76 research outputs found
Investigations into hydrodynamics and heat transfer in vacuum fluidised beds.
Investigations into vacuum fluidised beds has improved understanding of heat transfer and hydrodynamics which will enable optimization of various industrial processes. Significant contributions were made in theoretical and experimental analysis
Numerical model of cloud-to-ground lightning for pyroCb thunderstorms
This paper demonstrates a 2-D numerical model to represent two conceptual pyrocumulonimbus (pyroCb) thundercloud structures: i) tilted dipole and ii) tripole structure with enhanced lower positive charge layer, which are hypothesized to explain the occurrence of lightning flashes in pyroCb storms created from severe wildfire events. The presented model considers more realistic thundercloud charge structures to investigate the electrical states and determine surface charge density for identifying potential lightning strike areas on Earth. Simulation results on dipole structure-based pyroCb thunderclouds confirm that the wind-shear extension of its upper positive (UP) charge layer by 2-8 km reduces the electric field and indicates the initiation of negative surface charge density around the earth periphery underneath the anvil cloud. These corresponding lateral extensions have confined the probable striking zone of-CG and +CG lightning within 0-23.5 km and 23.5-30 km in the simulation domain. In contrast, pyroCb thundercloud possessing the tripole structure with enhanced lower positive charge develops a negative electric field at the cloud's bottom part to block the progression of downward negative leader and cause the surface charge density beneath the thundercloud to become negative, which would lead to the formation of +CG flashes. Later, a parametric study is conducted assuming a positive linear correlation between the charge density and aerosol concentration to examine the effect of high aerosol concentration on surface charge density in both pyroCb thunderclouds. The proposed model can be expanded into 3-D to simulate lightning leader movement, aiding wildfire risk management. © 2008-2012 IEEE
Frequency-Doubling of Femtosecond Pulses in “Thick” Nonlinear Crystals With Different Temporal and Spatial Walk-Off Parameters
We present a comparative study on frequency-doubling characteristics of femtosecond
laser pulses in thick nonlinear crystals with different temporal and spatial walk-off
parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at
1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of
β-BaB2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and
spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more
strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under
such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing
condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG
of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and
spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance
bandwidth for the SHG process, under different temporal and spatial walk-off conditions in
the two crystalsPeer ReviewedPostprint (author's final draft
Numerical modelling of radiative heat transfer in a polydispersion of ceramic particles under direct high-flux solar irradiation
The effects of polydispersity on radiative and interfacial convective heat transfer are investigated in particle–gas two-phase media for solar particle receiver applications. Non-grey radiative transfer is analysed using the collision-based Monte Carlo ray-tracing method. The Mie theory is employed to calculate radiative properties of particles. The finite volume method and the explicit Euler time integration scheme are used to solve the transient energy equations for the particle and gas phases. Three alternative approaches to modelling particle properties and thermal conditions are employed: (i) a novel discrete size model, in which particle groups within discrete size intervals are assigned individual properties and temperatures locally; (ii) a lumped size model, in which integral properties and a single temperature are assigned to the particle phase locally; and (iii) a monodisperse size model, in which properties are evaluated for the Sauter mean diameter of the polydispersion and a single temperature is assigned to the particle phase locally. Strongly size-dependent radiation absorption and interfacial convective heat transfer are predicted with the discrete size model for alumina particles. Particles smaller than 27.4μm located near the aperture absorb the solar irradiation and transfer heat to the gas phase most effectively. The angular spread of the incident solar radiation is found to have a negligible effect on the overall absorption, although the most uniform thermal conditions occur for the solar irradiation with the smallest confinement angle. The overall absorptance of alumina particles is higher by 3.4% and 2.7% than that of iron (III) oxide and mullite particles, respectively. The lumped and monodisperse size models allow for reduction of the computational time at the expense of lower accuracy and limited information about particle properties and thermal conditions. © 2021 The Author(s
