact of the uncertainty in the CO2 D coefficient on the oil recovery and gas storage efficiency was found in the high permeability case. Additionally, decreasing the gas injection rate or increasing the soaking time magnified the impact of the uncertainty of the CO2 D coefficient. Furthermore, the CO2 storage efficiency was found to be highly sensitive to the changes in the D coefficient in the high porosity case. To verify the results, the obtained simulation results were compared and found to be consistent with previous findings reported in the literature. This study adds accuracy to predicting CO2 EOR and storage projects through understanding the uncertainty of molecular diffusivity.

Solar fuel generation over nature-inspired recyclable TiO2/g-C3N4 S-scheme hierarchical thin-film photocatalyst

• In-situ construction of recyclable “nature-inspired biomimetic photocatalyst”. • TiO 2 /g-C 3 N 4 NAs exhibited enhanced photocatalytic CO 2 reduction performance. • Intimate interfacial contact is beneficial to the photogenerated charge transfer. • In-situ irradiated XPS measurements investigation on S-scheme mechanism. Preparation of efficient photocatalysts with ease of recovery in solar fuel generation is highly desired to achieve carbon neutralization in carbon dioxide (CO 2 ) emissions. Inspired from the forest with superior light penetration and fast gas transport, a TiO 2 /g-C 3 N 4 composite nanowire arrays (NAs) film with maximized light utilization is devised. It is achieved by in-situ coating a thin layer of g-C 3 N 4 (as the leaf) on the vertically-oriented TiO 2 arrays (as tree trunks) on Ti foil (as soil). Benefiting from the effective charge separation by S-scheme charge transfer, intimate contact by the in-situ growth as well as the ingenious structure, the composite, readily recyclable, displays exciting performance in photocatalytic CO 2 reduction. It is beyond doubt that the combination of heterojunction construction and “nature-inspired biomimetic photocatalyst” design promises practical applications and industrial use.

Permeability of Bituminous Coal to CH4 and CO2 Under Fixed Volume and Fixed Stress Boundary Conditions: Effects of Sorption

Permeability evolution in coal reservoirs during CO2-enhanced coalbed methane (ECBM) production is strongly influenced by swelling/shrinkage effects related to sorption and desorption of CO2 and CH4, respectively. Recent research has demonstrated fully coupled stress–strain–sorption–diffusion behavior in small samples of cleat-free coal matrix material exposed to a sorbing gas. However, it is unclear how such effects influence permeability evolution at the scale of a cleated coal seam and whether a simple fracture permeability model, such as the Walsh elastic asperity loading model, is appropriate. In this study, we performed steady-state permeability measurements, to CH4 and CO2, on a cylindrical sample of highly volatile bituminous coal (25 mm in diameter) with a clearly visible cleat system, under (near) fixed volume versus fixed stress conditions. To isolate the effect of sorption on permeability evolution, helium (non-sorbing gas) was used as a control fluid. All flow-through tests reported here were conducted under conditions of single-phase flow at 40°C, at applied Terzaghi effective confining pressures of 14–41 MPa. Permeability evolution versus effective stress data were obtained under both fixed volume and fixed stress boundary conditions, showing an exponential correlation. Importantly, permeability (κ) obtained at similar Terzaghi effective confining pressures showed κhelium > κCH4 >> κCO2, while κ-values measured in the fixed volume condition were higher than those in the fixed stress case. The results show that permeability to CH4 and CO2, under in situ conditions where free swelling of rock is not possible, is strongly influenced by the coupled effects of 1) self-stress generated by constrained swelling, 2) the change in effective stress coefficient upon sorption, 3) sorption-induced closure of transport paths independently of poroelastic effect, and 4) heterogeneous gas penetration and equilibration, dependent on diffusion. Our results also show that the Walsh permeability model offers a promising basis for relating permeability evolution to in situ stress evolution, using appropriate parameter values corrected for the effects of stress–strain–sorption.

Non-Cellular Layers of the Respiratory Tract: Protection against Pathogens and Target for Drug Delivery.

Epithelial barriers separate the human body from the environment to maintain homeostasis. Compared to the skin and gastrointestinal tract, the respiratory barrier is the thinnest and least protective. The properties of the epithelial cells (height, number of layers, intercellular junctions) and non-cellular layers, mucus in the conducting airways and surfactant in the respiratory parts determine the permeability of the barrier. The review focuses on the non-cellular layers and describes the architecture of the mucus and surfactant followed by interaction with gases and pathogens. While the penetration of gases into the respiratory tract is mainly determined by their hydrophobicity, pathogens use different mechanisms to invade the respiratory tract. Often, the combination of mucus adhesion and subsequent permeation of the mucus mesh is used. Similar mechanisms are also employed to improve drug delivery across the respiratory barrier. Depending on the payload and target region, various mucus-targeting delivery systems have been developed. It appears that the mucus-targeting strategy has to be selected according to the planned application.

Economies of Scale in City Gas Sector in Seoul, South Korea: Evidence from an Empirical Investigation

The city gas sector in Seoul, the capital of South Korea, consists of five locally monopolized companies. As the city gas penetration reaches 98% and city gas as cooking fuel and heating fuel is being converted to electricity and district heating system, respectively, the need to redefine the role of the city gas sector is being raised. In this respect, this study aims to analyze the economies of scale in the city gas sector using the translog variable cost function model over the period 2008–2020 and to compute the minimum efficient scale. The scale economy index ranged from 0.1 to 0.2, which was larger than 1.0. The results show that the city gas sector still enjoys economies of scale, although the economies of scale are gradually disappearing. The minimum efficiency scale was estimated to be 1.06 times the size of the total market, which is the total output of the five companies. This finding vividly suggests that reducing the number of city gas companies through mergers and acquisitions among five city gas companies is more desirable in terms of cost reduction. This study suggests that the business structure favorable to the city gas business, such as high population density and urbanization, can rapidly lose economies of scale under rapid electrification and a rigid wholesale market. The central and local governments, which have the authority to regulate the city gas business, need to promote mergers and acquisitions between city gas operators, and to normalize distorted energy rates in order to prevent excessive electrification.

