Liquid Water Formation Research Articles

Metastable liquid–liquid transition in a molecular model of water

Liquid water's isothermal compressibility and isobaric heat capacity, and the magnitude of its thermal expansion coefficient, increase sharply on cooling below the equilibrium freezing point. Many experimental, theoretical and computational studies have sought to understand the molecular origin and implications of this anomalous behaviour. Of the different theoretical scenarios put forward, one posits the existence of a first-order phase transition that involves two forms of liquid water and terminates at a critical point located at deeply supercooled conditions. Some experimental evidence is consistent with this hypothesis, but no definitive proof of a liquid-liquid transition in water has been obtained to date: rapid ice crystallization has so far prevented decisive measurements on deeply supercooled water, although this challenge has been overcome recently. Computer simulations are therefore crucial for exploring water's structure and behaviour in this regime, and have shown that some water models exhibit liquid-liquid transitions and others do not. However, recent work has argued that the liquid-liquid transition has been mistakenly interpreted, and is in fact a liquid-crystal transition in all atomistic models of water. Here we show, by studying the liquid-liquid transition in the ST2 model of water with the use of six advanced sampling methods to compute the free-energy surface, that two metastable liquid phases and a stable crystal phase exist at the same deeply supercooled thermodynamic condition, and that the transition between the two liquids satisfies the thermodynamic criteria of a first-order transition. We follow the rearrangement of water's coordination shell and topological ring structure along a thermodynamically reversible path from the low-density liquid to cubic ice. We also show that the system fluctuates freely between the two liquid phases rather than crystallizing. These findings provide unambiguous evidence for a liquid-liquid transition in the ST2 model of water, and point to the separation of time scales between crystallization and relaxation as being crucial for enabling it.

Searching for springtime zonal liquid interfacial water on Mars

We analyzed the spatial and temporal characteristics of the surface temperature at the northern water ice annulus on Mars that is left behind the receding seasonal carbon dioxide cap in springtime. Using OMEGA hyperspectral images we show that water ice without carbon dioxide ice coverage lasts for 10–30days between 55° and 70°N. The longest water ice coverage without CO2 ice is observed between 40–55°N and 300–330°E and lasts 80–110days in ideal case. Using TES temperature data, we show that thin interfacial liquid water may be present at the water ice annulus. Higher spatial resolution THEMIS temperature data shows that the above mentioned finding is relevant to a spatial scale of 100m. Although the exact near surface water vapor concentration is not known, beside the average 10pr-μm we used two elevated values and corresponding threshold temperatures for interfacial liquid water formation: 190 and 199K beside the average 180K. While the area of interfacial liquid water is substantially smaller in the case of higher threshold temperature values, even for 199K terrains exist at THEMIS and OMEGA scale of resolution where such thin interfacial liquid water could be present on the surface. Summarizing: good chance exists for the presence of liquid interfacial water in the warmest part of the day on at the northern hemisphere of Mars at extended areas – although firm evidence requires better targeted future observations.

Hazard Control of NOx in Hot Stove

Hazard control of NOx is very important for the long life of hot stove shell and environmental protection. NOx concentrations during different operation periods of hot stove were calculated according to thermodynamical and dynamical analyses. The results were verified by the previously measured data. Then, the influence of hot stove operation parameters on NOx concentration and the mechanism of liquid water formation in hot stove were studied. The results indicated that in gas period, the dome temperature should be controlled below 1420 °C in order to decrease NOx emission. In the case of banking operation, NOx concentration was about 40—60 times higher than that in gas period. Hence, reasonable measures should be taken to reduce banking operation, especially in the situation of large excess air ratio. Since NOx formed during the whole operation process, the most effective way of preventing liquid water and HNO3 generation is to control the pipe and shell temperature, which should be higher than the condensation temperature of water vapour. The condensation temperature should be considered as the design temperature of pipe and shell for hot stove.

