Evaporation Of Water Droplets Research Articles

Integrating hydrologic modeling and satellite remote sensing to assess the performance of sprinkler irrigation

Integrating hydrologic modeling and satellite remote sensing to assess the performance of sprinkler irrigation

THEORETICAL PERFORMANCE ASSESSMENT OF A PARABOLIC TROUGH HUMIDIFYING SOLAR COLLECTOR-BASED SOLAR STILL

In this paper, a parabolic trough humidifying solar collector-based solar still (PHSC-SS) is proposed. Its purpose is to apply some important performance improvement techniques to the flat plate humidifying solar collector-based solar still (flat plate HSC-SS), to significantly improve overall system performance. These included the use of parabolic trough solar concentrators and the design of humidifying solar collectors from evacuated tube collectors. The results reveal that, unlike flat plate HSC-SS, which must operate with a turbulent airflow regime to achieve optimum overall performance, PHSC-SS must operate with a laminar airflow regime and high inlet and outlet temperatures of air (at least 55 °C and less than 100 °C, at atmospheric pressure) in the heat collector element. For 900 W/m2 of incident solar irradiance, 2 m2 of solar collector area, and 0,00042 kg/s of air flow rate, the maximum energy efficiency, exergy efficiency and daily freshwater productivity of PHSC-SS were found to be 68,12%, 14,87% and 1,697 kg/h, respectively. Whereas for the same incident solar irradiance and solar collector area, and 0,1 kg/s of air flow rate, those of the flat plat HSC-SS were 72,9%, 1,12%, and between 1,07 – 2,923 kg/h (for inlet and outlet temperatures of air less than 30 °C, at atmospheric pressure), respectively. Although in some extreme cases freshwater productivity of flat plate HSC-SS can be higher than that of PHSC-SS, it should be noted that laminar airflow regime confers great advantages to PHSC-SS. These are higher air temperatures at condenser inlet (which ease water condensation process), no need of an auxiliary cooling device (needed in the flat plate HSC-SS), less mechanical vibrations of system, reduced condenser size, and less energy consumed by air blowers. Furthermore, the upper limit of the PHSC-SS is a PHSC-SS that operates without air flow, but rather by vaporization of water droplets at boiling point from absorber, followed by their suction to condenser, similarly to a flash evaporation.

Water droplet evaporation in varied gravity and electric fields

Sessile water droplet evaporation in varied gravity and electric fields has been experimentally studied. Specifically, the influences of gravity and electric fields are investigated in the context of the heat flux distribution beneath the droplets, as well as the droplet mechanics and resulting shapes. Experimental testing was carried out during a European Space Agency (ESA) Parabolic Flight Campaign (PFC 66). The droplets tested evaporated with a pinned contact line, a single wettability condition, and varied droplet volume and substrate heat flux. The peak heat transfer was located at the contact line for all cases. The peak heat flux, average heat flux, and droplet evaporation rate were shown to vary strongly with gravity, with higher values noted for hypergravity conditions and lower values in microgravity conditions. The droplet thermal inertia was shown to play a significant role, with larger droplets taking more time to reach thermal equilibrium during the parabolic testing period. No significant impact of the electric field on the droplet evaporation was noted for these test conditions.

Construction of an Au/g-C3N4/GO/Au substrate by a layer-by-layer assembly approach for surface-enhanced Raman spectroscopy

