Vapor Phase Change Research Articles

Reverse capillary flow of condensed water through aligned multiwalled carbon nanotubes

Molecular transport through nanopores has recently received considerable attention as a result of advances in nanofabrication and nanomaterial synthesis technologies. Surprisingly, water transport investigations through carbon nanochannels resulted in two contradicting observations: extremely fast transport or rejection of water molecules. In this paper, we elucidate the mechanism of impeded water vapor transport through the interstitial space of aligned multiwalled carbon nanotubes (aligned-MWCNTs)—capillary condensation, agglomeration, reverse capillary flow, and removal by superhydrophobicity at the tip of the nanotubes. The origin of separation comes from the water’s phase change from gas to liquid, followed by reverse capillary flow. First, the saturation water vapor pressure is decreased in a confined space, which is favorable for the phase change of incoming water vapor into liquid drops. Once continuous water meniscus is formed between the nanotubes by the adsoprtion and agglomeration of water molecules, a high reverse Laplace pressure is induced in the mushroom-shaped liquid meniscus at the entry region of the aligned-MWCNTs. The reverse Laplace pressure can be significantly enhanced by decreasing the pore size. Finally, the droplets pushed backward by the reverse Laplace pressure can be removed by superhydrophobicity at the tip of the aligned-MWCNTs. The analytical analysis was also supported by experiments carried out using 4 mm-long aligned-MWCNTs with different intertube distances. The water rejection rate and the separation factor increased as the intertube distance decreased, resulting in 90% and 10, respectively, at an intertube distance of 4 nm. This mechanism and nanotube membrane may be useful for energy-efficient water vapor separation and dehumidification.

Performance review of a novel combined thermoelectric power generation and water desalination system

A novel combined thermoelectric power generation and water desalination system is described with a system schematic. The proposed system utilises low grade thermal energy to heat thermoelectric generators for power generation and water desalination. A theoretical analysis presents the governing equations to estimate the systems performance characteristics combined with experimental validation. Experimental set-up consists of an electric heat source, thermoelectric modules, heat pipes, a heat sink and an evaporator vessel. Four heat pipes are embedded in a heat spreader block to passively cool the bottom side of the thermoelectric cells. The condenser of these four heat pipes is immersed in a pool of saline water stored in an evaporation vessel which is maintained at sub-atmospheric pressure. The liquid to vapour phase change cooling method achieve low saturation temperature and offers a high heat transfer coefficient for the cooling of the thermoelectric generators. At the same time this method utilises the low temperature heat extracted from the cold side of the thermoelectric generator for water desalination. It was observed that at low saturation temperatures greater heat flux could be supplied to the thermoelectric generators with less heat losses to the atmosphere.

Water transport in gas diffusion layer of a polymer electrolyte fuel cell in the presence of a temperature gradient. Phase change effect

The gas diffusion layer (GDL) is a crucial component as regards the water management in proton exchange membrane fuel cells. The present work aims at discussing the mechanisms of water transport in GDL on the cathode side using pore network simulations. Various transport scenarios are considered from pure diffusive transport in gaseous phase to transport in liquid phase with or without liquid–vapor phase change. A somewhat novel aspect lies in the consideration of condensation and evaporation processes in the presence of a temperature gradient across the GDL. The effect of thermal gradient was overlooked in previous works based on pore network simulations. The temperature gradient notably leads to the possibility of condensation because of the existence of colder zones within the GDL. An algorithm is described to simulate the condensation process on a pore network.

Simulation of complete liquid–vapour phase change process inside porous evaporator using local thermal non-equilibrium model

The present article demonstrates the necessity and the usefulness of a newly proposed smoothing algorithm for the effective diffusion coefficient in order to avoid non-physical “jump” in the predicted properties during numerical simulations of the complete liquid–vapour phase change process within porous media. The formulation is based on the two-phase mixture model (TPMM) along with the assumption of local thermal non-equilibrium (LTNE) condition where the governing equations have been solved using the finite volume method. For the purpose of demonstration, one-dimensional phase change problem of water has been considered. Comparison between results obtained with the local thermal equilibrium (LTE) and LTNE models indicates that wherever applicable, particularly for lower inlet Reynolds number and higher heat input, the latter should be used. The effect of employing different models for the partitioning of wall heat flux using LTNE model has been found negligible owing to the very high convective heat transfer coefficient between the solid and the fluid phases. A representative parametric study suggests that the diffusive energy transport process in the upstream direction through the solid and the fluid phases has a strong influence on the initiation and the termination of the phase change process.

