Droplet-wall Interaction Research Articles

Enhanced coalescence-induced droplet jumping on superhydrophobic surfaces with stepped structures

The coalescence-induced droplet jumping is a self-propelled water removal phenomenon on superhydrophobic surfaces, which has attracted considerable attention due to its potential in a wide range of applications such as self-cleaning and anti-icing/frosting. Improving the energy conversion efficiency, from the excessive surface energy to the kinetic energy, is pivotal to facilitate droplet jumping. In this study, we numerically investigated the dynamics of droplet coalescence on superhydrophobic surfaces with macro-stepped structures, with particular interest in understanding the role of the stepped structure on the droplet jumping process. Three-dimensional simulations were performed by using the lattice Boltzmann method, with the pseudopotential multiphase model and the multiple-relaxation-time collision operator being adopted to achieve high liquid–gas density/viscosity ratios. A wide range of nondimensional height difference of the stepped structure (0–1.5) and droplet radius ratio (0.5–2) was covered. Results show that adding macro-stepped structures can significantly enhance the droplet-wall interaction, thus yielding increased droplet velocity. The enhancement of droplet jumping is more remarkable for droplets of similar sizes, and the dimensionless height difference of the stepped structure is required to exceed a threshold of approximately 0.5. Among the present simulations, the maximum dimensionless droplet jumping velocity reaches 0.66, corresponding to an energy conversion efficiency of 35%. The present findings are helpful for the development of novel superhydrophobic surfaces that pursue efficient droplet removal.

Impact of Surface Roughness on the Impingement of Urea–Water Solution Droplets

The understanding of impingement processes is crucial for optimizing automotive selective catalytic reduction (SCR) systems. An accurate description of this behavior helps design exhaust systems and increases the validity of modeling approaches. A component test bench was set up, featuring a droplet chain generator for producing droplet sizes typically found in the urea–water solution sprays of SCR systems. A heatable impingement plate with an interchangeable surface enabled investigation of the influence of surface roughness. Data were acquired using a high-speed camera and image postprocessing. The droplet–wall interaction could be described using different regimes. An approach to characterizing impingement behavior based on weighted-regime superposition enabled gradual transitions between regimes, instead of step-like changes. It was observed that the surface roughness increased the droplet–solid contact area and generated thermal-induced secondary droplets at lower temperatures. A region of enhanced mechanical disintegration of the droplet was found, caused by peaks of the surface shearing off parts of the droplet. The probability of a droplet rebounding from the wall was reduced on a rough surface, due to the interference of the surface spikes with the droplet’s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface’s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed.

Determination of the single droplet post-impingement pattern on a dry wall: A data-driven approach

Basic droplet-wall interaction such as post-impingement patterns is of great importance since it can quantitatively provide an estimation of energy transformation in spray-wall interaction dynamics. Most existing studies have developed physical-based empirical criteria to determine the droplet-wall post-impingement patterns. However, these approaches are subject to common issues such as scalability and low accuracy. In this study, a data-driven approach utilizing machine learning (ML) classification models is proposed for the identification of droplet-wall post-impingement patterns. An experimental data pool with 1093 observations of a single droplet impinging on a dry wall is established. 16 input features describing liquid properties and wall characteristics are first determined based on domain knowledge. Feature engineering methods are adopted to investigate the most informative and influential features of the splash phenomenon. The selected feature subsets/spaces are then fed into six well-known classification models (i.e., classifiers) individually for the model tuning. The performance of the classifiers is evaluated by metrics such as accuracy, precision, recall, and F1 score. The results show that ML classifiers achieve a good performance in identifying splash cases, providing a higher prediction performance than existing criterion-based approaches. More importantly, this study demonstrates the applicability of ML method on droplet/spray-wall interaction research and proposes a feasible workflow schematic for future studies.

