Local Evaporation Rate Research Articles

Frontal pyrometric snapshot measurements of the keyhole wall temperature in laser welding of pure aluminium

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

Water and heat exchanges in mammalian lungs

A secondary function of the respiratory system of the mammals is, during inspiration, to heat the air to body temperature and to saturate it with water before it reaches the alveoli. Relying on a mathematical model, we propose a comprehensive analysis of this function, considering all the terrestrial mammals (spanning six orders of magnitude of the body mass, M) and focusing on the sole contribution of the lungs to this air conditioning. The results highlight significant differences between the small and the large mammals, as well as between rest and effort, regarding the spatial distribution of heat and water exchanges in the lungs, and also in terms of regime of mass transfer taking place in the lumen of the airways. Interestingly, the results show that the mammalian lungs appear to be designed just right to fully condition the air at maximal effort (and clearly over-designed at rest, except for the smallest mammals): all generations of the bronchial region of the lungs are mobilized for this purpose, with calculated values of the local evaporation rate of water from the bronchial mucosa that can be very close to the maximal ability of the serous cells to replenish this mucosa with water. For mammals with a mass above a certain threshold (simeq 5 kg at rest and simeq 50 g at maximal effort), it appears that the maximal value of this evaporation rate scales as M^{-1/8} at rest and M^{-1/16} at maximal effort and that around 40% (at rest) or 50% (at maximal effort) of the water/heat extracted from the lungs during inspiration is returned to the bronchial mucosa during expiration, independently of the mass, due to a subtle coupling between different phenomena. This last result implies that, above these thresholds, the amounts of water and heat extracted from the lungs by the ventilation scale with the mass such as the ventilation rate does (i.e. as M^{3/4} at rest and M^{7/8} at maximal effort). Finally, it is worth to mention that these amounts appear to remain limited, but not negligible, when compared to relevant global quantities, even at maximal effort (4–6%).

Effect of temperature and surfactants on evaporation and contact line dynamics of sessile drops

Researches on droplet evaporation has a vast range of applications in different fields. The mechanisms of droplets evaporation have been a topic of investigation for decades. The droplet after spreading on the substrate exhibits constant evaporation rate for a constant base diameter and decreasing contact angle; followed by another evaporation mode with constant contact angle and decreasing base diameter; and finally, evaporation with decreasing of both contact angle and base diameter. In the present study, the evaporation rate and mechanisms influenced by surfactant and temperature effects are analysed. The evaporation was demonstrated to be related to the dynamics of the sessile droplet spreading ability and the depinning event. It remains complex to predict the evaporation of the sessile droplet due to the continuous change in surfactant local concentration. Moreover, the temperature change modifies the surface tension and the induced inner flows. These two questions find answers in the triple line dynamic which controls the evaporation rate. The forces controlling the depinning with surfactant was quantified. A clear constant force was found following depinning. Such dynamic equilibrium demonstrates also the effect of the friction, surface roughness and moreover the limit of surfactant concentration in the triple line direct vicinity. The evaporation rate shows two particularities: one is the classical decrease of the evaporation rate after depinning; the second in case of surfactant, there is an enhancement appearing at later droplet lifetime, which could be a consequence of film evaporation contributing to the evaporation near the triple line. The local evaporation does not change significantly despite the change in the contact angle with respect to surfactant concentration. The substrate temperature enhances evaporation rate. The coupled temperature and surfactant effect show a local evaporation rate decreasing with surfactant concentration increase at ambient temperature (around 20 °C) and enhancement with surfactant concentration at higher temperature.

Evaporation and fluid flow near the boundary of a stationary dry patch.

We consider an isolated circular dry patch formed in an evaporating liquid layer and investigate local viscous flows in both liquid and air near the contact line which is the boundary of the dry patch. Flow patterns in the liquid deviate significantly from the predictions of single-phase models even when the air-to-liquid dynamic viscosity ratio is small. In particular, the separatrices in the liquid flow patterns at large contact angles disappear completely for a range of realistic viscosity ratios when the shear stress on the air side of the interface is accounted for. Experimentally observed motion of microdroplets near the contact line under combined action of gravity and moist air flow is described using our local flow model. We demonstrate that analysis of droplet trajectories leads to unambiguous determination of the local evaporative flux profile. A numerical finite-element approach for the steady diffusion equationis then used to extract the same flux profile from the global solution for concentration field in the limiting case of very thin liquid layer and small contact angle. The local evaporation rate is underpredicted by the numerical method, most likely due to the neglect of convective mass transfer in the air.