Effect of modified flow field on non-isothermal transport characteristics and cell performance of a PEMFC

This study analyzes how modified flow field affects non-isothermal transport characteristics and cell performance of the proton exchange membrane fuel cell (PEMFC) by the finite-volume SIMPLE-C method coupled with preconditioned conjugate gradient methods. The modified flow field is obtained by rectangular obstacles opposite or staggered with the protuberant catalyst layer surface or narrowed flow channel, with ribs opposite or staggered with the protuberant catalyst layer surface. This study also considered the effect of liquid water formation on the fluid flow field. The cell performance of the modified flow field is compared with that of the conventional flow channel. The comparison result demonstrates that the narrowed flow channel with ribs opposite the protuberant catalyst layer surface has the best increase rate in cell performance. The increase rate is approximately 8%.

Effect of Water Transport on the Electrical Performance of PEM Fuel Cell

This study investigates the electrical performance of a polymer electrolyte membrane fuel cell (PEMFC) by using the two-phase model taking into account the formation of liquid water. Mass transfer resistances from a water flooding problem are also considered to analyze the fuel cell performance. The effect of important operating conditions, such as temperature and humidity of fuel and air on PEMFC in terms of the distribution of gas composition and the electrical characteristics of fuel cell is investigated. The results show that the performance of PEMFC can be improved when increasing operating temperatures and fuel and air humidity. Water flooding at the cathode side increases with increased current density. The presence of the water flooding causes high concentration loss, degrading the PEMFC performance.

Numerical modeling of liquid water motion in a polymer electrolyte fuel cell

A three dimensional transient model fully coupling the two phase flow, species transport, heat transport, and electrochemical processes is developed to investigate the liquid water formation and transport in a polymer electrolyte fuel cell (PEFC). This model is based on the multiphase mixture (M2) formulation with a complete treatment of two phase transport throughout the PEFC, including gas channels, enabling modeling the liquid water motion in the entire PEFC. This work particularly focuses on the liquid water accumulation and transport in gas channels. It is revealed that the liquid water accumulation in gas channels mainly relies on three mechanisms and in the anode and cathode may rely on different mechanisms. The transport of liquid water in the anode channel basically follows a condensation–evaporation mechanism, in sharp contrast to the hydrodynamic transport of liquid water in the cathode channel. Liquid water in the cathode channel can finally flow outside from the exit along with the exhaust gas. As the presence of liquid water in gas channels alters the flow regime involved, from the single phase homogeneous flow to two phase flow, the flow resistance is found to significantly increase.

μPIV measurements of two-phase flows of an operated direct methanol fuel cell

In direct methanol fuel cells (DMFCs), two-phase flows appear in the channels of the anode side (CO2 bubbles in a liquid water–methanol environment) as well as of the cathode side (water droplets or films in an ambient air flow). CO2 bubbles or water droplets may almost completely fill the cross-section of a channel. The instantaneous effect of the formation of two-phase flows on the cell performance has not been investigated in detail, yet. In the current project, the micro particle image velocimetry (μPIV) technique is used to elucidate the corresponding flow phenomena on the anode as well as on the cathode side of a DMFC and to correlate those phenomena with the performance of the cell. A single-channel DMFC with optical access at the anode and the cathode side is constructed and assembled that allows for μPIV measurements at both sides as well as a detailed time-resolved cell voltage recording. The appearance and evolution of CO2 bubbles on the anode side is qualitatively and quantitatively investigated. The results clearly indicate that the cell power increases when the free cross-section area of the channel is decreased by huge bubbles. Methanol is forced into the porous gas diffusion layer (GDL) between the channels and the membrane is oxidized to CO2, and hence, the fuel consumption is increased and the cell performance rises. Eventually, a bubble forms a moving slug that effectively cleans the channel from CO2 bubbles on its way downstream. The blockage effect is eliminated; the methanol flow is not forced into the GDL anymore. The remaining amount of methanol in the GDL is oxidized. The cell power decreases until enough CO2 is produced to eventually form bubbles again and the process starts again. On the other hand under the investigated conditions, water on the cathode side only forms liquid films on the channels walls rather than channel-filling droplets. Instantaneous changes of the cell power due to liquid water formation could not be observed. The timescales of the two-phase flow on the cathode side are significantly larger than on the anode side. However, the μPIV measurements at the cathode side demonstrate the ability of feeding gas flows in microchannels with liquid tracer particles and the ability to measure in two-phase flows in such a configuration.

