Vapor Phase Densities Research Articles

Modeling hydrogeological conditions of noble gases in water and aqueous electrolyte solutions

Dissolved noble gases in water hold significant potential as tracers and precursors in various hydrogeological and geological applications, particularly in understanding surface water-groundwater interactions. These applications necessitate precise solubility predictions. Employing the fugacity-fugacity approach, our study models the solubilities of Neon, Argon, Krypton, Xenon, and Radon in both pure water and electrolyte solutions. Leveraging the Cubic-Plus-Association Equation of State, we achieve remarkable accuracy, with average deviations below 2.5 % for vapor pressure, liquid, and vapor-phase densities. Additionally, in water–gas binary systems, deviations in gas solubilities within pure water remain under 5.0 %. Extending our analysis to water–gas-salt ternary systems, our model demonstrates deviations smaller than 5.6 %. Alongside discussing the gas dissolution mechanism, this work emphasizes the adaptability of the model, informed by robust experimental and modeling data. The interpretations derived from modeling hydrogeological conditions are poised to enhance our understanding of hydrological processes, thereby assisting planners and water managers in achieving sustainable development and management of water resources.

Comparative analysis of condensation heat transfer and pressure drop of low GWP near-Azeotropic mixture and R-290 in a plate heat exchanger

Downward condensation of a near-azeotropic low GWP refrigerant mixture of R290/R1270 (65%/35% wt.), named HC01, was investigated inside an offset strip-fin (OSF) flow-structured plate heat exchanger (PHE). This study tested the prototype heat exchanger with HC01 and compared the condensation heat transfer coefficient (CHTC) and pressure drop (CPD) with the pure R-290 for working conditions of refrigerant mass flux (G) = 20–50 kg/m2s, inlet vapor quality (xin) = 0.2–0.9, saturation temperature (Tsat) = 40–55 °C, and heat flux (q) = 4–13 kW/m2. It was observed that the condensation of HC01 occurred entirely within the forced convection condensation flow regime, despite a slight change in the CHTC at a lower xin as the G increased. The CHTC demonstrated an increase in response to higher values of G and q, but decreased with increasing Tsat. Similarly, the CPD exhibited an increase with higher G and a decrease with lower Tsat, whereas the q had minimal influence. Comparison results showed that the HC01 exhibited 14.2% higher average CHTC but 6.5% lower CPD than that of R-290 because of the differences in properties, such as liquid thermal conductivity, viscosity, surface tension, latent heat, and discrepancies in liquid and vapor phase densities. Correlations were generated to predict the values of the Nusselt number (Nu) and friction factor (FF) of HC01, with mean absolute errors of 12.1% and 12.7%, respectively.

АВТОМАТИЗИРОВАННАЯ ОЦЕНКА ЗОН ВОЗДЕЙСТВИЯ ОПАСНЫХ ФАКТОРОВ ПОЖАРА ПРИ РАЗЛИВЕ СЖИЖЕННОГО ПРИРОДНОГО ГАЗА В УСЛОВИЯХ ЧРЕЗВЫЧАЙНОЙ СИТУАЦИИ

The main stages of the development of an emergency situation during the bottling of liquefied natural gas are presented. A tree of events has been formed in the event of the occurrence and development of a fire hazardous situation associated with the destruction of a tank or process pipeline with liquefied natural gas. Possible regimes of liquefied natural gas spreading on a smooth underlying surface and the formation of a variable power vapor source are identified. Models of liquefied natural gas evaporation are studied in relation to the composition and possible thickness of the filling layer. It is shown that under the assumption that the two phases are in thermodynamic equilibrium. The vapor phase density of liquefied natural gas can be obtained by solving the Peng-Robinson equation using the Klosek-McKinley method for a given pressure and temperature, which is based on an empirical correlation for the molar volume of the liquefied natural gas mixture. A model of a fast phase transition is formed based on the solution of the Van den Berg equation and an algorithm that automates the procedure for assessing the zones of influence of dangerous fire factors during an liquefied natural gas spill.

