Thesis (M.S. in Meteorology)--Naval Postgraduate School, March 2005. / Thesis Advisor(s): Philip Durkee. Includes bibliographical references (p. 79-83). Also available online.

This dissertation presents a methodology to simulate the dispersion of water droplets in the air flow typical of an Icing Tunnel. It is based on the understanding the physical parameters that influence the uniformity and the distribution of cloud of droplets in the airflow and to connect them with analytical parameters which may be used to describe the dispersion process. Specifically it investigates the main geometrical and physical parameters contributing to the droplets dispersion at different tunnel operative conditions, finding a consistent numerical approach to reproduce the local droplets dynamic, quantifying the possible limits of commercial CFD methods, pulling out the empirical parameters/constant needing to simulate properly the local conditions and validating the results with calibrated experiment. An overview of the turbulence and multiphase flow theories, considered relevant to the Icing Tunnel environment, is presented as well as basic concepts and terminology of particle dispersion. Taylor’s theory of particle dispersion has been taken as starting point to explain further historical development of discrete phase dispersion. Common methods incorporated in commercial CFD software are explained and relative shortcomings underlined. The local aerodynamic condition within tunnel, which are required to perform the calculation with the Lagrangian particle equation of motions, are generated numerically using different turbulent models and are compared to the historical K-ε model. Verification of the calculation is performed with grid independency studies. Stochastic Separated Flow methods are applied to compute the particle trajectories. The Discrete Random Walk, as described in the literature, has been used to perform particle dispersion analysis. Numerical settings in the code are related to the characteristics of the local turbulent condition such as turbulence intensity and length scales. Cont/d.

abstract: Durable, cost-effective, and environmentally friendly anti-icing methods are desired to reduce the icing hazard in many different industrial areas including transportation systems, power plants, power transmission, as well as offshore oil and gas production. In contrast to traditional passive anti-icing surfaces, this thesis work introduces an anti-icing coating that responds to different icing conditions by releasing an antifreeze liquid. It consists of an outer porous superhydrophobic epidermis and a wick-like underlying dermis that is infused with the antifreeze liquid. This bi-layer coating prevents accumulation of frost, freezing fog, and freezing rain, while conventional anti-icing surfaces typically work only in one of these conditions. The bi-layer coating also delays condensation on the exterior surface at least ten times longer than identical system without antifreeze. It is demonstrated that the significant delay in condensation onset is due to the integral humidity sink effect posed by the hygroscopic antifreeze liquid infused in the porous structure. This effect significantly alters the water vapor concentration field at the coating surface, which delays nucleation of drops and ice. It was demonstrated that with a proper design of the environmental chamber the size of the region of inhibited condensation and condensation frosting around an isolated pore, as well as periodically spaced pores, filled by propylene glycol can be quantitatively predicted from quasi-steady state water vapor concentration field. Theoretical analysis and experiments revealed that the inhibition of nucleation is governed by only two non-dimensional geometrical parameters: the pore size relative to the unit cell size and the ratio of the unit cell size to the thickness of the boundary layer. It is demonstrated that by switching the size of the pores from millimeters to nanometers, a dramatic depression of the nucleation onset temperature, as well as significantly greater delay in nucleation onset can be achieved. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2017

