Supercooled Large Droplet Research Articles

A diffusion-enhancing icing theory for the freezing transition of supercooled large water droplet in impact

The freezing behavior of supercooled large droplet(SLD) in impact causes severe threats to civil aircraft. However, the mechanism by which the impact dynamics of SLD accelerates the icing still remains unclear. In this paper the impact and freezing of water droplets with different supercooling on inclined surfaces of different wettability were investigated experimentally and theoretically. Compared to one/two freezing morphologies and linear supercooling-freezing time relation reported previously, four freezing morphologies were observed in different supercoolings, and the transition of freezing morphologies resulted in the piecewise nonlinear supercooling-freezing time relation. By comparing the characteristic times of droplet dynamics and horizontal icing, it was found that the dynamics-icing interaction could lead to higher heat conduction area in higher supercooling. Theoretical analysis shows that the unsteady heat conduction enhances the freezing rate in a short time initially. The coupled effect of large contact area and initial heat flux greatly enhances the diffusion of latent heat in freezing of droplet with high supercooling. Based on this machanism, a diffusion-enhancing icing theory is proposed. Compared with previous icing theories, this theory can give much better predictions about the freezing rate of impinging supercooled water droplets, especially in conditions of high supercooling and hydrophobic surface.

E-MPS法を用いた薄膜を有する壁面に対する水滴衝突による二次液滴の数値シミュレーション

The ice accretion on an aircraft threatens navigation safety because it causes deformation of the wing shapes and reduces the aerodynamics performance. In the glaze ice conditions, where the ambient temperature is about -10 to -3 ℃, impinged droplets do not freeze instantly, and they runback along the wing surface as a liquid film. In addition, splashing and rebounding occur, which generate secondary droplets, if supercooled large droplets (SLD) whose diameters are more than 40 μm impinge on an aircraft. Furthermore, liquid film on an aircraft wing at glaze ice conditions significantly affects these phenomena. The purpose of this study is to model the droplet impact behavior and the formation of secondary droplets on a wall with a thin water film in terms of SLD icing under glaze ice conditions by using the three-dimensional E-MPS method. In a vertical impact case, the crown is formed by the impingement, and no secondary droplet is generated by the finger jets. The crown height decreases with decreasing water temperature, which is attributed to the decrease in Weber number. In an oblique impact case, the total mass of secondary droplets increases as the impact angle decreases. The secondary droplet mass generation attains the maximum at about 39 degrees impact angle. Moreover, the secondary droplets do not appear as the impact angle decreases around 30 degrees, resulting from Kelvin-Helmholtz instability. The velocities of secondary droplets decrease as the secondary droplets are generated later.

Flight Characteristics with Different Supercooled Large Droplet Ice Configurations

AbstractAn evaluation of the effects of different ice configurations on the flight characteristics of a fixed-wing aircraft is presented. Within a joint research project of German Aerospace Center (DLR) and Brazilian aircraft manufacturer Embraer simulation models of three supercooled large droplet (SLD) ice configurations were developed for one Phenom 300 prototype. A specific flight test campaign with artificial SLD ice shapes on the aircraft was conducted to gather the relevant flight data. The simulation models for the different ice configuration were obtained by system identification, and specific results for the SLD-ice simulation models are provided. The analysis of aircraft characteristics was based on the results of these high-quality simulation models: the icing-induced changes of the flight performance characteristics evaluated by the interpretation of several parameters like thrust-to-weight ratio and specific excess power. The typical flight performance degradation was found for all ice configurations with different magnitude. The change of aircraft eigenmodes was investigated in detail by analysing the system matrix of the linearised models at a specific trim points. In addition, the diverse effects found for different ice configurations (App. C and SLD) are discussed and the change of root locus is analysed. Furthermore, ice-induced changes of the handling qualities are evaluated using numerical criteria of flying qualities standard “MIL-STD-1797 A”: no significant deterioration was found for the investigated ice configurations.

Impingement and freezing of a supercooled large droplet on an ice surface

In the present study, we carried out an experimental investigation of the impinging and freezing processes of a supercooled large water droplet on an ice surface. One high speed camera was used to measure the dynamic motions of the water droplet while two charge coupled device (CCD) cameras were adopted to obtain the images of the freezing process and the freezing morphologies, respectively. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were carefully evaluated. The results showed that the subcooling degree of the water droplet had an apparent influence not only on the spreading process but on the freezing morphology as well. When the subcooling temperature of the water droplet was high (e.g., Tw = –3.0 °C), a triangle cross-section profile formed. However, once the subcooling temperature of the water droplet was relatively low (e.g., Tw = –9.0 °C), the final ice morphology consisted of two parts: one part was a cone-like bead in the center while the other part was an ice ring at the periphery. Moreover, at the same water droplet temperature, raising the ice surface temperature led to an increase in the maximum spreading factor.

