Browsing by Author "Hammond, David W."
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Item Open Access Analytical and experimental investigation into the thermal aspects of droplet impingement(Cranfield University, 2006-03) Quero García, Manuel; Hammond, David W.The mechanics and heat transfer of droplet impact is studied in the range of parameters interest for Super-cooled Large Droplet icing. The investigation explores the development of the splash produced experimental and numerically. A Navier-Stokes solver has been developed in order to compare experiments and modelling. Heat transfer is included in the simulations making possible the analysis of the thermal history during the impact of a Super-cooled droplet into a warm and running thin water film. Also a theoretical and numerical study has been undertaken in order to simulate the first stages of ice formation on the critical surfaces of aircraft during the droplet impact under freezing conditions due to super-cooled icing. The parameters considered experimental and numerically are: • Droplet size: 100-700Jlm. • Droplet impact velocity: 18-80m/s. • Angles of impact: 70°,45° and 20°. • Airflow (droplet) temperature: 200 e and _lOoe. • Water film thicknesses: 150Jlm and 50Jlm. • Water film temperature: 15°e and lOoe. • Water film velocity: 5m/s. The simulations are compared to the experiments run under the same conditions. Results for the parameters at the early stages of the splash agree well but as the splash process continues there are more differences between the two sets of results.Item Open Access Car engine breather icing(Cranfield University, 2012-11) Horoufi, Aryan; Hammond, David W.Icing in an engine breather system can block the engine breather pipe, cause excessive crankcase pressure and degrade the engine performance. In this project, a numerical study, experimental tests and CFD analysis are employed in order to understand condensation and the extent of freezing inside a vertical pipe, a horizontal pipe and a T-joint pipe which are exposed to an external convective cooling. The pipe internal flow is assumed to be a vapour/air mixture. This study has led an evaluation of freezing in an engine breather pipe. The finding in this project highlighted the effects of the pipe internal flow condition (vapour mass fraction, relative humidity, mixture gas flow rate, and inlet relative humidity), the pipe external cooling condition (temperature and air velocity) and pipe thermal conductivity on condensation and extents of ice formation in the pipe. In the experimental study, a test rig has been designed and the condensation and freezing in the pipe have been tested at the Cranfield Icing Tunnel. The local pipe temperatures are measured to validate the numerical and the CFD analysis. The numerical study has led to develop a one dimensional code which used heat and mass analogy to model condensation and freezing in a vertical pipe exposed to a cold air flow (-20C). This code satisfactory predicts the trend and magnitude of the local temperatures and heat transfer coefficient along the vertical pipe at available test condition within an acceptable uncertainty of 25%. This study proposes an empirical correlation based on a degradation factor to evaluate heat transfer coefficient inside a vertical pipe. Its results fit with the experimental data within 15% uncertainty. The CFD methodology developed in this study is capable of predicting condensation rates, local temperatures, heat transfer coefficients and extent of freezing in the pipes with good agreement with the experimental results. The CFD model over predicts the breather pipe ice blockage time due to disparities between an actual engine operating condition and the CFD model. Therefore, an adjustment factor of 1.7 is proposed in this study to correlate the predicted blockage time. The results of this study can help Jaguar to establish guideline for future design of engines breather pipes.Item Open Access Character and interface shear strength of accreted ice on subcooled surfaces submerged in fuel(Cambridge University Press, 2016-01-27) Lam, Joseph K.-W.; Lao, Liyun; Hammond, David W.; Power, J. P.Sudden release of accreted ice in fuel systems could pose a serious challenge in aircraft operation. The resultant snowshower may reach the filter and fuel-oil heat exchanger, causing a restriction in fuel flow to the engine. It is fundamental to have an appreciation of the character and the interface shear strength of the accreted ice in aircraft fuel systems. This helps to recognise factors for the sudden release of the accreted ice and the intensity of the consequential snowshower. An experimental study was carried out to quantify the character and the interface shear strength of accreted ice on subcooled surfaces submerged in jet fuel. Ice was accreted on naked aluminium, painted aluminium and carbon fibre composite surfaces at various subcooled temperatures. The accreted ice was akin to fresh snow and exhibited soft and fluffy attributes. The character may be expressed quantitatively in terms of the porosity and was found to be c. 0·95. The ice weakly adhered to the substrate surfaces, and the interface shear strength was found to be c. 0·36Pa and c. 2·19Pa at the top surface and at the vertical surface of a specimen block, respectively. It was not possible to detect any variation in the porosity and the interface shear strength for different types of surface finishes and differences in water affnity in fuels due to the crude approach in the estimation of these parameters.Item Open Access Experimental and modelling investigation of the deformation, drag and break-up of drizzle droplets