ДИФФУЗИОННОЕ ГОРЕНИЕ СЖИЖЕННОГО ПРИРОДНОГО ГАЗА. СКОРОСТЬ ВЫГОРАНИЯ И РАЗМЕРЫ ПЛАМЕНИ

Исследование направлено на изучение параметров горения сжиженного природного газа (СПГ), необходимых для оптимального выбора средств обеспечения пожарной безопасности при его производстве, хранении, транспортировке и использовании. Актуальность данной работы связана с существенным ростом масштабов производства и использования СПГ, являющегося одним из важнейших средств обеспечения энергобезопасности страны и ее конкурентоспособности на внешнем рынке. The study is devoted to investigation in the size of the flames of liquefied natural gas (LNG). A detailed analysis of accidents at LNG storage and processing facilities shows the potential dangers arising from its use, as well as the relevance of their scientific study. The data on the fires that have occurred, presented in the scientific literature, indicate that the height of the flame of flammable liquids (including liquefied gases) depends largely on the rate of burnout and the typical size of the spill area (side of the square, diameter), as a rule, limited by the dike. The empirical Thomas equation for determining the dimensionless flame height of flammable liquids is given in the scientific literature. The proposed dependence summarizes the data on the size of the flames of the following series of flammable liquids: gasoline, hexane, methanol. The application of this dependence to generalize the available literature data on the size of LNG flames undertaken by domestic authors showed that the calculated and experimental data differ by about 3 times. According to the authors of such an assessment, the difference indicates that the Thomas equation is not optimal for a reliable estimation of the size of LNG flames, which differ significantly in their physicochemical properties from known combustible liquids. Considering this, as well as the fact that the burnout rate depends to varying degrees on many factors (evaporation area, surface type, thickness of the combustible liquid layer, etc.), the most reliable data can be obtained by direct experiments which results are provided in this paper. Based on the results of the experiments, a comparative assessment of the dimensionless height of the flame, measured in experiments when burning LNG in steel baking trays and a concrete bath, was carried out with calculations according to the Thomas formula. In this formula, the density of the LNG vapor phase directly above the combustible liquid layer was used to estimate the burnout rate. A graphical representation of the experimental results and the calculated ones (according to the Thomas formula) using the vapor phase density parameter showed that the generalization of the literature and the present experimental data gives a more reliable result. According to the results of the study, we can conclude about the possibility of using the data obtained for design estimates of the size of LNG flames and the feasibility of further large-scale studies to obtain more complete information.

Liquid-Phase Speed of Sound and Vapor-Phase Density of Difluoromethane.

Difluoromethane (HFC-32, DFM), with a global warming potential (GWP) of 677, is of interest as a pure refrigerant and as a component in low-GWP refrigerant mixtures. Additionally, difluoromethane has recently been identified as a safe, liquefied-gas electrolyte material in batteries. Using state-of-the-art instruments for measurements, this paper presents new liquid-phase speed of sound and vapor-phase density data for difluoromethane. Two hundred and nine liquid-phase speed of sound values were measured using a dual-path pulse-echo instrument at temperatures from 230 to 345 K and pressures from 2.1 to 70 MPa. Accounting for all sources of uncertainty, the relative expanded combined uncertainty (k = 2) in the speed of sound ranged from 0.035 to 0.17%. One hundred and thirty-eight vapour-phase density values were measured using a two-sinker densimeter at temperatures from 240 to 340 K and pressures from 0.1 to 1.61 MPa with an uncertainty of 0.011 to 0.12%. These experimental data will be valuable in the ongoing development of a new fundamental thermodynamic equation of state for difluoromethane.

Molecular Dynamics Simulations of the Vapor-Liquid Equilibria in CO2/n-Pentane, Propane/n-Pentane, and Propane/n-Hexane Binary Mixtures.

Molecular dynamics (MD) simulations were used to study vapor-liquid equilibrium interfacial properties of n-alkane and n-alkane/CO2 mixtures over a wide range of pressure and temperature conditions. The simulation methodology, based on CHARMM molecular mechanics force field with long-range Lennard-Jones potentials, was first validated against experimental interfacial tension (IFT) data for two pure n-alkanes (n-pentane and n-heptane). Subsequently, liquid-vapor equilibria of CO2/n-pentane, propane/n-pentane, and propane/n-hexane mixtures were investigated at temperatures from 296 to 403 K and pressures from 0.2 to 6 MPa. The IFT, liquid and vapor phase densities, and molecular compositions of the liquid and vapor phases and of the interface were analyzed. The calculated mixture IFTs were in excellent agreement with experiments. Likewise, calculated phase densities closely matched values obtained from the equation of state (EOS) fitted to the experimental data. Examination of the density profiles, particularly in the liquid-vapor transition regions, provided a molecular-level rationalization for the observed trends in the IFT as a function of both molecular composition and temperature. Finally, two variants of the empirical parachor model commonly used for predicting the IFT, the Weinaug-Katz and Hugill-Van Welsenes equations, were tested for their accuracy in reproducing the MD simulation results. The IFT prediction accuracies of both equations were nearly identical, implying that the simpler Weinaug-Katz model is sufficient to describe the IFT of the studied systems.