Icing Condition Predictions Using FBGS

Icing is a hazard which is important for the aerospace industry and which has grown over the last few years. Developing sensors that can detect the existence not only of standard icing conditions with typically small droplet size, but also of Supercooled Large Droplet (SLD) conditions is one of the most important aims in order to minimize icing hazards in the near future. In the present paper a study of the Fiber Bragg Grating Sensors’ (FBGSs) performance as a flight icing detection system that predicts the conditions of an icing cloud is carried out. The test matrix was performed in the INTA Icing Wind Tunnel (IWT) with several icing conditions including SLD. Two optic fibers with 16 FBGS in total were integrated in the lower and upper surface of an airfoil to measure the temperature all over the chord. The results are compared with a Messinger heat and mass balance model and the measurements of the FBGS are used to predict the Liquid Water Content (LWC) and Ice Accretion Rate (IAR). Finally, the results are evaluated and a sensor assessment is made. A good correlation was observed between theoretical calculations and test results obtained with the FBGS in the IWT tests. FBGS proved to detect the beginning and end of ice accretion, LWC and IAR quickly and with good precision.

Effects of surface roughness on splashing characteristics of large droplets with digital inline holographic imaging

Ice accretion on aircraft wings due to Supercooled Large Droplet (SLD) has a significant impact on flight safety. The dynamic behaviours of large droplet, such as deformation, breakup and splashing, have significant effects on icing accretion. The corresponding studies have been carried out experimentally and numerically, and empirical models to predict the dynamic behaviours of large droplets have been proposed. However, few empirical models involve surface roughness effects, especially its effects on the secondary droplets during icing accretion. In this paper, an experimental investigation on the impact of droplets onto surfaces with different roughness was conducted by using the high-speed Digital Inline Holography (DIH). The rapid splashing processes of droplets with Weber number around 570 and Mundo impact parameter K around 240 were captured and reconstructed based on a wavelet-based algorithm. Results show that DIH is suitable to diagnose the secondary droplets with a wide size range and record their velocities and locations. The roughness of the impact surface has a positive effect on the dynamics of splashing droplets, and the parameters including diameter, velocity, splash angle, splash height and mass-loss rate of splashing droplets rise with the increase of roughness. The new empirical correlations of the splashing characteristics of secondary droplets were fitted and developed, which could be used to update the current icing simulation models to take into account the effect of impact surface roughness on large droplets dynamics. This fundamental study may also be applied to the industrial applications including wind turbine icing, inkjets, sprays, as well as the erosion and formation of ice layers.

SPH simulation of supercooled large droplets impacting hydrophobic and superhydrophobic surfaces

Hydrophobic and superhydrophobic coatings are of great interest to the aerospace community as a measure to mitigate performance losses due to ice formation in flight. To numerically investigate the supercooled large droplets (SLDs) impacting such coated surfaces, a multiphase smoothed particle hydrodynamics (SPH) solver is developed by solving the Euler momentum equations for fluid dynamics, the energy equation for heat transfer and a latent heat model for phase changes. A continuum surface tension term is included in the momentum equations and a robust contact angle model that corrects the surface tension near triple-phase points is proposed to calculate the non-wetting effects of surfaces. The present solver is capable of modeling droplets impacting, spreading, retracting on and/or rebounding from hydrophobic/superhydrophobic surfaces, with heat transfer and phase change. It is first validated against the evolution of droplets on solid surfaces of various contact angles and then applied to droplets impacting hydrophobic and superhydrophobic surfaces at different temperatures. This work investigates SLD impingement and solidification on hydrophobic and superhydrophobic coatings and provides a base for the numerical experimentation of a single SLD icing behavior at in-flight speeds.

An improved splashing model for supercooled large droplets based on minimum mass loss ratio

Supercooled large droplets widely exist in clouds, which have poor aerodynamic follow-up and strong dynamic characteristics, making the relevant experiments and numerical simulation more difficult. Splash is the main influencing factor of droplet collection efficiency, and it is easier to be recorded and quantified. In the LEWICE model, little or no splash happened around stagnation point. Focus on this problem, an improved large droplet splashing model is proposed based on the idea of minimum mass loss ratio. In the numerical simulation part, the advantages and disadvantages of the proposed model are analyzed by comparing with several classical splashing models and experimental data. Based on the experimental data of airfoil MS317, the validity of the model is verified. Moreover, airfoil NACA23012 and ARJ cruise configuration are chosen to obtain the droplet collection efficiency, which shows the feasibility of the proposed model.