In this work, an Au/g-C3N4/GO/Au hybrid nanofilm for surface-enhanced Raman scattering (SERS) substrate application was prepared by a facile layer-by-layer assembly technique using nanosheet-structured graphitic carbon nitride (g-C3N4), graphene oxide (GO) and 40 nm-gold nanoparticles (Au NPs). The effects of composition, assembly sequence and assembly cycles on the SERS performance were optimized to achieve the SERS-active substrate. The detection limit of Au/g-C3N4/GO/Au substrate for crystal violet and rhodamine 6 G probe molecules reached as high as 10−8 M, with good stability and uniformity. Its further application in food colorants detection (erythrosine, carmine and temptation red) had successfully met the national food safety standards and the quantitative detection for erythrosine and temptation red had also been established. Based on finite-difference time-domain element simulation and density functional theory quantum chemical calculation, the mechanism of the SERS effect of the substrate was unveiled, which is attributed to the combination of electromagnetic enhancement effect of Au nanoparticle aggregates and chemical enhancement effect of GO and g-C3N4. Notably, water droplet evaporation induced Raman enhancement was observed on the Au/g-C3N4/GO/Au substrate. This can be ascribed to the inhomogeneous evaporation of the droplet that promotes the enrichment of the probe molecules into the center of the droplet. This novel enhancement strategy provides a new idea for further improving detection sensitivity of SERS substrate materials.

Effects of liquid water injection on flame surface topology and propagation characteristics in spray flames: A direct numerical simulation analysis

The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as they approach the flame even though some droplets can survive until the burnt gas side is reached, whereas water droplets do not significantly evaporate ahead of the flame and the evaporation of water droplets starts to take place in the reaction zone and is completed within the burnt gas. However, the gaseous-phase combustion occurs predominantly in fuel–lean mode although the overall equivalence ratio remains equal to unity. The water injection has been found to suppress the fuel droplet-induced flame wrinkling of the progress variable isosurface under the laminar condition, and this effect is particularly strong for small water droplets. However, turbulence-induced flame wrinkling masks these effects, and thus, water injection does not have any significant impact on flame wrinkling for the turbulent cases considered here. The higher rate of evaporation and the associated high latent heat extraction for smaller water droplets induce stronger cooling effects, which weakens the effects of chemical reaction. This is reflected in the decrease in the mean values of density-weighted displacement speed with decreasing water droplet diameter. The weakening of flame wrinkling as a result of injection of small water droplets is explained through the curvature dependence of the density-weighted displacement speed. The combined influence of cooling induced by the latent heat extraction of water droplets and flame surface flattening leads to a decrease in volume-integrated burning rate with decreasing water droplet diameter in the laminar cases, whereas the cooling effects are primarily responsible for the drop in burning rate with decreasing water droplet diameter in the turbulent cases.

Study on the mechanism of sessile droplets evaporation enhanced by the electric field

The evaporation characteristics of a sessile droplet under a direct current electric field were numerically investigated, and a numerical model was developed based on the arbitrary Lagrange-Euler method, combining the flow, heat transfer, vapour transport, and electrostatic equations. The simulations were validated to be in good agreement with previously reported experiments. The results demonstrate that the electric field can enhance the evaporation of the sessile droplet with different evaporative cooling numbers (initial electric field intensity E0 = 280 kV·m−1; 32.9% decrease in the evaporation time for water) by the following three methods: increasing the surface area of the droplet evaporation (2.6%) through the electric-field-induced deformation, improving the heat transfer efficiency between the droplet and substrate (droplet average temperature increases by 2.3 K) through the internal vortex, and by enhancing the shearing effect on the vapour layer (450.3% increase in vapour diffusion distance dc) through the external vortex. Additionally, the evaporation of a sessile water droplet undergoing thermocapillary convection is further enhanced under the electric field (E0 = 280 kV·m−1; 20.0% decrease in the evaporation time). As the inhomogeneous temperature distribution inside the droplet was eliminated by thermocapillary convection, the evaporation efficiency was dominated by the increased surface area of the droplet evaporation and the enhanced shearing effect on the vapour layer. These results will provide a better understanding of the electro-hydrodynamic enhanced droplet evaporation.