Combined infrared and hot air drying (IR-HAD) of sweet potato explored using a multiphase model: application of reaction engineering approach

For better understanding of transport phenomena, local evaporation rate and water vapor concentration should be taken into consideration when predicting drying behavior. However, to the best of our knowledge, there is no model which explicitly represents this. Predictive modeling of combined infrared-heating and hot air drying (IR-HAD) is useful to assist in designing new dryer units and manufacturing the products with desirable quality. As the quality changes are basically local phenomena, the model should be able to capture the local evaporation rate. In this study, for the first, time, spatial reaction engineering approach (S-REA) was developed and used to describe infrared-heating drying of the food materials by using the reaction engineering approach (REA) to represent local evaporation rate. Benchmarks against the single-phase approach and the results showed that the S-REA gave high accuracy toward the experimental data (R2 = 0.978–0.999). The S-REA successfully provided reasonable predictions of two-dimensional (2 D) profiles of moisture content, concentration of water vapor and temperature. This highlights the accuracy of the REA framework to describe the local evaporation rate during the infrared-heating. The REA-based predicted variables can then be applied to project the local quality parameters for quality assurance in food processing. The model is also ready to be used to audit energy demands in drying systems.

Simulation of Mass and Heat Transfer in an Evaporatively Cooled PEM Fuel Cell

Evaporative cooling is a promising concept to improve proton exchange membrane fuel cells. While the particular concept based on gas diffusion layers (GDLs) modified with hydrophilic lines (HPILs) has recently been demonstrated, there is a lack in the understanding of the mass and heat transport processes. We have developed a 3-D, non-isothermal, macro-homogeneous numerical model focusing on one interface between a HPIL and an anode gas flow channel (AGFC). In the base case model, water evaporates within a thin film adjacent to the interfaces of the HPIL with the AGFC and with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell via the AGFC. The transport to the cathode side is shown to be partly limited by the ab-/desorption into/from the membrane. The cooling due to the latent heat has a strong effect on the local evaporation rate. An increase of the mass transfer coefficient for evaporation leads to a transport limited regime inside the MEA while the transport via the AGFC is limited by evaporation kinetics.

Response of spray number density and evaporation rate to velocity oscillations

A theoretical investigation of the effect of gas velocity oscillations on droplet number density and evaporation rate is presented. Oscillations in gas velocity cause a number density wave, i.e. an inhomogeneous, unsteady variation of droplet concentration. The number density wave, as it propagates downstream at the mean flow speed, causes modulation of the local evaporation rate, creating a vapour wave with corresponding oscillations in equivalence ratio. The present work devises an analytical formulation of these processes. Firstly, the response of a population of droplets to oscillations in the gas velocity is modelled in terms of a number density wave. Secondly, the formulation is extended to incorporate droplet evaporation, such that an analytical expression for the evaporation rate modulation is obtained. Subsequently, the droplet 1D convection-diffusion transport equation with the calculated evaporation source term is solved using an appropriate Green’s function to determine the resulting equivalence ratio perturbations. The dynamic response of equivalence ratio fluctuations to velocity oscillations is finally characterized in terms of a frequency-dependent transfer function. The aforementioned analytical approach relies on a number of simplifying approximations, nevertheless it was validated with good agreement against 1D Euler-Lagrange CFD simulations.

Interaction of vapor cloud and its effect on evaporation from microliter coaxial well

The evaporation of volatile heavy hydrocarbon liquid from coaxial well configuration is studied to understand the vapor phase transport, vapor cloud interaction and its effect on the local evaporation rate of two evaporating coaxial cavities of microliter volume. The gap between the two coaxial cavities is varied without varying the dimensions (width and depth) of the coaxial cavities for studying the vapor cloud interaction. Digital holographic Interferometry (DHI) is used to decipher the vapor mole fraction field above the coaxial well. Gravimetric measurement has been carried out for measuring the evaporation rate. Diffusion-limited simulation of evaporation process has been carried out using COMSOL Multiphysics software to understand the role of convective motion. IR thermography measurement of liquid interface temperature has been carried out to correlate the vapor cloud interaction with local evaporation rate and evaporation induced cooling. A flat-disk shaped vapor cloud surrounds the coaxial cavities. Total evaporation rate measured from DHI shows good agreement with the gravimetric measurement having maximum deviation of 3%. Good match between the evaporation rate measured from gravimetry and holography with diffusion limited model is observed at low Grashof number. The mismatch in evaporation rate with the diffusion limited model is observed at high Grashof number due to higher radius of the coaxial cavity indicating the dominance of convective motion. The evaporation rate from individual well decreases when liquid simultaneously evaporates from the inner and outer coaxial cavities. This reduction in evaporation rate increases with decrease in gap between the two coaxial cavities due to increase in interaction between the vapor cloud. The decrease in local evaporation rate from individual cavity due to presence of a neighboring evaporating cavity correlates with the evaporative cooling effect using temperature measurement of the interface from IR thermography. Vapor cloud interactions of microliter volume coaxial cavities can influence the evaporation rate of individual coaxial cavity and the convection inside the liquid phase. The present study demonstrates the capability to precisely control the evaporation process by appropriate design of coaxial well configuration. The results from the present study can be useful for controlling the evaporation rate in several applications i.e. protein crystal growth, DNA/RNA sequencing and microfabrication etc. by suitable design of coaxial well.