Numerical investigation of AdBlue film formation and NH3 conversion in generic SCR system using Eulerian stochastic fields method

In this paper, the Eulerian stochastic fields (ESF) method in a LES framework is applied to a generic selective catalytic reduction (SCR) configuration in order to retrieve seamlessly the effect of turbulence–chemistry interaction on the NH3-conversion and AdBlue film formation. The ESF method is based on the transport equation of a joint scalar filtered density function. The injection of AdBlue and relevant spray dynamics are modeled by the Lagrange-particle description where a computational parcel represents a cluster of real AdBlue droplets. Owing to the low injection pressure, weak atomization of AdBlue leads to intense droplet wall-impingement and film formation on the SCR duct wall and/or on the mixer elements, if present. A 2-D thin film approach is adopted to model the film formation and dynamics in combination with multi-regimes droplet–wall interaction model. After verification tasks against the reference data for simple gas phase reactive cases, the verified ESF method is coupled with a one-equation based LES turbulence model together with the Lagrange particle-tracking which is combined with the 2-D thin film approach to simulate the AdBlue injection and the film formation in the generic SCR configuration. At first, the assessment of the adopted LES mesh is carried out in terms of the so-called LES quality of index to determine the optimal mesh resolution. Next, to ensure the convergence with respect to the number of required ESF and mesh resolution, SCR simulations are performed by using various (2, 4, 6, 8, 12 and 16) stochastic fields and also with refined mesh. Thereby, the sensitivity of the predictive capability of the numerical tool is evaluated for the production of NH3 and HNCO with and without ESFs. This clearly indicates the importance of an accurate description of the turbulence–chemistry interaction to retrieve reliably the conversion of NH3. Finally, the film dynamics especially the evolution of the film thickness described with six required ESF and without ESF is compared with experimental data from the generic SCR configurations. To further demonstrate the potential of suggested numerical approach, a detailed numerical analysis is provided in terms of spray-impingement dynamics, scalar uniformity index, droplet life time and characteristics of the droplets prone to form solid deposits in the monolith channel.

Study on the adhesion behaviour of oil droplets in water on solid surfaces with different wettability and inclination

AbstractMicroscopic adhesion of oil droplets is a hot research topic within multiphase flow systems in the petrochemical field in recent years, and it is also the key to revealing the mechanism of oil droplet–wall interaction. In this paper, a high‐speed camera was used to capture the dynamic process of oil droplets impacting stainless steel, polyethylene, brass, and other materials in water, deeply analyze the influence mechanism of material wettability, droplet size, and inclination on adhesion behaviour in this process, and draw the Re~θeq phase diagram. The experimental results show that properly enhancing the lipophilicity of the material can prolong the drainage time and effectively inhibit adhesion. Too much lipophilicity can dramatically shorten the drainage time instead, which easily leads to the rapid adhesion of oil droplets. An increase in droplet size can also prolong the drainage time; at Re ≥46.31, the excessive initial kinetic energy is consumed in the form of oil droplet rebound, which is not conducive to adhesion, and the more lipophilic the material is, the more significant the rebound effect. In addition, although an increase in the inclination can inhibit oil droplet bounce off, it also decreases the wall restraint to oil droplets, resulting in lateral and normal displacements, which greatly increases the drainage time. The findings can facilitate the understanding of oil droplet–wall interaction and provide a scientific basis for the design and development of efficient separation equipment and the optimization of the low‐temperature gathering and transportation of high water‐bearing crude oil.

Dynamic response of turbine blade considering a droplet-wall interaction in wet steam region

The high-speed impact of liquid droplets on blades in wet steam region seriously affects the safe operation of steam turbine. For blade stability and response characteristics, most studies utilized ideal gas as working medium and few studies focused on the role of droplets in the flow field. In this study, both the distribution information of droplet particles in the flow field and the impact of droplet particles on blades were comprehensively considered. First, the droplet distribution at the last stage inlet was calculated, and then the droplet-wall interaction model was determined and loaded into calculation process. The pressure distribution on blade surface, blade force variation, mechanical properties, and further dynamic response characteristics of blade were analyzed with ideal gas and wet steam models, respectively, by using one-/two-way fluid-structure interaction. Results show that the blade displacement of leading and trailing edges under wet steam model reaches the maximum at near 80% blade height, and there is a low frequency amplitude corresponding to 460 Hz–470 Hz with the peak value in the range of 22Pa–26Pa. The variations of the maximum displacement and maximum equivalent stress of the blade with time are more intense under wet steam model compared with those under ideal gas model.

