799 research outputs found

    Multi-scale Eulerian-Lagrangian simulation of a liquid jet in cross-flow under acoustic perturbations

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    [EN] The design of modern aeronautical propulsion systems is constantly optimized to reduce pollutant emissions while increasing fuel combustion efficiency. In order to get a proper mixing of fuel and air, Liquid Jets Injected in gaseous Crossflows (LJICF) are found in numerous injection devices. However, should combustion instabilities appear in the combustion chamber, the response of the liquid jet and its primary atomization is still largely unknown. Coupling between an unstable combustion and the fuel injection process has not been well understood and can result from multiple basic interactions. The aim of this work is to predict by numerical simulation the effect of an acoustic perturbation of the shearing air flow on the primary breakup of a liquid jet. Being the DNS approach too expensive for the simulation of complex injector geometries, this paper proposes a numerical simulation of a LJICF based on a multiscale approach which can be easily integrated in industrial LES of combustion chambers. This approach results in coupling of two models: a two-fluid model, based on the Navier-Stokes equations for compressible fluids, able to capture the largest scales of the jet atomization and the breakup process of the liquid column; and a dispersed phase approach, used for describing the cloud of droplets created by the atomization of the liquid jet. The coupling of these two approaches is provided by an atomization and re-impact models, which ensure liquid transfer between the two-fluid model and the spray model. The resulting numerical method is meant to capture the main jet body characteristics, the generation of the liquid spray and the formation of a liquid film whenever the spray impacts a solid wall. Three main features of the LJICF can be used to describe, in a steady state flow as well as under the effect of the acoustic perturbation, the jet atomization behavior: the jet trajectory, the jet breakup length and droplets size and distribution. The steady state simulations provide good agreement with ONERA experiments conducted under the same conditions, characterized by a high Weber number (We>150). The multiscale computation gives the good trajectory of the liquid column and a good estimation of the column breakup location, for different liquid to air momentum flux ratios. The analysis of the droplet distribution in space is currently undergoing. A preliminary unsteady simulation was able to capture the oscillation of the jet trajectory, and the unsteady droplets generation responding to the acoustic perturbation.Thuillet, S.; Zuzio, D.; Rouzaud, O.; Gajan, P. (2017). Multi-scale Eulerian-Lagrangian simulation of a liquid jet in cross-flow under acoustic perturbations. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 782-789. https://doi.org/10.4995/ILASS2017.2017.4697OCS78278

    THERMAL EVALUATION OF ADVANCED LEADING EDGE FOR ROTATING GAS TURBINE BLADE: NUMERICAL AND EXPERIMENTAL INVESTIGATIONS

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    Gas turbine engines play a vital role in our life. Our power demand is significantly and continuously growing. One approach to improve thermal efficiency in gas turbine engines requires a higher turbine inlet gas temperature. Advanced gas turbine engines operate at high temperatures, around 2000 K. Since operating at high temperatures may compromise the blade structure integrity, different cooling systems are used in a turbine blades. One of the most efficient cooling techniques is impingement cooling, mostly used in the leading edge. The leading edge experiences the highest temperature in the blade exposed to the hottest gas. Researchers studied different factors over the years to identify and optimize jet impingement on the leading edge of the blade. Nevertheless, publications on jets impingement under rotation are limited in the public literature. Hence, the objective of this study is to evaluate blade cooling via jet impingement on a rotating semi-circular internal channel. The study is initially carried out numerically and experimentally for validation purpose. After validation, a parametric numerical model is developed to understand the effect of internal jets impingement on a rotating leading edge. By comparing the experimental results and the numerical results, all features of the temperature distribution over the target surface are precisely captured. A good agreement between the numerical analysis and the experimental measurements has been established. The parametric numerical model is used to test higher jet Reynolds numbers, varying between 7,500 to 30,000, and a higher rotating speed, ranging from 0 to 750 rpm. The results show that jets impingement with high Reynolds numbers is an efficient method of cooling a rotating leading edge. The jets impingement cooling performance is strongly influenced by the individual jet location, the crossflow from other jets, and the blade rotation speed. The effect of rotation is diminished at high jet Reynolds numbers. The cooling performance improves as jet Reynolds number increases and as rotating speed decreases

    Multi-scale simulation of the atomization of a liquid jet in cross-flow in the presence of an acoustic perturbation

