Vapor Phase Change Research Articles

Biomimetic bilayer phase change hydrogels with enhanced water adsorption and desorption for evaporative cooling

Hygroscopic hydrogels, reliable carriers for water-based evaporative materials, undergo shrinkage after evaporative cooling, and the consequent reduction in hygroscopic sites can significantly impair their recyclability in cooling period. To realize cyclic evaporative cooling for rapid regeneration and evaporation, bilayer phase change hydrogel (BPH) concept combining an upper layer of hygroscopic polymer film (CaCl2@ carboxymethyl cellulose/konjac glucomannan films, CaCl2@CMC/KGM, MAF) with a lower layer of hygroscopic hydrogel (CaCl2@poly(vinyl alcohol)/sodium tetraborate decahydrate hydrogel, CaCl2@PVA/Borax, PCH), is presented in this paper. The high liquid–vapor phase change enthalpy of BPH (1274 J/g) enables it to continuously and rapidly remove heat. Benefiting from the MAF hygroscopic porous skeleton, the saturated BPH exhibited a faster water evaporation rate (2.15 kg m−2 h−1) and comparable water vapor transmission rate (1.61 kg m−2 h−1) at 50 °C. After being heated at 50 °C for 1 h, BPH requires 9h to regain its initial mass at 25 °C, 90 % RH. Fifteen moisture adsorption–desorption cycles demonstrate the excellent cyclic regeneration capability of BPH. COMSOL Multiphysics simulation results indicate that evaporative cooling in BPH provides excellent heat dissipation (reduced by about 27 °C). BPH reduces the peak temperatures of a mobile phone by 7.8 °C while charging and 6.5 °C while gaming. This design will contribute to the future cooling of heat-related devices and global sustainability.

Molecular kinetic modelling of non-equilibrium evaporative flows

Recent years have seen the emergence of new technologies that exploit nanoscale evaporation, ranging from nanoporous membranes for distillation to evaporative cooling in electronics. Despite the increasing depth of fundamental knowledge, there is still a lack of simulation tools capable of capturing the underlying non-equilibrium liquid–vapour phase changes that are critical to these and other such technologies. This work presents a molecular kinetic theory model capable of describing the entire flow field, i.e. the liquid and vapour phases and their interface, while striking a balance between accuracy and computational efficiency. In particular, unlike previous kinetic models based on the isothermal assumption, the proposed model can capture the temperature variations that occur during the evaporation process, yet does not require the computational resources of more complicated mean-field kinetic approaches. We assess the present kinetic model in three test cases: liquid–vapour equilibrium, evaporation into near-vacuum condition, and evaporation into vapour. The results agree well with benchmark solutions, while reducing the simulation time by almost two orders of magnitude on average in the cases studied. The results therefore suggest that this work is a stepping stone towards the development of an accurate and efficient computational approach to optimising the next generation of nanotechnologies based on nanoscale evaporation.

A Molecular Dynamics Perspective on the Impacts of Random Rough Surface, Film Thickness, and Substrate Temperature on the Adsorbed Film’s Liquid–Vapor Phase Transition Regime

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.

Microscopic insights on liquid–vapor phase change phenomena of R1336mzz(Z) nanofilm through molecular dynamics simulations

The ever-growing advances in electronics pose a critical cooling challenge. Two-phase heat transfer technology, with effective heat removal and no extra energy consumption, is emerging as a potential solution for sustainable thermal management. However, questions regarding phase change mechanisms are seemingly not addressed yet due to the limitation in time and space scales. In this work, a series of molecular dynamics simulations are performed where the R1336mzz(Z) liquid film rests over a smooth copper substrate. The molecular system starts with a nanofilm thickness of 4–10 nm and then is subjected to non-equilibrium heating (350, 400, 450, and 500 K) to initiate liquid-to-vapor phase change. Depending on the impacts of film thickness and wall temperature, four distinctive phase change scenarios are identified, i.e., diffusive evaporation, pseudo-diffusive evaporation, explosive boiling, and stable explosive boiling. The characteristics of these phase change modes are explicated from the perspectives of molecular motion, vaporization rate, temperature evolution, heat flux, and interfacial resistance. The phase change mode has a strong dependence on two aspects: improving heat transfer to breakup potential barrier and slowing energy dissipation to support formation of bubble embryos. The insight may deliver better understandings of phase change mechanism and applications of liquid refrigerants.

