The dynamic performance of wind power plants is difficult to model because of the large number of turbines that operate stochastically, with little information exchanged between the wind power plants ...and the serving utilities. In this study, we develop a linear dynamic equivalent model for large-scale wind power plants on the basis of data obtained at the point of interconnection in simulation. The modeling technique is based on adaptive parameter estimation of an equivalent model representing the dynamic performance of the plant. The developed model is tested against a large-scale plant model to confirm the effectiveness of the proposed technique.
In this paper, a feedback linearization technique is proposed to control the output voltage control of three-phase uninterruptible power supply systems. First, a nonlinear model including the output ...LC filters is derived from the power balance condition between the inverter output terminal and the load side. Then, input-output feedback linearization is applied to the nonlinear model to make it linear. The controller of the linearized model is designed by linear control theory. The tracking control law is obtained with a pole placement technique. It is shown experimentally that the proposed control scheme gives high dynamic responses in response to load variation as well as a zero steady-state error.
•Water and ethanol droplet dynamics on superheated surfaces are investigated.•Effects of micron-sized surface textures on droplet physical behaviors are analyzed.•Droplet Leidenfrost temperature, ...residence time, and spreading dynamics are measured.•Droplet behavior based on fluid types and geometry of surface textures is explained.
In this study, we investigated droplet (water and ethanol) dynamic behaviors on superheated surfaces with micro-textures. The textures have a circular pillar shape (∼9.0 μm diameter and ∼22 μm height) and the pitch between the pillars varied from 15 to 120 μm. Quantitative data on parameters governing the heat transfer process at droplet impact, including sequential droplet impact behavior, droplet residence time tR, spreading dynamics, and dynamic Leidenfrost temperature TDLT, were obtained experimentally and analyzed. Through the experimental observations, the droplet behaviors could be categorized into contact boiling, partial boiling, and film boiling regimes, and boiling regime maps for various fluids and surfaces were generated. tR remained almost constant under the same fluid conditions regardless of the fluid types, and the surface textures did not significantly affect the time scale. The spreading dynamics of water and ethanol droplets also demonstrated universal behaviors on smooth and textured surfaces. These results may indicate that no direct interaction between the droplet and surface occurs at the Leidenfrost point. However, TDLT was strongly affected by the surface textures. For water droplets, TDLT on the textured surfaces with pillar pitches of 60–120 μm increased by ∼60% compared with that on the smooth surface. However, for ethanol droplets, especially at relatively higher Weber number, TDLT was lower on all textured surfaces than on the smooth surface. To provide reasonable explanations for these observations, we performed approximate scaling analysis considering downward and upward pressures acting on a droplet at the impact and related parameters, such as vapor permeability and effective thermal conductivity, on the textured surface.
•A state of art of boiling heat transfer on micro/nano structured surface was introduced.•The technique of micro/nano structures fabrication was introduced.•The enhanced boiling heat transfer was ...reviewed by the physical mechanism.•The enhanced critical heat flux was reviewed by the physical mechanism and model.•The further research and new insight was proposed.
In the recent decades, the rapid growth of surface modification and fabrication technologies has facilitated the achievement of boiling heat transfer enhancement on micro/nanostructured surfaces. In this paper, several researches on the micro/nanostructured surfaces that have been designed to enhance boiling heat transfer are introduced and closely reviewed. Firstly, theoretical and experimental researches on nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) are introduced in the outline. The fabrication techniques for achieving these engineered surfaces, which are technically classified into machining, coating, chemical process, and micro/nanoelectromechanical systems, are described in detail in the paper. Explanations and analysis of the results of boiling heat transfer enhancement tests are presented in view of NBHT and CHF. Finally, the special features of the existing surfaces capable of enhancing boiling heat transfer are summarized, and the need for future research is also presented.
In this study, we developed a dynamic equivalent model for large-scale wind power plants based on an aggregation technique. The model is obtained in steps that include wind turbine clustering, ...development of slow dynamic models for the clusters, and aggregating of the models with the rest of wind power plant network. To confirm the effectiveness of the proposed method, the developed model is tested against the original large-scale wind power plant connected to the power system through computer simulations.
•The Venturi nozzle design with entry and exit angles, which leads the self-sucked air flow.•Microbubbles generation regarding to entry and exit angles.•High speed visualization study to track the ...self-sucked air flow stream broken into microbubbles.
