Unsteady Source Research Articles

Fluctuations of angle of attack and lift coefficient and the resultant fatigue loads for a large Horizontal Axis Wind turbine

Unsteady loads are a major limiting factor for further upscaling of HAWTs considering the high costs associated to strict structural requirements. Alleviation of these unsteady loads on HAWT blades, e.g. using active flow control (AFC), is of high importance. In order to devise effective AFC methods, the unsteady loading sources need to be identified and their relative contribution to the load fluctuations experienced by blades needs to be quantified. The current study investigates the effects of various atmospheric and operational parameters on the fluctuations of α and CL for a large HAWT. The investigated parameters include turbulence, wind shear, yawed inflow, tower shadow, gravity and rotational imbalances. The study uses the DTU's aeroelastic software HAWC2. The study identifies the individual and the aggregate effect of each source on the aforementioned fluctuations in order to distinguish the major contributing factors to unsteady loading. The quantification of contribution of each source on the total fatigue loads reveals >65% of flapwise fatigue loads is a result of turbulence while gravity results in >80% of edgewise fatigue loads. The extensive parametric study shows that the standard deviation of CL is 0.25. The results support to design active load control systems by highlighting the magnitude of CL and α variations experienced by HAWTs, and thus the dCL that needs to be delivered by an AFC system.

Multiple-relaxation-time lattice Boltzmann simulation for flow, mass transfer, and adsorption in porous media.

In this paper, to predict the dynamics behaviors of flow and mass transfer with adsorption phenomena in porous media at the representative elementary volume (REV) scale, a multiple-relaxation-time (MRT) lattice Boltzmann (LB) model for the convection-diffusion equation is developed to solve thetransfer problem with an unsteady source term in porous media. Utilizing the Chapman-Enskog analysis, the modified MRT-LB model can recover the macroscopic governing equations at the REV scale. The coupled MRT-LB model for momentum and mass transfer is validated by comparing with the finite-difference method and the analytical solution. Moreover, using the MRT-LB method coupled with the linear driving force model, the fluid transfer and adsorption behaviors of the carbon dioxide in a porous fixed bed are explored. The breakthrough curve of adsorption from MRT-LB simulation is compared with the experimental data and the finite-element solution, and the transient concentration distributions of the carbon dioxide along the porous fixed bed are elaborated upon in detail. In addition, the MRT-LB simulation results show that the appearance time of the breakthrough point in the breakthrough curve is advanced as the mass transfer resistance in the linear driving force model increases; however, the saturation point is prolonged inversely.

Tapping Sun Energy coupled with affordable Energy Storage – Future Game Changer

Sun provides abundant source of renewable energy that can be integrated with the electrical grid. Climate change issues have compelled policy makers to look into various ways to reduce carbon footprint and use green, renewable energy. Solar power, along with other alternative sources for energy, is quite popular these days. Talking about Solar, the primary disadvantage of solar power is that it obviously cannot be created during the night and power generated is also reduced during times of cloud cover. Energy Storage is a flexible asset that provides unprecedented flexibility in grid optimization. Cost effective solar energy storage methods are urgently needed due to the increased demand for solar power and due to its variability. But in today’s scenario, energy storage systems are not commercially economic for all customers, and that to more work needs to be done by industry, government, and regulators to support the continuing cost reductions. It is expected that the Energy Storage costs would slide to 41% by 2020.The value that solar and storage can together add to the energy system is leading to a more efficient, cleaner, and more secure future. However, solar energy storage becomes critical when unsteady sources of energy provide. Thus, affordable energy storage system along with the cheaper Solar energy would be lethal combination making an ultimate Game Changer for the Power Industry and Sustainability.

Numerical simulation of thermoelectric performance of linear-shaped thermoelectric generators under transient heat supply

A two-dimensional, dynamic finite element model is developed to study the transient characteristics of linear-shaped thermoelectric generators (L-TEG) in this paper. The model considers the effect of an unsteady heat source, which is regarded as a time-varying thermal boundary condition. The temperature-dependent TE material properties are also taken into account. For two loading processes (i.e. heating process and cooling process), we investigate transient TE behaviors of L-TEG under different loading ways and duration of thermal load as well as geometric dimensions of the TE legs, individually. The results indicate that, in heating process, the responses of the power and the absorbed heat to the thermal load present obvious time delay under some certain conditions; in the cooling process, interestingly, an internal heat source emerges in the L-TEG, caused by the delay of thermal diffusion from the hot end of the TE legs to cold end. This heat source renders a heat release from the generator to the ambient. A process relying on its own heat to generate electricity is thus formed. This study further helps the understanding of TE behavior of TE generators in service.