Turbulent flow boiling simulation based on pseudopotential lattice boltzmann method with developed wall boundary treatments and unit conversion

The lattice Boltzmann method (LBM) is widely studied for complex flow simulations, including liquid–vapor phase change phenomena. Although previous studies extended the LBM simulation capability from single-phase to two-phase boiling simulations, most of them are restricted to no-flow (pool boiling) or low-flow-rate (laminar flow) boiling conditions owing to the stability problem in collision at low viscosity, representing high Reynolds numbers. However, a high Reynolds number flow is essential for investigating the effects of turbulence on bubble dynamics. Recently, the central moment-based collision method is used to enable high Reynolds numbers for bulk flow; however, extending the method for multiphase-flow simulation with high Reynolds numbers is limited owing to the boundary treatment near the wall. In this study, we augment the previous boundary treatment in LBM simulation with a forcing term, which increases the stability of the simulation even under two-phase conditions. With the improved boundary treatment, the flow boiling phenomena at high Reynolds numbers (up to Re = 107,000) are successfully reproduced. Moreover, in this study, unit conversion is reconsidered with the relationship between conversion factors because the poor outcomes of bubble dynamics are attributed to misinterpreted unit conversion. The new unit conversion method satisfies the necessary conditions for mapping the physical bubble behavior to lattice frame, reproducing bubble dynamics effectively following the Reynolds number, with reasonable spatial and temporal resolution. Consequently, the flow boiling simulation can accurately predict the bubble departure diameter from the real experimental results and the heat transfer trend as a function of the Reynolds number.

Data-driven diagnostics of boiling heat transfer on flat heaters from non-intrusive visualization

Rankine-based power generation cycles require the use of boiling heat exchangers for liquid–vapor phase change. However, operating boilers at too high heat fluxes can lead to heater meltdown due to onset of the boiling crisis. Despite recent advances in increasing this critical heat flux, the lack of fast and accurate diagnostics hinders boiling heat transfer safety at high heat fluxes. Here we show that neural networks fed on direct observation of bubble dynamics can be used to measure heat flux during nucleate boiling. The boiling heat transfer data was obtained with a horizontally oriented, circular flat heater with saturated water as the heat transfer fluid. A deep convolutional neural network (CNN) was trained using frames from videos acquired from 15 different heat dissipation values on the boiling curve to learn how these images correlate to the underlying heat flux. The CNN predicted heat fluxes with a mean square error of 14.6 W/cm2. Furthermore, the CNN presented high sensitivity to pixel intensity in image regions relevant to boiling heat transfer physics (e.g. bubble boundaries, contextual clues about bubble image distortion). Understanding this approach may allow future studies in unstable and critical boiling regimes that require fast transient heat flux diagnostics and cannot rely on slower thermohydraulic measurements.

Thermopneumatic Liquid Metal Radiofrequency Switch Actuated by Low‐Boiling‐Point Fluid

Existing radiofrequency (RF) switches used in reconfigurable antennas often encounter performance issues such as high insertion loss, low isolation, and nonlinear effects of the switch. To address these issues, this article presents a liquid metal RF switch that utilizes a heated low‐boiling‐point fluid (LBPF) to drive the movement of a eutectic gallium‐indium alloy (EGaIn) plunger to turn the switch on/off. The operating principle of the switch is to use the pressure difference generated by the liquid–vapor phase change of the LBPF to make the EGaIn plunger to contact with, or disconnect from the copper electrodes of the switch. The performances of the RF switches actuated by different LBPFs are respectively investigated, and the results show that all RF switches have millisecond response times, and the contact resistances are as low as 0.14 mΩ. In addition, good RF performances are achieved, with an insertion loss of less than 3 dB and isolation of more than 17 dB in the frequency band from 0 to 2 GHz, and no nonlinear effects. This study also validates the performance of the proposed switch for frequency reconfiguration of a folded monopole antenna.