Two-phase flow distribution in a refrigerant distributor having four indoor-unit connections of a variable refrigerant flow system

• Multiple indoor units are connected to a refrigerant distributor of a VRF system. • Distribution of two-phase flow was experimentally and numerically investigated. • Maldistribution of two-phase flow is significantly influenced by void fraction. • CFD simulation was performed to estimate distribution of R410A. • Maldistribution of R410A appeared to be less than maldistribution of air-water. In a variable refrigerant flow (VRF) system, the refrigerant is supplied from one outdoor unit to multiple indoor units through a header or refrigerant distributor. When the two-phase refrigerant flows into the distributor, uneven distribution occurs and deteriorate system performance. An experimental apparatus simulating the refrigerant distributor of a VRF system was constructed and air-water two-phase flow distribution characteristics were investigated. The air-water two-phase flow was more mal-distributed as the void fraction increased. A change in void fraction affected the distribution more than a change in mass flow rate in the experimental range. The flow distribution in the header was little affected by variation of the downstream flow resistance. Previous empirical correlations were investigated to predict the air-water two-phase flow distribution measured in this experiment, but performance was poor. To estimate the refrigerant two-phase flow distribution, computational fluid dynamics (CFD) simulations were performed using R410A as a working fluid. The mal-distribution of R410A appeared to be mitigated compared with air-water flow due to the large vapor phase density of R410A.

Attainment of rigorous thermodynamic consistency and surface tension in single-component pseudopotential lattice Boltzmann models via a customized equation of state.

The lack of thermodynamic consistency is a well-recognized problem in the single-component pseudopotential lattice Boltzmann models which prevents them from replicating accurate liquid and vapor phase densities; i.e., current models remain unable to exactly match coexisting density values predicted by the associated thermodynamic model. Most of the previous efforts had attempted to solve this problem by introducing tuning parameters, whose determination required empirical trial and error until acceptable thermodynamic consistency was achieved. In this study, we show that the problem can be alternatively solved by properly designing customized equations of state (EOSs) that replace any cubic EOS of choice during the computation of effective mass used in Shan-Chen forces. A two-parameter cubic-shaped customized EOS is introduced. Contrary to previous efforts, customization parameters in the new EOS are nonempirical and are rather derived from solving the integral mechanical stability equation, which neglects the need for any type of tuning for the attainment of rigorous thermodynamic consistency. The proposed approach reduces the errors of the coexisting densities and saturated pressure in the simulation to a maximum of 0.01% within the liquid-vapor density ratio range from O(1) to O(10^{4}), which had not been achieved in any of the previous tuning-based efforts. A straightforward way for achieving the desired surface tension via the customized EOS is also provided.

An explicit expression of the empirical factor in a widely used phase change model

Many boiling problems have been simulated by a phase change model (Lee model) involving an empirical mass transfer intensity factor. It is challenge to determine the value of this factor. An explicit expression of the mass transfer intensity factor is derived, and the accuracy of the expression is validated by comparing simulation results with theoretical solutions for two benchmark problems. The influences of various fluid properties (liquid phase density and thermal conductivity, latent heat, saturation temperature, vapor phase density and thermal conductivity) and mesh size on the mass transfer intensity factor are discussed. Results demonstrate that the mass transfer intensity factor depends on density and thermal conductivity of liquid phase, latent heat, saturation temperature and mesh size, but is independent of the density and thermal conductivity of vapor phase. Effects of vapor density and thermal conductivity on the phase interface movement are taken into account by the temperature of the interfacial cell. The expression reveals that a constant value of the mass transfer intensity factor throughout the computational domain is not reasonable, and which is the reason for divergence issue. In a specific computational cell, the value of the mass transfer intensity factor depends on the volume fraction of the liquid phase, and extremely large value of the factor should be consistent with extremely small volume fraction of the liquid phase.