subjected to strong aerodynamics forces in relation to SLD aircraft icing(Cranfield University, 2005-03) Luxford, Geoffrey; Hammond, David W.The distortion, drag and break-up of drizzle droplets subjected to strong aerodynamic forces was investigated to understand the pre-impact behaviour of droplets in aircraft icing from supercooled freezing drizzle. The objective was to obtain a formulation and data for the drag properties of droplets distorted by the aerodynamic forces, which were beyond the scope of available experimental and modelling methods. A practical and efficient semi-empirical computer model was developed for small water droplets in air, 100µm < D < 500µm, at moderate Reynolds numbers, 350 < Re < 1500, and high Weber numbers 3 < We < 20. This used available experimental terminal velocity data for free-falling droplets, extrapolated to higher Weber numbers, and the numerical solution for sessile droplets on a horizontal unwettable surface, with corrections for the Reynolds number. A theory for bag break-up was developed based on the Rayleigh-Taylor instability of the windward droplet surface. The critical Bond number was 13.7, with a critical diameter of 10.1mm for free-falling water droplets, compared to the experimental value of 10mm diameter. The equivalent Weber number was 14.2 for free falling water droplets. Aerodynamic interaction between the closely-spaced droplets from a vibrating nozzle droplet generator resulted in irregular spacing and coalescence of droplets. In an alternative design a laminar jet impinged on a rotating slotted disk to achieve the necessary droplet spacing, but the significant size variability of the droplets degraded the experimental measurements. High-speed videos, to 50,000pps, and photographs were obtained of droplet distortion, break-up, coalescence and splashes using a high-intensity LED strobe flash. A specially-designed convergent wind tunnel was developed for experimental measurements, to validated the drag model and provide data for droplets distorted by aerodynamic forces. The convergent profile produced a rapidly-increasing Weber number at a sufficiently slow rate to avoid transients or droplet vibrations. A special instrument was developed, with three equispaced parallel laser beams and photo detectors, to determine the droplet velocity and acceleration. Droplet drag characteristics were measured up to Weber numbers of 16. Good agreement was obtained between droplet drag model and experimental results. The greatest discrepancy was about 20% at a Weber number of about 8.Item Open Access Experimental and modelling investigation of the deformation, drag and break-up of drizzle droplets subjected to strong aerodynamics forces in relation to SLD aircraft icing(Cranfield University, 2005-03) Luxford, Geoffrey; Hammond, David W.The distortion, drag and break-up of drizzle droplets subjected to strong aerodynamic forces was investigated to understand the pre-impact behaviour of droplets in aircraft icing from supercooled freezing drizzle. The objective was to obtain a formulation and data for the drag properties of droplets distorted by the aerodynamic forces, which were beyond the scope of available experimental and modelling methods. A practical and efficient semi-empirical computer model was developed for small water droplets in air, 100μm < D < 500μm, at moderate Reynolds numbers, 350 < Re < 1500, and high Weber numbers 3 < We < 20. This used available experimental terminal velocity data for free-falling droplets, extrapolated to higher Weber numbers, and the numerical solution for sessile droplets on a horizontal unwettable surface, with corrections for the Reynolds number. A theory for bag break-up was developed based on the Rayleigh-Taylor instability of the windward droplet surface. The critical Bond number was 13.7, with a critical diameter of 10.1mm for free-falling water droplets, compared to the experimental value of 10mm diameter. The equivalent Weber number was 14.2 for free falling water droplets. Aerodynamic interaction between the closely-spaced droplets from a vibrating nozzle droplet generator resulted in irregular spacing and coalescence of droplets. In an alternative design a laminar jet impinged on a rotating slotted disk to achieve the necessary droplet spacing, but the significant size variability of the droplets degraded the experimental measurements. High-speed videos, to 50,000pps, and photographs were obtained of droplet distortion, break-up, coalescence and splashes using a high-intensity LED strobe flash. A specially-designed convergent wind tunnel was developed for experimental measurements, to validated the drag model and provide data for droplets distorted by aerodynamic forces. The convergent profile produced a rapidly-increasing Weber number at a sufficiently slow rate to avoid transients or droplet vibrations. A special instrument was developed, with three equispaced parallel laser beams and photo detectors, to determine the droplet velocity and acceleration. Droplet drag characteristics were measured up to Weber numbers of 16. Good agreement was obtained between droplet drag model and experimental results. The greatest discrepancy was about 20% at a Weber number of about 8.Item Open Access An experimental and numerical study on the effect of some properties of non-metallic materials on the ice adhesion level(Cranfield University, 2013) Piles Moncholi, Eduardo; Hammond, David W.The rise of the Environmentalism in every sector of the Industry has lead the aircraft and engine manufacturing companies to develop new generations of more environmentally friendly engines. The companies, encouraged to this purpose, are in