(Digital Presentation) Porous Silicon Formed by Electrochemical, Chemical and Combined Etching

This work is devoted the thermophysical characteristics (TPC) of PS formed by Electrochemical, Metal-assisted chemical and combined etching methods. The TPC of PS/MACE+EC formed by the MACE+EC– method is higher than those of the PS/EC and PS/MACE samples formed by the EC and MACE methods. The energy activity of water splitting on the surface of PS/MACE+EC is higher than the energy activity of the PS/EC and PS/MACE samples. The combined (MACE+EC) method is the best method for obtaining PS, which has the highest energy activity in water splitting processes. The activation energy of water splitting processes Ea(PS/MACE+EC) is higher than the energy activity of the Ea(PS/EC) and Ea(PS/MACE) samples. The presence of Ni particles in the pores of PS<Ni> stabilizes the them TPC. The TPC of PS were studied in comparison with the TPC of crystalline (c-Si) and powdered silicon (powder-Si). It is known that PS has a high catalytic activity of hydrogen evolution during photoelectrochemical splitting of water. For example, the use of a surface enriched Ni layer on the surface of a PS photocathode PS<Ni> made it possible to increase its catalytic activity two times higher than the catalytic activity of the Pt- electrode in the reaction of hydrogen evolution [1]. The TPC of PS samples formed by EC, MACE and combined (MACE+EC)- etching methods are studied in comparison with the TPC of c-Si and powder-Si samples, Table. Method Electrolyte Composition Marked I Et1 HF:H2O2=1:1 EC II Et2 HF:H2O2: Ni(NO3)2 =1:1:1 МАСЕ III Et3 HF:H2O2: Ni(NO3)2 =1:1:1 EC+МАСЕ According of electron microscopy research the highest porosity is observed on the PS/MACE+EC samples, the smallest - on the PS/MACE sample formed by the MACE- method. The Raman spectra testify this, PS/MACE+EC(Et3) samples have a higher intensity of the Raman peak at a high-frequency shift due to electron-phonon scattering, the intensity and frequency of which depend on the concentration of scattering centers and, consequently, on the conditions for the formation of pores [2]. The TPC of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) spectra record changes in the mass Δm of the sample and heat flux ΔQ with an isobaric temperature change ΔT at the points of T-characteristic. An analysis of the calorimetric TPC and the TGA and DSC spectra shows that c-Si and powder have almost the same patterns of change in thermal conductivity, in contrast to the PS-spectra. The PS/EC, PS/MACE samples have the maximum change in heat flux Q(PS/EC)=187.6 mW, Q(PS/MACE)=231.6 mW, in contrast to powder-Si Q(powder-Si)=107.1 mW and c-Si ΔQ(c-Si)=65.02 mW. This dependence, which is responsible for the heat flux ΔQ change in the high T-range is associated with the maximum developed surface PS. Due to the fact that the presence of the Ni in the pores of PS<Ni>/MACE and PS<Ni>/MACE+EC stabilizes the TPC of PS, and thereby prevents significant changes in the TPC of Ni-containing PS<Ni>, in contrast to PS that does not Ni-contain in pores. Also have carried out experiments on the evaporation of water droplets at room temperature from the surface of c-Si and PS/EC, PS/MACE PS/MACE+EC samples. Experiments on the evaporation of water droplets at Troom from the surface of crystalline and PS samples obtained by various etching methods showed that the highest energy activity is observed in PS/MACE+EC samples, Ea(PS/MACE+EC) = 0.2087 eV, which is much higher than Ea(PS/EC) = 0.070 eV, Ea(PS/MACE) = 0.050 eV and Еа(c-Si) = 0.025 eV,The porous surface of the PS/MACE+EC sample has the highest energy activity compared to the PS/EC and PS/MACE samples, which is due to the high porosity of PS/MACE+EC. The PS/MACE and PS/MACE+EC samples showed high energy stability over 1 year in water evaporation experiments. The values of the heat capacities of various EC-, MACE-, EC+MACE - etching methods are established: - Cp(c -Si) = 20.7 KJ/gK, Cp(powder-Si) = 13.11 KJ/gK, Cp(PS/MACE) = 12.79 KJ/gK, Cp(PS/AE) = 5.62 KJ/gK.Combination of electrochemical etching with Ni-stimulated and chemical etching (EC+MACE) makes the possibility Ni-containing nanoporous silicon with high energy activity in water splitting processes: - Еа(PS/MACE+EC) = 0.2087 eV, Еа(PS/MACE) = 0.070 eV , Еа(PS/АЕ) =0.050 eV.The Ni- presence in the pores stabilizes the TPC of PS/MACE and PS/MACE+EC. The stability of Ni-containing samples PS/MACE+EC) and Ea(PS/MACE) is maintained for one year. [1] K.B. Tynyshtykbayev et al. Russian Microelectronics, 2016, Vol. 45, Nos. 8–9, pp. 603–612. https://doi.org/10.1134/S106373971608014X[2] K. Tynyshtykbayev et al. ECS Journal of Solid State Science and Technology, 2021 10 013009 DOI: 10.1149/2162-8777/abdd86