Super-Cooled Large Droplet Experimental Reproduction, Ice Shape Modeling, and Scaling Method Assessment

Super-cooled large droplets (SLDs) present a unique experimental challenge for conventional horizontal spray icing wind tunnels due to gravity effects on the droplets and coalescence between the particles. Limited ice accretion shapes in this regime exist, making the verification of modeling tools and the validation of the applicability of scaling methods difficult. Furthermore, SLD accretions often occur in nature in a bimodal icing cloud, providing an additional facility challenge related to the reproduction of such clouds. The objective of this research effort is to expand the available SLD dataset to assess the capability of an existent ice accretion tool (LEWICE 2D) to predict such ice shapes. The effort also attempts to further verify the applicability of ice shape scaling methods in the SLD regime. Finally, bimodal SLD cloud reproduction and ice shape prediction is investigated. The Adverse Environment Rotor Test Stand (AERTS) at Penn State was assessed as an alternative facility to icing wind tunnels for ice accretion testing in the SLD regime. Laser diffraction measurement data analysis demonstrated that the icing nozzles used in the facility can produce an SLD within of requested values. Techniques to measure liquid water content (LWC) in the facility are also presented. LWC was controllable within . LEWICE ice shapes predicted were obtained using legacy empirical heat transfer models as well as an updated empirical heat transfer function developed at Penn State. When comparing modeling results to experimental shapes from the literature and the AERTS, LEWICE with the updated heat transfer function provided stagnation thicknesses within 5.6% of experimental values and horn protrusion within 16%. The modified Ruff scaling ice shape method was effective in the SLD regime, providing a mean deviations of 2.5% between reference and scaled ice shape characteristics. Finally, characteristics of a bimodal ice shapes were predicted by LEWICE. When comparing modeling results to experimental shapes, LEWICE with the updated heat transfer function provided predictions within 10.3% of the experimental stagnation thickness measured. It also provided horn angles with discrepancies of less than 12%. Icing limits of a bimodal icing cloud were observed to be that of a single mode SLD cloud of the same MVD, with an overall deviation of when comparing the experimental database to LEWICE.

Large-eddy simulations of complex aerodynamic flows over multi-element iced airfoils

Large-eddy simulations (LESs) of flows over two types of iced airfoils with three multi-elements are performed to investigate the aerodynamic characteristics and complex interactions between flows generated from slat, main, and flap elements. The two iced airfoils are considered under supercooled large droplet (SLD) and non-SLD conditions. A good agreement of the mean properties between our numerical and previous experimental data demonstrates that our LES method provides an accurate solution of the complex flows around iced airfoils., whereas it is not for unsteady Reynolds-averaged Navier–Stokes (URANS) data that is simulated independently. For the iced airfoils under the SLD and non-SLD conditions, the aerodynamic degradation is found compared to that of a clean airfoil because the separation bubbles (SBs) induced by ice accretion change shear layer (SL) trajectory shed from the slat cusp, leading to a severe reduction in mass flow. Furthermore, we show that the flow interactions near the slat gap play a crucial role in determining the flow characteristics on main and flap elements (e.g., flow separation). Although strong flow interactions are observed for the non-SLD case because of the presence of upwind horn-shaped ice, the smaller gap distance of the SLD case leads to a larger lift loss. The unsteady features of SBs on the upper surfaces of the slat and main elements under the non-SLD condition are characterized by the power spectral density (PSD) of the pressure fluctuations with multiple peaks at low and high frequencies.

Experimental statistics of micrometer-sized water droplet deformation and breakup behavior in continuous air jet flow

Fundamental study on breakup of the supercooled large droplet is important for understanding the microphysical phenomenon in aircraft icing. The deformation and breakup processes of water droplets with diameters in the range of 700 to 1000 microns injected into continuous air jet flow are investigated experimentally. Evolutions of the bag, the bag and stamen and the dual breakup morphology are recorded through the high-speed photography. Experiments are repeated 319 times to determine the statistics based dynamic behavior, including the mean deformation rate, velocity and duration. As concluded, minimum value of the droplet deformation rate in the x-direction increases from 0.46 to 0.72 when Weber number increases from 18 to 56. When the initial droplet breaks, the velocity of droplet tip is more than two times of the velocity of droplet terminal. Duration of the deformation stage accounts for about half of the total duration. Besides, a new interpretation of physical mechanism, combining effects of the aerodynamic drag, motion hysteresis and inertia, is proposed to explain formation and development of different deformation and breakup morphologies. This work is meaningful for the development of numerical simulation and theoretical models.

