Liquid-vapor Interface Research Articles

Phase change and fluid transport in porous evaporators

Porous evaporators have become the core component of energy systems such as thermal management of electronic equipment, solar steam power generation, and wet power generation by the advantages of their large specific surface area, high-efficiency heat transfer, and liquid self-transportation. How direct observation of the heat and mass transfer process inside the porous evaporator has always puzzled researchers, resulting in a lack of a theoretical foundation for the optimization of the porous structure. This limitation has significantly hindered improvements in the performance of energy systems. To address this challenge, this study proposes a novel visualization approach that integrates three-dimensional (3D) printing and Indium-Tin Oxide (ITO) glass observation of the phase change region in the evaporator. The key novel findings are that: (1) By controlling the pore size to match the bubble nucleation size, the phase change mode can transition from boiling to thin film evaporation, forming a continuous easy vapor escape channel and enhancing heat transfer. (2) The larger the capillary force of the porous structure, the stronger the locking force at the vapor–liquid interface, which ensured the stability of vapor escape and liquid transport at higher heat flux. (3) An ideal porous evaporator should possess strong liquid locking ability, feature pore structures at the phase change point within the size range of vapor nucleation, and simultaneously exhibit large pore size conducive to vapor escape. This investigation casts a new light on the fluid transport and phase change characteristics of porous evaporators, laying a theoretical foundation for the design of porous evaporators.

The surface tension of Martini 3 water mixtures.

The Martini model, a coarse-grained forcefield for biomolecular simulations, has experienced a vast increase in popularity in the past decade. Its building-block approach balances computational efficiency with high chemical specificity, enabling the simulation of organic and inorganic molecules. The modeling of coarse-grained beads as Lennard-Jones particles poses challenges for the accurate reproduction of liquid-vapor interfacial properties, which are crucial in various applications, especially in the case of water. The latest version of the forcefield introduces refined interaction parameters for water beads, tackling the well-known artifact of Martini water freezing at room temperature. In addition, multiple sizes of water beads are available for simulating the solvation of small cavities, including the smallest pockets of proteins. This work focuses on studying the interfacial properties of Martini water, including surface tension and surface thickness. Employing the test-area method, we systematically compute the liquid-vapor surface tension across various combinations of water bead sizes and for temperatures from 300 to 350K. These findings are of interest to the Martini community as they allow users to account for the low interfacial tension of Martini water by properly adjusting observables computed via coarse-grained simulations to allow for accurate matching against all-atom or experimental results. Surface tension data are also interpreted in terms of local enrichment of the various mixture components at the liquid-vapor interface by means of Gibbs' adsorption formalism. Finally, the critical scaling of the Martini surface tension with temperature is reported to be consistent with the critical exponent of the 3D Ising universality class.

Study on the instability of FC-72 vapor–liquid interface in a rectangular channel under different gravity conditions

Study on the instability of FC-72 vapor–liquid interface in a rectangular channel under different gravity conditions

Temperature and pressure jump coefficients at a liquid–vapor interface

The temperature and pressure jump coefficients at a liquid–vapor interface are calculated from the solution of the Shakhov kinetic model for the linearized Boltzmann equation. Complete and partial evaporation/condensation at the vapor–liquid interface are assumed as the boundary condition. The discrete velocity method is used to solve the problem numerically. The jump coefficients are tabulated as functions of the evaporation/condensation coefficient. The profiles of the vapor temperature and pressure deviations from that values at the interface corresponding to the liquid temperature and saturation pressure are plotted, and the solutions obtained from kinetic theory and continuum approach are shown to underline the effect of the jumps at the interface. The obtained results have been compared to those given by other authors, who applied the linearized Boltzmann equation as well as the model proposed by Bhatnagar, Gross, and Krook to it, and it was found that the pressure and temperature jump coefficients are relatively insensitive to the collision laws.

