Liquid Vapor Phase Transitions: Modeling, Riemann Solvers and Computation

Work transfer theory for mechanical non-equilibrium, quasi-equilibrium, and equilibrium

Effect of pore morphology on vapor–liquid phase transition and crossover behavior of critical properties from 3D to 2D

Methanol in oil was used to indirectly evaluate the aging status of cellulosic paper insulation for oil-immersed transformers. However, the distribution of methanol in oil is affected by many factors, not only related to the oil–paper adsorption equilibrium, but also may be affected by the vapor–liquid phase transition. In this study, the adsorption experiments of mineral oil and standard cellulosic Kraft paper for methanol were designed to explore the adsorption equilibrium process at different oil–paper mass ratios under room temperature. The vapor–liquid equilibrium experiments of methanol between oil and gas were carried out to explore the effects of temperature and phase volume ratios. It turned out that: firstly, compared with mineral oil, the Kraft paper has a stronger adsorption capacity for methanol, which makes most of methanol adsorbed in paper when equilibrium at room temperature. Secondly, the increase of oil–paper mass ratios reduces the adsorption of Kraft paper for methanol and makes the equilibrium time longer. Particularly, at a higher oil–paper mass ratio, due to the less paper mass, methanol would “reverse diffusion” from paper to oil standard solution in the early stage of equilibrium. Finally, the presence of methanol in gas phase cannot be ignored. The increase of temperature makes more liquid methanol in oil transferred to the gas phase by the phase transition process, while increasing the oil–gas phase volume ratios in constant volume system can inhibit this process.

We develop a Riemann solver for transport problems related to oil recovery. We consider one dimensional incompressible flow in porous media involving several chemical components, namely $H_2O$, $H^ $, $OH^-$, $CO_2$, $CO_3^{2-}$, $HCO_3^-$ and decane, which are in chemical equilibrium in aqueous and oleic phases. As there is mass transfer between phases and the partial molar volume differs between aqueous and oleic phases leading to a variable total Darcy velocity, fractional flow theory does not easily apply. Recall that for upscaled equations the convection terms completely dominate the diffusion terms; this is why our basic model considers the limit of zero diffusion coefficients. The Riemann solution for this model can therefore be applied for upscaled transport processes in enhanced oil recovery involving geochemical aspects. We formulate three conservation equations, in which we substitute regression expressions that are obtained by geochemical software (PHREEQC). Gibbs phase rule together with charge balance shows that compositions can be rewritten in terms of the pH only. We use the initial and boundary conditions for carbonated aqueous phase injection in an oil reservoir containing connate water with some carbon dioxide. We compare the Riemann solution with a numerical solution, which includes capillary and diffusion effects. The structure of the Riemann solution for constant oil viscosity, from left (upstream) to right (downstream), consists of two rarefaction waves connected by a chemical shock; the latter is continued with a constant state and finally a fast Buckley-Leverett saturation shock. In the first rarefaction wave only the saturation changes, while in the second one both saturation and composition change. The connection point between the rarefaction waves can be constructed from a curve of states where the two characteristic velocities coincide. The significant new contribution is the effective Riemann solver we developed to obtain solutions for oil recovery problems including geochemistry and a space dependent total Darcy velocity.

We consider a system of two nonhyperbolic conservation laws modelling incompressible two-phase flow in one space dimension. The purpose of this paper is to justify the use of singular shocks in the solution of Riemann problems. We prove that both strictly and weakly overcompressive singular shocks are limits of viscous structures. Using Riemann solutions we solve Cauchy problems with piecewise constant data for the nonhyperbolic two-fluid model.

We present a numerical method, based on the Dafermos regularization,for computing a one-parameter family of Riemann solutions of a system ofconservation laws. The family is obtained by varying either the left orright state of the Riemann problem. The Riemann solutions are requiredto have shock waves that satisfy the viscous profile criterion prescribedby the physical model. The system is not required to satisfy strict hyperbolicity or genuine nonlinearity; the left and right states need not be close; and the Riemann solutions may contain an arbitrary number of waves, including composite waves and nonclassical shock waves. The method uses standard continuation softwareto solve a boundary-value problem in which the left and right states ofthe Riemann problem appear as parameters. Because the continuation methodcan proceed around limit point bifurcations, it can sucessfully computemultiple solutions of a particular Riemann problem, including ones thatcorrespond to unstable asymptotic states of the viscous conservation laws.