Lorsqu’un hélicoptère opère dans des conditions givrantes extrêmes, l’accumulation de glace sur les pales peut considérablement impacter les performances de l’appareil. De nombreuses recherches portant sur le développement d’un système de dégivrage à faible consommation et moindre coût ont été initiées ces dernières années. Dans cette thèse, une technique ondulatoire de protection contre la formation de glace sur les surfaces des pales d’hélicoptères est étudiée. La stratégie proposée repose sur l’utilisation d’ondes guidées d’ordre élevé spécifiques pour créer des cisaillements dépassant la force d’adhésion surfacique d’un profil de glace. Des essais ont d’abord été menés pour réaliser le modèle E.F. d’un tronçon de pale, puis une stratégie de réduction de modèle est développée pour la Méthode des Éléments Finis Ondulatoires. Cette formulation s’appuie sur la projection des vecteurs d’état sur une base réduite, constituée des formes d’ondes progressives. Elle permet de réaliser des analyses ondulatoires large-bande dans les structures complexes, 1D ou 2D périodiques. Les ondes guidées sont d’abord examinées dans la pale d’hélicoptère, puis les effets de localisation et de conversion des ondes sont interprétés dans divers guides d’ondes 1D et 2D. Les interactions de ces ondes d’ordre élevé avec les profils d’accrétion de glace, ainsi qu’avec plusieurs types de singularités structurelles, sont analysées au moyen d’une Méthode des Matrices de Diffusion. Une formulation ondulatoire temporelle est ensuite proposée pour l’analyse rapide de la propagation d’un train d’ondes dans les guides d’ondes couplés. Enfin, un réseau d’actionneurs est conçu pour la génération de trains d’ondes d’ordre élevé, et des validations temporelles sont réalisées dans une plaque composite ainsi que dans une pale de Super Puma. / When helicopters fly through extreme conditions, ice can aggregate on their blades and seriously affect the aircraft performances. Recently, an increasing research effort was devoted to the development of affordable low power de-icing solutions. In this thesis, a wave-based approach is adopted to prevent and/or remove ice aggregates from the surfaces of helicopter rotor blades. The de-icing strategy uses specific high-order guided waves to exceed the shear adhesion strength of ice accretion profiles. Experiments are conducted in order to update the FE model of a realistic rotor blade, then a Model Order Reduction strategy is developed for the Wave Finite Element Method. It involves a projection of the state vectors on a reduced basis of propagating waves shapes, and enables broadband wave analysis in structurally advanced 1D and 2D periodic structures. Guided wave propagation is studied within a helicopter rotor blade, and wave localization and conversion effects are discussed in various 1D and 2D composite waveguides. The interactions of high-order waves with ice aggregates and other types of structural singularities are also examined by means of a Diffusion Matrix Method. Then, time-domain propagation in coupled waveguides subjected to a wave pulse is analysed through a computationally efficient wave-based formulation. Finally, a smart actuator network is designed for the generation of high-order wave pulses and validations are conducted in a composite plate and a Super Puma rotor blade using time simulation.

The study involves development of a methodology to simulate a fin-tube evaporator under icing conditions using CFD in Ansys® Academic Fluent 2021R1. It aims to build on previous studies performed on heat pumps. It was performed by Abhay M. Hervatte in collaboration with Bosch Thermoteknik AB, Tranås, SE during the spring term of the year 2021. The thesis is published by Linköping University. Initially, experiments were conducted to measure the ice growth on the fins of the evaporator as a function of time. A CAD model of the evaporator was then generated. The evaporator geometry was scaled down and simplified to reduce the simulation time. Due to restrictions in the software, the simulations were split into two parts - one for the flow of the refrigerant through the evaporator pipes and another for flow of air over the fins. The internal flow simulation was a steady state simulation consisting of the phase-change of the refrigerant after absorbing heat from the ambient. through the pipes and a transient simulation for the external flow over the fins. The internal flow consisted of multi-phase simulation of the evaporation of the refrigerant - propane - after absorbing heat through the pipe walls. The external flow involved the multi-phase simulation of ice being deposited from humid air on the surface of the fins. The inner surface of the evaporator pipes was used as a bridge, and surface profiles from the internal simulation would be used to transfer the boundary conditions to the other simulation. Results of the ice-film thickness over the fins were obtained and compared to the experimental value and found to be in reasonable agreement with each other, with scope for improvement in the future.

In order to study inlight ice adhesion at the droplet-scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquid-solid interface. Calibration of a simulation parameter, Amush, is performed in order to match experimental data for different surface types and surface temperatures. The calibrated simulation strategy is applied to low-speed, inlight icing conditions. The effects of surface variation and droplet diameter variation are investigated, providing insight into the icephobicity of superhydrophobic surfaces. Numerical results suggest that large droplets (approximately 200 micron-diameter) will freeze and adhere to a superhydrophobic surface.

No description available.

No description available.