Heat and Mass Transfer Processes and Evaporation of a Liquid Droplet on a Structured Surface

The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated.

Experimental Investigation of the Water Droplet Evaporation on Inclined Surfaces by Taguchi and ANOVA Optimization Analysis

Abstract We experimentally investigated the evaporation characteristics of a sessile water droplet on a glass substrate with different surface roughness levels. The influence of five parameters is evaluated for the evaporation process: substrate temperature (30 °C, 45 °C, and 60 °C), surface roughness (P0, P600, and P60), droplet volume (3, 5, and 8 µL), water droplets initial temperature (30 °C, 40 °C, and 60 °C), and inclination angle (0 deg, 45 deg, and 75 deg) of the glass substrate. The Taguchi orthogonal array design of L27 is utilized to establish minimal candidate trial points for experimental works, and more trials have been conducted to quantify the effects accurately. Then, analysis of variance (ANOVA) has been used to evaluate the evaporation times for the sessile droplets. The results indicate that evaporation times decrease with rising substrate temperatures, increasing substrate inclination angle, and increasing initial water droplet temperatures. In contrast, evaporation times rise with increasing surface roughness and droplet volumes. After evaluation of the ANOVA analysis, surface roughness levels and droplet volumes are considered the most influential parameters after substrate temperatures, which is the most effective parameter on the evaporation times. On the other hand, initial water droplet temperatures and substrate inclination angle are less effective considering droplet evaporation times. A linear regression fit was derived via ANOVA analysis for the evaporation time, and the best mean deviation was found to be 10% from the experiments. The experimental outcomes were compared to previous research, and correlations were derived. The proposed correlation has given good results considering experimental and literature data.

Application of acoustic levitation for studying convective heat and mass transfer during droplet evaporation

The observation of acoustically levitated droplets offers great potential for studying their evaporation characteristics under well-defined ambient conditions. In contrast to alternatives such as sessile droplets or droplets suspended by a tiny fiber, the key advantage of acoustic levitation is the contact-free nature of the experimental method. However, acoustic streaming effects or droplet deformation by acoustic forces may affect the heat and mass transfer considerably. Consequently, the droplet evaporation process is usually dominated by these intrusive effects of the acoustic field. The main objective of this study is to present an experimental setup, which minimizes the disturbing effects of the levitation technique in order to enable a solid investigation of convective heat and mass transfer during droplet evaporation. To achieve this goal, a high resonance frequency of 100kHz is chosen and the investigation is limited to substantially smaller droplets compared to other studies in the literature. Moreover, an air flow is injected through a hole pattern in the reflector in order to control the relative velocity between the droplet and the gas phase. Particle Shadow Velocimetry is used to characterize the gas flow field in a stepwise approach, initially without any levitated droplet. Subsequently, the vortex structures around non-evaporating droplets are discussed. Eventually, evaporating water droplets are studied. The applied optical diagnostics offer the distinct advantage that the droplet size can be measured simultaneously with the surrounding gas flow field. Hence, the evaporation rate can be evaluated for different combinations of droplet size and relative velocity to the gas phase. For the entire range of resulting Reynolds numbers, the experimentally obtained evaporation rates were found to agree very well with results of a numerical evaporation model when a well-established correlation is used to account for convective heat and mass transfer.