Lattice Boltzmann modeling of boiling heat transfer: The boiling curve and the effects of wettability

A hybrid thermal lattice Boltzmann (LB) model is presented to simulate thermal multiphase flows with phase change based on an improved pseudopotential LB approach (Li et al., 2013). The present model does not suffer from the spurious term caused by the forcing-term effect, which was encountered in some previous thermal LB models for liquid–vapor phase change. Using the model, the liquid–vapor boiling process is simulated. The boiling curve together with the three boiling stages (nucleate boiling, transition boiling, and film boiling) is numerically reproduced in the LB community for the first time. The numerical results show that the basic features and the fundamental characteristics of boiling heat transfer are well captured, such as the severe fluctuation of transient heat flux in the transition boiling and the feature that the maximum heat transfer coefficient lies at a lower wall superheat than that of the maximum heat flux. Furthermore, the effects of the heating surface wettability on boiling heat transfer are investigated. It is found that an increase in contact angle promotes the onset of boiling but reduces the critical heat flux, and makes the boiling process enter into the film boiling regime at a lower wall superheat, which is consistent with the findings from experimental studies.

Three dimensional liquid and vapour distribution in the wick of capillary evaporators

Heat and mass transfer with liquid–vapour phase change in a representative unit cell of a capillary evaporator is studied using a mixed pore network model. The model combines the computation of temperature and pressure fields in vapour and liquid pores according to mean field approaches with pore scale invasion rules depending on the capillary pressure thresholds associated with each local constriction between two pores. The metallic body through which heat is transferred to the porous wick is also taken into account in the simulations. After comparisons with a visualisation experiment, numerical simulations performed in three dimensional pore networks lead to the identification of three main regimes depending on the applied heat load. Compared with previous works using the so-called vapour pocket assumption, the 3D simulations reveal a regime where the phase distribution within the wick is fundamentally different. This regime is characterised by the coexistence of both the liquid and vapour phases underneath the casing within the wick. This regime is shown to correspond to the best evaporator performance.

Channeled coolant plate: A new method to enhance freshwater production from an air gap membrane distillation (AGMD) desalination unit

Air gap membrane distillation (AGMD) desalination process incorporates phase change and transport of vapor from saline water through a hydrophobic membrane and an air gap due to difference in vapor pressure across the membrane and a coolant plate. In this study, an attempt was made to observe the effect of a channeled coolant plate instead of a flat plate for distillation of saline water. Since the AGMD process has limited distillate production due to the resistance offered by the air gap, a specially designed coolant plate was used to manipulate the mass transfer by enhancing the heat transfer during condensation. For the same equivalent air gap, the freshwater production enhanced maximum up to 50% compared to a flat coolant plate for a coolant temperature of 25°C. A one dimensional heat transfer model is developed and validated using the experimental data. The results obtained from the model matched well with experiments and it was used to predict freshwater production from various designs of finned coolant plates.

Numerical modeling of annular flow with phase change in a microchannel

Phase change phenomena in microchannels have been projected as an effective option for thermal management of various microscale systems. However, for the design of heat sinks which utilize these phenomena, a clear understanding of the physical mechanisms involved, in the microscale domain, is required. In the present study, a numerical simulation is carried out to predict the characteristics of two phase flow with liquid to vapor phase change, in rectangular microchannels for a range of hydraulic diameters. The annular flow pattern, reported to be the most common in microchannel, has been investigated, with water as the flowing medium. The analysis considers, among other parameters, the effect of surface tension on the flow dynamics, which could be significant in microchannels. The mathematical model is numerically solved using a quasi-three dimensional approach, incorporating the variations in the cross-sectional areas of both the phases along the flow direction. Since it is expected that the continuum approximation may be insufficient in the vapor domain in channels of very small dimensions, an analysis incorporating slip flow at the liquid vapor interface analysis has been performed, where the channel dimensions produce Knudsen number values in the range 0.001–0.1, as suggested in the literature. The liquid film thickness along the flow direction is determined utilizing mass conservation, and the velocity distributions at different locations along the flow direction are obtained by solving the governing differential equations numerically. The two phase pressure drop is calculated for cases demanding slip and no slip conditions. A comparison has been made of the fluid flow characteristics, between the slip and no slip cases for the cases where slip flow is expected. The effects of the imposed heat flux, volume flow rate and channel geometry on the velocity distribution and pressure drop are examined. The predicted pressure drop values are found to increase with increasing heat flux, and decrease with an increase in the flow rate and the channel height. The two phase heat transfer coefficient and the wall temperature of the channel are also determined. The two phase heat transfer coefficient is found to increase with an increase in the vapor quality. It is also noticed that the wall temperature decreases along the length of the channel and the wall temperature at the exit of the channel decreases with decrease in volume flow rate, within the range of parameters used, which is in accordance with observations reported in the literature. The results are found to be in good agreement with the experimental and theoretical results available in the literature.