Numerical analysis on plume temperature properties formed above a harmonically oscillating fire source

Large eddy simulation (LES) of turbulent buoyant plumes formed above an oscillating fire source has been carried out so as to clarify their temperature properties in the offshore environment. For the sake of simplicity, we are concerned here only with a rolling environment among six-degree-of-freedom system of complex ship motion. Furthermore, under the assumption of a small rolling angle, unsteady fire source movement associated with the rolling motion can be approximated by simple harmonic oscillation in the horizontal direction. Numerical results obtained under several sets of oscillation conditions, especially under different amplitude conditions, are compared with temperature variations of conventional axisymmetric and two-dimensional fire plumes above stationary sources. Consequently, it has been clarified that in the case where the amplitude is four times longer than the side length of a square source, the present buoyant plumes exhibit almost the same feature as the two-dimensional fire plume in the lower part of the buoyant plume region. On the other hand, it has been found that for any value of amplitude, the temperature properties are close to those of axisymmetric plumes in the upper part of the buoyant plume region. Based on these findings, a simple theoretical model to estimate the variation in mean excess temperature as a function of elevation has been developed by the use of a well-recognized, semi-analytical solution of the axisymmetric plume formed above a virtual point source. It has been confirmed that this simple model can reasonably reproduce the same plume features as obtained by the LES simulations.

Optimized Reduction of Unsteady Radial Forces in a Singlechannel Pump for Wastewater Treatment

A single-channel pump for wastewater treatment was optimized to reduce unsteady radial force sources caused by impeller-volute interactions. The steady and unsteady Reynolds- averaged Navier-Stokes equations using the shear-stress transport turbulence model were discretized by finite volume approximations and solved on tetrahedral grids to analyze the flow in the single-channel pump. The sweep area of radial force during one revolution and the distance of the sweep-area center of mass from the origin were selected as the objective functions; the two design variables were related to the internal flow cross-sectional area of the volute. These objective functions were integrated into one objective function by applying the weighting factor for optimization. Latin hypercube sampling was employed to generate twelve design points within the design space. A response-surface approximation model was constructed as a surrogate model for the objectives, based on the objective function values at the generated design points. The optimized results showed considerable reduction in the unsteady radial force sources in the optimum design, relative to those of the reference design.

Inhomogeneity and Lagrangian unsteadiness in turbulent thermal convection

An experimental particle tracking is used to distinguish two sources of Lagrangian unsteadiness in turbulent thermal convection. The first one, induced by the large-scale circulation inhomogeneity, can be suppressed by separating the mean flow and the turbulent fluctuations. This provides a subtle way to study the second unsteadiness, which is linked to the large-scale circulation sloshing.

Linearized Euler Equations for the Prediction of Linear High-Frequency Stability in Gas Turbine Combustors

A novel methodology for linear stability analysis of high-frequency thermoacoustic oscillations in gas turbine combustors is presented. The methodology is based on the linearized Euler equations (LEEs), which yield a high-fidelity description of acoustic wave propagation and damping in complex, nonuniform, reactive mean flow environments, such as encountered in gas turbine combustion chambers. Specifically, this work introduces three novelties to the community: (1) linear stability analysis on the basis of linearized Euler equations. (2) Explicit consideration of three-dimensional, acoustic oscillations at screech level frequencies, particularly the first-transversal mode. (3) Handling of noncompact flame coupling with LEE, that is, the spatially varying coupling dynamics between perturbation and unsteady flame response due to small acoustic wavelengths. Two different configurations of an experimental model combustor in terms of thermal power and mass flow rates are subject of the analysis. Linear flame driving is modeled by prescribing the unsteady heat release source term of the linearized Euler equations by local flame transfer functions, which are retrieved from first principles. The required steady-state flow field is numerically obtained via computational fluid dynamics (CFD), which is based on an extended flamelet-generated manifold (FGM) combustion model, taking into account heat transfer to the environment. The model is therefore highly suitable for such types of combustors. The configurations are simulated, and thermoacoustically characterized in terms of eigenfrequencies and growth rates associated with the first-transversal mode. The findings are validated against experimentally observed thermoacoustic stability characteristics. On the basis of the results, new insights into the acoustic field are discussed.