Experimental Investigation and Modeling: Considerations of Simultaneous Surface Steel Droplets’ Evaporation and Corrosion

The present work focuses on the problem of steel surface corrosion as a kinetic expression when water droplets are repeatedly deposited and evaporated on/from its surface. This process, together with the rainwater film corrosion process, belongs to the theoretical foundations of the problem of atmospheric corrosion. It was considered that the formation of water droplets on surfaces is a random but repetitive process, as well as the fact that experimental and theoretical observations show that the droplet corrosion front of a metal surface is located in its zone circumference. We thus aimed to establish how the corrosion process evolves on a steel plate when many drops are deposited and removed repeatedly. An experimental setup and working procedure were used to obtain data characterizing the simultaneous process of steel surface corrosion and water droplet evaporation. For natural convection conditions with a variable relative humidity and temperature environment, an extensive data set consisting of the dynamics of individual droplet evaporation coupled simultaneously with the corrosion of the steel surface under the droplet was obtained. The mathematical models for evaporation and corrosion under the droplet have the same dynamic transfer surface for water evaporation and oxygen supply in the droplet. An approach for determining this surface depending on the momentary droplet mass was considered. Several simultaneous measurements of evaporation–corrosion dynamics were used to calibrate the coupled models, which were then used to show their compatibility with experimental data.

Independent microscale sensing of phase interface and surface temperature during droplet evaporation

Despite the important and pervasive nature of evaporative heat transfer phenomena, fundamental questions still remain about the microscopic processes that occur in and around individual droplets on a surface. In order to understand the underlying physics behind evaporative heat transfer, it is critical to have information at the individual droplet level regarding the surface temperature distribution with time as well as the location and speed of the moving multiphase contact line (MCL). In this work, a multifunctional microscale device comprised of a resistance heater, an array of spatially distributed thin-film resistance temperature detectors, and a phase interface sensing capacitance micro-sensor array has been utilized to measure the local heat transfer characteristics and MCL behavior simultaneously for the evaporation of individual sessile water droplets on a horizontal heated surface. The resistance- and capacitance-based operating principles of the micro-device means that it is capable of detecting temperature changes and tracking MCL at the microscale in real time even for applications with limited or no visibility such as within thermal management hardware or processing equipment. Importantly, having knowledge of the MCL’s location and speed with microscale precision allows for its influence on surface temperature and heat transfer to be directly studied rather than inferred. Results show that the MCL passage precedes the change in local surface temperature and the duration of the time difference between these events depends on the MCL’s speed. In addition, the passage of the MCL accounts for more than 70% of the overall temperature change during the evaporation process.

Evaporation of water droplets and nucleation at the interface

Evaporation of water droplets and nucleation at the interface

High solar water droplet evaporation rates from Actinocyclus sp. diatom frustules decorated with silver nanoparticles

Solar vapour generation technology has emerged as a promising approach for reducing the global problem of water shortage issues via the solar water interfacial evaporation technique utilised for water purification. However, achieving superior light absorption and a high evaporation rate in a solar evaporator remains challenging. Here, we show that high solar interfacial water evaporation rates can be achieved using a natural and sustainable biomaterial, diatom frustules, decorated with silver nanoparticles made using a one-pot synthesis method. The species of diatom selected for this study was Actinocyclus sp. that has complex nano- and micro-structured surfaces and air pores that lead to multiple light scattering. The absorptance before and after silver nanoparticle coating was measured after creating a thin frustule layer on a hydrophobically treated quartz substrate. It was found that the cleaned and uncoated frustules had zero absorptance from 400 nm to 1200 nm. However, silver nanoparticles coated frustules showed a large broadband absorptance up to 90 % from 400 nm to 1200 nm, covering most of the solar spectrum, due to localised surface plasma resonance and multiple light scattering in the air pores. The frustules were floated on the surface of a water droplet for solar water interfacial evaporation rate measurements. A high evaporation rate of 5.4 kg m−2 h−1 under 1 sun irradiation was found for silver nanoparticles decorated frustules and for a droplet volume of 10 mm3. This is significantly larger than that found for frustules without silver nanoparticles or bare water where the evaporation rates were 1.7 kg m−2 h−1 and 1.2 kg m−2 h−1, respectively. Moreover, increasing the number of floating frustules per drop resulted in a factor of 1.67 enhancement. Our results show that research into silver nanoparticle decorated frustules can potentially provide a novel route for fabricating a high-performance solar vapour generator.