We studied the effect of varying the entry and exit angles of Venturi nozzles on the formation of microbubbles in Venturi nozzle-type microbubble generators. We 3D-printed nozzles with five entry angles (15, 22, 30, 38 and 45°) and five exit angles (15, 22, 30, 38 and 45°). For the visualization experiment, we inserted the nozzles into a cover case made of aluminum and transparent acrylic. We measured the pressure drop and the air flow rate with respect to the entry and exit angles, determined the diameters of the bubbles using a digital camera, and analyzed bubble breakage by observing the behavior of the bubbles using a high-speed camera. We confirmed that the exit angle (not the entry angle) is dependent on the pressure drop and found that the air flow rate did not vary linearly with the fluid flow rate, as expected according to Bernoulli's theorem. Instead, it tended to remain constant or decrease as the fluid flow rate increased due to the abnormal flow. The sizes of the bubbles decreased as the exit angle increased, except in cases where the outlet angle was greater than 30° at high flow rates (260–300 LPM). We observed a change in bubble size with respect to exit angle. According to our visualization, the bubbles were broken by the flow separation at the beginning of the divergence at the exit.
•Liquid–vapor phase distribution and temperature of dry spots were simultaneously observed.•The relation between dry spot behaviors and CHF was explained based on the measured data.•Dynamics and ...thermal evolution of irreversible dry spot were presented as the most crucial factor.
It is generally accepted that the dry area underneath growing bubbles plays a vital role in understanding and modelling nucleate boiling heat transfer phenomena, including the critical heat flux (CHF). This study presents an investigation of the nucleate boiling phenomena under various surface heat flux conditions using a high-speed infrared thermometry technique called DEPIcT. The observation results for the liquid–vapour phase distribution on a boiling surface showed the formation, coalescence, and dynamic behaviours of dry spots on a boiling surface, and as the surface heat flux increased, the frequency and density of the dry spots significantly increased. Intensive nucleation behaviour near the triple contact line of a dry spot interrupting the wetting of the liquid was observed at a high heat flux. The liquid–vapour phase distribution and the temperature evolution of the dry spot area indicated the existence of an irreversible dry spot, which leads to surface overheating and consequent CHF triggering by rapid spreading on a boiling surface. The surface temperature initiating the formation of the irreversible dry spot was measured to 134°C, which is far lower than the Leidenfrost temperature and the maximum liquid-contact temperature reported by other studies. To provide qualitative explanations of the CHF triggering mechanism related to the irreversible dry spot, a conceptual boiling curve was presented. It can consider the hydrodynamic and thermal effects postulated on the boiling surface in the time-varying manner of dry spots and their re-wetting.
Experiments on turbulent heat transfer by supercritical CO
2 in a vertical upward flow were conducted in a tube with an inner diameter of 4.5
mm. The experiments were performed for bulk fluid ...temperatures ranging from 29 to 115
°C, pressures ranging from 74.6 to 102.6
bar, local wall heat fluxes ranging from 38 to 234
kW/m
2, and mass fluxes ranging from 208 to 874
kg/m
2
s. The wall temperature distributions were significantly influenced by wall heat flux and mass flux. The wall temperature had a noticeable peak value when the wall heat flux was moderate and the mass flux was low. To determine the buoyancy and flow acceleration effects on heat-transfer characteristics, the ratios of the Nusselt numbers obtained from the experimental data and from a reference correlation are compared with
Bo* and
q
+ along the test section. To analyze the changes in the shear stress distribution due to flow acceleration and buoyancy effects, the ratios of the shear stress reduction to wall shear stress due to flow acceleration and buoyancy effects are derived from approximate considerations. A new heat-transfer correlation is proposed, which could be used to predict heat transfer phenomena in a vertical upward flow of a supercritical fluid. The correlation is assessed by comparison with various experimental data.
•Explored the effects of liquid subcooling on droplet-wall collision in film boiling.•Liquid subcooling remarkably affected the extent of droplet collision heat transfer.•Only the local heat flux ...correlated strongly with liquid subcooling.•Subcooling effect was well predicted with heat flux partitioning at the liquid–vapor interface.
The effects of subcooling on single droplet-wall collision heat transfer characteristics during film boiling were experimentally investigated. Water droplet subcooling was varied from 0 to 60 K under atmospheric conditions, while the collision velocity and substrate temperature remained constant at 0.7 m/s and 500 °C. The instantaneous and localized heat transfer properties associated with dynamic collision of a water droplet were measured by integrated high-speed shadowgraph and infrared thermometry. Liquid subcooling greatly affected heat transfer. The residence time, effective heat transfer area, and local heat flux were measured. Only the local heat flux correlated strongly with liquid subcooling. When heat flux partitioning at the liquid–vapor interface was considered, i.e., conduction heat transfer into the subcooled droplet and the heat required for evaporation, heat transfer effectiveness was accurately predicted under various subcooling conditions.
