Transition and control in rotating-disk boundary-layer

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Etude des instabilités et de la transition de la couche limite produite par un disque en rotation

Ce travail de thèse expérimental étudie les instabilités et la transition de la couche limite produite par un disque en rotation. Pour l écoulement naturel (c.-à-d. sans forçage extérieur), les mesures des profils de vitesse moyenne, de spectres en fréquence et de moyennes de phase des séries temporelles de vitesse ont permis de distinguer différents régimes en fonction de la distance adimensionnelle R à l axe du disque. Pour les faibles valeurs de R, les profils de vitesse moyenne suivent la solution de von Kármán. Pour des valeurs plus importantes de R, des écarts à cette solution analytique sont observés et augmentent avec R. Ces écarts sont dus à la croissance spatiale de modes instables de la couche limite (vortex .cross-flow.), et la mesure du taux de croissance spatiale de ces modes correspond bien aux prédictions théoriques de l analyse de stabilité linéaire. Dans cet écoulement, la transition se produit vers R 530 et la turbulence pleinement développée s installe vers R 600. Les profils dans la région pleinement turbulente suivent la loi logarithmique des couches limites turbulentes et les spectres de vitesse présentent une loi en puissance de type Kolmogorov. Pour étudier la réponse au forçage, un dispositif expérimental a été mis au point qui permet d exciter des perturbations stationnaires (dans le référentiel du laboratoire) ou en rotation à une fréquence qui peut être réglée indépendamment de la fréquence de rotation du disque. La réponse de l écoulement à ces deux types de forçage et avec deux formes différentes pour l élément de forçage a été étudiée. Un forçage stationnaire produit un sillage qui décroît avec la distance à l élément de forçage, en accord avec la théorie. Le forçage avec des éléments en rotation peut produire un paquet d ondes amplifié qui, bien que non linéaire, suit des trajectoires proches de celles prédites par la théorie linéaire.This dissertation concerns experimental work on the instability and transition of the rotating-disk boundary-layer flow. In the case of the natural flow (i.e. without forcing), measurements of mean-flow profiles, frequency spectra and phase-locked averages of the velocity time series allow us to distinguish different flow regimes as a function of nondimensional distance, R, from the disk axis. As R increases, the mean-velocity profiles initially follow the von Kármán solution. At higher R, departures arise and increase with R. These departures are due to the spatial growth of boundary-layer instability modes (cross-flow vortices), whose radial growth rates are found to match linear-theory predictions. The flow becomes transitional at R 530 and fully turbulent by R 600. The profiles in the fully turbulent region follow the log law of turbulent boundary layers and the velocity spectra exhibit Kolmogorov-type power laws. To study the response to forcing, an experimental apparatus has been designed which allows the excitation of stationary (in thelaboratory frame of reference) disturbances or disturbances which rotate with a frequency which can be varied independently of the disk rotation rate. The flow response to both types of forcing and two forcing-element geometries was studied. Stationary forcing produces a wake which decays with distance from the element, in agreement with theory. Forcing due to rotating elements can generate growing wavepacket-like disturbances, which although nonlinear, follow trajectories close to linear-theory predictions.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Energy and Exergy Assessment of S-CO2 Brayton Cycle Coupled with a Solar Tower System