Preventing ice accumulation on aircraft surfaces is important to maintain safe operation during flight. Ice accumulation on aero-engine nose cones is detrimental as large pieces may break off and be ingested into the engine damaging the compressor blades. Currently, hot bleed air is taken from the compressor and blown over the inside and outside surfaces of the nose cone to prevent ice formation on the surface. Although effective, this technique reduces the efficiency of the aero-engine. This investigation evaluates the performance of a novel anti-icing system that uses a rotating heat pipe to transfer heat from the engine to the nose cone. Rotating heat pipes are effective two-phase heat transfer devices capable of transporting large amounts of heat over small temperature differences and cross-sectional areas. In this system, waste heat that is generated in the engine would be transferred to the rotating heat pipe at an evaporator and then transferred into the critical areas of the nose cone at a condenser preventing ice accumulation on the outside surface. In this investigation, the heat is transferred into the heat pipe from a fluid heated by the engine that would pass through a small annular gap between the rotating heat pipe and a stationary wall. The heat transfer for this configuration and the effect of passive heat transfer augmentation on the outside of the rotating heat pipe in the jacket was investigated experimentally for a range of Taylor numbers of 10^6 < Ta < 5x10^7 and for axial Reynolds numbers of 900 < Re_x < 2100, characteristic of this configuration when engine lubricant was used as the working fluid. It was found that by using an array of three-dimensional cubical protrusions, the heat transfer in the evaporator could be increased by 35% to 100%. This result was better than that found using two-dimensional rib roughness. It was also found that the evaporator performance was a limiting factor in the heat transfer performance of the system under most conditions, so further optimization of the evaporator is important. In the proposed condenser design, the condenser section of the rotating heat pipe would be encased in a lightweight, high conductivity polycrystalline graphite or similar composite material and the end of the heat pipe would be in direct contact with the nose cone. It was found that the end-wall of the heat pipe was not a source of high heat transfer, however it provided an effective means for heating the tip of the nose cone. The effect of using heating channels on the inside of the nose cone was also considered. Here, the condensate from the rotating heat pipe was driven through small radially spaced channels on the inside surface of the nose cone. The heating channels were found to be ineffective due to the small contact area that could be made with the nose cone. This was a result of the limited condensate flow that occurs in rotating heat pipes. The heat transfer through the proposed system was 700W to 1100W using water and 400W to 800W using ethanol in the heat pipe. It was found that 50% to 75% of the arclength of the nose cone could be maintained above 0°C using water in the heat pipe at an ambient temperature of -30°C and an airplane speed of 300 km/h. This arclength decreased to approximately 25% when ethanol was used as the working fluid. An increase in airplane speed reduced this arclength maintained above 0°C significantly. / Thesis / Master of Applied Science (MASc)

The most ubiquitous mode of frost formation on substrates is condensation frosting, where dew drops condense on a supercooled surface and subsequently freeze, and has been known since the time of Aristotle. The physics of frost incipience at a microscopic scale has, nevertheless, eluded researchers because of an unjustified ansatz regarding the primary mechanism of condensation frosting. It was widely assumed that during condensation frosting each supercooled droplet in the condensate population freezes in isolation by heterogeneous nucleation at the solid-liquid interface, quite analogous to the mechanism of icing. This assumption has very recently been invalidated with strong experimental evidence which shows that only a single droplet has to freeze by heterogeneous nucleation (typically by edge effects) in order to initiate condensation frosting in a supercooled condensate population. Once a droplet has frozen, it subsequently grows an ice bridge towards its nearest neighboring liquid droplet, freezing it in the process. Thus ensues a chain reaction of ice bridging where the newly frozen droplets grow ice bridges toward their nearest neighbor liquid droplets forming a percolating network of interconnected frozen droplets. Not always are these ice bridges successful in connecting to their adjacent liquid droplets. Sometimes the liquid droplet can completely evaporate before the ice bridges can connect, thus forming a local dry region in the vicinity of the ice bridge. In this work, we first formulate a thermodynamic framework in order to understand the localized vapor pressure gradients that emerge in mixed-mode phase-change systems and govern condensation and frost phenomena. Following this, we study droplet pair interactions between a frozen droplet and a liquid droplet to understand the physics behind the local ice bridge connections. We discuss the emergent scaling laws in ice bridging dynamics, their relative size dependencies, and growth rates. Thereafter, we show how with spatial control of interdroplet distances in a supercooled condensate and temporal control of the first freezing event, we can tune global frost propagation on a substrate and even cause a global failure of all ice bridges to create a dry zone. Subsequently, we perform a systematic study of dry zones and derive a scaling law for dry zones that collapses all of our experimental data spanning a wide parameter space. We then show that almost always the underlying mechanism behind the formation of dry zones around any hygroscopic droplet is inhibition of growth and not inhibition of nucleation. We end with a discussion and preliminary results of our proposed anti-frosting surface that uses ice itself to prevent frost. / Master of Science