Study on condensation of oily particles under the influence of water vapor phase transition

To address the problem that oily fine particles are too small to be easily captured by traditional dust removal equipment, which hinders improving the purification efficiency, this paper proposes using the technical principle of water vapor phase transition to conduct experiments and simulations. A set of water vapor experimental devices for fine particles of oil mist is designed, and the data are analyzed by a Grimm spectrometer. The experimental results show the phase change of water vapor has a remarkable effect on the agglomeration of oily fine particles. Phase change condensation makes the particle size of fine particles increase by 5–10 times in a truly brief time, and the polymerization and removal efficiency are greatly improved. To analyze the changes of the coalescence characteristics in a larger range of parameter changes, the Euler multiphase flow model is adopted, the particle condensation growth rate function is introduced by a user-defined function to establish a coalescence model and the heterogeneous condensation growth characteristics of oily particles are analyzed in a supersaturated water vapor environment. The simulation results show that the condensing chamber temperature of the condensing chamber is 283 K, and the lower the air flow rate of the condensing chamber, the more favorable the heterogeneous condensation growth of oil particles. The change rule of agglomeration characteristics of oily particles obtained by experiment and simulation proves the feasibility of improving the removal efficiency of oily fine particles.

A thermal lattice Boltzmann model for evaporating multiphase flows

Modeling thermal multiphase flows has become a widely sought methodology due to its scientific relevance and broad industrial applications. Much progress has been achieved using different approaches, and the lattice Boltzmann method is one of the most popular methods for modeling liquid–vapor phase change. In this paper, we present a novel thermal lattice Boltzmann model for accurately simulating liquid–vapor phase change. The proposed model is built based on the equivalent variant of the temperature governing equation derived from the entropy balance law, in which the heat capacitance is absorbed into transient and convective terms. Then a modified equilibrium distribution function and a proper source term are elaborately designed in order to recover the targeting equation in the incompressible limit. The most striking feature of the present model is that the calculations of the Laplacian term of temperature, the gradient term of temperature, and the gradient term of density can be simultaneously avoided, which makes the formulation of the present model is more concise in contrast to all existing lattice Boltzmann models. Several benchmark examples, including droplet evaporation in open space, droplet evaporation on a heated wall, and nucleate boiling phenomenon, are carried out to assess numerical performance of the present model. It is found that the present model effectively improves the numerical accuracy in solving the interfacial behavior of liquid–vapor phase change within the lattice Boltzmann method framework.

An immersion flow boiling heat dissipation strategy for efficient battery thermal management in non-steady conditions

Aiming at the heat dissipation requirements of future high-power batteries under complex operating conditions, this paper adopts immersion flow boiling as the strategy to investigate battery thermal management performance under Nürburgring circuit condition of Porsche Taycan. A multi-filed coupled model is proposed to investigate its flow and heat transfer performance. The results show that batteries have a higher heat generation in non-steady conditions, which means that it is more crucial to investigate the battery thermal management performance under non-steady state. Immersion flow boiling can meet the thermal management requirements of batteries with high heat generation rate, which has significant advantages over single-phase liquid cooling, and the temperature uniformity can be further improved by means of roughing surface. Besides, flow velocity will completely change the heat transfer and flow mechanism of immersion flow boiling. The optimum flow regime can both exchange heat by liquid–vapor phase change and replenish the vaporized working medium by convection, achieving the lower temperature rise and better temperature uniformity in battery. Moreover, the boiling heat transfer in battery thermal management is in the partial nuclear boiling state, which is much lower than the critical heat flux, and thus can effectively inhibit high heat flux conditions such as thermal runaway.

