Liquid-vapor Phase Change Research Articles

Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit

Nanoporous structures including single nanopores and nanoporous membranes have been utilized as a platform to study fundamental liquid-vapor phase change heat transfer (PCHT) processes as well as a promising candidate for high flux heat dissipation. Both thin film boiling and evaporation through nanoporous structures have been demonstrated to achieve high heat flux, but they are usually considered two mutually exclusive regimes operated under vastly different conditions, and the factors dictating how close the PCHT process is to the kinetic limit are elusive. In this work, we utilized a unique transition between thin film boiling and evaporation through nanoporous membranes to clarify the factors determining the heat flux and heat transfer coefficient (HTC) with respect to the kinetic limit conditions. We unambiguously showed the controllable transition from boiling to evaporation, when the liquid receded into the nanopores and provided additional driving force from capillary pumping sustained in the nanoscale pores. We showed that this transition is universal and can be understood from a simple fluid transport model for all the four types of fluids we studied, which cover a wide span of surface tension (water, ethanol, isopropanol (IPA), FC-72). More importantly, PCHT conditions at the transition points between boiling and evaporation were close to those of the kinetic limit of all these fluids. However, further increase of the heat flux beyond the transition points led to decreasing HTC and deviation from the kinetic limit, which can be attributed to the increasing vapor resistance in the vapor space and inside the nanopores. This increasing vapor resistance was also confirmed by experiments on IPA with different vapor pressures.

Evaluation of Level Set reinitialization algorithms for phase change simulation on unstructured grids

In this study, we present different numerical methods for the simulation of liquid-vapor phase change (boiling). We use a Level Set formalism to capture the liquid-vapor interface. Such a formalism requires a reinitialization (aka redistancing) step of the Level Set function after advection. This step is critical for phase change simulation as it must neither move the interface nor induce perturbations in the Level Set function profile, otherwise the normal vector to the interface and its curvature, two quantities that are crucial to define respectively the interface velocity due to phase change and the pressure jump at the interface due to surface tension, would be in turn affected by too large numerical errors. Here we present a comparison of different reinitialization algorithms of the Level Set function for boiling simulations, on structured and unstructured grids. These methods are then validated against the analytical case of a static growing bubble with a fixed mass transfer rate. In particular, we observe that at the time corresponding to a doubled bubble radius, the error on the bubble radius decreases with the grid cell size for all presented methods.

4D Printing of High-Performance Thermal-Responsive Liquid Metal Elastomers Driven by Embedded Microliquid Chambers.

Four-dimensional (4D) printing of swellable materials have been viewed as an ideal approach to build shape morphing architectures. However, there is less variety in high-performance swellable materials, limiting its development. To address this challenge, we proposed a new strategy for designing high-performance thermal-responsive swellable materials. The reversible liquid-vapor phase change of embedded low boiling point liquid chambers and functional liquid metal fillers endows the designed elastomer with the reversible thermal-responsive swellable property with high stability, fast response speed, and large equilibrium deformation. Notably, liquid metal fillers play a crucial role in improving the thermal-responsive property via improving the thermal conductivity and fracture toughness and decreasing the stiffness. To demonstrate the feasibility of constructing shape morphing architectures with proposed thermal-responsive liquid metal elastomers, typical bilayer structures were printed and investigated. By altering the key design parameters, the response speed and equilibrium deformation can be adjusted as needed. Therefore, complex shape morphing architectures can be printed. This study could provide a new avenue to design swellable material systems for 4D printing of shape morphing architectures.

Continuum soft actuators based on reprogrammable geometric constraints

Although numerous soft actuators have been developed to generate diverse and dexterous morphing such as branching, bending, and twisting, continuum manipulation with increased degrees of freedom still relies on the coactivation of multiple actuators. We demonstrate continuum soft actuators that exhibit a higher number of deformation modes in a single actuating unit. Our devices consist of an elastomer composite embedded with micro liquid droplets and an array of resistive heating elements to locally drive the liquid–vapor phase change of adjacent liquid droplets. The elastomer composite body only inflates in the vicinity of a resistive heating element in the on-state while other segments, including those near inactive elements, serve as rigid constraints, such that the bending direction can be systemically changed depending on the state of the individual resistive heating elements in the embedded array. We show that our continuum soft actuators permit fully continuous motions, including rotations, via a simple operating scheme.