Multiscale Evaporation Rate Measurement Using Microlaser-Induced Fluorescence

Abstract As the heat generation at device footprint continuously increases in modern high-tech electronics, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures enhance evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale. Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures by using microlaser-induced fluorescence (μLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating the effects of temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes the overall and local evaporation rates by measuring the solution drying time. The second approach employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures, which calculates the submicron-level evaporation rates. Using these methods, we reveal that the local evaporation rate between microstructures is high but is balanced with a large capillary-feeding. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures.

Heat and mass transfer in the boundary layer during evaporation of aqueous solutions of ethanol and acetone into air and superheated vapor components of solutions

A stationary flow of three-component gas in a laminar boundary layer on a flat plate is considered at adiabatic evaporation of a two-component liquid film. Numerical modelling were carried out at atmospheric pressure, air flow velocity of 2 m/s and temperature from 20 to 500 °C in the whole range of concentrations of a low-boiling component of binary solutions: ethanol/water, acetone/water, and acetone/ethanol. Data on the local specific evaporation rate of ethanol/water and acetone/water solutions into dry air and superheated mixture of ethanol, acetone and water vapors were obtained depending on the temperature of the inlet flow. Composition of superheated vapor mixture was taken to be equilibrium relative to the liquid solution. With an increase in the molar fraction of ethanol or acetone in the water solution, the evaporation rate increases, and when evaporated into superheated vapor, the growth of evaporation rate with increasing vapor temperature is significantly higher than at evaporation into air. With an increase in ethanol or acetone concentration in the liquid solution, the inversion temperature decreases from down to 155…165°C.

Numerical Investigation of Shape Effect on Microdroplet Evaporation

Abstract As electronic devices continue to shrink in size and increase in functionality, effective thermal management has become a critical bottleneck that hinders continued advancement. Two-phase cooling technologies are of growing interest for electronics cooling due to their high heat removal capacity and small thermal resistance (<0.1 k cm2/W). One typical example of a two-phase cooling method is droplet evaporation, which can provide a high heat transfer coefficient with low superheat. While droplet evaporation has been studied extensively and used in many practical cooling applications (e.g.,, spray cooling), the relevant work has been confined to spherical droplets with axisymmetric geometries. A rationally designed evaporation platform that yields asymmetric meniscus droplets can potentially achieve larger meniscus curvatures, which gives rise to higher vapor concentration gradients along the contact line region, and therefore, yields higher evaporation rates. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on a porous micropillar structure. The equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes are explored using the volume of fluid (VOF) method. The evaporative mass transport at the liquid–vapor interface is modeled using a simplified Schrage model. The results show highly nonuniform mass transport characteristics for asymmetrical microdroplets, where a higher local evaporation rate is observed near the locations where the meniscus has high curvature. This phenomenon is attributed to a higher local vapor concentration gradient that drives faster vapor diffusion at more curved regions, similar to a lightning rod exhibiting a strong electric field along a highly curved surface. By using contact line confinement to artificially tune the droplet into a more curved geometry, we find that the total evaporation rate from a triangular-based droplet is enhanced by 13% compared to a spherical droplet with the same perimeter and liquid–vapor interfacial area. Such a finding can guide the design and optimization of geometric features to improve evaporation in advanced microcooling devices.