In this research, we performed energy and exergy assessments of a solar driven power plant. Supercritical carbon dioxide (S-CO2) Brayton cycle is used for the conversion of heat to work. The plant runs on solar energy from 8 a.m. to 4 p.m. and to account for the fluctuations in the solar energy, the plant is equipped with an auxiliary heater operating on hot combustion gases from the combustion chamber. The capital city of Saudi Arabia (Riyadh) is chosen in this study and the solar insolation levels for this location are calculated using the ASHRAE clear-sky model. The solar collector (central receiver) receives solar energy reflected by the heliostats; therefore, a radially staggered heliostat field is generated for this purpose. A suite of code is developed to calculate various parameters of the heliostat field, such as optical efficiencies, intercept factors, attenuation factors and heliostat characteristic angles. S-CO2 Brayton cycle is simulated in commercial software, Aspen HYSYS V9 (Aspen Technology, Inc., Bedford, MA, USA). The cycle is mainly powered by solar energy but assisted by an auxiliary heater to maintain a constant net power input of 80 MW to the cycle. The heliostat field generated, composed of 1207 rows, provides 475 watts per unit heliostat’s area to the central receiver. Heat losses from the central receiver due to natural convection and radiation are significant, with an average annual loss of 10 percent in the heat absorbed by the receiver. Heat collection rate at the central receiver reveals that the maximum support of auxiliary heat is needed in December, at nearly 13% of the net input energy. Exergy analysis shows that the highest exergy loss takes place in the heliostat field that is nearly 42.5% of incident solar exergy

Energy Analysis of the S-CO₂ Brayton Cycle with Improved Heat Regeneration

Supercritical carbon dioxide (S-CO2) Brayton cycles (BC) are promising alternatives for power generation. Many variants of S-CO2 BC have already been studied to make this technology economically more viable and efficient. In comparison to other BC and Rankine cycles, S-CO2 BC is less complex and more compact, which may reduce the overall plant size, maintenance, and the cost of operation and installation. In this paper, we consider one of the configurations of S-CO2 BC called the recompression Brayton cycle with partial cooling (RBC-PC) to which some modifications are suggested with an aim to improve the overall cycle’s thermal efficiency. The type of heat source is not considered in this study; thus, any heat source may be considered that is capable of supplying temperature to the S-CO2 in the range from 500 °C to 850 °C, like solar heaters, or nuclear and gas turbine waste heat. The commercial software Aspen HYSYS V9 (Aspen Technology, Inc., Bedford, MA, USA) is used for simulations. RBC-PC serves as a base cycle in this study; thus, the simulation results for RBC-PC are compared with the already published data in the literature. Energy analysis is done for both layouts and an efficiency comparison is made for a range of turbine operating temperatures (from 500 °C to 850 °C). The heat exchanger effectiveness and its influence on both layouts are also discussed

Etude des instabilités et de la transition de la couche limite produite par un disque en rotation

Ce travail de thèse expérimental étudie les instabilités et la transition de la couche limite produite par un disque en rotation. Pour l écoulement naturel (c.-à-d. sans forçage extérieur), les mesures des profils de vitesse moyenne, de spectres en fréquence et de moyennes de phase des séries temporelles de vitesse ont permis de distinguer différents régimes en fonction de la distance adimensionnelle R à l axe du disque. Pour les faibles valeurs de R, les profils de vitesse moyenne suivent la solution de von Kármán. Pour des valeurs plus importantes de R, des écarts à cette solution analytique sont observés et augmentent avec R. Ces écarts sont dus à la croissance spatiale de modes instables de la couche limite (vortex .cross-flow.), et la mesure du taux de croissance spatiale de ces modes correspond bien aux prédictions théoriques de l analyse de stabilité linéaire. Dans cet écoulement, la transition se produit vers R 530 et la turbulence pleinement développée s installe vers R 600. Les profils dans la région pleinement turbulente suivent la loi logarithmique des couches limites turbulentes et les spectres de vitesse présentent une loi en puissance de type Kolmogorov. Pour étudier la réponse au forçage, un dispositif expérimental a été mis au point qui permet d exciter des perturbations stationnaires (dans le référentiel du laboratoire) ou en rotation à une fréquence qui peut être réglée indépendamment de la fréquence de rotation du disque. La réponse de l écoulement à ces deux types de forçage et avec deux formes différentes pour l élément de forçage a été étudiée. Un forçage stationnaire produit un sillage qui décroît avec la distance à l élément de forçage, en accord avec la théorie. Le forçage avec des éléments en rotation peut produire un paquet d ondes amplifié qui, bien que non linéaire, suit des trajectoires proches de celles prédites par la théorie linéaire.This dissertation concerns experimental work on the instability and transition of the rotating-disk boundary-layer flow. In the case of the natural flow (i.e. without forcing), measurements of mean-flow profiles, frequency spectra and phase-locked averages of the velocity time series allow us to distinguish different flow regimes as a function of nondimensional distance, R, from the disk axis. As R increases, the mean-velocity profiles initially follow the von Kármán solution. At higher R, departures arise and increase with R. These departures are due to the spatial growth of boundary-layer instability modes (cross-flow vortices), whose radial growth rates are found to match linear-theory predictions. The flow becomes transitional at R 530 and fully turbulent by R 600. The profiles in the fully turbulent region follow the log law of turbulent boundary layers and the velocity spectra exhibit Kolmogorov-type power laws. To study the response to forcing, an experimental apparatus has been designed which allows the excitation of stationary (in thelaboratory frame of reference) disturbances or disturbances which rotate with a frequency which can be varied independently of the disk rotation rate. The flow response to both types of forcing and two forcing-element geometries was studied. Stationary forcing produces a wake which decays with distance from the element, in agreement with theory. Forcing due to rotating elements can generate growing wavepacket-like disturbances, which although nonlinear, follow trajectories close to linear-theory predictions.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Performance Analysis of Organic Rankine Cycle with Internal Heat Regeneration: Comparative Study of Binary Mixtures and Pure Constituents in Warm Regions

