Volumetric Evaporation Research Articles

Numerical Modeling of Water Transport in a Microporous Layer-Coated Porous Transport Layer for a Polymer Electrolyte Membrane Water Electrolyzer with an Interdigitated Flow Field for Internal Gas-Liquid Separation

A novel design for interdigitated flow fields in polymer electrolyte membrane electrolyzers has been developed for both ground and space applications. This design separates oxygen and liquid water internally, eliminating the need for water circulators to remove bubbles and external gas-liquid separators with buoyancy. The capillary pressure in the hydrophobic microporous layer (MPL) of the anode porous transport layer facilitates internal separation of oxygen gas and pressurized liquid water. A finite element model (COMSOL Multiphysics) simulates water transport in the MPL, while electrochemical impedance spectra determine the electrochemical kinetic parameters for the model. The model takes into account the oxygen bubble coverage of the cathode, liquid water saturation in the MPL, and the current ratio between liquid water and water vapor at the MPL-cathode layer interface. The vapor from liquid water in the MPL is modeled to mix with oxygen for diffusion. A volumetric water evaporation rate as a linear function of liquid water saturation in the MPL is assumed, where the rate constant is estimated from vapor permeation tests.

Numerical Modeling of Water Transport in a Microporous Layer-Coated Porous Transport Layer for a Polymer Electrolyte Membrane Water Electrolyzer with an Interdigitated Flow Field for Internal Gas-Liquid Separation

A novel design for interdigitated flow fields in polymer electrolyte membrane electrolyzers has been developed for both ground and space applications. This design separates oxygen and liquid water internally, eliminating the need for water circulators to remove bubbles and external gas-liquid separators with buoyancy. The capillary pressure in the hydrophobic microporous layer (MPL) of the anode porous transport layer facilitates internal separation of oxygen gas and pressurized liquid water. A finite element model (COMSOL Multiphysics) simulates water transport in the MPL, while electrochemical impedance spectra determine the electrochemical kinetic parameters for the model. The model takes into account the oxygen bubble coverage of the cathode, liquid water saturation in the MPL, and the current ratio between liquid water and water vapor at the MPL-cathode layer interface. The vapor from liquid water in the MPL is modeled to mix with oxygen for diffusion. A volumetric water evaporation rate as a linear function of liquid water saturation in the MPL is assumed, where the rate constant is estimated from vapor permeation tests.

Перспективные конструкторские решения для сушки жидких пищевых систем

An assessment of promising alternative design solutions for drying liquid food systems is given. Drying technology and techniques for removing moisture from liquid materials in a dispersed state have found widespread use in the food industry. The almost instantaneous drying of the sprayed particles in the coolant flow causes the formation of a developed phase interaction surface and acceleration of heat and mass transfer procedures, as well as the exclusion of overheating of the material due to intensive volumetric evaporation of moisture. A number of dryer designs have been developed to eliminate disadvantages (such as the large size of the dryers and their relatively high energy consumption and complexity of structural elements) or to reduce their impact on the course of the dehydration operation. The design specifics of devices for dewatering in the atomized state are due to the methods of supplying thermal energy, technological conditions and limitations, parameters of the dried product and the final powder substance, as well as its specific yield, peculiarities of the spraying operation, etc. The justification of the method of spray drying of liquid food material and the constructive use of the apparatus should be based on the study of a set of parameters and characteristics of the dehydration object, as well as on the construction and solution of a mathematical model of heat and mass transfer and exchange processes at the boundary of the phase section, as well as their mechanism in the development of a rational method of moisture removal to ensure the quality of finished products and energy- and resource saving. The proposed plant designs are recommended for organizing the drying process of liquid products, in particular for drying broths in fish production technologies.

Water Transport Modeling in a Microporous Layer for a Polymer Electrolyte Membrane Water Electrolyzer Having a Gas-Liquid Separating Interdigitated Flow Field