Modeling the cathode pressure dynamics in the Buckeye Bullet II 540 kW hydrogen PEM fuel cell system

The Buckeye Bullet 2 (BB2) is the world's fastest hydrogen fuel cell vehicle, with an international speed record of 302.9 mph. In order to achieve the power levels necessary for reaching the top speed, a unique gas supply system was designed to feed the PEM fuel cell modules. Stored Heliox with 40% oxygen content was used as the oxidizer and supplied to the cathode at high pressure. The high oxygen concentration at the cathode leads to a high rate of water formation in the GDL, with considerable influence on the pressure dynamics. For this reason, a precise monitoring of the pressure and water formation is required so that the cathode can operate at the maximum allowable pressure.This paper presents a novel control-oriented modeling approach to predict the cathode pressure dynamics of the BB2 PEM fuel cell system, developed for system optimization, monitoring and control. A distributed-parameter model was designed to characterize the liquid water formation and transport in the cathode channels, starting from the conservation laws for viscous fluid flow.The model was validated against a set of laboratory tests and actual race data. In this context, the proposed model is compared to a well known control-oriented PEM fuel cell model, to illustrate how the ability to predict the water transport at high reaction rates allows for an improved prediction of the pressure dynamics.

Water distribution and removal along the flow channel in proton exchange membrane fuel cells

Distribution expressions of total gas pressure and partial water vapor pressure along the channel direction were established based on lumped model by analyzing pressure loss in the channel and gas diffusion in the layer. The mechanism of droplet formation in the flow channel was also analyzed. Effects of the relative humidity, working temperature and stoichiometry on liquid water formation were discussed in detail. Moreover, the force equilibrium equation of the droplet in the flow channel was deduced, and the critical flow velocity for the water droplet removal was also addressed. The experimental results show that the threshold position of the liquid droplet is far from the inlet with the increase of temperature, and it decreases with the increase of the inlet total pressure. The critical flow velocity decreases with the increase of the radius and the working pressure.

A passive method of water management for an air-breathing proton exchange membrane fuel cell

Water management in an AB-PEMFC (air-breathing proton exchange membrane fuel cell) poses a big challenge due to its passive operation. The issue is addressed in a passive way by designing the fuel cell with low thermal conductivity materials. Use of low thermal conductivity materials for cell fabrication led to higher cell temperatures. The liquid water formation was delayed to high current densities due to increase in saturation pressure and higher buoyancy induced flow. Peak power density was increased by 36% and the limiting current density was increased by 37.5% when the cell was redesigned with low thermal conductivity materials. Fabricating the cells with low thermal conductivity materials can be very effective method of water management for air-breathing single PEM (proton exchange membrane) fuel cells and stacks of low capacity.

The effect of flow distributors on the liquid water distribution and performance of a PEM fuel cell

Water management is one of the important factors which determine the performance of a Proton Exchange Membrane (PEM) fuel cell using hydrogen as fuel. For developing efficient water management systems, it is important to know the potential locations of formation and the nature of distribution of liquid water in the fuel cell. In the present study a PEM fuel cell with three different types of flow distributors are modeled and numerically simulated to find out the water formation and distribution characteristics. The model is validated by comparing the simulated polarization curve to experimental data. It is found that the type of flow distributor used plays a major role in determining the distribution of liquid water in the cell. A parallel flow distributor exhibits poor water removal capabilities whereas a serpentine flow distributor exhibits better water removal. A mixed flow distributor is found to give better water distribution characteristics compared to the parallel and serpentine distributors. Further the effect of liquid water formation and distribution on the species transport, temperature distribution and current generation are also investigated.