Localization of unsteady heat source in a tube from pressure measurements with the inverse method

This paper presents work to identify the position of an unsteady heat source in a one-dimensional tube from acoustic pressure measurements with the inverse method. The relationship between the oscillation heat release rate and the pressure can be represented as a Volterra integral equation of the first kind. The discretization method was applied to transform the integral equation into matrix form. To stabilize the solution of the matrix equation, the Tikhonov regularization method was proposed. Experiments were performed to validate the inverse method. A semi-infinite probe system was used to measure the pressure perturbations in the tube, to avoid the high temperature damaging the microphone. Before the pressure measurements were taken, calibration was performed for the semi-infinite probe system to obtain accurate pressure data in the tube. The experiments were performed in three steps. First, the localization of a pure sound source in the tube at ambient temperature was studied. Second, localization of a pure sound source at hot conditions was considered. Third, the pure sound source was replaced with an unsteady heat source, and pressure data were used to determine the position of the unsteady heat source. Results show that calibration and the regularization are both necessary for the determination of the sound source position in the tube. Meanwhile, at the hot and heat release rate conditions, with the consideration of the temperature distribution in the thermoacoustic model, the position of the sound source and the unsteady heat release source can be determined successfully.

Heat source estimation in low diffusive materials

In this work, a method to estimate unsteady 2D heat sources is developed. The sources are estimated from Infra-Red (IR) temperature mapping on the front face of thermally thin material. A comparison of the estimation accuracy as well as the evaluation of time cost is also carried out between a direct method (coupled with filtering techniques) and an iterative method (of conjugate gradient type). The present work was realised on experimental data.

Validation and comparison of aerodynamic modelling approaches for wind turbines

The development of large capacity Floating Offshore Wind Turbines (FOWT) is an interdisciplinary challenge for the design solvers, requiring accurate modelling of both hydrodynamics, elasticity, servodynamics and aerodynamics all together. Floating platforms will induce low-frequency unsteadiness, and for large capacity turbines, the blade induced vibrations will lead to high-frequency unsteadiness. While yawed inflow conditions are still a challenge for commonly used aerodynamic methods such as the Blade Element Momentum method (BEM), the new sources of unsteadiness involved by large turbine scales and floater motions have to be tackled accurately, keeping the computational cost small enough to be compatible with design and certification purposes. In the light of this, this paper will focus on the comparison of three aerodynamic solvers based on BEM and vortex methods, on standard, yawed and unsteady inflow conditions. We will focus here on up-to-date wind tunnel experiments, such as the Unsteady Aerodynamics Experiment (UAE) database and the MexNext international project.

System design and control for waste heat recovery of automotive engines based on Organic Rankine Cycle

In this study, a novel and practical design for an ORC (Organic Rankine Cycle) system is presented. It can be applied to waste heat recovery of unsteady heat sources, such as exhaust gas of automobile engines. According to the variation characteristics of exhaust gas temperature and mass flow rate, the EGMR (Exhaust Gas Mixture Recirculation) loop is designed. The exhaust gas flowing out of the evaporator is partially recovered and mixed with the real-time engine exhaust gas, and then led to recirculation in the evaporator. Moreover, an energy storage module is also introduced, which is simplified as a metal block with internal tubes. A control approach based on ADRC (Active Disturbance Rejection Control is established. It is composed of an outer sub-loop controller for the evaporating pressure, a control allocator for mass flow rate allocation of the exhaust gas, and an inner sub-loop controller for the valve openings. The system is evaluated according to the numerical simulations. The results show that it can work steadily and reliably. The control error of the evaporating pressure is lower than 0.1%, and the fluctuation of the superheating is less than 1 °C. Compared with the conventional ORC system, both the cycle and the overall energy efficiencies are increased significantly. Furthermore, since no extra thermodynamic cycle or working fluid is demanded, the system architecture is more practical and simpler than most previous designs of system improvement.