A novel interdigitated flow field design for polymer electrolyte membrane electrolyzers (proton exchange membrane water electrolysis cells), which has oxygen exhaust channels separated from pressurized liquid water feeding channels, has been developed for ground and space applications 1), 2). This structure separates oxygen and liquid water inside the anode of the cell. Thereby it dispenses with water circulators for bubble removal from the cell and external separators by natural or centrifugal buoyancy. We thus aim to develop a numerical model for further optimization of the cell design.Finite element modeling (COMSOL Multiphysics) of water transport is three-dimensionally conducted for the anode porous transport layer coated with a hydrophobic microporous layer (MPL) (SIGRACET 29BC, SGL Carbon Inc.) assembled with the interdigitated flow field. The MPL works to separate evolved oxygen gas exhausted in the through-plane direction and pressurized liquid water injected in the in-plane direction owing to the capillary pressure 2), 3). Prominent leakage of the liquid water to the oxygen channels has not been detected.Electrochemical kinetic parameters for the model are determined by electrochemical impedance spectra. Current-voltage measurement of the cell is also performed to validate the numerical modeling.We model the current densities and the current ratio between the reactant liquid water and water vapor at the interface between the MPL and catalyst layer (CL). The model involves fractional bubble coverage of the CL with produced oxygen gas and liquid water saturation in the MPL. The vapor evaporating from the liquid water in the MPL is assumed to be mixed with the evolved oxygen for diffusive water transport. The volumetric evaporation rate is assumed as a function of the liquid water saturation in the MPL.References1) Y. SONE, O.S. HERNANDEZ-MENDOZA, A. SHIMA, M. SATO, H. NAKAJIMA, H. MATSUMOTO, Water Electrolysis by the Direct Water Supply to the Solid Polymer Electrolyte through the Interdigitated Structure of the Electrode, Electrochemistry. 89 (2021) 138–140. https://doi.org/10.5796/electrochemistry.20-00145.2) H. NAKAJIMA, V. VEDIYAPPAN, H. MATSUMOTO, M. SATO, O.S. MENDOZA-HERNANDEZ, A. SHIMA, Y. SONE, Water Transport Analysis in a Polymer Electrolyte Electrolysis Cell Comprised of Gas/Liquid Separating Interdigitated Flow Fields, Electrochemistry. 90 (2022) 017002. https://doi.org/10.5796/electrochemistry.21-00097.3) H. Nakajima, S. Iwasaki, T. Kitahara, Pore network modeling of a microporous layer for polymer electrolyte fuel cells under wet conditions, J. Power Sources. 560 (2023) 232677. https://doi.org/10.1016/j.jpowsour.2023.232677.AcknowledgmentsThis study is based on results obtained from a project, JPNP21014, commissioned by the New Energy and Industrial Technology Development (NEDO).

Coupled transport and poromechanics model to understand quality evolution during sequential drying

Sequential drying provides opportunities to achieve quality by combining individual drying modes. Many process and product parameters affecting multiple drying modes make it complex. We studied quality evolution in a sequence of microwave, impingement, and hot air drying of the mashed vegetable chip using a poromechanics and transport model with volumetric evaporation, moisture-based microwave absorption, pressure-driven expansion, moisture loss-driven shrinkage and glass transition. It was validated for temperature, moisture, size and porosity obtained in an industrial setting. Gas porosity is higher at the center during microwave and at the surface during impingement drying. Moisture loss-driven shrinkage was more dominant than pressure-driven expansion. Shrinkage followed the moisture loss when the chip was rubbery, but shrinkage reduced after the glass transition occurred. Sequential drying was able to achieve a quality that would be hard to achieve using a single drying method. The mechanistic framework, successful for three drying modes and their sequence, is useful to understand drying-like processes.

Modulating Photothermal Properties of Carbon Dots through Nitrogen Incorporation Enables Efficient Solar Water Evaporation

As a new family in carbon nanomaterials, carbon dots (CDs) are potential candidates for solar water evaporator, owing to their cost-effectiveness, non-toxicity, high solubility, and tunable optical properties. Despite such potentials, however, CDs mainly absorb solar spectrum in the ultraviolet region while their absorption in the visible region is limited, the characteristics that hinder their functionality in generating steam from solar energy. Herein, the optical and photothermal properties of CDs, derived from urea and citric acid, can be modulated by controlling their surface stoichiometry through varying the molar ratio of the precursors. Our approach is simple, fast, and highly scalable by utilizing a microwave irradiation technique. We found that increasing the nitrogen content results in broadening of the absorption spectra into the visible region due to more functional groups introduced on the CD surface that reduce the band gap, as confirmed both by X-ray photoelectron spectroscopy and theoretical calculation. Employing the CDs as photothermal materials in the volumetric solar evaporator, we demonstrate a remarkable evaporation efficiency of up to 70% along with a volumetric evaporation rate of 1.11 kg m–2 h–1 under 1 sun illumination, superior to direct bulk water heating. Furthermore, the CDs show excellent durability and stability, as demonstrated by their stable evaporation rate for 10 days, with no significant decrease in the optical and photothermal properties. This finding provides a pathway to design and functionalize CDs with controllable optical and photothermal properties for an efficient solar evaporation system.