Effect of humidified water vapor on heat balance management in a proton exchange membrane fuel cell stack

Summary Thermal management has been considered as one of the most important issues for the operation of proton exchange membrane fuel cells (PEMFCs). Phase change affects the performance and even the heat balance of the stack during operation. A 46 single cell PEM stack with anode and cathode humidification is developed to investigate, both theoretically and experimentally, the effect of phase change on the heat generation and removal characteristics of the stack. The results show that the heat removed by the coolant water is greater than that generated by the electrochemistry reaction, and heat released due to the phase change of water vapor cannot be neglected. Heat generated in the stack can be removed completely by the coolant water, which need to be forced cooling for recycling use when the current density reaches 1000 mA·cm−2. The arithmetic product of the specific heat capacity and mass of the stack can be used as a novel criterion to evaluate the validity of the heat balance in the system. The exothermic reaction is very fast in the stack, which consequently requires bipolar plates with high heat conductivity coefficient to improve the temperature uniformity at the elevated operational current density. Copyright © 2014 John Wiley & Sons, Ltd.

Dynamic model of heat and mass transfer in an unsaturated porous wick of capillary pumped loop

This paper presents a numerical study of a Capillary Pumped Loop evaporator. A two-dimensional unsteady mathematical model of a flat evaporator is developed to simulate heat and mass transfer in unsaturated porous wick with phase change. The liquid–vapor phase change inside the porous wick is described by Langmuir's law. The governing equations are solved by the Finite Element Method. The results are presented then for a sintered nickel wick and methanol as a working fluid. The heat flux required to the transition from the all-liquid wick to the vapor–liquid wick is calculated. The dynamic and thermodynamic behavior of the working fluid in the capillary structure are discussed in this paper.

Experimental study of flexible, unstructured metal foams as condensation structures

This paper experimentally considers the use of non-structured (i.e., flexible) metal foams as a way to increase the amount of water vapor recovered from a humidified air stream. For that, a dedicated experimental setup consisting of three sub-systems (i.e., air pumping, air humidification, and air dehumidification) was developed. In the dehumidification section, different test sections (condensation structures) were installed and the amount of water recovered from the saturated air stream was recorded and correlated with each of them. Four main types of test sections were tested: (i) bare copper tube (used as base line), (ii) copper tube assisted by metal foam, (iii) finned tube and (iv) finned tube assisted by metal foam. Refrigerated fluid was circulated through copper tubes within a close loop and served as heat sink, allowing vapor phase change. The results show that the amount of water recovered increases with the surface area of the test section and with the temperature difference between the saturated air stream and the cooled surface. Also, it was shown that, above a certain quantity of metal foam within the test section, the amount of condensate produced is basically unaffected. Differently, the results indicate that the use of fins, which were properly brazed to the copper tubes, increases significantly the amount of water recovered from the saturated air stream.

Two-phase flow over flooded micro-pillar structures with engineered wettability pattern

Flooding caused by excessive droplet feeding on a heat dissipation area periodically occurs for droplet-based thermal management including spray cooling and electro-wetting. The conventional surface with highly wettable texture, which is designed for thin film evaporation, has negligible effect on improving thermal performance during flooding. This work examines a combination of micro-pillar structures and an engineered wettability pattern to improve the liquid–vapor phase change intensity and heat dissipation rate during flooding. Numerical simulations have been carried out to investigate the thermal and dynamic impact of the proposed combination structure on boiling and evaporation. A transient 3-D volume-of-fluid (VOF) model has been developed to analyze the behaviors of bubble growth, coalescence, and departure processes. Results including volumetric liquid–vapor mass transfer rate, heat source temperature and heat transfer coefficient are discussed. It has been demonstrated that the surface wettability gradient in the direction along the pillar’s height can effectively facilitate bubble departure and removal within the pillar forest. Thus smaller bubble size and lower thermal resistance in the fluid domain can be achieved. Also, the smoothness and coherence of the bubble departure will be enhanced. As for the wettability pattern on the micro-pillar structure, the resulting heat transfer performance is determined by a trade-off between the bubble departure improvement within the pillar forest and the bubble pinning at the hydrophobic pillar tops.