Fundamental Equations of State for cis-1,3,3,3-Tetrafluoropropene [R-1234ze(Z)] and 3,3,3-Trifluoropropene (R-1243zf)

New fundamental equations of state are presented for cis-1,3,3,3-tetrafluoropropene [R-1234ze(Z)] and 3,3,3-trifluoropropene (R-1243zf). The new equation for R-1234ze(Z), which is the main topic of this paper, is applicable to ranges at lower temperatures and higher pressures compared to the first published equation, and uncertainties in liquid-phase sound speeds are greatly improved. The equation is valid at temperatures from 238 to 440 K at pressures up to 34 MPa. Estimated uncertainties in this range are 0.1% for vapor pressures at temperatures above 300 K and 0.3% at lower temperatures, 0.1% for liquid-phase densities, 0.3% for vapor-phase densities, 0.02% for vapor-phase sound speeds, and 0.05% for liquid-phase sound speeds, except in the critical region where larger deviations up to about 1% are observed in density. The equation shows reasonable extrapolation behavior at extremely low and high temperatures, and at high pressures. The new equation for R-1243zf has a similar range of validity and unce...

Molecular simulation of the vapor-liquid equilibria of xylene mixtures: Force field performance, and Wolf vs. Ewald for electrostatic interactions

This article explores how well vapor-liquid equilibria of pure components and binary mixtures of xylenes can be predicted using different force fields in molecular simulations. The accuracy of the Wolf method and the Ewald summation is evaluated. Monte Carlo simulations in the Gibbs ensemble are performed at conditions comparable to experimental data, using four different force fields. Similar results using the Wolf and the Ewald methods can be obtained for the prediction of densities and the phase compositions of binary mixtures. With the Wolf method, up to 50% less CPU time is used compared to the Ewald method, at the cost of accuracy and additional parameter calibration. The densities of p-xylene and m-xylene can be well estimated using the TraPPE-UA and AUA force fields. The largest differences of VLE with experiments are observed for o-xylene. The p-xylene/o-xylene binary mixtures at 6.66 and 81.3 kPa are simulated, leading to an excellent agreement in the predictions of the composition of the liquid phase compared to experiments. The composition of the vapor phase is dominated by the properties of the component with the largest mole fraction in the liquid phase. The accuracy of the predictions of the phase composition are related to the quality of the density predictions of the pure component systems. The phase composition of the binary system of xylenes is very sensitive to slight differences in vapor phase density of each xylene isomer, and how well the differences are captured by the force fields.

Thermodynamic properties of hydrofluoroolefin (R1234yf and R1234ze(E)) refrigerant mixtures: Density, vapour-liquid equilibrium, and heat capacity data and modelling

Liquid-phase and vapour-phase densities are reported for the binary refrigerant mixtures (R125 + R1234ze(E)), (R134a + R1234ze(E)), (R143a + R1234ze(E)), (R1234ze(E) + R1234yf), (R125 + R1234yf), (R143a + R1234yf) and (R125 + R152a). The measurements span temperatures from (252 to 294) K and pressures from (0.8 to 4.2) MPa. Vapour-liquid equilibria (VLE) and liquid isobaric heat capacities are also reported for some mixtures. These measurements and previously published data were used to tune binary interaction parameters in existing Helmholtz energy models. Significant improvements in the predicted densities were achieved, for example the root mean squared relative deviation decreased from 0.33% to 0.021% for (R143a + R1234yf). The most significant improvement in the description of VLE occurred for (R1234yf + R1234ze(E)) where the root mean squared deviation in the predicted vapour phase compositions decreased from 0.010 to 0.00084 (a factor of 12).

Activity Coefficients of Aqueous Sodium, Calcium, and Europium Nitrate Solutions from Osmotic Equilibrium MD Simulations.

Osmotic and activity coefficients of three aqueous electrolyte solutions with cations of similar ionic radius, but different charges, are described by molecular dynamics with the help of the osmotic equilibrium method using polarizable force fields up to high concentration. Simulations of vapor-liquid interfaces of aqueous solutions of NaNO3, Ca(NO3)2, and Eu(NO3)3 at different concentrations and at 298.15 K provide time-averaged number density profiles and consequently the quantity of solvent molecules in the vapor phase. These three cations of similar ionic radii exhibit an increasing amount of water in their first coordination sphere due to their increasing charge. The solvent activity is directly determined by the vapor phase density at different salt concentrations with respect to the vapor phase density of the pure solvent. The obtained densities of the liquid bulk and the osmotic and activity coefficients for the three different nitrate salts are in good agreement with the experimental results. Time-averaged concentration profiles and the interpretation of radial distribution functions are used to explain the role of coordination on the thermodynamic properties of aqueous electrolyte solutions.