Investigation of sessile droplet evaporation using a transient two-step moving mesh model

The evaporation of droplets on surfaces is a ubiquitous phenomenon essential in nature and industrial applications ranging from thermal management of electronics to self-assembly-based fabrication. In this study, water droplet evaporation on a thin quartz substrate is analyzed using an unsteady two-step arbitrary Lagrangian-Eulerian (ALE) moving mesh model, wherein the evaporation process is simulated during the constant contact radius (CCR) and contact angle (CCA) modes. The numerical model considers mass transfer in the gas domain, flow in the liquid and gas domains, and heat transfer in the solid, liquid, and gas domains. Besides, the model also accounts for interfacial force balance, including thermocapillary stresses, to obtain the instantaneous droplet shape. Experiments involving droplet evaporation on unheated quartz substrates agree with model predictions of contact radius, contact angle, and droplet volume. Model results indicating temperature and velocity distribution across an evaporating water droplet show that the lowest temperatures are at the liquid-gas interface, and a single vortex exists for the predominant duration of the droplet's lifetime. The temperature of the unheated substrate is also significantly reduced due to evaporative cooling. The interfacial evaporation flux distribution, which depends on heat transfer across the droplet and advection in the surrounding medium, shows the highest values near the three-phase contact line. In addition, the model also predicts evaporation dynamics when the substrate is heated and exposed to different advection conditions. Generally, higher evaporation rates result from higher substrate heating and advection rates. However, substrate heating and advection in the surrounding gas have minimal effects on the relative durations of CCR and CCA modes for a given receding contact angle. Specifically, in this case, a 40× increase in substrate heating rate or 7.5× increase in gas velocity can only change these relative durations by 3%. This study also highlights the importance of surface wettability, which affects evaporation dynamics for all the conditions explored by the numerical model.

The theoretical approach of how to predict the critical rotational speed of the rotating nozzle in flashing purification to increase the evaporation rate

We developed a new flashing method in this study to improve the evaporation rate in flashing purification. In this case, a stationary nozzle is replaced by a rotating nozzle during the flashing process. The use of a rotating nozzle may distribute water droplets in previously unreachable spaces; the more droplets, the greater the evaporation rate. The use of a rotating nozzle during flashing causes problems if the rotational speed causes the water droplets to collide. To avoid a collision, a rotating nozzle must rotate at an appropriate rate. The kinematics of drifting water droplets after being sprayed by a rotating nozzle can be used to calculate this speed. It could be calculated by periodically observing droplet movement to determine the critical rotational speed of a rotating nozzle. By studying the trajectories of water droplet movement after being sprayed by a rotating nozzle, we can obtain no droplet collision circumstances. Water droplet evaporation during drifting reduces droplet size, resulting in a lower nozzle rotational speed to avoid a collision. For a water particle diameter of 153.5 µm, the smallest nozzle rotational speed predicted is 85.4 rpm. A droplet with a diameter of 80 µm produces 82 rpm of nozzle rotational speed, while a droplet with a diameter of 40 µm produces 80 rpm of nozzle rotational speed. Evaporation may reduce the size of the water droplets during the flashing process. To avoid droplet collision and increase evaporation rate, finer water droplets require a lower nozzle rotational speed.