a constant research for new manufacturing and production techniques, in order to improve their products, from the environmental point of view, by gaining efficiency in the manufacturing techniques and reduce the fuel consumption and emissions in-flight. Having in mind this scenario, the sponsor of this Project is interested in understanding how changing the materials of the blades, titanium alloys currently, for other lighter materials, such as composites, is going to have an effect in the overall gas turbine efficiency. In the particular case of this Project, it will be studied the influence of the Stiffness and coating Thickness of those non-metallic materials suitable to be employed as coatings on gas turbine fan blades, from the icing point of view. The work procedure will be based on a study of Linear Elastic Fracture Mechanics of bi-material junctions and will extrapolate the general problem to the ice-coatings case, by getting experimental data from tests carried out in an Icing Tunnel. It will be observed that the coating Stiffness has an influence on the Adhesion Level of ice to less stiff materials, if compared with the Adhesion Level of ice to metals. Besides, it will be described how a 0.5 millimetres thin polymeric coating placed over a metallic substrate is enough to reduce the Adhesion Level of ice, hiding any effect that the underneath materials might have on the Adhesion Level.Item Open Access Full-scale runback ice: accretion and aerodynamic study(Cranfield University, 2010-12) Alegre, Nathalie; Hammond, David W.The runback ice phenomenon is well-known for anti-icing or de-icing systems when the system is not evaporating 100% of the water impinging the surface. The water runs back to the point where the added heat no longer raises the surface temperature above freezing. The water freezes behind this limit. No runback ice is tolerated for some flight configurations, but not for all. Then for o.-design cases, some runback ice may grow on the wings surface. However, data from full-scale realistic runback ice is not very well-known by aircraft manufacturers and they are not sure what thickness is allowed before the e.ect of the ice on the flow becomes too adverse. To better understand full-scale high-fidelity runback ice growth and how it can be simulated with simplistic shapes, test campaigns and CFD studies were undertaken. First of all, tests in the Cranfield icing tunnel were performed. In this work, full-scale runback ice shapes were grown on a model with a full-scale leading edge equiped with an electrical heating system. An innovative moulding and casting technique has been introduced which allowed the production of 3D planarised full-scale realistic runback ice castings. In parallel to the icing tunnel tests, a mass and energy balance has been computed on Excel. This energy and mass balance can predict the heat and mass fluxes involved in the runback ice accretion mechanism. Following this, aerodynamic tests of the ice castings were lead in one of the low speed wind tunnels at Cranfield University. The aerodynamics of simplistic shapes such as geometrical shapes or ballotini layers were also studied. The e.ects of the ice castings on the flow were compared to the e.ects of the simplistic shapes. The tests were done on a flat surface and not on an airfoil due to technical complications. The boundary layer displacement thickness was the parameter used to quantify the e.ect of the shapes on the flow. 2D CFD simulations were performed as a support to the testing but as well to compare with the experimental data. The CFD simulations were for steady or unsteady flow. It has been possible to grow full-scale ice shapes in a relatively small icing tunnel. The shapes have been successfuly moulded and cast using silicone and plaster mixed with polymer. A catalogue of runback ice shapes for different liquid water content, heat inputs and positions along the chord has been recorded. Following the wind tunnel tests, it has been possible to find a relationship between the real ice and the simplistic shapes. Thin runback ice shapes (4 mm) has a similar e.ect on the flow as a layer of 1 mm ballotini. It was found that thicker ice shapes, of the order of 1 cm, is equivalent to a rectangle with rounded corner, associated with 1mm ballotini. The triangle shape which is usually used to simulate runback ice by the aircraft manufacturers, was found to be the most aerodynamically penalising simplisitc shape that has been investigated in this PhD project. It was found that rounded corners greatly improve the representativeness of the simplistic shapes, such as triangle or rectangle.Item Open Access The influence of erosion and wear on the accretion and adhesion of ice for nano reinforced polymetric composites used in aeronautics(Cranfield University, 2011-12) Gohardani, Omid; Hammond, David W.The usage of polymeric matrix composites in aerospace applications has been significantly prevalent based on their desired material characteristics, which include higher strength, lower weight and heat resistance. With current advancements in nanotechnology, carbon nanotube reinforced polymeric matrix composites may enhance the operational usage of these advanced materials even further. In this study, a set of novel aerospace material candidates are characterized based on their mechanical properties, resilience to liquid erosion, wettability and ice adhesion. The experimental evaluations presented, allow for a preliminary ranking of the polymeric matrix composites and assessment of the influence of reinforcing carbon nanotubes. The role of erosion in particular is highlighted from both a historical viewpoint and