Effect of impurities in captured CO2 on liquid–vapor equilibrium

The capture of large amounts of CO2 from power plants and other large CO2 point sources has become relevant within the concept to mitigate CO2 emissions via carbon capture and storage. The objective of this study was to investigate the impact of some major impurities found in the CO2 stream captured from power plants and other large point sources on the liquid–vapor phase change compared to pure CO2. The study contributes to a better understanding of how and within which boundary conditions transmission of impure CO2 from an emission and capture point to a storage location can be realized.A set up was constructed to provide the experimental data needed to trace the phase diagrams of the CO2-impurities mixtures. The data show an expansion of the liquid–vapor phase envelope and a start of boiling (in liquid) or condensation (in vapor) at relatively higher pressures. The experimental data were later used to assess the capabilities of three mixture models: the Soave–Redlich–Kwong cubic equation of state, the GERG-2008 model, and the EOS-CG model.

Passive, internal thermal management system for batteries using microscale liquid–vapor phase change

Conventional thermal management systems for lithium-ion batteries remove heat from the exterior surface of the battery, causing undesirable temperature increase and thermal gradients inside the battery. Internally cooling batteries can reduce both of these effects, improving safety and durability. In the present study, a novel internal cooling system that utilizes passive, liquid–vapor phase change processes is investigated using representative geometry and a surrogate heat source. Frictional and heat transfer characteristics of the representative cooling system with buoyancy driven flow are reported over a range of net heat inputs (94–6230 W L−1) and saturation temperatures (24 °C–33 °C). The results show that the mass flow rate increased to a maximum near a heat input of 1350 W L−1, and there was a slight influence of saturation temperature on the performance of the system. In addition, the calculated two-phase frictional pressure drops in the microchannel evaporator (3.175 mm × 160 μm channels) are compared to the representative correlation database (Dh < 1 mm) and used to develop a new frictional pressure drop model with improved accuracy over the tested mass flux range (45 < G < 112 kg m−2 s−1). The results presented here are utilized in a subsequent investigation to determine the performance improvement in large lithium-ion battery packs intended for electric and hybrid electric vehicular applications through internal cooling.

Isothermal vapor–liquid equilibrium data for the decafluorobutane (R3110) + 1,1,1,3,3-pentafluorobutane (R365mfc) system at temperatures from 333 K to 441 K

The organic Rankine cycle (ORC) is one solution to recover energy from high temperature hot sources. It requires working fluid with high critical coordinate values. It is named for its use for an organic high molecular masse fluid with a liquid–vapor phase change. The knowledge of phase diagram of working fluid is a crucial task in refrigeration process. In this context, a new mixture of decafluorobutane (R3110) and 1,1,1,3,3-pentafluorobutane (R365mfc), is studied by means of experimental measurements and modeling procedures. The purpose is to obtain the isothermal vapor–liquid equilibrium data (R3110+R365mfc) of binary system ranging from (333.26 to 441.61)K temperature, and (0.2016 to 3.0927)MPa pressure. The data were obtained with accurate within u(T, k=2)=0.02K, u(P, k=2)=0.0003MPa and u(z)=0.004 for molar composition. In the present work, the static analytic method is another time investigated to achieve a high quality data as it was largely demonstrated previously by the team of our laboratories. The experimental data are correlated using the classical equations: Peng–Robinson equation of state, the Mathias–Copeman alpha function, and the Wong–Sandler mixing rules involving the NRTL model. The predicted critical line is well reproduced.

Experimental investigation of the heat transfer characteristics of a helium cryogenic thermosyphon

The heat transfer performance of a cryogenic thermosyphon filled with helium as the working fluid is investigated experimentally with a G–M cryocooler as the heat sink in this study. The cryogenic thermosyphon acts as a thermal link between the cryocooler and the cooled target (the copper evaporator with a large mass). Helium is charged in different filling ratios, and the cooling down process and the heat transfer characteristics of the cryogenic thermosyphon are investigated. The cooling down process of the cooled target can be significantly accelerated by the presence of helium in the cryogenic thermosyphon and the cooling down period can be further shortened by the increase of filling ratio. The heat transfer mode changes from the liquid–vapor phase change to natural convection as the increase of the heating power applied on the evaporator. The heat transfer limit and thermal resistance are discussed for the liquid–vapor phase change heat transfer, and they can be estimated by empirical correlations. For the natural convection heat transfer, it can be enhanced by increasing the filling ratio, and the natural convection of supercritical helium is much stronger than that of gaseous helium.