A robust three-dimensional numerical model for heat transfer characteristics in sintered–grooved thin flat heat pipes

In the current research, a robust three-dimensional numerical model is developed for thin flat heat pipes (TFHPs) with a hybrid sintered–grooved wick structure. Numerical simulations for laminar incompressible flow in liquid wick and ideal gas incompressible flow in the vapor core are performed to predict temperature, pressure, and velocity profiles. The model utilized non-Darcy transport through a porous wick to determine liquid flow in the liquid-wick section. The mass flow rate of the fluid at the liquid–vapor interface is modeled using kinetic theory. Furthermore, the hybrid wick structure is modeled as an inhomogeneous porous medium. Additionally, this formulation enhances the stability and convergence of the numerical solution and accelerates the solving time. The developed model is validated with experimental data, showing very good agreement with axial wall temperatures, with a maximum error of about 2% in steady-state conditions. The numerical results, including system pressure, wall temperature, mass transfer at the liquid and vapor interface, and velocity magnitude streamlines, are investigated for a comprehensive understanding of the flow physics and performance of the hybrid wick. The results show that, at heat inputs of 5, 10, 20, and 30 W, the thermal efficiency of hybrid wick TFHP is 4.9%, 10.4%, 34.38%, and 23.3%, respectively, greater than that of the grooved wick. The TFHP with a hybrid wick indicates the best thermal efficiency at a heat input of 20 W, with a thermal resistance of 0.95 K/W.

Dynamic and interfacial properties for dichloromethane absorption in ionic liquids: Experimental and molecular dynamic insights

In this study, a strategy for dichloromethane (DCM) capture using ionic liquids (ILs) was systematically investigated from the perspective of dynamic analysis. The diffusion coefficient, Henry’s law constant (H), saturated absorption capacity, and overall volumetric liquid-side mass-transfer coefficient (KLao) of four types of ILs were obtained. Among all the ILs studied, tetrabutylphosphonium caproate ([P4444][C5COO]) demonstrated superior DCM absorption capabilities, and its corresponding H value (3.13 Pa m3/mol at 25 °C) was markedly lower than those of solvents documented in the literature. Furthermore, its KLao value was determined to be 2.77 × 10−3/s at 25 °C, which was measured in a customized cylindrical absorption bottle with a height of 150 mm and an inner diameter of 16 mm. The absorption bottle, fitted with a gas inlet tube having an inner diameter of 5 mm and positioned 5 mm above its base, utilized a simple bubbling technique to introduce gas into the liquid phase, eliminating the need for gas-dispersion equipment. Nevertheless, ILs with high affinities showed relatively high initial mass-transfer rates for DCM. Based on molecular dynamics (MD) simulations, the vapor–liquid interface properties, including mass and number density profiles, molecular orientation, and surface tension, were investigated. This revealed that in the pure [P4444][C5COO] system, the alkyl chain of the anion was oriented toward the gas phase, while the carboxyl group was oriented toward the bulk. The tilt angle of [C5COO]− with respect to the interface was within the range of 37° < θ′ < 70°. For the [P4444][C5COO]/DCM mixture, at the interface, the anion exhibits an enhanced ordered orientation, while the cation shows a more disordered orientation.

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.

Mass transfer at vapor-liquid interfaces of H2O + CO2 mixtures studied by molecular dynamics simulation

Abstract In many industrial applications as well as in nature, the mass transfer of CO2 at vapor-liquid interfaces in aqueous systems plays an important role. In this work, this process was studied on the atomistic level using non-equilibrium molecular dynamics simulations. In a first step, a molecular model of the system water + CO2 was developed that represents both bulk and interfacial equilibrium properties well. This system is characterized by a very large adsorption and enrichment of CO2 at the vapor-liquid interface. Then, non-equilibrium mass transfer simulations were carried out using a method that was developed recently: CO2 is inserted into the vapor phase of a simulation box which contains a liquid slab. Surprising effects are observed at the interface such as a net repulsion of CO2 particles from the interface and a complex time dependence of the amount of CO2 adsorbed at the interface.