There are various organic compounds that can be utilized in the organic Rankine cycle as working fluids. The selection of a suitable working fluid is complicated due to the large number of options and factors affecting the choice, such as thermodynamic properties, environmental impact, cost, etc. This study evaluates seven different pure organic compounds and twenty-one of their binary zeotropic mixtures as potential working fluids for the organic Rankine cycle powered by a heat source at 200 °C. The pure organic fluids show higher exergy efficiency, higher specific net power output, and lower heat exchange area requirements compared to the binary mixtures. Among the pure fluids, RE347mcc performs the best in terms of exergy efficiency, followed by neopentane, isopentane, and pentane. Cyclopentane exhibits the highest power production capacity per unit mass flow rate of the working fluid. Two mixtures, pentane/Novec 649 and cyclopentane/Novec 649, showed significantly higher exergy efficiency than their individual components, but at significantly lower specific power production capacity. The study presents an interesting trade-off between exergy efficiency and heat exchange area, indicating that a small increase in exergy efficiency can lead to a large decrease in the required heat exchange area. The outcomes of this study can help in selecting suitable working fluids for ORC operation with a heat source at 200 °C

Energy and Exergy Analysis of the S-CO2 Brayton Cycle Coupled with Bottoming Cycles

Supercritical carbon dioxide (S-CO2) Brayton cycles (BC) are soon to be a competitive and environment friendly power generation technology. Progressive technological developments in turbo-machineries and heat exchangers have boosted the idea of using S-CO2 in a closed-loop BC. This paper describes and discusses energy and exergy analysis of S-CO2 BC in cascade arrangement with a secondary cycle using CO2, R134a, ammonia, or argon as working fluids. Pressure drop in the cycle is considered, and its effect on the overall performance is investigated. No specific heat source is considered, thus any heat source capable of providing temperature in the range from 500 °C to 850 °C can be utilized, such as solar energy, gas turbine exhaust, nuclear waste heat, etc. The commercial software ‘Aspen HYSYS version 9’ (Aspen Technology, Inc., Bedford, MA, USA) is used for simulations. Comparisons with the literature and simulation results are discussed first for the standalone S-CO2 BC. Energy analysis is done for the combined cycle to inspect the parameters affecting the cycle performance. The second law efficiency is calculated, and exergy losses incurred in different components of the cycle are discussed