An efficient thermal lattice Boltzmann method for simulating three-dimensional liquid–vapor phase change

In this paper, a multiple-relaxation-time lattice Boltzmann (LB) approach is developed for the simulation of three-dimensional (3D) liquid–vapor phase change based on the pseudopotential model. In contrast to some existing 3D thermal LB models for liquid–vapor phase change, the present approach has two advantages: for one thing, some gradient terms, such as the gradient of volumetric heat capacity and the Laplacian term of temperature, that must be computed with finite-difference scheme in previous works is not needed for the present thermal model. For another, the current model is constructed based on the seven discrete velocities in three dimensions (D3Q7), making it more efficient and much easier to be implemented. Also, based on the scheme proposed by Zhou and He (1997), a pressure boundary condition for the D3Q19 lattice is proposed to model the multiphase flow in open systems. This model is then validated by considering the temperature distribution in a 3D saturated liquid–vapor system, the d2 law, the droplet evaporation on a heated surface and the nucleate boiling. It is observed that the numerical results fit well with the analytical solutions, and the results of the finite difference method or the experimental data. Our numerical results indicate that the present approach is reliable and efficient in dealing with the 3D liquid–vapor phase change.

Comprehensive analysis of shutdown purge influencing factors of proton exchange membrane fuel cell based on water heat transfer and water vapor phase change mechanism

A two-dimensional multiphase proton exchange membrane fuel cell model is established to study the effect mechanism of temperature, pressure, humidity, and flow rate on the purge rate and ionomer dissolved water content at the end of purging on the basis of water heat transfer and water vapor phase change mechanism. The numerical model is analyzed by COMSOL Multiphysics 6.0 software, and the polarization curve and high-frequency impedance curve obtained under the same operating conditions are used to verify the validity of the model. Results show that ionomer dissolved water content increases and then decreases slightly due to the liquid water dissolved phase transfer in the slow-change step of the purge. Temperature, pressure, and humidity can effectively improve the purge rate and reduce the ionomer dissolved water content of the purge end. However, flow rate has little impact on both, but it can shorten the duration of the first stage and improve the effectiveness of the purge by effectively removing liquid water. This study provides a basis for revealing the influence of shutdown purge and formulating a reasonable shutdown purge strategy.

Ultrathin liquid film phase change heat transfer on fractal wettability surfaces

Surface wettability is significant for the liquid–vapor phase change heat transfer. In this paper, patterned wettability surfaces are constructed from the perspective of fractal theory to comprehensively investigate the microscopic mechanism of surface wettability patterns affecting evaporation and bubble nucleation. The evaporation heat transfer rates of the hydrophilic fractal wettability surfaces are close to that of the pure hydrophilic surface. The solid–liquid interfacial heat transfer dominates the evaporation heat transfer. In contrast, the liquid–vapor interfacial heat transfer’s effect is insignificant. The hydrophobic points of the hydrophilic fractal wettability surfaces provide stable and abundant nucleation sites for the bubble nucleation process. The nucleation times of the hydrophilic fractal wettability surfaces are earlier than that of the pure hydrophilic surface. Hydrophobic fractal wettability surfaces can improve bubble nucleation. With increased hydrophilic points, nucleation sites are mainly concentrated in hydrophobic regions near the hydrophilic/hydrophobic contact boundary. The expanding low potential energy region near the contact boundary is the main reason for improving bubble nucleation. Critical heat flux (CHF) increases as the area proportion of the hydrophilic part increases, and surface superheats decrease when CHF occurs. The fractal wettability surfaces improve CHF, and the CHF of the hydrophilic fractal wettability surface is close to that of the pure hydrophilic surface.