A novel coaxial heat pipe with an inner vapor tube for cooling high power electronic devices

Improving heat transfer limit of heat pipe is important for their application in high-power equipment system. Shear stress at the liquid–vapor interface affects the heat transfer capacity of heat pipe. In this study, a novel coaxial heat pipe is designed to eliminate vapor entrainment from interfacial shear stress for the first time. An inner thin-walled tube is sintered with the powder wick in the adiabatic section by one-step sintering process. The effects of filling ratio and powder size of sintered wick on the thermal performance of coaxial heat pipe are investigated experimentally. The results indicate that the optimal powder size range for the sintered wick of coaxial heat pipe is 106–125 μm, while the optimal filling ratio is about 125 %. It means that the sintered wick with 106–125 μm powder can achieve better trade-off between capillary performance and permeability. The coaxial heat pipe can operate at heat loads of 70 ∼ 140 W with the total thermal resistance less than 0.06 °C /W. Moreover, compared with the normal heat pipe, the heat transfer capacity of coaxial heat pipe is increased by 57 %, while the evaporation temperature is decreased by 9.6 ∼ 19.1 °C and the total thermal resistance is decreased by 41 %∼320 %. The inner tube can improve the replenishment efficiency of working liquid by eliminating interfacial shear stress, resulting in the significant heat transfer improvement. The design of coaxial heat pipe is an effective and low-cost solution to improve the thermal performance of heat pipe.

Coupled modeling and simulation of tank self-pressurization and thermal stratification

Numerical analysis of thermo-fluid dynamics for cryogenic propellant storage primarily consists of nodal and computational fluid dynamics (CFD) modeling approaches. While nodal approaches prioritize faster computation over fidelity, CFD approaches promise accuracy and fidelity at the expense of significant computational resources. This paper presents an efficient coupling methodology developed to facilitate the simulation of a long-term self-pressurization process of a cryogenic propellant tank as well as associated thermal stratification phenomenon under both normal and reduced gravity conditions. Key highlight of the methodology is the development of a coupling scheme based on a domain decomposition approach that separates the tank into two computational regions at the liquid-vapor interface. SINDA/FLUINT, the nodal code, is utilized to simulate the liquid region, while ANSYS Fluent, the CFD code, handles the vapor region. A data exchange algorithm was developed to compute required boundary conditions for each region using the local thermodynamic properties of assumed infinitely thin interface. The coupling between the nodal and CFD codes was demonstrated by simulating a self-pressurization process in a small size tank using hydrogen as the working fluid. A sensitivity study on the grid sizing and coupling time step was conducted to determine appropriate spatial and ensure a divergent-free explicit data exchange time step size. Using the coupling approach, two numerical case studies were performed to study tank self-pressurization by considering two initial hydrogen fill levels. The effectiveness and efficiency of the coupling methodology was assessed by comparing the temperature and pressure evolution results of the coupled simulation with those obtained solely from the CFD simulation. Results showed that the coupling scheme and data exchange algorithm were successfully implemented, and the coupled simulation agreed well with the CFD simulations. In addition, a stratification number was used to characterize the transient dynamic development of the thermal stratification. Lastly, the computational costs associated with both approaches were compared. Significant reductions in simulation time were achieved for both cases.

Numerical simulation for pressure fluctuations caused by the growth of a single boiling bubble

The pressure fluctuations caused by the rapid volume changes during the initial growth stage for a single bubble are crucial excitations of boiling induced vibration. Therefore, it is necessary to accurately model the dynamic behavior of bubbles during this stage to obtain reasonable excitation forces, which can be used to estimate the vibration response of structures. The present study develops a dynamics model, which is verified and validated to be applicable to the growth of spherical bubbles in infinite superheated liquid and hemispherical bubbles growing on a vertical heated surface. The effects of mass transfer are considered for the vapor-liquid interface motion, and the temperature variations at the liquid side are determined by the finite difference method. The numerical data match well with the theoretical and experimental results, and the characteristics of the excitation forces caused by a single boiling bubble are obtained and analyzed. In addition, the influences of superheat, accommodation coefficient for phase change, and the temperature gradient at the liquid side on excitation forces caused by bubble growth are studied. The results show that the temperature distributions near the heated surface strongly affect the frequency domain characteristics of the excitation forces.