Multiphase SPH Modelling of Supercooled Large Droplets Freezing on Aircraft Surfaces

Solidification is an important issue that needs to be addressed when modelling supercooled large droplets (SLD) impacting aircraft surfaces. This work develops a 3D triple-phase smoothed particle hydrodynamics (SPH) solver that can simultaneously simulate SLD impingement and solidification on cold surfaces by solving the Euler equations for fluid dynamics, the energy equation for heat transfer, and a latent heat model for phase changes. The solver is validated against a Stefan problem in which a liquid is cooled and solidified by a cold boundary, and against a still droplet freezing on a cold plate. It is then applied to droplets impacting on cold surfaces at aeronautical speeds, analysing the freezing time and ice particle distributions as the droplet velocity and impact angle are varied. This is an important step towards a macroscopic numerical model of SLD post-impact behaviour for in-flight icing simulations.

Interferometric laser imaging for droplet sizing method for long range measurements

A recent appendix in the aircraft regulations comprises testing supercooled large droplets impinging on its surfaces. For those tests, the size and distributions of droplets need to be characterized in icing wind tunnels. In this paper, the applicability of implementation of the “Interferometric Laser Imaging for Droplet Sizing” technique inside a wind tunnel with a 3 m × 2 m open elliptical test section has been discussed. Experiments have been conducted in the laboratory for the discussion at object distance of 1.6 m and 2.29 m and droplets diameters between 360 µm and 850 µm. All the streams were previously characterized by means of the shadowgraph imaging technique. A novel approach of the Interferometric Laser Imaging for Droplet Sizing technique where droplets are not fully defocused to avoid excessive overlapping is presented. Two new image processing approaches provide in general good results as compared to previous methods.

Effect of Characteristic Phenomena and Temperature on Super-Cooled Large Droplet Icing on NACA0012 Airfoil and Axial Fan Blade

Icing simulations involving super-cooled large droplets (SLDs) on a NACA0012 airfoil and a commercial axial fan were performed considering the characteristic behavior of SLD icing (i.e., splash-bounce, deformation, and breakup). The simulations were performed considering weak coupling between flow field and droplet motion. The flow field was computed using the Eulerian method, wherein the droplet motion was simulated via the Lagrangian method. To represent the ice shape, an extended Messinger model was used for thermodynamic computation. The ice shape and collection efficiency of the NACA0012 airfoil derived using the icing simulation exhibited a reasonable agreement with the existing experimental data. The icing simulation results for the axial fan, in terms of distribution of ice on the blade and its influence on the flow field, indicated that flow separation occurred, and the mass flow rate of the flow passage decreased. Moreover, the splash and bounce phenomena considerably influenced the icing process; however, the effect of the deformation and breakup phenomena was negligibly small. In terms of the effect of the SLDs on the icing phenomena, it was noted that, with the decrease in the SLD temperature (from −5 °C to −15 °C), the number of adhering SLDs increased, whereas the number of splashing and bouncing SLDs decreased.

MPI Parallelisation of 3D Multiphase Smoothed Particle Hydrodynamics

This paper addresses the MPI parallelisation of a 3D multiphase smoothed particle hydrodynamics (SPH) solver, focusing on supercooled large droplets (SLD) impingement. SPH uses moving particles to represent fluid flows, demanding particular parallelisation strategies. A cell system provides a spatial reference to generate dynamical neighbour lists, and to pack, send and receive particles. A re-indexing method is introduced to accommodate migrating particles and a sequential particle collecting technique is proposed to reduce the complexity of communications for 3D partitioning. The scalability is tested on up to 1024 processors with 110 million particles, and the effects of particle load on computing speeds are also investigated. The solver is then applied to droplet impingement at various impact speeds and diameters to study the consequent splashing. This work provides a generic approach for highly multi-threaded SPH solvers, allowing massive computations for improved accuracy and/or numerous runs for parametric studies.