Modeling liquid–vapor phase change experiments: Cryogenic hydrogen and methane

Mass accommodation coefficients are essential inputs to kinetic models of liquid–vapor phase change, yet after nearly 100 years there remains significant discrepancy in reported values. These discrepancies have been attributed to a wall material or geometric dependency resulting in the need for an empirical correction factors. The lack of experimental results for cryogenic fluids poses a serious impediment to modeling/predicting propellant behavior for long term space missions. Using a combination of neutron imaging experiments and multi-scale modeling, mass accommodation coefficients for liquid hydrogen and methane are determined. When the local variation in thermophysical properties are accounted for, the experimentally derived accommodation coefficients for hydrogen are invariant to container size, material and evaporation rate. The discrepancy in prior measurements of the accommodation coefficient for other fluids can be alleviated by a multi-scale analysis that incorporates local variation in thermophysical properties. The values of accommodation coefficients for hydrogen and methane are consistent with generalized transition state theory. This suggests that a mass accommodation coefficient is a solely a function of the liquid–vapor density ratio, making it a fluid-independent property easily determined without the need for empirical correction factors as reported in previous investigations.

High performance vapor chamber enabled by leaf-vein-inspired wick structure for high-power electronics cooling

The boom of the advanced technologies, such as microprocessors, 5G, and big data, has led the thermal management to be a crucial factor in determining the operating speed, reliability, and efficiency as well as lifetime of electronic devices. As passive heat transfer devices utilizing the liquid–vapor phase change, vapor chambers are greatly attractive due to the high thermal conductivity and passive operation. Here, we present a novel vapor chamber with leaf-vein-inspired wick structure, which could effectively facilitate the condensate to flow back to the evaporation region. Besides, the capillary wick functioned as the supporting structure instead of traditional solid supporting columns to withdraw the deformation of the vapor chamber. The heat transfer performance of the proposed vapor chamber was experimentally investigated under forced water cooling condition, and the effects of the cooling water temperature and mass flow rate were analyzed systematically. The results showed that the vapor chamber could effectively tolerate a wide heat load range from 20 W to 500 W with no noticeable performance degradation, and a minimum thermal resistance of 0.029 °C/W was attained at 200 W. Additionally, increasing the cooling water temperature and decreasing the cooling water mass flow rate could lead to the increase of the total thermal resistance at all heat loads. Finally, compared to other reported works, the vapor chamber featured a lowest thickness and vapor chamber thermal resistance, demonstrating a promising solution for cooling high-power and miniaturized electronics.

Nanoscale liquid-vapor phase change characteristics of binary mixtures from molecular dynamics perspective

Nanoscale thin liquid film boiling of a binary liquid mixture subjected to extremely rapid heating within molecular dynamics framework is the focus of our study. A three-phase MD simulation domain is considered where liquid argon-krypton (Ar-Kr) binary mixture rests over a solid Platinum (Pt) like substrate. The molecular system starting from freezing state is subjected to non-equilibrium boundary heating following a sufficient equilibration period to induce liquid–vapor phase change process. Depending on the mixture composition and boundary heating rate, four distinct phase change scenarios have been observed. Characteristics of different phase change scenarios have been studied in terms of important system parameters such as transient atomistic distribution, temperature, pressure, evaporation history, onset of liquid cluster splashing from the wall along with atomic kinetics parameters like COM (Center of Mass), and MSD (Mean Square Displacement). Addition of Kr with Ar that is less volatile compared to Ar, has been found to result in a delay in the evaporation onset and decrease in interaction energy. As a consequence, with the increase of Kr fraction in the binary mixture, the mobility of the evaporated atoms is hindered and the onset temperature of liquid splashing from the wall that resembles generation of “Leidenfrost Film” changes. Also, the near wall atomistic energetics is significantly influenced by mixture composition. Finally, a contour of various phase change modes for Ar-Kr binary mixtures has been presented as a function of mixture composition and boundary heating rate. Present work has been found to be aligned with contemporary works in the context of the derived heat transfer properties.