Performance Estimation of a Capillary-Fed Evaporative Microthruster

Nanosatellites are important for carrying out short-term and cost-effective communication and surveillance missions. Their small size necessitates the need for propulsion systems that are lightweight, compact, and capable of delivering accurate reaction and attitude control while allowing for seamless integration with the satellite. This paper reports on a numerical analysis to determine the performance of a micro-electromechanical system (MEMS)-based vaporizing liquid microthruster that utilizes microtextured substrates for passive feeding of the propellant (water) using capillary force and subsequent thin film evaporation by localized heating. The generated vapor flows through a converging nozzle to produce thrust. The performance of the propulsion device is evaluated in terms of the mass flow rate, thrust, and specific impulse. The model demonstrates a unique way of integrating the evaporation characteristics at the liquid–vapor interface to real nozzle flow dynamics. The evaporation phenomenon at the liquid–vapor interface is captured by utilizing kinetic theory of the gases, and real nozzle flow is analyzed by considering compressible-slip flow through the converging nozzle. It is shown that the microthruster can generate a thrust of ∼60 μN and an specific impulse of ∼67 s with a power input of approximately 3 W. The thrust and specific impulse efficiencies, when compared to quasi-one-dimensional isentropic values, are determined to range between ηthrust∼12 and 40% and between ηISP∼55 and 92%, respectively, for a power input of 0.2–3 W.

Experimental study on flow optimization and thermal performance enhancement of an ultrathin silicon-based loop heat pipe

Excessive heat flux of integrated circuits (ICs) leads to the failure of electronic devices and the consumption of extra energy. Silicon-based loop heat pipe (sLHP) offers a simple, reliable and easily-integrate method for IC thermal management. In this study, the influence of two common wicks (micropillar arrays (MP) wick and microchannel (MC) wick) on fluid flow and performance of sLHP are systematically compared and studied through visual experiment. And two advanced wicks (in-line long rib (IR) wick and staggered long rib (SR) wick) are proposed to further optimize the sLHP. The results show that the fluctuant vapor-liquid interface in MP wick is beneficial to the supplement of working fluid from adjacent flow channel, but it also introduces the uncertainty of flow. The MC wick features the preset flow channel and stable fluid flow but limited heat transfer capacity. Notably, the IR wick and SR wick effectively enhance that capability of supplementing fluid by combining the advantages of the MP wick and MC wick showing better thermal performance. The effective thermal conductivity of SR-sLHP and IR-sLHP is better than that of MP-sLHP and MC-sLHP, with the maximum values of 860 W/(m·K) and 848 W/(m·K), respectively.

Numerical and Experimental Investigation of Heat Transfer in the Porous Media of an Additively Manufactured Evaporator of a Two-Phase Mechanically Pumped Loop for Space Applications

Two-phase pumped cooling systems are applied when it is required to maintain a very stable temperature for heat dissipation in a system. A novel additively manufactured evaporator for two-phase thermal control was developed at NASA Jet Propulsion Laboratory (JPL). The Two-Phase Mechanically Pumped Loop (2PMPL) allows to manage the heat transfer with much wider breadth of control authority compared to capillary-based systems, while alleviating the system's sensitivity to pressure drops. The focus of this work is the understanding and capturing the micro-scale evaporation occurring in the porous structure of the evaporator. The Boiling and Phase Change Heat Transfer Laboratory at the University of California, Los Angeles (UCLA) developed an all-encompassing numerical simulation tool to predict the operational thermal behavior of the evaporator considering the effect of the liquid-vapor interface at the wick-to-vapor boundary. The numerical model incorporated the behaviour of the liquid-vapor meniscus at particle level located along the evaporative boundary between the wick structure and the vapor chamber. The numerical model allowed to study the effect of different parameters, such as boundary conditions, geometry, wick and fluid properties. An experimental setup was built at UCLA in order to characterize the heat transfer within an additively manufactured porous sample fabricated at JPL and in particular its evaporative heat load under certain heat inputs. The experimental efforts served as validation for the numerical results and aided in the characterization of the transient phenomena, such as dry-out.