Investigation of the confinement effect on the evaporation behavior of a droplet pinned on a micropillar structure

Evaporation of sessile droplet suffers from reduced evaporation rate due to the confinement of vapor diffusion imposed by the bottom substrate. However, it is possible to change the evaporation behavior of a droplet by suspending it from the bottom substrate, in particular, supporting the droplet on a micropillar. This is expected to enable diffusion transport in the downward direction that will subsequently enhance evaporative transport. In this study, we investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. The approximate solutions for total evaporation rate and local evaporative flux were subsequently derived from the total evaporation rate predicted by the simulation results. The simulation results, agreeing within 5% with the experimental measurements, show that increasing the micropillar height enhances the total evaporation rate from the suspended hemispherical droplet. This enhancement is due to a dramatic improvement of the local evaporation rate near the contact line region as micropillar heights increase. The micropillar heights examined for maximum evaporation rates were observed under substrate temperatures from 60–98 °C. The increasing pillar height leads to smaller vapor diffusion resistance but greater conduction resistance.

Experimental and numerical study on low-frequency oscillating behaviour of liquid pool fires in a small-scale mechanically-ventilated compartment

The unstable oscillatory behaviour, with frequency in the order of few mHz, that has been occasionally observed in mechanically-ventilated compartment fires, is studied experimentally and numerically. First, a series of experiments using a small-scale compartment have been conducted using heptane and dodecane as fuels. Results show that unstable and stable combustion regimes can occur depending on fuel type, pool size, air renewal rate of the compartment (ARR), and ventilation conditions. For a certain range of these factors, unstable low-frequency (LF) oscillatory combustion, accompanied by thermodynamic pressure and ventilation flow rate variations and displacement of the flame outside the pan, is observed. The occurrence and persistency of LF oscillations result from the competition between oxygen supply and fuel vapor supply due to the heat feedback from the flame and enclosure to the fuel tray. Whatever the fuel type, it is found that i) the range of ARR where LF oscillations appear and the oscillation amplitude increase with the pool size, and ii) the frequency increases, while amplitude decreases, with increasing ARR, independently of the pool size. It is also found that the more flammable the fuel, i) the smaller pool size for which LF oscillations appear and the higher the frequency for the same ventilation conditions, and ii) the wider the range of ARR where LF oscillations appear for a given pool size. The effects of air inlet position and blowing direction on the oscillations properties is also investigated. Second, predictive CFD simulations have been performed using the in-house SAFIR software. Although SAFIR does not correctly describe the displacement of the flame outside the fuel pan, it satisfactorily reproduces the LF oscillatory fire behaviour, especially its dominant frequency. Information about inaccessible or difficult-to-measure local quantities, such as the local evaporation rate, temperature and heat flux at the liquid surface, and species concentrations, are provided from the numerical simulation.

An analytical model of the heating and evaporation of bi-component wall film

A novel analytical model for the heating and evaporation of a bi-component fuel wall film is presented, in which the unsteady one-dimensional characteristics along the film thickness direction are considered. The energy equation and the species equation of the liquid phase are coupled with a model of predicting the local evaporation rate. Moreover, the variations of physical properties of each component corresponding to the local temperature are taken into account, thus the temperature evolution and species diffusion in the fuel film can be reproduced by the new model. The model is validated using previous experimental data and one-dimensional numerical simulation. The predicted film thickness evolution is in good agreement with the experimental data. The surface temperature and mass fraction of the components in the liquid film are also in good agreement with the numerical solutions. The results show that the mass fraction gradient exists inside the liquid film. This reflects the fact that the consideration of the finite diffusivity is important for accurate prediction of the wall film evaporation process. The results also indicate that the increase of the mass fraction of the more volatile component at the initial moment results in a decreased wall film lifetime and peak surface temperature.

Change of evaporation rate of single monocomponent droplet with temperature using time-resolved phase rainbow refractometry

Droplet evaporation characterization, although of great significance, is still challenging. The recently developed phase rainbow refractometry (PRR) is proposed as an approach to measuring the droplet temperature, size as well as evaporation rate simultaneously, and is applied to a single flowing n-heptane droplet produced by a droplet-on-demand generator. The changes of droplet temperature and evaporation rate after a transient spark heating are reflected in the time-resolved PRR image. Results show that droplet evaporation rate increases with temperature, from −1.28×10−8 m2/s at atmospheric 293 K to a range of (−1.5, −8)×10−8 m2/s when heated to (294, 315) K, agreeing well with the Maxwell and Stefan–Fuchs model predictions. Uncertainty analysis suggests that the main source is the indeterminate gradient inside droplet, resulting in an underestimation of droplet temperature and evaporation rate. With the demonstration on simultaneous measurements of droplet refractive index as well as droplet transient and local evaporation rate in this work, PRR is a promising tool to investigate single droplet evaporation in real engine conditions.