Combined experimental and numerical study on fuel deposition characteristics of an impinging PFI spray with different impingement geometries

This paper presents a combined experimental and numerical study on the fuel deposition characteristics during the Port fuel injection (PFI) spray impingement process. The numerical models mainly consisted of a newly developed atomization model, the Senda spray impingement model, and the Bai-Gosman liquid film model. A group of measurement methods were used to hierarchically calibrate and validate the numerical models. It was found that the calibrated models were able to predict the time-resolved morphology of the impinging spray, as well as the shape, area, and thickness distribution of the deposited fuel film with a good reliability. Then the calibrated models were used to study the effects of impingement distance ( Lw) and impingement angle ( θw) on the fuel film deposition. The results showed that as Lw increases, the film area ( Af) and film mass ( mf) rise faster and reach higher final values. As θw increases, Af and mf grow slower, and their final values are hugely reduced. Increasing Lw and decreasing θw can effectively reduce the average film thickness ( hf,a). Af is mainly affected by the contact area between the spray plumes and the impinging wall, while mf is also affected by the dominant droplet-wall interaction regime.

Aerodynamic performance investigation of an axial flow compressor under water ingestion

This paper presents numerical simulations of the effects of water ingestion in the axial flow compressor. A self-compiled droplet-wall interaction model is used to study the impingement of the droplets and the film fragmenting process on the surfaces of the compressor blades, which also included the splashing effects. The blade profile has been divided into small regions to effectively apply the rough wall model to capture the film-related flow losses, which has shown flow characteristics with better accuracy. The formation of water film, its shedding, the size, and the movement of droplets after film peeling have been investigated using unsteady simulations at different time intervals. The increased loss of characteristics, shortening of the operating range, and change of flow velocity angle were observed with increasing water content. Moreover, the current research work shown that an increase in the amount of water has severe aerodynamic effects that caused the shock waves to move an upstream location with an increase in intensity. A higher blockage is generated in the tip section flow as the amount of the water increased. Tip losses were very dominant as water injection rate increased, resulting an additional energy loss and performance degradation.

The spray distribution of a liquid jet in supersonic crossflow in the near-wall region

The gas–liquid mixing process of a liquid jet in supersonic crossflow with a gas–liquid momentum ratio of 7.67 in the near-wall region is investigated numerically. The two-phase flow large eddy simulation is based on the Eulerian–Lagrangian approach and considers the droplet–wall interaction. The results indicate the penetration depth and the lateral extension width, which are in good agreement with the experimental data. The Ω shape, especially the spray foot structure of spray in the cross-sectional plane, is captured well. The transport process of spray toward the wall and the formation of spray foot are systematically studied. Under the influence of the upper CVP (counter-rotating vortex pair), partial droplets in the center region of the spray are transported to the near-wall region and move toward both sides when encountering the wall CVP. Under the current gas–liquid momentum ratio, droplets collide with the wall mainly in the central region at the bottom, which will produce splashed droplets. Affected by the horseshoe vortex, the instantaneous distribution of droplets on both sides near the wall shows stripes shape. The spray foot structure forms the shape that is narrow on the top and wide on the bottom and is mainly formed by splashed droplets. Some splashed droplets in the low-speed boundary layer constitute the lower half of the spray foot; meanwhile, some splashed droplets enter mainstream and constitute the upper half of the spray foot. Moreover, the spray is mainly distributed in the core region, and the spray concentration is very sparse in the spray foot region.

Dynamical interaction between a droplet and a wall heated beyond the Leidenfrost temperature

During the reflood of the reactor core after a large break loss of coolant accident (LOCA), the fuel cladding temperature exceeds the Leidenfrost temperature, hindering the rewetting of the fuel rods. Under this condition, droplet dynamics becomes important to restrict the temperature rise and prevent damages to the fuel rods. In this work, a simple model is proposed to study the dynamic interaction between a droplet and a wall heated beyond the Leidenfrost temperature, with an improved lumped approach adopted for the transient heat transfer and a modified lubrication approximation for the vapor flow. The model was compared favorably with experimental data from the literature and used to study the influence of key parameters of the droplet-wall interaction. A heat removal of 69.7 mJ per droplet was obtained under typical reflood conditions, confirming that droplets contribute to heat removal by superheated vapor in the order of one tenth.