A horizontal refined piecewise curve interface reconstruction (HOPCIR) algorithm for reconstructing the vapor-liquid interface

The vapor-liquid interface can be characterized as a circle or sphere affected by surface tension. However, linearization or planarization processing is commonly employed to simplify the reconstruction of the vapor-liquid interface within a grid cell, introducing errors in numerical simulations of two-phase flow. Therefore, this paper proposes a straightforward and accurate Horizontal Refined Piecewise Curve Interface Reconstruction (HOPCIR) algorithm to reconstruct the curve vapor-liquid interface for the two-dimensional structured grid. In this method, a standardized circle segment is employed to represent the vapor-liquid interface segment within each mixed grid cell, uniquely defined by its radius and center. The radius of this circle is determined by the curvature of the interface segment, and an iterative procedure is employed to calculate the circle's center using the known fluid volume fraction and initial interface obtained by linear reconstruction. Another integral aspect of the HOPCIR algorithm involves solving the reconstructed distance function to enhance the accuracy of curve reconstruction. Comparative analysis with the coupled volume-of-fluid and level set (VOSET) method demonstrates the superior performance of the HOPCIR algorithm in reconstructing vapor-liquid interfaces and solving the reconstructed distance function, interface curvature, surface tension, and fluid volume fraction.

A mesoscopic approach for nanoscale evaporation heat transfer characteristics

We propose a mesoscopic approach for investigating steady/transient nanoscale evaporation heat transfer characteristics, whose kinetic nature makes it an ideal tool to model the evaporation kinetics at the liquid-vapor interface and the microscopic wall-fluid interactions. Simulation of the evaporation of a flat nanoscale thin liquid film demonstrates its capability of resolving kinetically limited evaporation and liquid film adsorption. For the simulation of an evaporating meniscus in a nanochannel, our proposed mesoscopic approach considers the conservation of mass, momentum and energy of both liquid and vapor phases, thereby naturally incorporating both the multi-dimensional heat transfer effect and vapor transport resistance in nano-confined space. We demonstrate that solid-fluid interaction plays dominant roles on the interfacial transport during nanoscale evaporation, and vapor transport resistance has nonnegligible influences on the evaporation heat/mass transport in nano-confined spaces. By handling statistical behavior of molecule ensembles, this mesoscopic approach not only preserves microscopic physics but also has high computational efficiencies, enabling the simulation of space and time domains on the practical application level. This work paves the way for quantitative investigations of steady/transient nanoscale evaporation heat transfer characteristics using mesoscopic approach.

Solvent Cavitation during Ambient Pressure Drying of Silica Aerogels.

Ambient-pressure drying of silica gels stands out as an economical and accessible process for producing monolithic silica aerogels. Gels experience significant deformations during drying due to the capillary pressure generated at the liquid-vapor interface in submicron pores. Proper control of the gel properties and the drying rate is essential to enable reversible drying shrinkage without mechanical failure. Recent in operando microcomputed X-ray tomography (μCT) imaging revealed the kinetics of the phase composition during drying and spring-back. However, to fully explain the underlying mechanisms, spatial resolution is required. Here we show evidence of evaporation by hexane cavitation during the ambient-pressure drying of silylated silica gels by spatially resolved quantitative analysis of μCT data supported by wide-angle X-ray scattering measurements. Cavitation consists of the rupture of the pore liquid put under tension by capillary pressure, creating vapor bubbles within the gels. We found the presence of a homogeneously distributed vapor-air phase in the gels well ahead of the maximum shrinkage. The onset of this vapor/air phase corresponded to a pore volume shrinkage of ca. 50 vol % that was attributed to a critical stiffening of the silica skeleton enabling cavitation. Our results provide new aspects of the relation between the shape changes of silica gels during drying and the evaporation mechanisms. We conclude that stress release by cavitation may be at the origin of the resistance of the silica skeleton to drying stresses. This opens the path toward producing larger monolithic silica aerogels by fine-tuning the drying conditions to exploit cavitation.