Experimental characterization of transition region in rotating-disk boundary layer

International audienceThe three-dimensional boundary layer due to a disk rotating in otherwise still fluid is well known for its sudden transition from a laminar to a turbulent regime, the location of which closely coincides with the onset of local absolute instability. The present experimental investigation focuses on the region around transition and analyses in detail the features that lead from the unperturbed boundary layer to a fully turbulent flow. Mean velocity profiles and high-resolution spectra are obtained by constant-temperature hot-wire anemometry. By carefully analysing these measurements, regions in the flow are identified that correspond to linear, weakly nonlinear or turbulent dynamics. The frequency that dominates the flow prior to transition is explained in terms of spatial growth rates, derived from the exact linear dispersion relation. In the weakly nonlinear region, up to six clearly identifiable harmonic peaks are found. High-resolution spectra reveal the existence of discrete frequency components that are deemed to correspond to fluctuations stationary with respect to the disk surface. These discrete components are only found in the weakly nonlinear region. By systematically acquiring low- and high-resolution spectra over a range of narrowly spaced radial and axial positions, it is shown that while the transition from laminar to turbulent regimes occurs sharply at some distance from the disk surface, a complex weakly nonlinear region of considerable radial extent continues to prevail close to the disk surface

Thermodynamic Analysis of Partitioned Combined Cycle using Simple Gases

In combined cycle gas turbines, most of the energy loss is usually due to the high temperature of the exhaust gases. Different heat recuperation methods are used. In this study, a novel direct method for heat recovery is investigated. Confidence in the results is established by accounting for all the losses and simulation errors while comparing with the conventional cycle. Aspen HYSYS and MATLAB are the simulation tools used. The General Electric (GE) 9HA.02 combined cycle is taken as a base case. Five gases, air, argon, hydrogen, nitrogen, and carbon dioxide, are studied with the proposed modification. The efficiency maximization function is updated and the pressure and temperature ratios of individual Brayton and Rankine cycles are discussed. The combustor/heat exchanger is modified and simulated according to the known principles of heat and momentum transfer. The whole simulation algorithm is provided. Equation of state (EOS-PR) is used to calculate the properties at every discretized step (for H2, critical properties are modified/HYSYS inbuilt feature). Different gases are analyzed according to their property profiles over the whole cycle. The effect of fluid properties on efficiency is discussed as a guideline for any tailored fluid

Proposal and Investigation of a New Tower Solar Collector-Based Trigeneration Energy System

These days, the low efficiency of solar-based thermal power plants results in uneconomical performance and high-cost uncompetitive industries compared with conventional fossil fuels. In order to overcome such issues, a novel combined cooling–power–heating (trigeneration) system is proposed and analyzed in this paper. This system uses an ammonia–water binary mixture as a working fluid and a solar heat source to produce diverse types of energy for a multi-unit building in a sustainable fashion. In addition to the basic cooling–power cogeneration cycle, a flashing chamber that will boost the flow rate of refrigerant without any additional heat supply is employed. By developing a mathematical model, the system performance is analyzed using varying parameters of solar irradiation, hot oil temperature, process heat pressure, and ambient temperature to investigate the influence on electrical power, cooling capacity, refrigeration exergy, energy utilization factor (EUF), and system exergy efficiency. Increasing direct normal irradiation (DNI) from 500 W/m2 to 1000 W/m2 reduces the system EUF and exergy efficiency from 53.62% to 43.12% and from 49.02% to 25.65%, respectively. Both power and refrigeration exergy increase with increasing DNI and ambient temperature, while heating exergy remains constant. It is demonstrated that of 100% solar energy supplied, 46.03% is converted into energetic output and 53.97% is recorded as energy loss. The solar exergy supplied is distributed into 8.34% produced exergy, 29.78% exergy loss, and the remaining 61.88% is the destructed exergy. The highest destruction of solar exergy (56.89%) occurs in the central receiver