High Heat Flux Evaporation of Low Surface Tension Liquids from Nanoporous Membranes.

Water is often considered as the highest performance working fluid for liquid-vapor phase change due to its high thermal conductivity and large enthalpy of vaporization. However, a wide range of industrial systems require using low surface tension liquids where heat transfer enhancement has proved challenging for boiling and evaporation. Here, we enable a new paradigm of phase change heat transfer, which favors high volatility, low surface tension liquids rather than water. We utilized a nanoporous membrane of ≈600 nm thickness and <140 nm pore diameters supported on efficient liquid supply architectures, decoupling capillary pumping from viscous loss. Proof-of-concept devices were microfabricated and tested in a custom-built environmental chamber. We used R245fa, pentane, methanol, isopropyl alcohol, and water as working fluids with devices of total membrane area varying from 0.017 to 0.424 cm2. We realized a device-level pure evaporation heat flux of 144 ± 6 W/cm2 for water, and the highest evaporation heat flux was obtained with pentane at 550 ± 90 W/cm2. We developed a three-level model to understand vapor dynamics near the interface and thermal conduction within the device, which showed good agreement with experiments. We then compared pore-level heat transfer of different fluids, where R245fa showed approximately 10 times the performance of water under the same working conditions. Finally, we illustrate the usefulness of a figure of merit extracted from the kinetic theory for evaporation. The current work provides fundamental insights into the evaporation of low surface tension liquids, which can impact various applications such as refrigeration and air conditioning, petroleum and solvent distillation, and on-chip electronics cooling.

3D lattice Boltzmann simulation of droplet evaporation on patterned surfaces: Study of pinning–depinning mechanism

The liquid–vapor phase change lattice Boltzmann method is used to investigate the pinning–depinning mechanism of the contact line during droplet evaporation on the stripe-patterned surfaces in 3D space. Considering the curvature of the contact line and the direction of the unbalanced Young's force, the local force balance theory near the stripe boundary is proposed to explain the steady state of the droplets on the stripe-patterned surfaces. An equation is proposed to evaluate the characteristic contact angle of the stabilized droplets. During the evaporation of the droplet, the stick-slip-jump behavior and the CCR-Mixed-CCA mode can be well captured by the lattice Boltzmann simulation. When the contact line is pinned to the stripe boundary, the contact line in the direction perpendicular to the stripes is slowly moving while the curvature of the contact line is gradually increasing. The gradually increasing curvature of the contact line accelerates the movement of the contact line, and the final contact line is detached from the stripe boundary. The research results provide theoretical support and guidance for the design, improvement and application of patterned surfaces in the field of micro-fluidic and evaporation heat transfer.

Synergistic Tandem Solar Electricity-Water Generators

Summary Energy and clean water are two intertwined fundamental elements for human civilization and sustainable development. Here, we devise a monolithic tandem solar electricity-water generator that synergistically produces electricity and clean water by utilizing the full spectrum of solar irradiance. The tandem generator has two components: a top infrared-transparent photovoltaic device using above-band-gap photons, and a bottom solar water purifier using the below-band-gap photons. More strikingly, a well-designed water-proof thermal interconnecting layer (WTIL) makes these two components work synergistically: the bottom purifier serves as an evaporative cooler of the top solar cell to increase its efficiency, whereas the thermalization energy of the top cell is reutilized by the bottom purifier for producing more clean water. We experimentally demonstrate that a prototype hybrid tandem solar device with WTIL can generate electricity with a power output of 204 W m−2 and purify water at a rate of 0.80 kg m−2 h−1 under 1-sun illumination.