Numerical Study of Super-cooled droplet impingement on Aeroengine

In this work, a large bypass ratio engine was taken as the research object. The impingement property of droplets with various diameters including SLD (Super-cooled Large Droplet) was obtained by Euler method based on Mundo model. The grid models of nose cone, fan, guild vanes in bypass duct and core duct were established in segments firstly. The mixed boundary was used to realize the data exchange between different flow fields. Then the Spalart-Allmaras turbulence model was applied to calculate the three dimensional engine flow field. Based on the flow field result, the droplet trajectory was calculated by Euler multiphase flow method. The LWC (Liquid Water Content), droplet collection efficiency and the effect of droplet diameter on impingement law were obtained. The method used in this paper and results could provide some references for subsequent engine icing calculation and anti-icing system design.

Effect of nucleation and icing evolution on run-back freezing of supercooled water droplet

Supercooled large droplet (SLD), which can cause run-back ridged ice, will affect the aerodynamic performance of aircraft and poses a serious threat to aircraft safety. However, there is little knowledge about the abnormal run-back icing mechanism, which is vital for the development of aircraft anti-icing technologies. The flow and freezing of supercooled droplet impinging on inclined surface can help us better understand the run-back icing of SLD, especially the coupling of water flow and phase change in the freezing process. This paper experimentally investigates the freezing behavior of supercooled water droplet impinging on inclined surface. By observing the processes of water flow, ice formation and growth with a high-speed camera, the overflow distance and the freezing time are recorded with different temperatures. Different from previous discoveries that droplet freezes as the form of two smaller droplets on an inclined surface, we found two new frozen morphologies under the condition of short nucleation time: ellipse and slender strip, when the supercooling is high and low, respectively. The overflow distance and freezing time will increase with the decrease of ice growth rate, when supercooling decreases. And the freezing time will increase dramatically when the supercooling is low enough. Finally, the mechanism of run-back freezing of droplet based on the nucleation and icing evolution is discussed. Droplet impact on the inclined surface will result in large overflow distance when it nucleates after its retraction stage.

Impact freezing modes of supercooled droplets determined by both nucleation and icing evolution

Supercooled large droplet (SLD), which can cause abnormal icing phenomenon like run-back ridged ice, poses a serious threat to aircraft safety. In impact freezing of a supercooled droplet, the coupling of impact dynamics and ice growth determines its freezing rate and heat transfer efficiency, which is responsible for the abnormal icing phenomenon. The knowledge of supercooled droplet impact freezing is vital for the development of aircraft anti-icing and safety technologies. However, the effect of icing evolution on impact freezing is not well understood yet, especially on surfaces with different properties. This work experimentally investigates the freezing process of supercooled droplets impinging on surfaces with different heat conduction properties. By observing the phenomenon of droplet impact-nucleation and ice growth with high-speed camera, the frozen morphology, spreading ratio and freezing time are recorded with different temperatures and surface heat conduction properties. The mechanism of impact freezing of supercooled droplet is analyzed in different nucleation and ice growth conditions. In experiment, two frozen morphologies of droplets are found: irregular shape on plexiglass and poached egg shape on metal surface, besides the previously discovered frozen morphologies (basin, pancake, semisphere). The former shape exists when the horizontal growth rate of ice is lower than retraction rate of droplet, while the latter one exists when the vertical growth rate of ice is low enough. These two morphologies (irregular shape and poached egg) would evolve into shapes of basin or pancake with the increase of horizontal and vertical growth rate of ice, when the supercooling and surface thermal conductivity rise. During this transformation, the freezing of droplet is mainly influenced by nucleation rather than ice growth. Theoretical analysis shows that the freezing of supercooled droplet is determined by both nucleation and icing evolution. The freezing mode is nucleation-dominated if the growth rate of ice is higher than retraction rate, and the freezing mode is ice growth-dominated in the opposite case.

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.

A multiphase SPH framework for supercooled large droplets dynamics

Purpose This paper aims to introduce a three-dimensional smoothed particle hydrodynamics (SPH) framework for simulating supercooled large droplets (SLD) dynamics at aeronautical speeds. Design/methodology/approach To include the effects of the surrounding air, a multiphase model capable of handling high density-ratio problems is adopted. A diffusive term is incorporated to smooth the density field and avoid numerical instabilities. Additionally, a particle shifting technique is used to eliminate anisotropic particle distributions. Findings The framework is validated against low-speed droplet impingement experimental results and then applied to the droplet impingement at high speeds typical of SLD conditions. Preliminary parametric studies are conducted to investigate the post-impact splashing. It is observed that a thicker water film can decrease the crown diameter and a smaller impact angle can suppress upward and forward splashing. Originality/value A three-dimensional multiphase SPH framework for SLD dynamics at a wide range of impact speed is developed and validated. The effects of particle resolution, water film thickness and impact angle on the post-impact crown evolution are investigated.