Direct Observation of Anionic Yields from the Liquid-Vapor Interface by Electron Irradiation.

Dissociative electron attachment (DEA) is widely believed to play a high-profile role in ionizing radiation damages of bioorganic molecules, and its fundamentals are mainly learned from the gas-phase studies. However, the DEA process in aqueous solution is still in debate. Here we provide experimental evidence about the DEA processes of liquid methanol by using electron-impact-time-delayed mass spectrometry. In contrast to the gas- and solid-phase DEAs, methoxide ion CH3O- is the predominant product from the liquid interface. Furthermore, this anion can be produced with both the primary low-energy electrons and the inelastically scattered and secondary low-energy electrons. On the contrary, the primary low-energy electrons in the liquid bulk are more likely to be solvated, rather than directly participating in the DEA process. Our study provides new insights into radiation chemistry, particularly of bioorganic relevance.

Integrating machine learning and image processing for void fraction estimation in two-phase flow through corrugated channels

There is a substantial amount of information embedded in images of two-phase flow captured through high-speed video (HSV) or high-resolution photography. However, accurate image segmentation is necessary to unlock a meaningful analysis of the data. In this study, we discuss how to estimate the flow void fraction in chevron-type corrugated channels typical of compact plate heat exchangers (CPHE) from back-lit front-view HSV images, using machine learning (ML) algorithms and data processing techniques. A U-Net neural network was employed for image segmentation, demonstrating robust performance with evaluation metrics consistently exceeding 0.9. The binary masks (indicating gas or liquid phases) derived from segmentation were processed in MATLAB® to estimate void fraction through a 3D reconstruction algorithm of the gas cluster’s volume. In contrast to conventional void fraction estimates based on the area ratio of binary masks, this algorithm models the curvature of the liquid-vapor interface through the corrugated channel. When compared to other methods, our algorithm predicts very similar void fraction contour maps. However, the average discrepancy between our algorithm and the area-ratio approach can be as high as 80%, underscoring the importance of the processing method in analyzing the data and developing correlations. Finally, a drift flux model was introduced to predict the void fraction distribution using a two-part equation accommodating the dependency of the distribution coefficient C0 on the liquid flow rate for a corrugation Froude number Fr∼ larger than 1. The proposed model can predict the void fraction dataset with a mean average percentage error of 8.17%. In summary, U-Net’s pixel-level accuracy facilitates deep and precise post-processing of HSV images, enabling meaningful void fraction measurements. Thanks to its generality and minimal training effort requirements, the discussed methodology can be applied to estimate void fractions in various two-phase flow experiments and operating conditions.

Influence of the thermophysical properties of working liquids on heat transfer performance during flow boiling in microchannels

A thorough understanding of the governing interfacial interactions during flow boiling in a microchannel would provide important insights into the underlying physics of fluid dynamics and heat transfer mechanisms. In this study, flow-boiling experiments were performed in a single rectangular microchannel with hydraulic diameter of 0.18 mm. Three working fluids (water, ethanol, and HFE7100) with significantly different thermo-physical properties were tested under an inlet subcooling of 25 °C, mass flux of 50–330 kg·m−2·s−1, and effective heat fluxes up to 400 kW·m−2. Non-dimensional groups were used to quantify the relative effects of the forces governing the liquid–vapor interface. The bubble dynamics, flow pattern transition, heat transfer, and pressure drop were compared for the three different working fluids, and a mechanistic explanation of the flow boiling performance was proposed based on a force scaling analysis. Meanwhile, through investigating the influence of dimensionless parameters on heat transfer coefficient, a new basic heat transfer correlation that applies to a wide range of working fluids was developed, with 92.5 % of data points falling within ± 30 % error bands and with a mean absolute error of 14.7 %. HFE7100 exhibited the minimum increase in pressure drop with heat flux owing to its smallest liquid-to-vapor density ratio, low viscosity, and lowest interfacial tension effect. This study lays the foundation for the development of more reliable theoretical correlations and provides important insights into optimal design principles.