Molecular simulation of binary phase diagrams from the osmotic equilibrium method: vapour pressure and activity in water–ethanol mixtures

ABSTRACTHerein, an approach for simulating phase diagrams of binary mixtures is presented, where a bulk liquid and its corresponding vapour phase are simulated by means of molecular dynamics using explicit polarisation. Time-averaged density profiles for the pure compounds and mixtures at different mole fractions provide information about the spatial distribution in the bulk liquid and the amount of evaporated species in the adjacent vapour phase. The activities in the liquid phase are calculated from the mean vapour phase densities at a given composition, providing a good qualitative agreement compared to experimental data and the precision of the method follows a previously developed Poisson model of evaporation. With the Redlich–Kister approach for the activities in a binary mixture, the directly obtained activities are fitted providing corrected activity coefficients of the two species. This method is applied to ethanol water mixtures at different mole fractions. The obtained structural data are in good agreement with experimental data and time-averaged density profiles provide a detailed insight into the composition of the liquid–vapour interface. An azeotropic point is obtained for an excess concentration of ethanol at 87% as percentage by mass compared to the experimental value of 95%.

Technology Application of Environmental Friendly Refrigeration (Green Refrigeration) on Cold Storage for Fishery Industry

The application of refrigeration technology to postharvest fishery products is an very important. Moreover, Indonesia is a tropical region with relatively high temperatures. Fish storage age can be prolonged with a decrease in temperature. Frozen fish can even be stored for several months. Fish freezing means preparing fish for storage in low-temperature cold storage. The working fluid used in cold storage to cool low-temperature chambers and throw heat into high-temperature environments is refrigerant. So far refrigerant used in cold storage is Hydrochloroflourocarbons (HCFC) that is R-22. Chlor is a gas that causes ODP (Ozone Depleting Potential), while Flour is a gas that causes GWP (Global Warming Potential). Government policy began in 2015 to implement Hydrochloroflourocarbons Phase-Out Management Plan. Hydrocarbon (HC) is an alternative substitute for R-22. HC-22 (propane ≥ 99.5%) has several advantages, among others: environmentally friendly, indicated by a zero ODP value, and GWP = 3 (negligible), thermophysical property and good heat transfer characteristics, vapor phase density Which is low, and good solubility with mineral lubricants. The use of HC-22 in cold storage is less than R-22. From the analysis results obtained, cold storage system using HC-22 has better performance and energy consumption is more efficient than the R-22.

Simulating Osmotic Equilibria: A New Tool for Calculating Activity Coefficients in Concentrated Aqueous Salt Solutions.

Herein, a new theoretical method is presented for predicting osmotic equilibria and activities, where a bulk liquid and its corresponding vapor phase are simulated by means of molecular dynamics using explicit polarization. Calculated time-averaged number density profiles provide the amount of evaporated molecules present in the vapor phase and consequently the vapor-phase density. The activity of the solvent and the corresponding osmotic coefficient are determined by the vapor density at different solute concentrations with respect to the reference vapor density of the pure solvent. With the extended Debye-Hückel equation for the activity coefficient along with the corresponding Gibbs-Duhem relation, the activity coefficients of the solutes are calculated by fitting the osmotic coefficients. A simple model based on the combination of Poisson processes and Maxwell-Boltzmann velocity distributions is introduced to interpret statistical phenomena observed during the simulations, which are related to evaporation and recondensation. This method is applied to aqueous dysprosium nitrate [Dy(NO3)3] solutions at different concentrations. The obtained densities of the liquid bulk and the osmotic and activity coefficients are in good agreement with the experimental results for concentrated and saturated solutions. Density profiles of the liquid-vapor interface at different concentrations provide detailed insight into the spatial distributions of all compounds.

Nondispersive Infrared Gas Analyzer for Vapor Density Measurements of a Carbonyl-Containing Organometallic Cobalt Precursor.