PEM 단위 연료전지 가시화 셀을 이용한 당량비 변화에 따른 플러딩 현상에 관한 연구

본 논문은 고분자 전해질 연료전지 공기극 유로 내부에서 당량비에 따라 발생하는 플러딩 현상을 가시화를 통해 확인하고, 전류 변화에 따른 물의 운송 특성에 관한 연구를 수행하였다. 공기극 당량비는 1.5, 2.0, 연료극 당량비는 1.5로 고정하여 실험을 수행하였다. 연료전지 공기측 당량비 2.0로 공급하였을 때 1.5와 비교하여 짧은 시간에 물이 생성되기 시작하였으며, 플러딩 영역이 빠르게 생성되는 결과가 나타났다. 또한, 공기극 당량비 1.5로 유지하는 경우 플러딩 영역 이후에 건조화 7.8A 이후 구간에서 건조화가 진행되며, 8A 이후구간에서 건조화가 시작되는 공기극 당량비 2.0에서 작동하는 연료전지와 비교하여 넓은 영역에서 물 생성이 활발하게 이루어져 MEA의 내구성과 수소이온전도도가 우수한 결과를 확인하였다.

An efficient method for numerical predictions of the performance characteristics of fuel cells. I. Model development and validation

This study presents the model development and validation of an efficient method for numerical predictions of the performance characteristics of proton exchange membrane (PEM) fuel cells. To efficiently execute extensive modeling in a short time period, the computational model uses a half-cell at the cathode with only one oxidant channel as the modeling domain. The original three-dimensional (3D) model is replaced with a reduced two-dimensional model. In terms of these computational models and reduced dimensions, mathematical formulations describing physical phenomena within fuel cells are developed. In addition, two-phase Darcy's laws, which are originally only applied to porous media, are further extended to describe the transport and formation of liquid water within channels. In this study, the Fortran programming language is used as a numerical program to implement iterative calculations and predict the overall performance characteristics for three different cases. By comparing the modeling and experimental polarization curves for case 1 and the modeling of points of current density vs. cell voltage ( IV) of different computational models for cases 2 and 3, this computational model is found to possess a certain degree of accuracy and reliability. In addition, the complete extensive modeling also works more efficiently and with fewer computational resources.

Artificial Cloud Seeding Using Liquid Carbon Dioxide: Comparisons of Experimental Data and Numerical Analyses

AbstractAn artificial seeding experiment was carried out over the Genkai Sea, Japan, using liquid carbon dioxide. The seeded cloud was followed by an aircraft and radar at Kyushu University. A radar-echo intensity of 19 dBZ was formed in the seeded cloud where no radar echo was observed previously. The results of the experiment are analyzed using a numerical model [Weather Research and Forecasting Model (WRF), version 3.1] to evaluate the effects of the artificial seeding. The numerically simulated radar echo has a slightly stronger intensity and wider area than that observed. The results of the experiments were similar to those produced by the numerical model, however. Evaporation of cloud liquid water; growth of snow particles that is due to deposition, autoconversion, aggregation, and collection; and convection caused by released latent heat are shown in the numerical results as effects of the seeding experiment. The falling snow particles cause downward flow, which is compensated for by upward flow that causes the formation of new cloud liquid water as a secondary effect of seeding, although this secondary effect is confirmed only in the numerical results.

Study on the effect of humidity and stoichiometry on the water saturation of PEM fuel cells

This paper investigates the effects of relative humidity (RH) and stoichiometry of reactants on the water saturation and local transport process in proton exchange membrane fuel cells. A two-dimensional model was developed, taking into account the effect of the formation of liquid water on the reactant transport. The results indicate that the reactant RH and stoichiometry significantly affect cell performance. At a constant anode RH = 100%, a lower cathode RH maintains membrane hydration to give better cell performance. At a constant cathode RH = 100%, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction but also increases the difference in the water concentrations between the anode and cathode. This enhances the back-diffusion of water from the cathode to the anode, reducing possible flooding for better cell performance. Higher anodic stoichiometry results in the reduction of cathodic water saturation by increasing water back-diffusion, thereby enhancing fuel cell performance. Higher cathodic stoichiometry also reduces water saturation by drying more liquid water to increase cathode local current density. Copyright © 2011 John Wiley & Sons, Ltd.