MENGGUNAKAN METODE DISKUSI UNTUK MENINGKATKAN KEMAMPUAN MENULIS PADA SISWA KELAS IV SD NEGERI NEUHEUN KABUPATEN ACEH BESAR
Banda Ace
Design boundaries of large-scale falling particle receivers
A free falling particle receiver has been studied to investigate the design boundary of large-scale falling particle receivers. Preliminary receiver geometry and condition of falling particle curtain were scoped according to the nominal receiver capacity (135 MWth), receiver outlet temperature (800 °C) and temperature difference (147 °C) recommended by the research program. Particle volume fraction and solar energy absorptivity were analyzed for two particle sizes (280 µm and 697 µm) in different flow range. The results were then converted to part load efficiency of the receiver. Ray tracing with a scoped receiver design provided the amount of spillage and overall performance of the receiver which comprises multiple cavities with different solar energy inputs. The study revealed and quantified some inherent problems in designing falling particle receivers such as, transmission energy loss caused by low solar energy absorption, efficiency decrease in part load operation, and uneven temperature distribution across falling particle curtain.This research was performed as part of the Australian Solar Thermal Research Initiative (ASTRI), a project
supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA)
An Efficient Deep Convolutional Neural Network Model For Yoga Pose Recognition Using Single Images
Pose recognition deals with designing algorithms to locate human body joints
in a 2D/3D space and run inference on the estimated joint locations for
predicting the poses. Yoga poses consist of some very complex postures. It
imposes various challenges on the computer vision algorithms like occlusion,
inter-class similarity, intra-class variability, viewpoint complexity, etc.
This paper presents YPose, an efficient deep convolutional neural network (CNN)
model to recognize yoga asanas from RGB images. The proposed model consists of
four steps as follows: (a) first, the region of interest (ROI) is segmented
using segmentation based approaches to extract the ROI from the original
images; (b) second, these refined images are passed to a CNN architecture based
on the backbone of EfficientNets for feature extraction; (c) third, dense
refinement blocks, adapted from the architecture of densely connected networks
are added to learn more diversified features; and (d) fourth, global average
pooling and fully connected layers are applied for the classification of the
multi-level hierarchy of the yoga poses. The proposed model has been tested on
the Yoga-82 dataset. It is a publicly available benchmark dataset for yoga pose
recognition. Experimental results show that the proposed model achieves the
state-of-the-art on this dataset. The proposed model obtained an accuracy of
93.28%, which is an improvement over the earlier state-of-the-art (79.35%) with
a margin of approximately 13.9%. The code will be made publicly available
Is the Elephant Flying? Resolving Ambiguities in Text-to-Image Generative Models
Natural language often contains ambiguities that can lead to
misinterpretation and miscommunication. While humans can handle ambiguities
effectively by asking clarifying questions and/or relying on contextual cues
and common-sense knowledge, resolving ambiguities can be notoriously hard for
machines. In this work, we study ambiguities that arise in text-to-image
generative models. We curate a benchmark dataset covering different types of
ambiguities that occur in these systems. We then propose a framework to
mitigate ambiguities in the prompts given to the systems by soliciting
clarifications from the user. Through automatic and human evaluations, we show
the effectiveness of our framework in generating more faithful images aligned
with human intention in the presence of ambiguities
Development of ASTRI high-temperature solar receivers
Three high-temperature solar receiver design concepts are being evaluated as part of the Australian Solar Thermal Research Initiative (ASTRI): a flux-optimised sodium receiver, a falling particle receiver, and an expanding-vortex particle receiver. Preliminary results from performance modelling of each concept are presented. For the falling particle receiver, it is shown how particle size and flow rate have a significant influence on absorptance. For the vortex receiver, methods to reduce particle deposition on the window and increase particle residence time are discussed. For the sodium receiver, the methodology for geometry optimisation is discussed, as well as practical constraints relating to containment materialsThis research was performed as part of the Australian Solar Thermal Research Initiative (ASTRI), a project
supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA)
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