Levitation of liquid microdroplets over a dry heated substrate near triple-phase contact line

Levitating droplets of liquid condensate are known to organize themselves into ordered arrays over hot liquid-gas interfaces. We report experimental observation of similar behaviour over a dry heated substrate in the vicinity of the triple-phase contact line with high local evaporation rate. It was found that there is an area near the contact line, where no levitating droplets are observed. The width of this area increases with the substrate temperature. Also, it was found that the distance between adjacent levitating droplets increases with the temperature.

Natural convection above circular disks of evaporating liquids

We investigate theoretically and experimentally the evaporation of liquid disks in the presence of natural convection due to a density difference between the vapor and the surrounding gas. From the analogy between thermal convection above a heated disk and our system, we derive scaling laws to describe the evaporation rate. The local evaporation rate depends on the presence of a boundary layer in the gas phase such that the total evaporation rate is given by a combination of different scaling contributions, which reflect the structure of the boundary layer. We compare our theoretical predictions to experiments performed with water in an environment controlled in humidity, which validate our approach.

Predicting the evaporation rate of stationary droplets with the VOF methodology for a wide range of ambient temperature conditions

This paper presents CFD predictions for the evaporation of nearly spherical suspended droplets for ambient temperatures in the range 0.56 up to 1.62 of the critical fuel temperature, under atmospheric pressures. The model solves the Navier-Stokes equations along with the energy conservation equation and the species transport equations; the Volume of Fluid (VOF) methodology has been utilized to capture the liquid-gas interface using an adaptive local grid refinement technique aiming to minimize the computational cost and achieve high resolution at the liquid-gas interface region. A local evaporation rate model independent of the interface shape is further utilized by using the local vapor concentration gradient on the droplet-gas interface and assuming saturation thermodynamic conditions. The model results are compared against experimental data for suspended droplet evaporation at ambient air cross flow including single- and multi-component droplets as well as experiments for non-convective conditions. It is proved that the detailed evaporation process under atmospheric pressure conditions can be accurately predicted for the wide range of ambient temperature conditions investigated.

Spatial distribution and temporal variability of stable water isotopes in a large and shallow lake

ABSTRACTStable isotopic compositions of lake water provide additional information on hydrological, meteorological and paleoclimate processes. In this study, lake water isotopic compositions were measured for more than three years in Lake Taihu, a large and shallow lake in southern China, to investigate the isotopic spatial and seasonal variations. The results indicated that (1) the whole-lake mean δ2H and δ18O values of the lake water varied seasonally from −48.4 ± 5.8 to −25.1 ± 3.2 ‰ and from −6.5 ± 0.9 to −3.5 ± 0.8 ‰, respectively, (2) the spatial pattern of the lake water isotopic compositions was controlled by the direction of water flow and not by local evaporation rate, and (3) using a one-site isotopic measurement to represent the whole-lake mean may result in unreasonable estimates of the isotopic composition of lake evaporation and the lake water residence time in poorly mixed lakes. The original data, documented here as an online supplement, provides a good reference for testing sensitivity of lake water budget to various isotopic sampling strategies. We propose that detailed spatial measurement of lake water isotopic compositions provides a good proxy for water movement and pollutant and alga transports, especially over big lakes.

Reaction Engineering Approach (REA) to Modeling Drying Problems: Recent Development and Implementations

Among the drying models available in the literature, the REA model (which was first proposed in 1996) is semi-empirical. It was described based upon a basic physical chemistry principle. The “extraction of water from moist material” is signified by applying the activation energy concept. The single expression of the extraction rate represents the competition between evaporation and condensation. It also encompasses the internal specific surface area and mass transfer coefficient, and thus is linked to material characteristics. The REA can be classified into two categories—Lumped (L) REA and Spatial (S) REA—which can be used to deal with drying a material as a whole or considering the local phenomena within the material, respectively. Both models have been proven to be very effective. The REA is effective for generating parameters since only one accurate drying run is required to establish the relative activation energy function. Both internal and external resistances are modeled by the REA. In its lumped format, the REA is employed to describe the global drying rate, while in the S-REA, the REA is used to model the local evaporation rate. This article covers fundamentals of the REA which have not been fully explained, as well as the most recent development and applications. The application of the S-REA as a non-equilibrium multiphase model is highlighted.