Modeling Two-Phase Water Transport in Hydrophobic Diffusion Media for PEM Fuel Cells

The prevailing method for modeling water transport in hydrophobic fuel cell diffusion media employs the same two-phase generalization of Darcy’s law widely used for analyzing the movement of oil and water through soil and porous rock. Recent experiments, however, have identified certain problems when the same approach is applied to the nonwoven carbon papers used in fuel cell diffusion media. These problems could be addressed by generalizing the Darcy formulation still more, but significant challenges, such as measuring the relevant material properties, still remain. Instead, this paper proposes a simpler model adapted specifically for fuel cells. Based on analysis of published experiments, the proposed one-dimensional model for heat and two-phase water transport assumes a uniform liquid pore saturation wherever the diffusion medium is wet and neglects the capillary pressure gradients required to transport water in liquid form. In addition to the properties for dry transport, the two-phase water transport model requires just one additional material parameter—the ratio of dry to wet effective gas diffusion coefficients—which can be measured by straightforward limiting current methods. Example half-cell solutions for a Toray diffusion medium show how the proposed model can reproduce changes in oxygen transport resistance observed in limiting current experiments.

Spatially inhomogeneous bimodal inherent structure of simulated liquid water

In the supercooled regime at elevated pressure two forms of liquid water, high-density (HDL) and low-density (LDL), have been proposed to be separated by a coexistence line ending at a critical point, but a connection to water at ambient conditions has been lacking. Here we perform large-scale molecular dynamics simulations and demonstrate that the underlying potential energy surface gives a strictly bimodal characterization of the molecules at all temperatures and pressures, including the biologically and technologically important ambient regime, as spatially inhomogeneous either LDL- or HDL-like with a 3 : 1 predominance for HDL under ambient conditions. The Widom line in the supercooled regime, where maximal structural fluctuations take place, coincides with a 1 : 1 distribution. Although our results are based on molecular dynamics force-field simulations the close agreement with recent analyses of experimental X-ray spectroscopy and scattering data indicates a unified description also of real liquid water covering supercooled to ambient conditions.

Simulation of Ice Accretion Based on Roughness Distribution

Predicted model of roughness was built based on three different forms of liquid water on icing surface observed in experiments. Some issues regards to the icing mechanism and mass and heat transfer in roughness conditions were addressed. The classical Messinger ice accretion model was revised in which the remaining liquid water on ice surface was added. Validation of a new icing code based on roughness was performed by comparing the predicted results with experimental data. Factors that affecting the roughness height and its distribution were also investigated.

Failure assessment of composite cooler tubes in a gas boosting station

The present study describes origin and failure mechanisms of air cooled heat exchangers tubes, in a gas pressure boosting station. Hydrocarbon gas was circulating within aluminum finned tubes and cooling was done by forcing ambient air over the exterior of the tubes, made of carbon steel A-214 material. The hydrocarbon gas was contained traces of H 2S and substantial level of CO 2. The investigation was carried out in a station located in southern part of Iran. The process involved condensation of water and hydrocarbon along the length of tubes, resulting in a wet gas multiphase flow situation. Such type of coolers is also called composite coolers. The failure of tubes was characterized on the bases of all the available evidences and metallurgical examinations, such as analysis of tube materials, feeding gas, condensate water, and the residue inside the tubes. The processing was also simulated by Hysis-3.1 software, in order to evaluate and compare various parameters such as gas flow rate, liquid water and hydrocarbon formation, in actual and design condition. The air cooled tubes showed highest corrosion rate and was experienced leakage regularly. The results indicated that, low velocity assisted sweet corrosion caused severe pitting inside the tubes, and led to failure.

Thermodynamics of phase transitions in the system ice VI-ice VII-water

The paper presents an analysis of experimental results on phase equilibria in the system ice VI-ice VII-water. The mathematical processing of the experimental data makes it possible to further constrain the location of phase transition boundaries between crystalline ices VI and VII and liquid water. Based on the standard thermodynamic properties of H2O, an equation was derived for the free energy of formation of liquid water in its stability fields. The equations thus obtained were then utilized to calculate the standard thermodynamic properties (free energy, enthalpy, and entropy) of water ice VI and VII and the free energy of their formation at pressures of 6.3–25 kbar.