A Joint Experimental and Large Eddy Simulation Characterization of the Liquid Fuel Spray in a Swirl Injector

Abstract The quality of liquid fuel spray injection determines to a large extent the performance of aero-engine combustors. This investigation focuses on the detailed characterization of the liquid fuel spray in a test rig targeted at aero-engine applications. The liquid fuel is injected as a hollow cone by a simplex atomizer and the injector comprises a radial swirler. Two features of the droplet distribution are less commonly found. First, the distributions of droplet diameters exhibit nonaxisymmetric patterns, which are investigated for three types of swirlers. Second, it is found that the size-conditioned velocity distributions feature a single wide peak for small droplets and become bimodal for the largest droplets, with a first peak at low velocities, and a second one at higher velocities. Experiments are complemented with large eddy simulations and Lagrangian particle tracking. The spray interacts with the lateral injector surface and requires a droplet–wall interaction model for the liquid film. Simulations do not retrieve the lack of rotational symmetry that is found experimentally indicating that this feature is not linked to the nature of the flow. This is also consistent with further experiments with a different atomizer confirming that this is due to imperfections in the atomizer geometry. Another result is that certain swirler designs are more robust to atomizer imperfections. Simulations accounting for the liquid film yield a bimodal distribution for the droplets' axial velocity distribution which is not obtained without this model, indicating that it is important to represent the droplet–wall interaction.

Water soluble polymer added high mass flux spray: A novel approach for the attainment of enhanced heat transfer rate in transition boiling regime

Although in the literature, the polyethylene glycol (PEG) added water dropwise evaporative cooling at very low temperature and single droplet evaporation mechanisms are revealed, the role of polyethylene glycol in spray evaporative quenching at very high initial surface temperature (900°C) has not been revealed so far. According to the reported information, PEG added water droplet evaporation favors high heat flux and therefore, in the current work, it is expected that by performing high mass flux polymer spray cooling at very high initial temperature, heat transfer rate can be augmented. The enhancement mechanism is based on the alteration of the surface properties in the favorable direction of heat transfer. The surface modification is achieved by adding water soluble polymer (Polyethylene Glycol) of different molecular weights (PEG 200 and PEG 600) as additives, which make the surface hydrophilic due to the deposition on the hot plate during evaporation. Before experimentation, the conditions and properties leading to the formation of coating were investigated by analyzing the polymer droplet wall interaction. This study depicts that due to the interaction of polymer with wall and very slow diffusive nature of polymer in water, the deposition of polymer is expected to occur during evaporation. The hydrophilic nature of the surface increases the spreading factor of the impinging droplet and this phenomenon increases heat transfer area, which in turn favors higher heat transfer rate. Spray evaporative cooling study was carried out on a hot AISI 304 steel plate using polymer solution of different concentrations as coolant and the enhancement intensity was identified by comparing with the result of pure water. The comparison illustrates significant enhancement i.e. 30% in CHF and 28% in IHF for 20 ppm concentration of polymer type I. For the verification of heat transfer mechanism and identification of the reason behind enhancement, different dimensionless numbers such as Reynold's number, Weber and Prandtl numbers were analyzed. The surface morphology of heat treated steel plate clearly indicates the deposition of the polymer over surface of the plate and the characteristics of the deposition confirm the superhydrophilic nature. In comparison with the literature, the developed coating on the surface of the plate is thermally, mechanically and chemically more stable.

A numerical–experimental assessment of wall impingement models for spark-ignition direct-injection engines