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    International audienceThe reduction of pollutant emissions is currently a major concern in the aerospace sector. Among the proposed solutions, lean combustion appears as an effective technology to reduce the environmental impact. However, this type of technology may also favour the appearance of combustion instabilities. These instabilities, resulting from a thermo-acoustic coupling, can lead to irreversible damage to the combustion chambers.Experimental studies previously conducted at ONERA on a multipoint injector by Apeloig highlighted the importance of atomization on the instabilities loop. Indeed, the fuel vapour concentration near the injection zone has been shown to fluctuate in accordance with the imposed acoustic perturbation. The driving mechanism would then result from a flapping motion of the liquid jets in the multiple injection points, induced by the gas flow oscillations. This would in turn affect the characteristic convective timescales of the fuel, in the form of a spray or even of thin liquid films on the duct walls.In order to characterize this interaction, this work focuses on the unsteady simulation of a round liquid jet in the presence of a transverse gas flow in a rectangular section duct. Following an experimental study, the multi-scale numerical approach for multi-phase flows, implemented in the ONERA CEDRE code, has been tested in presence of an imposed acoustic perturbation. This approach consists of the coupling of three models: a multi-fluid model able to capture the largest scales of the liquid column atomization; a dispersed phase approach for the atomized spray, and a “Shallow Water” approach for wall films. The coupling of these approaches is provided by dedicated atomization and impact models, which ensure liquid transfer between the three models.Simulation results show that the multi-fluid solver is able to correctly capture the largest scales of the liquid jet. The simulated liquid jet trajectories match the experimental ones, as well as their dynamic response to the imposed acoustic perturbation. As the liquid is transferred to the dispersed phase solver, the jet motion deeply affects the spray formation and behaviour. Good agreement was found on the particle resulting mean velocity, but only partial agreement on the phase delay. An important wall deposition has been detected for particular jet positions as well

    Numerical and experimental modelling of near-field overflow dredging plumes

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    Turbidity plumes are an important topic in the environmental aspects of dredging. Turbid sediment plumes can cause damage when they reach environmentally sensitive areas such as coral reefs, sea grass fields and wetlands. The main source of turbidity while employing Trailer Suction Hopper Dredgers is the release of excess water through the overflow shaft. In order to minimise environmental impacts of turbidity in early stages of planning as well as during project execution, turbidity prediction tools are necessary. To this end, numerical modelling tools are the most effective in the prediction of the sea currents and sediment dispersion. The near field plume dynamics below and directly behind the sailing hopper dredgers has always been the weakest link in these predictions, since accurate input of the vertical and horizontal distributions of sediment at the source location are paramount to obtain reliable results at the environmentally sensitive areas further away. In this research, physical and numerical modelling are used as a tool to determine the three-dimensional flows of water, sediment and air bubbles directly after release from the overflow shaft. A full dredger hull geometry and an actuator disk accounting for propeller action will add to the representation of the complexity of the flow. The goal of the research is to improve the predictions of dredging-induced turbidity in the planning phase. In this way, modifications of the dredging operations can be made in order to reduce the environmental impact.nrpages: 312status: publishe

    Jet Mixing Enhancement by High Amplitude Pulse Fluidic Actuation

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    Turbulent mixing enhancement has received a great deal of attention in the fluid mechanics community in the last few decades. Generally speaking, mixing enhancement involves the increased dispersion of the fluid that makes up a flow. The current work focuses on mixing enhancement of an axisymmetric jet via high amplitude fluidic pulses applied at the nozzle exit with high aspect ratio actuator nozzles. The work consists of small scale clean jet experiments, small scale micro-turbine engine experiments, and full scale laboratory simulated core exhaust experiments using actuators designed to fit within the engine nacelle of a full scale aircraft. The small scale clean jet experiments show that mixing enhancement compared to the unforced case is likely due to a combination of mechanisms. The first mechanism is the growth of shear layer instabilities, similar to that which occurs with an acoustically excited jet except that, in this case, the forcing is highly nonlinear. The result of the instability is a frequency bucket with an optimal forcing frequency. The second mechanism is the generation of counter rotating vortex pairs similar to those generated by mechanical tabs. The penetration depth determines the extent to which this mechanism acts. The importance of this mechanism is therefore a function of the pulsing amplitude. The key mixing parameters were found to be the actuator to jet momentum ratio (amplitude) and the pulsing frequency, where the optimal frequency depends on the amplitude. The importance of phase, offset, duty cycle, and geometric configuration were also explored. The experiments on the jet engine and full scale simulated core nozzle demonstrated that pulse fluidic mixing enhancement was effective on realistic flows. The same parameters that were important for the cleaner small scale experiments were found to be important for the more realistic cases as well. This suggests that the same mixing mechanisms are at work. Additional work was done to optimize, in real time, mixing on the small jet engine using an evolution strategy.Ph.D.Committee Chair: David Parekh; Committee Member: Ari Glezer; Committee Member: Jeff Jagoda; Committee Member: Richard Gaeta; Committee Member: Samuel Shelto