Development of a novel self-entry exploitation device for marine natural gas hydrate and the feasibility studies

Natural gas hydrate (NGH) is a potentially green energy resource for the future. Five trial production experiments of marine NGH have been carried out worldwide. However, the currently widely used drilling method has poor economic applicability, and there are still many critical technical problems that need to be solved. Based on the characteristics of marine NGH reservoir and the exploration principle, a novel self-entry exploration device (SEED) according to the penetration mechanism of torpedo anchor was proposed, and the working principle was described in detail. The SEED could significantly reduce the cost of NGH exploitation because it eliminates the need for deepwater drilling rigs. Moreover, unlike traditional cement wellbore, the SEED is an artificial rigid structure that withstands greater depressurization to increase efficiency. The penetration laws and gas production characteristics of the SEED were studied through numerical simulation. The research results show that the penetration depth into the seabed can reach more than 300 m, which can cover most of the NGH reservoirs' depth. The gas production efficiency is significantly improved compared to the traditional wellbore. The SEED has specific potential for exploring marine NGH, providing a new idea for commercial exploitation.

Microsized Pore Structure Determination in EPDM Rubbers Using High-Pressure 129Xe NMR Techniques.

Microsized pore parameters, such as pore size and distance between pores in a series of model EPDM rubbers, were determined in situ under the pressure of 500 psi using 129Xe nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) and spin-spin (T2) relaxation measurements, pulsed-field gradient (PFG) NMR, and two-dimensional exchange spectroscopy (2D EXSY). The T1/T2 (≫1) ratio for the xenon confined in the pores is larger than that for nonconfined free xenon. This suggests that almost the entire pore surface interacts with xenon atoms like a closed pore. While these pores still connect each other through very narrow diffusion/exchange channels, it is possible to observe the echo decay in PFG-NMR and cross-peaks in 2D EXSY. The results show that both diffusion (Dpore ≈ 2.1 × 10-10 m2/s) and exchange (exchange rate, τexch = a few tens of milliseconds) of xenon between a pore within the material and outer surface are prolonged. The exchange distances (l), which correspond to the xenon gas penetration depth, were estimated to be 70-100 μm based on the measured diffusion coefficients and exchange rate (1/τexch). NMR diffraction analysis reveals that pore size (a) and pore distance (b) are on the order of magnitude of micrometers and tens of micrometers, while the diffusion coefficients of xenon gas in the diffusion channels (Deff) are about 10-8 m2/s. Overall, this study suggests that the pores with a few micrometers connected through very narrow flowing channels with the length of several tens of micrometers are developed 70 to 100 μm below the rubber surface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately 2 orders of magnitudes, than the diffusion in the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that the properties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gas penetration by the addition of both carbon black and silica fillers.

Model to Balance an Acceptable Radon Level Indoors

A theoretical model is presented for balancing an acceptable radon concentration in indoor air. The infiltration of radon from the ground to the indoor air can be controlled by barriers or by lowering the air pressure at the lower zone of the ground slab. Indoor air with a radon concentration higher than that of outdoor air can further be controlled through the effective dilution of indoor air with outdoor air. The theory estimates the allowed radon infiltration from the ground to balance radon at an acceptable level indoors for a given ventilation rate, considering the radon contribution to the indoor air from indoor materials, building materials and the interior. A method using this theory is presented, identifying the necessary airtightness required for a radon barrier to balance the acceptable radon concentration for a building. Barriers include commercially used system solutions, such as bitumen-based radon blockers, wet-room membranes, reinforced fixed mortar pastes, and polyethene membranes. An acceptable indoor radon concentration of between 100 and 300 Bq/m3 in indoor air is used. Barriers are evaluated by their ability to prevent soil gas penetration from the ground in combination with their effect on the building durability, as barriers may create a far more vulnerable building.

Hydrogen separation of porous carbon nanotubes: A density functional theory study

First-principle density functional theory (DFT) is employed to investigate the hydrogen separation performance of porous carbon nanotubes (PCNTs). The calculation results indicate that PCNTs are an excellent material for H2 purification. The reaction path of the gas penetration through PCNTs demonstrates that the diffusion barriers of gas inside or outside the tube are various, 0.413 eV and 0.328 eV, respectively, indicating that the gas located outside the nanotubes is relatively easy to diffuse into the tube. Additionally, PCNTs are more selective to H2 than other gas molecules, up to 1031, superior to porous silicon and porous phosphorene, and making potential material for gas separation.

Gas Assisted Injection Molding of an Industrial Part: 3D Simulation Provides Virtual Molding Trials

Abstract Techniques developed for three-dimensional simulation of gas assisted injection molding of thermoplastics are here extended to a complex industrial application, a car door handle, providing uniquely detailed insights into the molding process. Handling of the three stages of the process – plastic filling, and primary and secondary gas penetration – is described in terms of the boundary and initial conditions applied. Predictions for plastic filling patterns are compared successfully with molding results, and provide detailed forewarning of potential problems due to weld surfaces and air-traps. Particular emphasis is placed on accurate modeling of secondary gas penetration, which is also validated against practical results. A further novel feature is the export of an STL model of the gas core, providing a CAD model of the finished molding for subsequent in-service analysis.