Liquid wall impingement in direct-injection engines can cause soot and hydrocarbon emissions as well as reduced combustion efficiency. This study focuses on detailed evaluations of numerical droplet impingement criteria that govern the onset of splash. The five selected splash criteria, which all extrapolate from single-droplet impacts to full sprays, are representative of those currently in use for spark-ignition direct-injection engines. The computations examined the sensitivity of impinging spray simulations to the splash criteria for a high-pressure, direct-injection swirl spray under atmospheric conditions impinging at a 45° angle onto a flat plate. The numerical results were compared to an unusually extensive set of experimental data: Mie scattering and light transmission imaging, plus quantitative refractive index matching measurements of the fuel film area and thickness, and phase Doppler interferometry measurements of droplet size and velocity near the plate. Good qualitative and at least fair quantitative agreement was obtained for the global spray impingement and wall film formation, especially for single-drop criteria that include the effect of viscosity. The film area and shape were insensitive to the splash criteria, illustrating the importance of film thickness measurements for validating simulations. The results also revealed the sensitivity of impingement calculations to droplet arrival frequency when that is taken into account. In general, the comparisons indicated the need to capture the effect of multiple droplets impinging on the wall at irregular frequencies in the criterion, as well as other important physics of the droplet–wall interaction that may mask the true effect of the impingement criterion.

Numerical Simulation of Supercooled Large Droplet Icing Phenomenon: A Review

The impingement and ice accretion of supercooled large droplets (SLD) on the aircraft surface is one of the crucial factors threatening flight safety. The movement and impingement of SLD have many unique characteristics that conventional small droplets do not own. Therefore, a large number of experimental and numerical studies about SLD have been carried out to explore its physical properties and simulation method. The distribution and motion characteristics of supercooled large droplet during the process of approaching to the aircraft are first reviewed in this paper. Then the governing equations of SLD under the framework of Lagrangian and Eulerian methods are analyzed and established. The unique phenomena of SLD such as water droplet deformation and breakup, droplet–wall interaction and re-impingement in the literature are analyzed. The research development and results of the droplet–wall interaction phenomenon have been discussed particularly, which is summarized and classified from three aspects: droplet splashing threshold, splashing model and the method of modification of governing equation. Finally, the establishment process and the corresponding modification of the icing model in SLD condition is given, and the related calculation results are exhibited to validate the numerical simulation methods of SLD. Some shortcomings in current research are presented and the aspects needed to be developed further in future studies for the acquisition of more accurate simulated results are also recommended.

Ice accretion and aerodynamic effects on a multi-element airfoil under SLD icing conditions

The impingement behavior of large water droplets, their interactions with the solid wall and the subsequent ice accretion and aerodynamic effects have become a key issue in in-flight aircraft icing. In this study, ice accretion and aerodynamic effects on a multi-element airfoil were investigated under the recently introduced Appendix O icing envelope. Supercooled large droplet (SLD) dynamics were taken into account by employing a unified computational approach. Ice accretion was simulated using a partial differential equation (PDE) based solver, instead of the commonly used control volume method. The numerical solver of the SLD impingement was built on the droplet deformation and droplet–wall interaction splash models. The unified solvers for clean air, large droplet impingement, ice accretion, and the aerodynamic analysis of ice effects—all of which are based on a single unstructured upwind finite volume framework—were first validated using available experimental data and then applied to investigate ice accretion and the resulting aerodynamic effects on multi-element airfoils for various flight conditions and, in particular, near-freezing SLD icing conditions. Interestingly, two counter-intuitive results were found when comparing the ice accretion and associated aerodynamic degradation for non-SLD and SLD cases. Moreover, considering runback ice was shown to be essential in the design of an ice protection system (IPS) for the multi-element wing.

Wetting phenomena and droplet-wall interaction modelling with Smoothed Particle Hydrodynamics approach

Smoothed Particle Hydrodynamics (SPH) is a meshless CFD method, remaining a promising alternative for multiphase flow calculations due to straightforward interface treatment. In the present work we particularly focus on the accurate modelling of wetting phenomena and contact angles. For that purpose we modify the surface tension model in the neighbourhood of the triple line. The necessity of proper boundary conditions in SPH is discussed along the preliminary validation of proposed approach. Results show a great potential of the chosen method.

Numerical simulation of ice accretion in supercooled large droplet conditions

The simulated methods for ice accretion on two-dimensional airfoil surface are established completely under the Eulerian framework in supercooled large droplet (SLD) conditions. The two-dimensional code to solve the partial differential equations (PDEs) of droplet phase is derived to simulate the impingement characteristic of SLD. Also, several semi-empirical models which explain the droplet-wall interaction are compared and discussed to show respective features when simulating the splashing phenomenon. In particular, a new boundary condition for wall called penetrable wall for splashing droplet (PWSD) is proposed to deal with the impingement of SLD on solid surface, which efficiently improves the accuracy of simulation. Then the improved impingement characteristic of SLD is input into the extended mass and heat transfer model to simulate the ice growth on airfoil surface. The multistep advanced method is carried out to better match the physical phenomenon of ice growth. At last, the simulated results of critical parameters: local droplet collection efficiency and the height of ice growth are compared with the experimental data which verify the applicability of proposed models.