based on empirical results for static and dynamic wettability and ice adhesion. Discussion of different ranking systems and fractography arising as a consequence of liquid impact are further addressed in this study. It is found that the candidate samples exhibit different physical parameters but nominally similar erosion resilience despite the presence of the reinforcing carbon nanotubes. The wettability of the experimental materials and their ice adhesion characteristics are further shown to be influenced by the presence of carbon nanotubes and largely dependent upon degradation of the material surfaces.Item Open Access Influence of flight-path variability of conditions upon in-flight icing(Cranfield University, 2008-10) Stanfield, Robin; Hammond, David W.; Carver, Stephen; Spooner, PaulIn design and development of aircraft, standard practice uses `the icing design envelopes' to select atmospheric conditions for modelling icing encounters. Over the duration of these encounters, atmospheric conditions are assumed to be constant and to exhibit no variability. In reality variability exists, to an extent where it may adversely affect the severity of ice accretions beyond that identified by ground-based modelling. Similarly, certain tools and systems employed by industry may sacrifice efficiency & effectiveness in neglecting the variability that exists. This project considered what operational and safety bene ts might be derived from an enhanced knowledge of ice accretion under more realistic, variable conditions; in contrast with a reference case identified to have equivalent constant conditions. In doing so, variable encounters were modelled experimentally in an icing tunnel to compare against a constant-condition reference; aerodynamic penalty was assessed numerically using CFD, allowing a comparison to be made between variable and constant-condition profiles; and desk-research considered variable conditions in the context of existing and emerging technology. Considerable differences were observed between variable profiles themselves and with the reference profile, with aerodynamic penalty being considerably enhanced for 25% of variable cases, and considerably reduced for a further 25%. Desk-research suggests that in understanding variability, to reduce costs asso- iciated with aircraft icing, more realistic ground-based modelling capabilities could reduce the need for natural flight-testing in the long term, though this would require substantial enhancement to current numerical prediction capabilities. Similarly, the power applied to ice protection systems could be tailored more speci cally to demand, enhancing e ciency. On the basis of current instrumentation, this would first require development of more accurate and robust LWC measurement systems. It was therefore recommended that specialists in meteorology, icing physics, ice protection systems and aerodynamics; conduct more extensive research towards understanding variability and assessing its potential to enhance flight-safety, whilst simultaneously reducing cost.Item Open Access Mechanics of ice detachment applied to turbomachinery(Cranfield University, 2012) Pervier, Marie-Laure; Hammond, David W.Flying in icing conditions is a real hazard for aircraft as they can undergo potential disastrous increase in drag, reduction in maximum lift which lead to an increase in fuel consumption. Additionally pitot tubes and other sensors can become blocked or their operation compromised. Ice shed from other parts of the aircraft can enter the engine and lead to blade damage. Whilst ice protection systems are comonly used on propellers, the potential benefit of applying them to a fan have not, as yet, been considered suffi cient to o set the cost and energy penalties of such system. As engines become larger, it is more diffi cult to contain ice and self-shedding becomes an increasing hazard for the nacelle and other parts downstream of the fan. The main objectives of the project were to determine the mechanical proper- ties of ice such as might form on an engine fan, in order to help Rolls-Royce in building a finite elements model able to simulate ice shedding from fan blades. Lots were written about ice however only little information about the mechanical properties of impact ice was available in the literature and the values which were, were generally not applicable in the case of aeroengine in icing conditions. According to the literature and from Rolls-Royce photos and films of ice shedding from fan blades, self-shedding mechanisms were ruled by adhesive shear strength and tensile strength. Therefore, the experimental part of the project consisted of measuring these two mechanical properties as well as the density, the sti ness and the grain size of ice grown on titanium substrate. Two test rigs were used to measure the mechanical properties: the \mode I" and the \shear" test rig. The mode I test rig was already available and was only modified in order to test more specimens during each run. This test al- lowed to measure the pressure needed to remove the ice from the substrate in a running icing tunnel. Using equations from the literature, fracture energy, fracture toughness and tensile strength were calculated. The influence of ambient total temperature, cloud liquid water content and tunnel wind speed were investigated. Tensile strength was found to be increasing as the total temperature is decreasing, decreasing as the LWC is increasing and going trough a maximum as the tunnel wind speed is increasing. Values obtained lied in the range from 0.6 to 1.5 J.m-2 (corresponding to between 2 and 10 MPa) which is, in general, higher than the ones reported by other authors. This difference can be explained by the fact that the mode I test was conducted in a running icing tunnel while the previous authors have conducted the mechanical test after the tunnel has been stopped. In parallel, finite element models have been developed and results similar to the experiments were obtained ...