Acoustic scattering by a finite rigid plate with a poroelastic extension

The scattering of sound by a finite rigid plate with a finite poroelastic extension interacting with an unsteady acoustic source is investigated to determine the effects of porosity, elasticity and the length of the extension when compared to a purely rigid plate. The problem is solved using the Wiener–Hopf technique, and an approximate Wiener–Hopf factorisation process is implemented to yield reliable far-field results quickly. Importantly, finite chord-length effects are taken into account, principally the interaction of a rigid leading-edge acoustic field with a poroelastic trailing-edge acoustic field. The model presented discusses how the poroelastic trailing-edge property of owls’ wings could inspire quieter aeroacoustic designs in bladed systems such as wind turbines, and provides a framework for analysing the potential noise reduction of these designs.

Mechanism of shock unsteadiness in separated shock/boundary-layer interactions

A large-eddy simulation-based study is presented and focuses on different unsteadiness-source features in a Mach 2.3 shock reflection with separation. The sources of unsteadiness are localized and the path taken by disturbance as it spreads out to the whole field is defined. It is shown that the phenomena arising inside the recirculation bubble govern the whole interaction, at both low and intermediate frequencies. Indeed, the shock motion appears to mirror phenomena found in the separated zone. Moreover, features of separated-flow unsteadiness bear some resemblance to those occurring in incompressible flows. An equivalent inviscid scheme of the unsteady interaction is established in order to describe the whole shock-system unsteadiness at low and intermediate frequencies and the downstream unsteady-pressure field.

Study of the vortex-induced pressure excitation source in a Francis turbine draft tube by particle image velocimetry

Francis turbines operating at part-load experience the development of a precessing cavitation vortex rope at the runner outlet, which acts as an excitation source for the hydraulic system. In case of resonance, the resulting pressure pulsations seriously compromise the stability of the machine and of the electrical grid to which it is connected. As such off-design conditions are increasingly required for the integration of unsteady renewable energy sources into the existing power system, an accurate assessment of the hydropower plant stability is crucial. However, the physical mechanisms driving this excitation source remain largely unclear. It is for instance essential to establish the link between the draft tube flow characteristics and the intensity of the excitation source. In this study, a two-component particle image velocimetry system is used to investigate the flow field at the runner outlet of a reduced-scale physical model of a Francis turbine. The discharge value is varied from 55 to 81 % of the value at the best efficiency point. A particular set-up is designed to guarantee a proper optical access across the complex geometry of the draft tube elbow. Based on phase-averaged velocity fields, the evolution of the vortex parameters with the discharge, such as the trajectory and the circulation, is determined for the first time. It is shown that the rise in the excitation source intensity is induced by an enlargement of the vortex trajectory and a simultaneous increase in the precession frequency, as well as the vortex circulation. Below a certain value of discharge, the structure of the vortex abruptly changes and loses its coherence, leading to a drastic reduction in the intensity of the induced excitation source.

Simulation of spray direct injection for compressed air energy storage

Integrating Compressed Air Energy Storage (CAES) to a variable and unsteady energy source can help provide a levelized power output. This is particularly attractive for off-shore wind turbines integrated with the energy storage that has high efficiency. Such efficiency is possible if the compression portion can be isothermal, and a novel approach has been developed to achieve this by employing water spray during compression to promote heat transfer. This concept has been previously investigated with one-dimensional simulations that indicated spray cooling with droplet heat transfer over a large total surface area allows high-efficiency compression. However, the actual application is more complicated, and therefore the present study examines this concept with detailed two-dimensional unsteady flow simulations. In particular, multi-phase computational fluid dynamics is implemented in an axisymmetric domain to investigate compression in a cylinder for first-stage and second-stage compression using a spray discharge within the cylinder at various mass loadings. The spray is based on a single pressure-swirl nozzle directed along the centerline and operating at the maximum liquid mass flux possible while retaining a mean droplet diameter of no more than 30 µm. The two-dimensional simulations uncovered flow characteristics such as vortex formation for the air-flow near the cylinder head and strong spatial variations in droplet size and concentration. Despite these effects, the overall two-dimensional efficiency was similar to that of one-dimensional predictions. The results also indicated that a single pressure-swirl nozzle injection resulted in an injected mass loading of 1.6 and yielded efficiency as high as 93% for a first-stage compression cycle. However, a second-stage compression cycle (with an intake pressure of 10 bar) using this same single nozzle resulted in reduced overall work efficiency indicating that a multi-nozzle configuration should be considered.