Lattice boltzmann simulation of boiling heat transfer in a shear flow

In this work a two-phase lattice Boltzmann method was used to numerically study the behavior of liquid-vapor phase change induced by a heated plate under the action of shearing. The effects of the action of shearing on the bubble growth and departure were investigated in terms of the flow features, the average heat flux and the bubble releasing period. It is shown that the shear flow significantly enhances the heat transfer of the system in two respects: increasing the average heat flux and decreasing the bubble releasing period. The effects of the intensity of the shear flow and the gravity force on the bubble releasing period were examined as well. The most striking finding is that there exists a sudden jump in the period at a critical shear intensity of the flow. The reason behind this abnormal behavior is that the residual part of bubble is nearly condensed after the bulk of bubble departs from the heated plate.

Lattice boltzmann simulation of boiling heat transfer in a shear flow

In this work a two-phase lattice Boltzmann method was used to numerically study the behavior of liquid-vapor phase change induced by a heated plate under the action of shearing. The effects of the action of shearing on the bubble growth and departure were investigated in terms of the flow features, the average heat flux and the bubble releasing period. It is shown that the shear flow significantly enhances the heat transfer of the system in two respects: increasing the average heat flux and decreasing the bubble releasing period. The effects of the intensity of the shear flow and the gravity force on the bubble releasing period were examined as well. The most striking finding is that there exists a sudden jump in the period at a critical shear intensity of the flow. The reason behind this abnormal behavior is that the residual part of bubble is nearly condensed after the bulk of bubble departs from the heated plate.

Eulerian modeling of aerosol transport and deposition in respiratory tract under thermodynamic equilibrium condition

A computational study is carried out using a purely Eulerian approach for investigating the inhaled e-cigarette aerosol transport and deposition in the human upper respiratory tract (URT). These aerosols consist of some volatile compounds and are generally hygroscopic with vapor liquid phase change at the particle–air interface. An Eulerian model is developed based on the thermodynamic equilibrium assumption, where the local masses of the constituents are partitioned into either the vapor or liquid phases subject to the local equilibrium constraints at each point in the computational mesh at the end of each time-step. The computational model is validated using available experimental data, with the results demonstrating good agreement. Given the fact that experimental determination of the deposition under human inhalation conditions is extremely difficult, this modeling approach can be considered as a practical method to estimate deposition of aerosol constituents in the upper respiratory tract.

Characteristics of thermal energy removal from heat-generating porous media considering liquid-vapour phase change

Heat-generating porous media is encountered in different industrial applications as well as natural processes - natural geysers, geothermal energy extraction, packed bed reactors, nuclear and thermal power plants etc. The present article discusses the characteristics of thermal energy removal from heat-generating porous media, considering phase change of the saturating fluid phases. The complex phenomena of multiphase flow through porous media, coupled with the associated heat and mass transfer processes, is modelled numerically and implemented in a general computational platform with the help of user-defined functions. The model is validated extensively with experimental data and utilised for analysing heat removal from a heat-generating porous bed submerged in a liquid pool. This is in contrast to the various specialised numerical codes that have been specifically developed for modelling these phenomena.The study analyses the fundamental impacts of bed porosity, particle size and bed configuration on heat transfer and multiphase flow mechanism in the heat-generating porous bed. Bed porosity and particle size are observed to influence both fluid transport and heat transfer mechanisms, while bed composition is observed to mainly affect the fluid transport process. These effects, as such, influence heat removal from the heat-generating porous bed, which is quantified in terms of the maximum temperature rise within the bed and the ultimate energy removal possible from the bed. The latter is represented in terms of dryout power density, which represents the minimum volumetric heat generation at which it becomes impossible to ensure adequate heat removal from the bed. Assessment of dryout power density, as such, is very important in the context of heat removal and safety of the related applications. Increase in particle size as well as porosity is observed to augment this limit. The influence of bed composition on this limit is observed to be determined by the interplay of two parameters bed height and interface area of the bed with the surrounding liquid pool.