• Frog-inspired soft robot achieves muscle-like actuation via phase-change. • Thermochromic camouflage enables biomimetic environmental adaptation. • Buoyancy control broadens motion range in aquatic environments. • Unidirectional skeletal fin mimics frog webbing, boosting swimming efficiency. Underwater robotics holds significant potential for marine exploration and ecological monitoring, yet conventional systems often face challenges such as noise, structural complexity, and limited adaptability. To address these limitations, this study presents a frog-inspired underwater robot driven by liquid–vapor phase transition technology, designed to emulate the agility and efficiency of biological locomotion. By leveraging the reversible volume change of low-boiling-point phase transition materials (HFO-1336mzz-Z), we developed a lightweight actuator system (≤ 7 g) capable of rapid response (≤0 . 5 s), high deformation (≥ 120°), and biomimetic flexibility. The robot integrates antagonistic leg mechanisms for dynamic propulsion, frog-inspired webbed feet for enhanced thrust-to-drag ratios (≈ 3), and thermochromic coatings for adaptive camouflage. Experimental results demonstrate a maximum joint angle change of 148°, an angular velocity of 0.88 rad/s, and a maximum propulsion speed of 127 mm/s. Furthermore, the robot successfully completes an ascent of 23 cm within 20 s. This compact, low-cost design overcomes traditional rigid or fluid-driven constraints, offering a novel framework for applications in underwater exploration, environmental sensing, and biomimetic research.

While recent studies have proven an unexpected liquid–vapor phase transition of adsorbed liquid films, a comprehensive description of the mechanisms of different types of phase change regimes over realistic representations of random rough surfaces is absent in the literature. The current comprehensive study investigates the effects of a gold random rough surface, liquid film thickness, and substrate temperature on the liquid–vapor phase change regime of an adsorbed sodium liquid film, considering the evaporator section of a wicked heat pipe (WHP) using a molecular dynamics (MD) simulation. At first, to generate a realistic random rough surface, a new and promising method is proposed that is entirely based on MD simulations. Then, to simulate the evaporator section of a WHP, a unique configuration for eliminating the vapor domain is developed. The simulation results reveal that three distinct regimes, namely, normal evaporation, cluster boiling, and film boiling, could be identified, which are presented on two-dimensional diagrams with the substrate temperature and liquid film thickness as coordinates for the ideally smooth and random rough surfaces. The results also manifest that even though using the random rough surface could lead to different phase transition regimes, the type of regime depends mainly on the substrate temperature and liquid film thickness. Furthermore, this study displays two different modes for normal evaporation. Also, it is shown that the impacts of the liquid film thickness and substrate temperature on the mode of normal evaporation are much more significant than the surface roughness.

The swim bladder is crucial to underwater robots to enhance their overall performance and to expand their range of motion. However, previous attempts to incorporate this function have failed or have adopted mechanical swim bladders with high-disturbances. This study presents an entirely soft swim bladder capable of controlling buoyancy selectively and noiselessly, making it applicable to sensitive underwater environments. The soft swim bladder, which consists of an elastic cover layer, flexible heating elements, and three expandable pouches filled with low boiling point fluid, can express four modes of motion by varying buoyancy: sinking, suspending, rising, and fast-rising. The varying buoyancy is achieved through liquid-vapor phase transition of the fluid in the selected pouches when Joule heated above its boiling temperature. Moreover, the swim bladder is integrated with a shape memory alloy-based fishtail to form a soft fish robot. The synergy between the bladder and the tail allows the robot to explore a total of ten disparate modes of maneuvers, and their dynamic performance has been evaluated. The results of this study present the potential for the soft swim bladder to be utilized in any underwater robotic applications to enhance their swimming performance.

The generalized Replica Exchange Method (gREM) is extended into the isobaric-isothermal ensemble, and applied to simulate a vapor-liquid phase transition in Lennard-Jones fluids. Merging an optimally designed generalized ensemble sampling with replica exchange, gREM is particularly well suited for the effective simulation of first-order phase transitions characterized by "backbending" in the statistical temperature. While the metastable and unstable states in the vicinity of the first-order phase transition are masked by the enthalpy gap in temperature replica exchange method simulations, they are transformed into stable states through the parameterized effective sampling weights in gREM simulations, and join vapor and liquid phases with a succession of unimodal enthalpy distributions. The enhanced sampling across metastable and unstable states is achieved without the need to identify a "good" order parameter for biased sampling. We performed gREM simulations at various pressures below and near the critical pressure to examine the change in behavior of the vapor-liquid phase transition at different pressures. We observed a crossover from the first-order phase transition at low pressure, characterized by the backbending in the statistical temperature and the "kink" in the Gibbs free energy, to a continuous second-order phase transition near the critical pressure. The controlling mechanisms of nucleation and continuous phase transition are evident and the coexistence properties and phase diagram are found in agreement with literature results.