Effect of Model Tear Film Lipid Layer on Water Evaporation.

A majority of in vitro models were incapable of reproducing the evaporation resistance of tear film lipid layer (TFLL) in vivo. The purpose of this research is to develop a novel in vitro model to study the effect of TFLL on water evaporation. A ventilated, closed-chamber, droplet evaporimeter with a constant surface area has been invented to study the evaporation resistance of TFLL. This evaporimeter ensures a rigorous control of environmental conditions, including the temperature, relative humidity, airflow rate, surface area, and surface pressure, thus allowing for reproducible water evaporation measurements over a time period of only 5 minutes. The volumetric evaporation rate of this droplet evaporimeter is less than 2.7 µL/min, comparable to the basal tear production of healthy adults. Together with direct film imaging using atomic force microscopy (AFM), we have studied the effect of a model TFLL on water evaporation, as a function of the lipid composition and surface pressure. A model TFLL composed of 40% wax esters, 40% cholesteryl esters, and 20% polar lipids was capable of reducing the water evaporation rate by 11% at surface pressure 47 mN/m. AFM revealed that the model TFLL at high surface pressures consists of discrete droplets/aggregates of the nonpolar lipids residing atop a polar lipid monolayer with phase separation. The TFLL may resist water evaporation with a combined mechanism by increasing film compactness of the polar lipid film at the air-water surface, and, to a lesser extent, by increasing film thickness of the nonpolar lipid film.

Сapillatry effects manifested in the process of draining a liquid film that evaporated into a gas cross flow

The aim of the work is to determine the regularities of changes in the width of the flowing liquid film evaporating into a gas cross flow resulting from thermocapillary and concentration-capillary effects (Marangoni effect).
 The research was carried out experimentally employing an installation with a heated (working) surface, on which a liquid film (water or aqueous glycerol solution) was flowing. A flow of air was fed into the space between the working surface and the enclosing wall located at a distance from it, evenly in height and perpendicularly to the film movement. The film width was measured along the height of the working surface and the obtained dependencies were presented as graphs for different parameters of the flowing process.
 In all experiments, the graphs have an initial section with a sharp decrease in the film width, below which its change occurs smoothly. The degree of reduction in the film width depending on the parameters of the flowing process can reach 80%. In laminar mode, the reduction of the film width is more uniform than in turbulent mode, when some rise of the curves beyond the initial section is observed and can be explained by intensive mixing of the liquid. As the initial liquid flow increases, the degree of reduction in the film width decreases. Significant influence on the change in the film width is exerted by the temperature of the working surface during the flow of water: with increasing temperature, the film width decreases. At values of this temperature greater than 100℃, the rise of the curves is observed, possibly associated with the transition to volumetric evaporation.
 When the glycerol solution is flowing, the temperature of the working surface less influences the change in film width, although its decrease along the height of the working surface is greater than for water. The process parameters such as initial liquid temperature, initial solution concentration, and velocity and air temperature have relatively little effect on the change in film width. To correct the area of the film evaporation surface, determined by technological calculation, the width of the average evaporation surface is introduced under capillary effects. Its value increases with the initial liquid flow. For water, this dependence is characterized by a jump-like decrease in the film width in the transition from laminar to turbulent flow mode. This effect is absent on the dependence for glycerol solution, which can be explained by its higher viscosity. As the temperature of the working surface increases, the width of the average evaporation surface of the film decreases, some decrease being also observed with an increase in air velocity. Empirical equations for estimation of the correction factor were obtained. Capillary effects in this case are expressed by decrease in the width of the film at the height of its runoff, which leads to the need to correct the area of the film evaporation surface determined by technological calculation.