[cont.].Item Open Access Methodology to analyse three dimensional droplet dispersion applicable to Icing Wind Tunnels(Cranfield University, 2009) Sorato, Sebastiano; Hammond, David W.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.Item Open Access Modelling, imaging and measurement of distortion, drag and break-up of aircraft- icing droplets(2005-01-01T00:00:00Z) Luxford, Geoffrey; Hammond, David W.; Ivey, Paul C.The distortion, drag and break-up of supercooled drizzle droplets in aircraft icing was investigated in ambient conditions. An effective efficient computer procedure was developed for the distortion and drag of small droplets, < 1mm, at low Reynolds numbers, < 1000, and high Weber numbers, > 10. High-speed videos and photographs were obtained with an improved high-intensity LED strobe. Experimental measurements validated the drag model for droplets distorted by a rapidly accelerating airflow in a convergent wind tunnel. To prevent droplet coalescence, due to wake interactions, droplets were generated with a steady jet impinging of a rotating slotted disk.Item Open Access Role of droplet distortion and break-up in large droplet aircraft icing(2004-01-01T00:00:00Z) Luxford, Geoffrey; Hammond, David W.; Ivey, Paul C.Experimental and analytical evidence is presented to illustrate droplet distortion and potential break-up in the flow conditions similar those of a drizzle droplet in the vicinity of an aerofoil. The droplet size range considered is 100u m to 500u m. The current research is at ambient temperature but with equivalent parametric conditions to SLD icing. While the results support the contention that smaller droplets, <100u m, remain essentially spherical, the larger droplets become sufficiently distorted to affect their drag characteristics and can break up due to the transient aerodynamic forces.Item Open Access Semi-empirical approach to characterize thin water film behaviour in relation to droplet splashing in modelling aircraft icing(Cranfield University, 2012-07) Alzaili, Jafar S. L.; Hammond, David W.Modelling the ice accretion in glaze regime for the supercooled large droplets is one of the most challenging problems in the aircraft icing field. The difficulties are related to the presence of the liquid water film on the surface in the glaze regime and also the phenomena associated with SLD conditions, specifically the splashing and re-impingement. The steady improvement of simulation methods and the increasing demand for highly optimised aircraft performance, make it worthwhile to try to get beyond the current level of modelling accuracy. A semi-empirical method has been presented to characterize the thin water film in the icing problem based on both analytical and experimental approaches. The experiments have been performed at the Cranfield icing facilities. Imaging techniques have been used to observe and measure the features of the thin water film in the different conditions. A series of numerical simulations based on an inviscid VOF model have been performed to characterize the splashing process for different water film to droplet size ratios and impact angles. Based on these numerical simulations and the proposed methods to estimate the thin water film thickness, a framework has been presented to model the effects of the splashing in the icing simulation. These effects are the lost mass from the water film due to the splashing and the re-impingement of the ejected droplets. Finally, a new framework to study the solidification process of the thin water film has been explored. This framework is based on the lattice Boltzmann method and the preliminary results showed the capabilities of the method to model the dynamics, thermodynamics and the solidification of the thin water film.Item Open Access Study of rate of dust build up on optical windows(Cranfield University, 2016-01) Ya-Alimadad, Maryam; Hammond, David W.The work presented in thesis is part of the DANIELA project which aims to replace the current air data system on civil aircrafts with a flush mounted Air Data System (ADS) built around a 3 axis Doppler LIDAR function as the primary data channel. This thesis is focused on the comparison of different window materials and their optical clarity by means of theoretical and experimental analysis. Four different window materials including BK7, Sapphire, Germanium and ZnS are placed in a wind tunnel. The samples are each exposed to flows of air and water for specific periods of time during which temperature, pressure and air speed are recorded. Subsequently, each sample is carefully observed under the microscope. This is followed by the measurement of the amount of back scatter via detecting the change in the voltage once it is placed in the optical station. The optical tests reveal the amount of dust adhered to the samples which results in increased voltage. Review of these samples under the microscope matches the results obtained from the optical test. The two sets of data obtained from the two tests determined that some samples collected more dust in comparison to others. It was established that under identical test conditions i.e. flow, temperature and moisture, BK7 and Sapphire collect considerably less dust compared to ZnS. Moreover it was impossible to test Germanium sample optically, under a microscope as it is a dark opaque glass.