Numerical simulations of the liquid-vapor phase change dynamic processes in a flat micro heat pipe

The performance of a flat micro heat pipe (FMHP) is highly dependent on the wick geometry. Understanding the dynamic behaviors of liquid-vapor phase change in the FMHP is of significance for its optimal design, especially the wick geometry. In this work, a pore-scale three-dimensional pseudopotential lattice Boltzmann model coupled with a thermal model is developed to study the transient behaviors of the FMHP without any empirical equations. In the developed model, the curvature of liquid-vapor meniscus can be automatically adjusted according to the external heat source and the receding behavior of liquid films can be captured. Besides, effects of detailed wick microstructures on the performance of the FMHP are systematically studied. The results show that a more hydrophilic grooved wick yields a higher capillary force to improve the working ability. The working temperature and liquid charge amount significantly affect the performance of the FMHP. The grooved-type wick with a trapezoid shape has the best performance, next by the rectangular shape and then inversed trapezoid shape. For the micro-pillar type wick, with a relatively small pillar pitch at the evaporation section and a large pitch at the condensation section can improve the working performance.

Two-phase flow pattern and heat transfer coefficients of R245fa/R134a under non-uniform heat flux

The non-uniform distribution of the heat flux commonly occurs in systems where vapor-liquid phase changes, such as nuclear reactors, and aerospace thermal control systems. This poses a great challenge on the study of pressure drop, heat transfer coefficient, and flow pattern prediction. In addition, flow patterns and heat transfer characteristics are usually affected by differences in fluid properties (viscosity, density, and surface tension), thermodynamic conditions (pressure, temperature), and geometric dimensions. At present, most of the researches on non-uniform heat flux are concentrated in systems with pure working fluids. However, compared to the pure working fluid, the mixed working fluid, particularly the zeotropic working fluid can improve the cycle efficiency and has great application potential, chiefly due to the temperature glide. There are few studies on the two-phase flow pattern and heat transfer characteristics of the zeotropic working fluid under non-uniform heat flux. Aiming at the problem of vapor-liquid phase change of zeotropic working fluid under non-uniform heat flux boundary conditions, a two-phase flow boiling experimental system in a horizontal tube with a 10 mm inner diameter under non-uniform heat flow was established. The studied zeotropic working fluid is R245fa/R134a with a mass fraction of 0.7:0.3. Experiments were performed at conditions where the mass flux ranges from 175 to 373 kg/(m2 s), while the heat flux ranges from 9.95 to 47.57 kW/m2. Using a high-speed camera and image processing technology, the gray values of the observed bubble flow, plug flow, stratified flow, and annular flow were analyzed, and the flow patterns under both non-uniform and uniform heat flux were demonstrated. Effects of the vapor quality, mass flux, heat flux and saturation temperature on the two-phase heat transfer coefficient were analyzed. And on the basis of experimental data, the prediction accuracy of four commonly used correlations was evaluated. The results show that the gray value of bubble flow and plug flow fluctuate widely, while the gray value fluctuation range of stratified flow and annular flow is relatively small. The typical flow pattern under non-uniform heat flux is the same as flow pattern under uniform heat flux. With the same amount of the additional heat, under the condition of non-uniform heat flux, the initial vapor quality of annular flow is advanced, and the proportion of the annular flow is larger than the non-annular flow. The main reason for this phenomenon is that the flow instability caused by non-uniform heat flux accelerates the transition of intermittent flow to annular flow. In addition, the heat transfer coefficient changes little at the lower vapor quality, and the heat transfer coefficient of the intermittent flow region is independent of vapor quality. In the annular flow region with higher vapor quality, the heat transfer coefficient increases with the increase of vapor quality. The heat transfer coefficient increases with the increase of mass flux and heat flux. The saturation temperature has little effect on the heat transfer coefficient in the convection boiling region and has a certain influence in the nucleate boiling region. Furthermore, the prediction ability of the four correlations is poor in the non-annular flow region. Relatively, the prediction accuracy of the Sun & Mishima correlation is the highest in the annular flow region.