Investigation on the oil–gas separation efficiency considering oil droplets breakup and collision in a swirling flow

In this study, the oil droplets separation process in a swirling flow was investigated using numerical simulations and verification experiments. Numerical models were established to investigate the effects that the collision and breakup of droplets and the collision between droplets and wall on separation efficiency, respectively. The separation efficiencies were calculated using different models under various inlet velocities, from 10 to 18m/s. The simulated results showed that droplets colliding could improve the separation efficiencies with up to 22.4% increments at the 10m/s inlet velocity for droplets with a diameter of 10μm. The oil–gas separation efficiency was reduced up to 17.4% due to the oil droplet–wall interaction in swirling flow when the simulation model involved droplet–wall collisions. The simulated separation efficiencies showed little change when oil droplet breakups were considered in the model under different inlet velocities and droplet diameters. A test rig was built to validate the simulation results. It showed good agreement between the experimental data and simulated results. Thus, the numerical simulation model could be used to predict separation performance and understand the separation process in a swirling flow.

A two-dimensional splashing model for investigating impingement characteristics of supercooled large droplets

In this article, a two-dimensional (2D) splashing model is proposed to investigate the dynamics when supercooled large droplets (SLD) impinging on a wall surface in the aircraft-icing field. Energy conservation for droplet motion and impingement is used to capture the properties of the splashed droplets. A new statistical treatment of the droplet impinging energy and angle during the droplet-wall interaction is introduced in order to calculate the average dynamics of the SLD within a micro-control volume on wall surface. Based on the LEWICE predictions of droplet collection efficiencies and the available experimental ones, a new criterion for droplet splashing/deposition as well as a new formulation for the splashed mass is suggested. Lagrangian approach is adopted to describe the movement and impingement of SLD. The proposed model together with the previously developed droplet tracking method (DTM) for calculating droplet collection efficiency with the effect of droplet reimpingement constitute a relatively complete predicting approach of SLD impingement characteristics. Comparisons between the current predictions and the experimental observations, including SLD impingement over clean and contaminated airfoil surfaces as well as shapes of ice accretion in typical icing conditions, are carried out. Further, results obtained with the LEWICE splashing model are also plotted on the same graphs in order to assess the accuracy of the current splashing model in predicting SLD impingement. Results show that good agreement is achieved between the current predictions, including SLD impingement and ice accretion shapes, and the experimental ones. The predictions of the impingement distribution over contaminated surfaces obtained with the current splashing model show a much closer agreement with the experimental results than the ones obtained with LEWICE splashing model. For further investigation of SLD impingement, the properties of the droplet splashing and reimpingement during the ice accretion process are also addressed.

Droplet–wall interaction upon impingement of heavy hydrocarbon droplets on a heated wall

Regime maps that predict the heavy hydrocarbon droplet impingement behavior on a heated wall (Weber number of the impinging droplet v/s wall temperature) are constructed based on CFD simulations using the Volume of Fluid model with the geo-reconstruct scheme. Based on the simulation results, maps are constructed for single-component droplets with a diameter of 50 and of 100µm. The applied CFD model is validated by comparing these with regime maps available in literature, constructed based on experimental data for model liquids and liquid mixtures. The impingement regimes of Splash, Stick, Rebound and Breakup are well-predicted. Two distinct types of Splash (Splash with ligament formation and Splash with ring detachment) are reported for the first time. Using the validated CFD model, regime maps are constructed for multi-component heavy hydrocarbon droplets with a diameter of 50 and of 100µm. Significant differences between the single-component and the multi-component droplet impact behavior are observed. Improved and new correlations for regime transitions, droplet stretching on the wall, droplet rebounding velocity and number of splashed droplets are derived based on energy balances. They are found to correlate well with CFD predictions.