A nondispersive infrared (NDIR) gas analyzer was demonstrated for measuring the vapor-phase density of the carbonyl-containing organometallic cobalt precurso μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA). This sensor was based on direct absorption by CCTBA vapor in the C≡O stretching spectral region and utilized a stable, broadband IR filament source, an optical chopper to modulate the source, a bandpass filter for wavelength isolation, and an InSb detector. The optical system was calibrated by selecting a calibration factor to convert CCTBA absorbance to a partial pressure that, when used to calculate CCTBA flow rate and CCTBA mass removed from the ampoule, resulted in an optically determined mass that was nominally equal to a gravimetrically-determined mass. In situ Fourier transform infrared (FT-IR) spectroscopy was performed simultaneously with the NDIR gas analyzer measurements under selected conditions in order to characterize potential spectroscopic interferences. Interference due to CO evolution from CCTBA was found to be small under the flow conditions employed here. A CCTBA minimum detectable molecular density as low as ≈3 × 1013 cm-3 was calculated (with no signal averaging and for a sampling rate of 200 Hz). While this NDIR gas analyzer was specifically tested for CCTBA, it is suitable for characterizing the vapor delivery of a range of carbonyl-containing precursors.

Spectroscopic Evaluation of the Molecular Structures of di-μ-Chlorobis(1,5-Cyclooctadiene) Iridium (I) and Rhodium (I) Complexes

Vibrational and H-NMR spectroscopic studies on di-μ-chlorobis(1,5-cyclooctadiene) of iridium(I) and rhodium (I) complexes have been carried out. In addition, the two D2h and D2 structures for both complexes have been fully optimized. It was expected from the single-molecule vapor-phase density functional theory (DFT) calculation that the D2 structure is more stable by 5 - 6 kcal/mol. While spectroscopic analysis study confirms that in the solid phase, the two complexes retain the higher D2h symmetry. The vibrational wavenumbers of certain modes associated to free 1,5-cyc- looctadiene were observed to be shifted to lower values upon coordination with rhodium or iridium metals. It was also found theoretically that the metal-olefin interaction is slightly more pronounced for iridium metal.

A determination of liquid–vapour interfacial properties for methanol using a linear-combination-based isotropic periodic sum

We performed molecular dynamics simulations of the methanol liquid–vapour interfacial systems for a wide temperature range of 200–350 K. The linear-combination-based isotropic periodic sum (LIPS) method was used for estimating Coulombic interactions of the systems. The temperature dependence of the liquid and vapour phase densities, surface thickness and surface tension using the LIPS method was almost equal to that of the Ewald sum method. The temperature dependence of the electrostatic potential profile using the LIPS method was very similar to that of the Ewald sum method. We conclude that the LIPS method provides a reasonable accuracy in determining the liquid–vapour interfacial properties of polar molecules for many temperature conditions.

Effect of Electric Field on Water Confined in Graphite and Mica Pores

We examined the influence of external electric field on the vapor–liquid coexistence curve and on structural properties of TIP4P/2005 water confined in hydrophobic and hydrophilic pores using all-atom molecular dynamics simulations. While the electric field increases the critical temperature of bulk water, the effect is contrary on the confined water. We clearly observe that the critical temperature of confined water decreases with increasing field strength. The current work strongly indicates that using electric field results in a decrease in the saturated liquid phase density and increase in the saturated vapor phase density of the confined water and can be used to modulate the evaporation rate of water under confinement. The effect of an electric field on the confined fluids is more pronounced at higher temperature. We also report that the critical density of water behaves differently in hydrophobic and hydrophilic pores. With increasing electric field, the critical density increases in hydrophobic pores; however, it is found to decrease in hydrophilic pores. We analyze the results using pair correlation functions and orientational, tetrahedral, and hydrogen bond distributions. Our investigation indicates that the presence of an electric field enhances the coordination number N(r) of the bulk phase. In contrast, the presence of an electric field reduces N(r) of the confined fluid. This is clearly reflected in the behavior of the critical temperature of bulk and confined water. Our structural analysis reveals that the application of an external field induces orientational order of dipole vector parallel to the field direction in bulk water, whereas its effects on dipole orientation is much less in confined systems. We also report that the hydrogen-bonding behavior in the vapor phase is responsible for the difference in critical density of water confined in hydrophobic and hydrophilic pores.