Numerical study on vapor–liquid phase change in an enclosed narrow space

Based on the multiphase lattice Boltzmann method, a transient model of vapor–liquid phase change in an enclosed narrow space during startup operation is developed and numerically analyzed to investigate the two-phase dynamics and heat transfer behaviors in the confined flat two-phase thermosiphon. The transient temperature distribution, vapor–liquid interface evolution and thermal performance are investigated over a wide range of heat load and liquid filling ratio. The results indicate that, as heat load increases, the intermittent nucleation boiling, fully developed nucleation boiling and film boiling sequentially takes place at the evaporator section of the confined flat two-phase thermosiphon. Correspondingly, the transient temperature evolution shows the periodic large-amplitude fluctuation, the random medium-amplitude fluctuation, and gentle small-amplitude fluctuation. The fully developed boiling is desirable for the heat transfer enhancement on the evaporator surface due to smaller temperature fluctuation and lower thermal resistance. The moderate liquid filling ratio ensures the desired interaction between the evaporator and condenser, resulting in superior thermal performance. To optimize the thermal performance, the optimal liquid filling ratio is about 40%.

Gravity scaling law of heat transfer in nucleate pool boiling

Boiling phenomenon can realize high performance heat exchange due to latent heat transportation accompanying the liquid-vapor phase change, resulting in its wide applications for high heat flux transfer both in normal gravity on the ground and in space microgravity environment. It is well known that gravity strongly affects boiling phenomenon by creating forces in the systems that drive motions, shape boundaries, and compress fluids. The study of the influence of gravity on boiling heat transfer is of great significance for relevant space applications. Our current knowledge on boiling phenomenon, however, is often composed of a great amount of empirical correlations and semi-mechanistic models for engineering applications, which are mainly depended upon empirical data obtained from elaborately designed experiments on the ground. Although many empirical correlations and semi-mechanistic models include gravity as a parameter, they usually fail when extended beyond the empirical range they were based on. Microgravity experiment offers a unique opportunity to study the complex interactions in boiling phenomenon without gravity, and thus to reveal the mechanism of gravity, or gravity scaling law, underlying boiling phenomenon by comparing microgravity experimental data with those obtained in normal gravity. Review of the up-to-data progress on gravity scaling law of nucleate pool boiling heat transfer is made in the present paper, especially focusing on the RKM (Raj-Kim-McQuillen) gravity scaling law which presents a unified framework for scaling behaviours of nucleate pool boiling heat transfer relevant to gravity based on the quasi-steady state, ground-based short-term reduced gravity experimental results acquired during transition periods of approximately 3–5 s when the acceleration varied continuously from hypergravity to low- g , or vice versa , in parabolic flights. The gravity scaling parameter for heat flux was updated based on high quality microgravity data aboard ISS (the International Space Station), and its robustness in predicting low gravity heat transfer is further demonstrated by predicting many of the trends in the pool boiling literature that cannot be explained by any single model. There are, however, some deficiencies and/or unsolved problems in the RKM gravity scaling law. The definition of the dimensionless temperature involves the temperatures of the boiling incipiency and of the CHF (critical heat flux). It was implicitly assumed that these two characteristic temperatures are constants in different gravity conditions. These assumptions, however, have no theoretical or empirical basis in fact. Moreover, recent numerical studies utilizing the lattice Boltzmann method, as well as some experimental evidences, showed that the temperature of CHF increases with the gravity level. Furthermore, the hypotheses on the asymptotic behaviours near the boiling incipiency and the critical heat flux confused the meaning of different gravity scaling parameters, which are defined clearly in the present paper. An important reason for the deficiencies of the RKM gravity scaling law is the lack of empirical data, especially experimental results in long-term, steady state pool boiling in different levels of reduced gravity. Thus, a variable gravity pool boiling experiment project utilizing CSS (the Chinese Manned Space Station) under construction, as well as systematic numerical simulations, are suggested in order to promote the research in this field.