Numerical simulation on gas-droplet flow characteristics and spray evaporation process in CFB-FGD tower

Nozzle arrangement in the nozzle spray system has a significant impact on the gas-droplet flow characteristics and the temperature distribution within the circulating fluidized bed flue gas desulphurization (CFB-FGD) tower, which is critical to the SO2 removal efficiency. The effects of spray direction, nozzle number and nozzle spray angle on gas-droplet distribution and temperature distribution inside the FGD tower are investigated with numerical simulation based on a Eulerian-Lagrangian mathematical model. An optimal nozzle arrangement scheme is proposed to improve the contact between gas and water droplets and the flue gas temperature distribution. Results show that upward spray direction is beneficial to the interaction between water droplets, improving gas-droplet flow characteristics and spray evaporation process, and water droplets number trapped by tower wall could be reduced in the water droplets evaporation. With the increase in nozzle number, it is conducive to the contact between flue gas and water droplets to increase the evaporation efficiency of water droplets, as well as the uniformity of temperature distribution inside the tower. With nozzle spray angle increases from 30° to 120°, flue gas velocity decreases, water droplets number trapped by the tower wall increases. The temperature distribution at different cross-section is the most uniform when the nozzle spray angle is 60°.

Evaporation of Suspended Nanofluid (SiO$$_{2}$$/Water) Droplets: Experimental Results and Modelling

The results of experimental studies and modelling of the evaporation of suspended water droplets containing silicon dioxide SiO $$_{2}$$ nanoparticles at mass fractions 0.02 and 0.07 are presented. The experimental results are analysed using the previously developed model for multicomponent droplet heating and evaporation. In this model droplets are assumed to be spherical and the analytical solutions to the heat transfer and species diffusion equations are incorporated into the numerical code. They are used at each timestep of the calculations. Silicon dioxide nanoparticles are considered to be a non-evaporating component. It is demonstrated that both experimental and predicted values of droplet diameters to the power 1.5 decrease almost linearly with time, except at the beginning and the final stages of the evaporation process, and are only weakly affected by the presence of nanoparticles. At the final point in this process, the effect of nanoparticles becomes dominant when their mass fraction at the droplet surface reaches about 40 % and a cenosphere-like structure is formed. Both predicted and observed droplet surface temperatures rapidly decrease during the initial stage of droplet evaporation. After about $$t=100$$ s the predicted surface temperature remains almost constant whilst its experimentally observed values increase with time. This might be related to a decrease in the temperature of ambient air in the vicinity of droplets, not taken into account in the model. Both observed and predicted values of the mass fraction of silicon dioxide at the droplet surfaces are shown to increase with time until they reach about 0.4.

Evaporation of heated droplets at different wetting modes: A decoupled study of diffusive and convective effects

The evaporation of water droplets on the heated substrate exhibits diverse wetting modes such as Constant Contact Angle (CCA) mode, Constant Contact Radius (CCR) mode, and mixed mode. Coupled with multiple driving forces of vapor mass transfer, these wetting modes, especially mixed mode, have complex but unclear effects on droplet evaporation. The present study uses moving-mesh numerical simulation based on Arbitrary Lagrangian-Eulerian method to realize the evaporation of droplets at different wetting modes, and the simulation results of mixed mode evaporation agree well with experiments. The variations of temperature field during droplet evaporation are investigated, and the resemblance among diverse wetting modes can be attributed to the shape and the fluid flow intensity of the droplet. Decoupled analyses are conducted on the two major driving forces of evaporation for heated droplets at different wetting modes: diffusion and convection, and the effect of internal and external flows on the distribution of multiple mass fluxes along the droplet surface is elucidated. During droplet evaporation, CCR mode is more dependent on the diffusive effect, CCA mode is more dependent on the convective effect, and mixed mode lies in between. The present study elucidates the effect of wetting modes on the variation of multiple physical fields for the evaporation of heated droplets, promising in industrial and technological fields such as thermal management, droplet microfluidics, and medical treatment.

The Experimental Study of Evaporation of Water and Nanofluid Droplets on the Surfaces of Materials with Different Thermal Conductivities

The Experimental Study of Evaporation of Water and Nanofluid Droplets on the Surfaces of Materials with Different Thermal Conductivities