    Large Eddy Simulations of complex turbulent flows

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    In this dissertation a solution methodology for complex turbulent flows of industrial interests is developed using a combination of Large Eddy Simulation (LES) and Immersed Boundary Method (IBM) concepts. LES is an intermediate approach to turbulence simulation in which the onus of modeling of “universal” small scales is appropriately transferred to the resolution of “problem-dependent” large scales or eddies. IBM combines the efficiency inherent in using a fixed Cartesian grid to compute the fluid motion, along with the ease of tracking the immersed boundary at a set of moving Lagrangian points. Numerical code developed for this dissertation solves unsteady, filtered Navier-Stokes equations using high-order accurate (fourth order in space) finite difference schemes on a staggered grid with a fractional step approach. Pressure Poisson equation is solved using a direct solver based on a matrix diagonalization technique. Second order accurate Adams-Bashforth scheme is used for temporal integration of equations. Dynamic mixed model (DMM) is used to model subgrid scale (SGS) terms. It can represent large scale anisotropy and back-scatter of energy from small-to-large scale through scale-similar term and maintain the energy drain through eddy viscosity term whose coefficient is allowed to change with in the computational domain. This code is validated for several bench-mark problems and is demonstrated to solve complex moving geometry problem such as stator-rotor interaction. A number of parametric studies on jets-in-crossflow are performed to understand complex fluid dynamics issues pertaining to film-cooling. These studies included effects of variation of hole-aspect ratio, jet injection angle, free-stream turbulence intensity and free-stream turbulence length scales on the coherent structure dynamics for jets-in-crossflow. Fundamental flow physics and heat transfer issues are addressed by extracting coherent structures from time-dependent three dimensional flow fields of film-cooling by inclined jet and studying their influence on the film-cooled surface heat transfer. A direct method to perform heat transfer calculations in periodic geometries is proposed and applied to internal cooling in rotating ribbed duct. Immersed boundary method is used to render complex geometry of trapped vortex combustor on Cartesian grid and fluid mixing inside trapped vortex cavity is studied in detail

    Evaporating crossflow sprays in gas-solid flows

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    Injection of evaporating sprays into gas-solid flows is encountered in many engineering processes such as energy production industry and chemical industry. The phenomenon involves phase change, three-phase interactions, heat and mass transfer. All of these characteristics control the process efficiency, pollutant production and product quality. However, very limited studies are available on the evaporating spray jets in gas-solid flows, especially on the spray evaporation rate within a gas-solid medium. A combined study of experiments and theoretical analysis has been carried out here to investigate the fundamental mechanism of evaporating Crossflow spray jets in gas-solid flows. In this study, in addition to a laboratory scale circulating fluidized bed to provide a continuous gas-solid flows, a laser/lamp-light assisted visualization and image analysis system and a computer aided temperature measurement system have been developed which enables measurement of spray trajectories and temperature distributions of mixture phases in dilute/dense gas-solid flows. All the experiments have been performed in the circulating fluidized bed with a simple rectangular column, controllable solids load and flow conditions, and well-defined liquid nitrogen sprays. The spray trajectory, spray penetration length, and flow pattern are investigated. The geometric and operating parameters, such as nozzle size, nozzle type, injection angle, jetting velocity, and solids loading are studied in the experiments. The experimental study shows that the loading of solid particles in mainstream can significantly shorten the penetration length and alter the spray structure. It is also shown that the quick evaporation of spray droplets leads to the dilution of solids concentration in the evaporation region as well as the reduction of phase temperatures. In this study, a fundamental parametric model for applications of an evaporating liquid jet into a gas-solid flow, which takes into account the three-phase interactions as well as phase changes. The model is focused on the study of the effects of spray parameters on the mixing characteristics such as spray penetration length, temperature and velocity of each phase, trajectories, and the phase volume fraction distributions. The governing equations are based on the conservation principles of mass, momentum and energy of all three phases. The model predictions have also been found in good agreement with the experimental results. Droplet evaporation rate is the most important factor to affect the phase interactions of the spray in gas-solid flows. The spray evaporation is dominated by the heat transfer through collisions between droplets and solid particles. Part of this study is focused on the Leidenfrost collisions between evaporating droplets and solid particles, which are involved in many multiphase flow applications, e.g., petroleum refinery, surface coating, and fire quenching. In this study, an analytical model has been developed to describe the Leidenfrost collision between a droplet and a hot solid sphere. The whole collision process, the maximum collision time, the maximum deformation area, and the evaporation rate are simulated. Effects of solid curvature and Weber number on the collision time and droplet evaporation rate are illustrated. Modeling predictions are validated by the available experimental results