Evaluation of several liquid–vapor phase change models for numerical simulation of subcooled flow boiling

The phase change model has recently attracted attention for use in flow-boiling numerical simulation research. Comparison and evaluation of the calculation accuracies and resource consumptions of phase change models are essential for developing an efficient and high-precision phase change model. In this study, the Dong and Chen models, were added to the FLUENT software as two new models by introducing a unique user-defined function (UDF). The two unique models and the Lee model served as the foundation for numerical research, and the values of dependabilities and properties for flow and heat transmission predicted by these models were compared. Visual studies and flow-boiling heat transfer correlations were used to validate the results. The results reveal that the Chen model, which considers the active nucleation density site of bubbles, has a significant advantage over the other two models in terms of predicting flow characteristics. The Lee and Dong models are better suited for forecasting the changing tendency of the heat transfer coefficient, whereas the values obtained with the Chen model are closer to the reference values for the Liu–Winterton correlation. Besides, the two novel models also have higher computational costs.

Drawbacks of phase change models in simulating flashing of steam with a subsaturation downstream boundary condition

AbstractThe development of phase change models applicable to a wide range of temperatures, pressures, and mass flow rates is primarily limited by the metastable or partially stable behaviour of the fluid. Due to this, the fluid does not change phase even after crossing saturation conditions. Most liquid–vapour phase change models have been developed primarily for the cavitation process where the phase change is not sustained, occurs in a very narrow region of space, or occurs under equilibrium conditions. In this paper, the mechanism of phase change is discussed along with the review of three different computational fluid dynamics (CFD)‐based phase change methods available in OpenFOAM, which are used to simulate flashing of steam, in applications related to steam assisted gravity drainage (SAGD) systems. The first method is based on barotropic compressibility (BC) used with the realizable turbulence model, the second combination is of mass transfer model (MTM) with the realizable turbulence model, and the third one is based on two fluid (TF) method along with a family of turbulence models. These methods are tested on a converging–diverging nozzle with pressure driven phase change. It is demonstrated that these methods are not able to adjust their physics to different depressurization rates, do not account for liquid to be in superheated conditions, and have significant discrepancies with experimental results. In the end, better approaches to model this category of phase change are discussed.

Spatiotemporal dynamics of water film confinement during spreading and evaporation in highly hierarchical wicking nano/microstructure on Si surface at 120 °C

Enhancing the wicking/evaporative functionality of materials by surface nano/microstructuring is a key approach in creating advanced technologies based on the liquid–vapor phase change, particularly in the field of power generation for substantial fuel savings and reducing global greenhouse gas pollution. Despite the technological importance, the capillary flow of a liquid undergoing intensive evaporation on a hot nano/microstructured surface is not well understood. During the capillary flow on a nano/microstructured surface, water confinement undergoes a dramatical spatiotemporal change. The evaporation mechanisms of water confined in capillary nano/microstructures fundamentally depend on the scale of liquid confinement, making the dynamics of water confinement one of the basic characteristics in spreading/evaporation behavior of water on a hot capillary surface. Here, we develop an experimental technique for studying the water film confinement dynamics based on different optical footprints of nanoscale and microscale water confinements found in our work. We study both water film confinement dynamics and traditional capillary flow/receding dynamics of a water drop in a highly hierarchical capillary surface nano/microstructure created in our work using femtosecond laser processing. For the first time, we obtain the spatiotemporal map of water nano/microstructural confinements that provides basic data for the identification of evaporation mechanisms. The obtained results give important guidelines for engineering advanced materials with an efficient wicking/evaporative functionality.

Modeling of Phase Change in Nanoconfinement Using Moment Methods

Abstract Accurate prediction of liquid–vapor phase change phenomena is critical in the design of thin vapor chambers and microheat pipes for the thermal management of miniaturized electronic systems. In view of this, we have considered the heat and mass transfer between two-liquid meniscuses separated by a thin gap of its own vapor. Assuming the heat and mass flow are to be steady and one-dimensional, analytic solutions are obtained to the linearized equations from the regularized 26-moment framework. Our analytic solutions provide excellent predictions for the effective heat conductivity of a dilute gas with those from the molecular dynamics (MD) and Boltzmann equation where Fourier's law fails. We also verified that the predicted heat and mass flow rates over the whole range of the Knudsen number are consistent with the kinetic theory of gases. Further, the model has been used to predict the effect of evaporation and accommodation coefficients on the heat and mass transfer between the liquid layers.