Unsteady Flow Evolution Through a Turning Midturbine Frame Part 1: Time-Resolved Flow

This paper identifies and analyzes the propagation of aerodynamic deterministic stresses through a two-spool counter-rotating transonic facility representative of modern and future turbine aeroengine sections. The test setup consists of a high-pressure stage, a diffusing turning midturbine frame with turning struts, and a counter-rotating low-pressure rotor. The flowfield downstream of the high-pressure stage is strongly influenced by the stator–rotor interaction. Such a mechanism interacts again with the downstream turning midturbine frame leading to a vane–rotor–vane interaction, which affects the behavior of the low-pressure stage. The results presented were obtained using a fast-response aerodynamic pressure probe for unsteady measurements as well as three-dimensional unsteady Reynolds-averaged Navier–Stokes calculations. The work is presented in two parts. This first part focuses on the explanation of the flow physics that governs the convection of unsteady three-dimensional flow through the midturbine duct. Viscous and inviscid mechanisms are discussed as main drivers for the convection of wakes, secondary vortices, and shocks. The flowfield in the duct is characterized by three superimposed effects: 1) duct diffusion and radial pressure gradient together with turning strut potential field, 2) rotor unsteady work source, and 3) vane/blade interaction phenomena. The understanding of these mechanisms will eventually help to control the unsteadiness content in future architectures where reduced engine component length will enhance the interaction effects.

Unsteady Wet Steam Flow Field Measurements in the Last Stage of Low Pressure Steam Turbine

Modern steam turbines need to operate efficiently and safely over a wide range of operating conditions. This paper presents a unique unprecedented set of time-resolved steam flowfield measurements from the exit of the last two stages of a low pressure (LP) steam turbine under various volumetric massflow conditions. The measurements were performed in the steam turbine test facility in Hitachi city in Japan. A newly developed fast response probe equipped with a heated tip to operate in wet steam flows was used. The probe tip is heated through an active control system using a miniature high-power cartridge heater developed in-house. Three different operating points (OPs), including two reduced massflow conditions, are compared and a detailed analysis of the unsteady flow structures under various blade loads and wetness mass fractions is presented. The measurements show that at the exit of the second to last stage the flow field is highly three dimensional. The measurements also show that the secondary flow structures at the tip region (shroud leakage and tip passage vortices) are the predominant sources of unsteadiness at 85% span. The high massflow operating condition exhibits the highest level of periodical total pressure fluctuation compared to the reduced massflow conditions at the inlet of the last stage. In contrast at the exit of the last stage, the reduced massflow operating condition exhibits the largest aerodynamic losses near the tip. This is due to the onset of the ventilation process at the exit of the LP steam turbine. This phenomenon results in three times larger levels of relative total pressure unsteadiness at 93% span, compared to the high massflow condition. This implies that at low volumetric flow conditions the blades will be subjected to higher dynamic load fluctuations at the tip region.

Investigation of the unsteady flow of a transonic compressor using the nonlinear harmonic method

In this paper, the nonlinear harmonic method implemented in Fine/Turbo CFD code has been used to analyze the unsteady flow field in a transonic compressor. The numerical accuracy and computational efficiency of this method have been demonstrated by comparing its predictions with experimental data and conventional time-domain unsteady simulation for the test case. The rotor–stator interactions are studied in detail to improve the understanding of the flow physics involved. The deterministic stresses in the nonlinear harmonic method have also been compared to the time-domain unsteady postprocessing results to analyze distinct sources of unsteadiness. For the present case, the nonlinear harmonic method can not only capture the unsteady results satisfactorily, but also gain a significant computational advantage over conventional unsteady method.

Pressure distribution in unsteady sink and source flows.

Basic flow generated in a viscous incompressible fluid by a "point" sink (source) of mass is revised. In practice, such flow can be modeled by sucking (pushing) fluid from a thin tube with a small porous sphere at one end. Intuitively, by sucking (pushing) fluid, one creates low (high) pressure near the origin and a positive (negative) radial pressure gradient drives the fluid to (from) the origin. A simple analysis, however, shows that the pressure distribution for both steady flows is the same. Then a question arises: How does the fluid "know" in what direction to flow? To explain this "paradox" an unsteady flow is considered and the pressure terms responsible for the flow direction are derived.