Novel nanofluid based efficient solar vaporization systems with applications in desalination and wastewater treatment

The current work proposes a new strategy to improve solar evaporation efficiency and explore the real applications of volumetric solar evaporation device in desalination and wastewater treatment. Nanofluid based volumetric solar evaporation system is efficient to enhance the absorbance of solar irradiation. However, the solar evaporation efficiencies are relatively low. There is a need to investigate the fundamentals of the limitation and find a new strategy to improve the solar evaporation efficiency. In this paper, a novel device containing tungsten carbide nanofluids as work fluids is designed for the first time to reduce heat loss towards the bulk water, improve the stability of nanofluids, and avoid contamination of nanoparticles. The results show that tungsten carbide nanofluids of 0.3 wt% can harvest 99% of the incident solar energy within 1 cm penetration distance. The photothermal conversion efficiency is 97.7%. The novel device gives an evaporation rate of 1.235 kg m−2 h−1 and reached solar evaporation efficiency of 74.9% under 1-sun irradiation. The applications in desalination and wastewater treatment show that the ion rejection rate of seawater is higher than 99.99%, and the content of heavy metal ion is significantly lower than that in the World Health Organization drinking-water standard.

Kinetics of convective drying during dehydration of poultry meat products

Convective drying is a common method of dehydration of meat snacks (chips), which largely forms the competitive properties of the finished product. The kinetic parameters of drying (for example, the change in moisture and mass of the drying object during the process, the intensity and duration of drying) are the basis for the development and selection of equipment for the drying section of the industrial production of dehydrated meat products and process control. These indicators were determined experimentally, since they largely depend on the physical properties of a particular drying object, and it is not yet possible to calculate them with an accuracy sufficient for engineering practice. The kinetic parameters of convective drying of salted samples in the form of plates with a thickness of 2, 3, 4, 6, 8, 10 mm, made of chicken fillet meat blocks, have been determined. In order to ensure the adequacy of the research results to the conditions of industrial production, the main stages of the industrial technological process and the characteristics of the raw material samples were physically modeled: the composition of the curing mixture, the dimensions and physical properties of the raw material samples, and drying modes. The presence of complete volumetric evaporation during a period of constant drying rate of samples of raw materials with a thickness of 2, 3, 4 mm was revealed, as well as partial volumetric evaporation (about 90%) for samples with a thickness of 6, 8, 10 mm. A change in the physical mechanism of the drying process in the period of decreasing speed (cessation of volumetric evaporation and a multiple decrease in the intensity of evaporation) has been established. Dependences of the duration of the complete drying process (the sum of the periods of constant and decreasing speed) on the thickness of the raw material samples have been determined (for example, a 4 mm thick sample dries 25% slower than a 2 mm sample). Based on the research results, the possibility of increasing the productivity of the drying equipment (up to 60%) and optimizing the drying agent supply system to the drying object is shown.

Improvement of the efficiency of volumetric solar steam generation by enhanced solar harvesting and energy management

Volumetric solar steam generation has attracted substantial interest due to its low cost, minimum carbon footprint and wide application in many areas including clean water production, desalination, and wastewater treatment. However, the efficiency of volumetric solar evaporation is still low and there is an urgent need to investigate the fundamental of the limitation of low efficiency and find a new strategy to improve the solar evaporation efficiency. In the current work, antimony doped tin oxide@carbon (ATO@C) nanofluids were prepared by a hydrothermal approach. The ATO@C nanofluids exhibit broad-band and high absorption in the solar spectrum due to the complementary effect of C (in visible region) and ATO (in the near infrared region). ATO@C nanofluids of 0.3 wt% could harvest 99.9% of the incident solar energy within 1 cm penetration distance. The photothermal conversion efficiency is 97.8%. The coupling relationship between the solar harvesting and the energy distribution was revealed. Increasing mass fraction and reducing thickness can localize the heat in the surface layer of nanofluids and thus minimize the energy consumption in heating water (internal energy) and therefore improve the solar evaporation efficiency. A high evaporation efficiency of 88.6% was achieved in this way.

Performance enhancement of a split air conditioner using porous material inside the evaporator pipes

AbstractIn this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop‐in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate.

A semi-empirical model for thermal resistance and dryout during boiling in thin porous evaporators fed by capillary action

Two-phase passive heat transport devices such as vapor chambers, loop heat pipes, and capillary pumped loops utilize porous evaporators for phase change and to drive fluid transport. Nucleate boiling can occur within such capillary-fed porous evaporators, especially under high-heat-flux operation, as has been visually observed in various experimental studies in the literature. However, prior modeling efforts have typically only considered single-phase flow of liquid through a completely saturated porous medium for characterizing the dryout limit and thermal performance. The present work offers a new semi-empirical model for prediction of thermal resistance and dryout during boiling in capillary-fed evaporators. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. The capillary pressure and the relative permeabilities (fraction of single-phase permeabilities) for two-phase flow in the porous medium are modeled as a function of the local liquid saturation. The heat flux at which the liquid saturation at the center of the evaporator becomes zero is defined as the dryout limit of the evaporator. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses.