Travelling waves of phase transitions in porous media

Liquid–vapour phase changes for a fluid flow through a porous medium are considered; in particular, the model allows for phase mixtures and includes an equilibrium pressure. Existence and uniqueness of travelling waves is established in a wide range of situations; the end states may be formed either by pure phases or mixtures; in the latter case the pressure equals the equilibrium pressure. A formal asymptotic analysis for vanishing relaxation time is carried out to show that the friction and reaction source terms have smoothing effect when the pressure is close to the equilibrium pressure and pure phases are avoided.

CFD investigation of thermal and pressurization performance in LH2 tank during discharge

Predictions of thermal and pressurization performance in a liquid hydrogen (LH2) tank during liquid discharge is of significance to the design and optimization of a rocket pressurization system. In this paper, a computational fluid dynamic (CFD) model is introduced to simulate the pressurized discharge event of LH2 tank. The wall region together with the fluid region is simultaneously considered as the computational domain, and low-Re k–ε model is applied to account for the fluid-wall heat exchange effect. Liquid–vapor phase change effect is also involved in the model. Comparison of the numerical results with existing experimental data suggests that the CFD model has a good adaptability in pressurization computation. Detailed characteristics, such as pressurant gas requirement, pressure altering history, and temperature distribution inside the tank, can be obtained by the model. The difference of pressurant gas, selecting helium or vapor H2, may result in the variations in pressure and temperature histories. Pressurization by vapor H2 supplies a higher pressure and also a temperature rise, which is significant to consider the selection of pressurant gas. The influences of phase change effect and injector structure on pressurization behaviors are also analyzed. The computational results show that liquid–vapor phase change has a slight influence on the pressurization behaviors. Significant pressure decay at the beginning stage of process may occur in the case of no-diffuser injector application since the incoming gas is excessively cooled by cold LH2. The results show that the present CFD model has a good adaptability in the prediction of pressurization behaviors and is a useful tool for the design and optimization of a pressurization system.

Vapor Phase of Thermally Unstable Sm, Eu, Yb Bromides

Vaporization of thermally unstable SmBr3, YbBr3, SmBr2, EuBr2, and YbBr2 is investigated by the high temperature mass spectrometry. It is found that the composition of vapor phase changes in the course of vaporization due to thermal decomposition of tribromides and disproportionation of dibromides. The neutral (Ln, Br, Br2, LnBr, LnBr2, LnBr3, Ln2Br4, Ln2Br5, and Ln2Br6) and charged (Br–, LnBr3 –, LnBr4 –) species are present in vapor at various vaporization stages. Eventually for each sample studied the vaporization approaches steadiness when the LnBr2 molecules are predominant in the vapor.

Evaporation analysis in sintered wick microstructures

Heat pipes offer passive transport of heat over long distances without incurring a significant drop in temperature. Topological and microstructural details of the wick material embedded in a heat pipe help determine its thermal performance. A good understanding of pore-scale transport phenomena is crucial to enhancing heat pipe performance. In this study, pore-scale analysis of thin-film evaporation through sintered copper wicks is performed. X-ray microtomography is employed to generate geometrically faithful, feature-preserving meshes. Commercial sintered wicks with particle sizes in the range of 45–60μm, 106–150μm and 250–355μm and with approximately 61% porosity are considered. The capillary pressure, characteristic pore radius, percentage thin film area and evaporative mass and heat fluxes are computed using a volume of fluid (VOF) approach. Two different solution strategies are employed to stabilize the numerical solution and to improve convergence. After verifying that these strategies yield the correct solution, the VOF model is used to obtain static meniscus shapes in the pore space of the sintered wick samples. The meniscus shape is then held fixed and steady-state, thin-film evaporation analysis is performed. Liquid–vapor phase change heat transfer is modeled using a modified Schrage equation. Based on the present analysis, the best performing sample (particle size range) is identified along with the optimum contact angle.