    The Design and Optimization of Jet-in-Cross-Flow (JICF) for Engineering Applications: Thermal Uniformity in Gas-turbines and Cavitation Treatment in Hydro-turbines

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    Jet-in-cross-flow (JICF) is a well-known term in thermal flows field. Ranging from the normal phenomenon like the volcano ash and dust plumes to the designed film cooling and air fuel mixing for combustion, JICF is always studied to understand its nature at different conditions. Realizing the behavior of interacting flows and importance of many variables lead to the process of reiterating the shapes and running conditions for better outcomes or minimizing the losses. Summarizing the process under the name of optimization, two JICF applications are analyzed based on the principles of thermodynamics and fluid mechanics, then some redesigns are proposed to reach the optimal statuses for the goals sought. Correlations and recommendations are given between the input variables and the outputs. In the first application, annular thermal mixing chamber, the cold stream penetrates the axial hot flow as circumferential inward jets. Thermal uniformity of the exit mixture is the target to maximize, and accordingly, a streamlined body is firstly suggested to be placed at the center of the chamber to divert the hot stream towards the cold one. Following the idea, the shape and dimensions (length and maximum diameter) are tested experimentally with four 3-D printed bodies expressing different aspect, blockage, and profile ratios. Later, an Analysis LED Design stage (numerical then experimental) checked the effect of adding swirlers on the best streamlined shape. Swirlers shape, number, and height are examined for the relation with the uniformity and pressure drop. By defining a decision-making variable (useful efficiency), the two contradicting variables were consolidated into one, and the swirlers performance was easier to be quantified and the most efficient one was nominated. At the final stage, a numerical study searched the optimal design(s) using design of experiment and optimization (Global and Hybrid) algorithms. The study sought the optimality of the dimensional aspects (diameter, length, and position) of the swirling streamlined body based on minimizing the contradicting objectives. The results were represented by Pareto curve, correlation matrix, parallel axes, and response surface model. It was understood that the optimization can offer improvement of 68% and 15% to the uniformity number and the pressure drop respectively. On the other hand, aeration treatment for cavitating flow in axial Kaplan turbine was considered for the second engineering application. Using CFD models of a 7.5-cm hydro-turbine, cavitation situation was simulated, then air is injected from the housing to redistribute around the blades of the rotor. The value of the vapor fraction is tracked over the blades and the hub areas throughout the time of turbine cycles. Comparison is achieved by evaluating an average value for the vapor fraction at each case. Air mass flowrate and ports distribution are found to be effective in reducing the cavitation phenomena. Proposed linear aeration distributor on the housing presented a promising technology for spreading the air over the blade chord in a better way than the circumferential distribution. The study allowed the understanding of the flow behavior (in terms of air flow, liquid pressure, and cavitation formations) and turbine performance (i.e. mechanical power) at different air injection locations and turbine rotational speeds. A broader view of research investigated the functionality of linear aeration distributor on the hub with an air supply going through a hollow shaft. The invention of the hub air injection targets the marine industry (i.e. propellers) where the housing/shrouds do not exist, but it also can be a competitor to the housing air injection technology as well. For the two aeration approaches (housing and hub), the conducted numerical investigations were based on the vapor mitigation and power regain in the Kaplan turbine, meanwhile the experimentation looked for the vapor and motor power reduction for the propeller operation. A good agreement (qualitatively and quantitatively) was found between matching cases created for such purpose using tools for high-speed imaging, statistical analysis for turbulent flow, image processing and power measurements. Finally, the dissertation sets some recommendations for the continuation of the researches on the two applications (thermal uniformity and aeration treatments) for better jets interaction with the cross flow by the consideration of the addition/orientation of guide vanes and the relocation of the jets on the turbine blades respectively
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