Estimating lake temperature profile and evaporation losses by leveraging MODIS LST data

Global lake evaporation is a critical component of the terrestrial water cycle. Accurate quantification of lake evaporation dynamics is of high importance for understanding lake energy budgets, land-atmosphere interactions, as well as regional water availability. However, the accurate quantification of lake evaporation has been hindered by the complexity involved with addressing the heat storage of water bodies. In this study, a new model—the Lake Temperature and Evaporation Model (LTEM)—was developed to simulate lake water temperature profiles, which were then used to calculate heat storage changes and evaporation rates. Inputs for the LTEM include the meteorological and bathymetric data, as well as the Moderate Resolution Imaging Spectroradiometer (MODIS) water surface temperature (WST)—which is the land surface temperature (LST) over water. The MODIS WST was leveraged to constrain the hydrodynamic simulations. Model results over 11 lakes around the world show robust performance of LTEM. The long term average temperature biases range from -0.5 °C to 0.5 °C, and the evaporation rate biases range from -0.19 mm/day to 0.28 mm/day. In particular, it is found that LTEM significantly improves the simulation of the seasonality of lake evaporation rates. The validation results suggest that the averaged coefficient of determination (R2) for the evaporation rate is 0.84, which is 0.28 higher than that obtained when the conventional Penman equation (without heat storage) is used. The volumetric evaporation time series was then calculated as a product of the monthly evaporation rate and lake surface area (derived from MODIS near-infrared image classifications). This study provides an end-to-end framework for quantifying volumetric evaporation for the world’s lakes and reservoirs. It also provides the capability to investigate the thermal dynamics of lake systems, and thus can benefit the various water resources applications across scales.

Magnetic MoS2 nanosheets as recyclable solar-absorbers for high-performance solar steam generation

To deal with water scarcity, solar steam generation has been widely regarded as one of the promising methods for freshwater production. In this work, for the first time, a novel recyclable MoS2-based nanocomposite (magnetic MoS2 nanosheets) was fabricated via the in-situ growth of nano Fe3O4 on the polydopamine (PDA)-functionalized MoS2 nanosheets, and subsequently applied as the solar absorber in the volumetric evaporation systems to generate solar steam. The magnetic MoS2 nanosheets showed a superhigh solar absorption of over 96% within the whole wavelength range of 200–2500 nm. As a result, a high evaporation efficiency of 79.2% under the low illumination of 2.5 kW m−2 was achieved. In addition, the magnetic MoS2 nanosheets not only showed the long-term well dispersion in aqueous solution due to the introduction of hydrophilic PDA, but also exhibited a fast and effective separation from aqueous solution with the help of the decorating nano Fe3O4, which much benefited to the continuously efficient solar steam generation and its good recyclability, respectively. Therefore, the MoS2-based nanomaterials in this work showed good prospects for solar energy harvesting.

Indirect Dryers for Biomass Drying—Comparison of Experimental Characteristics for Drum and Rotary Configurations

This paper focuses on indirect biomass drying. It compares the operating characteristics of a laboratory-scale drum dryer and a pilot-scale rotary dryer. Before the design of an industrial dryer for a specific material, it is important to experimentally prove the process and to determine the drying characteristics of the material. To verify the portability of experimental results for indirect dryers, a drum dryer with indirect electric heating in a laboratory scale was designed and built to test and study the process of indirect drying. Based on the results obtained on a small-scale device, a prototype of a pilot steam-heated rotary dryer was designed and manufactured. A broad range of experiments with green wood chips and wet bark from open-air storage with moisture contents of 50 to 65 wt % were carried out on both dryers. The drying curves indicating the process, the square and volumetric evaporation capacities, and the drying energy consumption were obtained and compared, and the feasibility of indirect drying for these tested types of biomass was confirmed.