We consider a nonlinear hyperbolic model describing phase transitions in nonlinear elastodynamics. The Riemann problem is solved uniquely, provided we supplement the fundamental conservation laws (mass, momentum) with a kinetic relation. The latter takes into account small-scale mechanisms, such as the viscosity and capillary effects in the material under consideration. Our construction generalizes, to solutions of arbitrary large amplitude of the model under study, an approach proposed by Hayes and LeFloch for general systems of conservation laws. The Riemann solutions may contain rarefaction waves, (compressive) classical shock waves, as well as (undercompressive) nonclassical shocks.

Traveling Waves, Riemann Problems and Computations of a Model of the Dynamics of Liquid/Vapor Phase Transitions

We have developed a new technique that utilizes microwave energy to study the phase behavior of mixtures at the extreme temperatures and pressures encountered in petroleum and natural gas reservoirs. Our technique measures shifts in a static microwave energy field caused by changes in sample properties on a phase boundary. The appearance of a second phase produces detectable changes in the microwave resonant frequency and power. The new method does not involve inaccurate or unsafe procedures such as visual observation of samples, interpolation of isochoric slopes, or compositional analysis of samples drawn from the sample cell. Only small sample cell volumes (<10 cm3) are required. The microwave technique is effective with all types of fluids, light and heavy, nonpolar and polar, inert and corrosive, and it performs well at extreme conditions of pressure and temperature. A magnetic mixing pump enables preparation of homogeneous mixtures of fluids within the apparatus at experimental conditions. Measurements can be made at constant volume or at constant temperature. At constant volume, the microwave technique can determine phase boundary temperatures to better than 0.1 K. Pressure accuracy depends on the method of measurement. Results for phase boundary determinations of acid-gas mixtures with small amounts of heavy oils at high pressures and temperatures are presented.

Water distributes ubiquitously among the solar system and outer space in a wide variety of solid forms, i.e. more than ten kinds of crystalline ice, two types of amorphous ice, and clathrate hydrates. These polymorphs often play crucial roles in the planetary geology. Diversity of the stable ices and hydrates also suggests the existence of the various kinds of stable and metastable phases yet to be discovered [1]. Computer simulations and the theoretical treatments are useful to explore them. In this talk, we introduce the phase transitions of ice VII, which is one of the highest-pressure ice phases. The melting curve of ice VII to high-pressure liquid water has not been settled by experiments. We have proposed the intervention of a plastic phase of ice (plastic ice) between ice VII and liquid water, based on molecular dynamics (MD) simulations and the free energy calculations [2], which enables to account for large gaps among the various experimental curves of ice VII. In plastic ice, the water molecules are fixed at the lattice points, while they rotate freely. Interestingly, our additional survey by large-scale MD simulations elucidates that the phase transition between ice VII and plastic ice is first-order at low pressure as it was already predicted, while it is found to be second-order at higher pressures, where a tricritical point joins these phase boundaries together [3]. The critical fluctuations may give a clue for determining the phase boundary experimentally. We also argue about the phase transition dynamics of liquid water to ice VII at their direct phase boundary where metastable plastic ice phase plays an important role.

(1-x-y)K0.5Na0.5NbO3- xBaZrO3- yBi0.5Na0.5TiO3 ceramics (abbreviated as KNN-x, y) were prepared by the conventional solid state reaction method. BaZrO3 results in the increase of T R-O, and Bi0.5Na0.5TiO3 leads to the decrease of T O-T. The ceramics co-modified with these two additives exhibit a new phase boundary. With the shrinkage of O phase in the R–O–T phase structure, the first order O-T phase transition evolved to the secondary order R-T phase transition according to the thermal hysteresis. Enhanced piezoelectric properties (d 33 = 348 pC/N and k P = 43.2%) were obtained near the new phase boundary when x = 0.04 and y = 0.025.