Effect of instantaneous change of surface temperature and density on an unsteady liquid–vapour front in a porous medium

This article presents a comprehensive analysis of time dependent condensation model embedded in a porous medium with variations in liquid–vapour densities. Both similarity and asymptotic solutions for the unsteady liquid–vapour phase change front are obtained with the manifestation of various pertinent parameters. The obtained results are compared which congregate well as depicted clearly in graphs. Results indicate that with different diffusivity and contrast ratios, the similarity front parameter is found to be gradually declining with variation in a density ratio. We have shown for the condensation process, the ratio of sensible to latent heat is independent of time and is equal to the half of the Stefan number of the liquid phase.

Fractal description of fouling deposits in boiling heat transfer modelling

A novel methodology is developed for predicting the thermal impact of fouling in Steam Generators (SG). The originality of this methodology is to resort to fractal and statistical theories to depict the porous structure of the deposits. The proposed Statistical Fractal methodology (SF) accounts for the heat transfer driven by the liquid-vapor phase change inside the deposits. It simulates the complex intricate networks of sinuous open pores of different scales, with liquid inflows (capillaries) and vapor outflows (steam-chimneys). The multi-layered representation of fouling deposits allows to mimic aging mechanisms such as densification which occur during SG operation.The SF predictions are consistent with experimental data. The deposit thickness and the profile of porosity are found to be the most influential fouling properties on the heat exchange. The methodology is capable to simulate the experimentally observed heat transfer enhancement for thin and porous deposit as well as the heat exchange decline for thick and dense deposit.

A continuum framework for modeling liquid-vapor interfaces out of local thermal equilibrium

Continuum numerical methods modeling liquid-vapor phase change processes typically assume local thermal equilibrium at the liquid-vapor interface – continuity of temperature at phase interfaces, and a relation between interfacial saturation pressure and temperature based on phase co-existence curves. Several standard macroscopic problems have been solved accurately by adhering to this assumption. However, in micro-scale and certain non-standard macro-scale applications, significant jumps in temperature are observed at liquid-vapor interfaces during phase change, and vapor pressure can be far from equilibrium values. In this paper, we present a locally discontinuous arbitrary Lagrangian Eulerian finite element formulation with temperature discontinuities at interfaces. A penalty-based approach is used to weakly enforce the temperature jump condition. Furthermore, we use kinetic-theory-based Schrage relationship to evaluate the rate of phase change. We apply our methodology to solve the problem of flowing vapor in a planar heat pipe. The flow rates predicted by our continuum simulations are in excellent agreement with recently published molecular dynamics simulation results of the same problem. Interestingly, accounting for temperature discontinuities leads to only a slight improvement in the prediction of mass fluxes as compared to the case when temperature continuity is assumed. This is in contrast to the large improvement in the prediction of temperature profiles and is a consequence of a weak dependence of the evaporation/condensation rates on the vapor temperature.

Experimental investigation of evaporation from asymmetric microdroplets confined on heated micropillar structures

Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously–fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross sections. The evaporation experiments are performed at substrate temperatures ranging from 60 °C to 98 °C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 °C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 °C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems.

Membrane desalination performance governed by molecular reflection at the liquid-vapor interface

Membrane-based separation processes that utilize liquid-vapor phase changes, such as membrane distillation (MD) and osmotic distillation (OD), form the basis of emerging desalination technologies. Although thin, hydrophobic membranes are required for optimal separation performance, current modelling approaches often neglect the transport resistances associated with phase-changes at liquid-vapor interfaces. We develop a new theoretical framework to analyze water transport across porous hydrophobic membranes by accounting for the transport resistances associated with both diffusion through the membrane and phase-changes at the liquid-vapor interfaces. Applying the developed framework to MD and OD reveals that for thick membranes (≫1μm) water flux is governed by a combination of molecular and Knudsen diffusion, showing an inverse proportionality to membrane thickness. However, the resistances associated with phase-changes, which arise from molecular reflection at liquid-vapor interfaces, become transport-limiting factors as membrane thickness decreases (<∼1μm). Our analysis identifies an optimal membrane thickness of between 1μm and 10μm to maximize the water flux in MD, while the water flux in OD is shown to increase monotonically as the membrane thickness decreases. The framework presented provides key design principles for membranes that utilize vapor transport for desalination, water treatment, and chemical concentration.