THE EFFECT OF THE FILLING RATIO ON THE OPERATING CHARACTERISTICS OF AN INDIRECT DRUM DRYER

This article investigates the effect of the filling ratio of the indirect rotary dryers on their operating characteristics. For moist biomass drying before combustion, the use of indirect drum dryers heated by a low pressure steam has proven to be highly suitable. Regarding the design of new dryers, it is necessary to experimentally verify the operating characteristics for specific materials and drying conditions. For this purpose, a set of experiments on a steam heated rotary drum dryer were carried out with green wood chips containing 60 to 66 wt% of moisture. The following operational characteristics of the dryer were experimentally determined: drying curves describing the process, square and volumetric evaporation capacities and drying heat consumptions. Based on the experimental results, the effect of various drum filling by dried material on the mentioned operating characteristics was analysed. On the one hand, higher drum filling ratio increases the drying time, on the other hand, the evaporation capacity also increases, while the specific energy consumption does not significantly alter. The maximum value of the evaporation capacity was reached when the drum was filled to 20 wt%. When the filling ratio was increased to 25 wt%, the evaporation capacity experienced almost no change.

Experimental study on transient heat/mass transfer characteristics during static flash of aqueous NaCl solution

The instant height/temperature of waterfilm were selected as a pair of characteristic parameters to study the transient heat/mass transfer properties during static film flash of aqueous NaCl solution though experiments with waterfilm concentration ranging between 0 and 0.15 (mass fraction), superheat between 5.1 and 35.0 K, and initial height of waterfilm between 0.10 and 0.20 m. The heat transfer mainly happened at bubble interface within waterfilm. The mass transfer included two steps of vaporization at bubble interface and vapor overflow from top free surface. The transient intensity of heat transfer was measured by volumetric evaporation coefficient. It shared the same experience with volumetric void fraction of quick increase to a peak value and gradual falling back. But its change always lagged behind that of void fraction. The transient intensity of overflow was measured by overflow speed of flash vapor. It was found to increase with rising superheat or initial height of waterfilm, but dropped significantly with increasing waterfilm concentration. On basis of these results, a calculation model for static film flash was set up. The evolution of waterfilm temperature/height could be computed simultaneously from given evolution of flash chamber pressure and other necessary initial conditions.

Fundamentals of radiation heat transfer in AlSi10Mg powder bed during selective laser melting

Purpose This paper aims to study numerically the influence of the applied laser energy density and the porosity of the powder bed on the thermal behavior of the melt and the resultant instability of the liquid track. Design/methodology/approach A three-dimensional model was proposed to predict local powder melting process. The model accounts for heat transfer, melting, solidification and evaporation in granular system at particle scale. The proposed model has been proved to be a good approach for the simulation of the laser melting process. Findings The results shows that the applied laser energy density has a significantly influence on the shape of the molten pool and the local thermal properties. The relative low or high input laser energy density has the main negative impact on the stability of the scan track. Decreasing the porosity of the powder bed lowers the heat dissipation in the downward direction, resulting in a shallower melt pool, whereas pushing results in improvement in liquid track quality. Originality/value The randomly packed powder bed is calculated using discrete element method. The powder particle information including particle size distribution and packing density is taken into account in placement of individual particles. The effect of volumetric shrinkage and evaporation is considered in numerical model.

Evaporation of ethanol-water sessile droplet of different compositions at an elevated substrate temperature

We experimentally investigate the evaporation dynamics of sessile droplets with different compositions of ethanol-water binary mixture at different substrate temperatures. At room temperature, a binary droplet undergoes two distinct evaporation stages unlike only the pinned stage evaporation for pure droplets. In the binary droplets, the more volatile ethanol evaporates faster leading to a nonlinear trend in the evaporation process. At elevated substrate temperature, we observed an early spreading stage, an intermediate pinned stage and a late receding stage of evaporation. Increasing the substrate temperature decreases the lifetime of binary droplets rapidly. At high substrate temperature, we found that the lifetime of the droplet exhibits a non-monotonic trend with the increase in ethanol concentration in the binary mixture, which can be attributed to the non-ideal behaviour of water-ethanol binary mixtures. Interestingly, the evaporation dynamics for different compositions at high substrate temperature exhibits a self-similar trend showing a constant normalised volumetric evaporation rate for the entire evaporation process. This indicates that the evaporation dynamics of a binary droplet of a given composition at high substrate temperature is equivalent to that of another pure fluid with a higher volatility at room temperature. The evaporation rates of pure and binary droplets at different substrate temperatures are compared against a theoretical model developed for pure and binary mixture droplets. The model predictions are found to be quite satisfactory for the steady evaporation phase of the droplet lifetimes.