4,272 research outputs found
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Combined Numerical Analysis of an Oil-free Twin Screw Compressor Using 3D CFD and 1D Multi-chamber Thermodynamic Model
The application of three-dimensional computational fluid dynamics in twin-screw compressors provides an outstanding opportunity for developers to gain an understanding of the complex internal flow phenomena occurring within the machine. Equipped with this knowledge, design parameters, such as clearances and port geometry, can be optimised, to enhance performance. However, as with all modelling, be it numerical or analytical, a high degree of certainty in the accuracy of the results is necessary.
This paper presents the results of a study of oil-free twin screw compressor in which the results of two modelling techniques are compared. The modelling techniques used are an analytical non-dimensional thermodynamic chamber model and a numerical computational fluid dynamic model. The paper presents an overview of an oil-free twin screw compressor machine, before describing important operating characteristics and the modelling techniques used. To validate, both models are compared against historical test data, this validation indicated the chamber model is more accurate. Following this, the focus will be on the comparison of key performance indicators, including, volume flow rate, volumetric efficiency, indicated power, and discharge temperature at varying duty points. The paper concludes that the difference between both models decreases as the compressor operating speed increases, although the level of variance is dependent on pressure ratio
Functional near-infrared spectroscopy for Hb and HbO₂ detection using remote sensing
The goal of the work presented in this thesis is to develop a wireless, near-infrared (NIR) imaging system to provide flexibility and functionality to clinicians and researchers who require monitoring of blood profusion to tissue, muscles, or the brain. The prototype device uses a single stimulus/detection unit composed of an Epitex NIR LED with three wavelength options: 730, 805, and 850 nm, and an OPT101 photodiode detector. The device can be used to detect changes in the levels of oxygenated and deoxygenated hemoglobin in the body by measuring the amounts of absorbed and backscattered light at the wavelength associated with the correct compound. The backscattered light collected by the optical sensor is converted to a digital, serial bit stream for wireless transmission to a base station computer. The usefulness of this design may significantly change the way in which researchers and clinicians study the human body. Without the need to attach a subject to bulky equipment and confine them to a laboratory setting, the investigator can gather data unrestricted by the experimental setting. This advantage permits a vital metabolic indicator to be studied in many different and extremely difficult situations
A software approach to defeating side channels in last-level caches
We present a software approach to mitigate access-driven side-channel attacks
that leverage last-level caches (LLCs) shared across cores to leak information
between security domains (e.g., tenants in a cloud). Our approach dynamically
manages physical memory pages shared between security domains to disable
sharing of LLC lines, thus preventing "Flush-Reload" side channels via LLCs. It
also manages cacheability of memory pages to thwart cross-tenant "Prime-Probe"
attacks in LLCs. We have implemented our approach as a memory management
subsystem called CacheBar within the Linux kernel to intervene on such side
channels across container boundaries, as containers are a common method for
enforcing tenant isolation in Platform-as-a-Service (PaaS) clouds. Through
formal verification, principled analysis, and empirical evaluation, we show
that CacheBar achieves strong security with small performance overheads for
PaaS workloads
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Experimental Investigation of the Transient Flow in Roots Blower
Rotary positive displacement machines are common method to pump flow in various process industries. Their performance highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding and reducing internal leakage flows. However, Developments of grid generating tools for use of CFD in rotary positive displacement machines have not yet been fully validated. Thereby arising a need to validate these models that help in better understanding of the leakage flows. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access to analyse flows in in such machines. This paper describes the setup of the experimental test rig with the optical Roots blower in the Centre for Compressor Technology at City, University of London and the first results obtained using three different flow visualization methods. These are namely i) the high-speed camera (HC), ii) the continuous time resolved PIV (CPIV) and iii) the instantaneous PIV obtained with double pulse PIV laser and double shutter camera (IPIV). Test results from these three tests are compared and discussed in the paper. The CPIV test shows the movement of the vortex and the general shape of the flow field clearly but is not sufficient to calculate velocity vectors of high-velocity particles due to the limitation of the laser and camera. The IPIV test can produce quantitative velocity vector images of the internal flow but needs improvement to look into the leakage flow. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flows. The objective is to enable further improvements in 3D CFD analysis of leakage flows in rotary positive displacement machines and ultimately lead to the improvement in the performance of rotary positive displacement machines
A search for rotating radio transients and fast radio bursts in the Parkes high-latitude pulsar survey
Discoveries of rotating radio transients and fast radio bursts (FRBs) in
pulsar surveys suggest that more of such transient sources await discovery in
archival data sets. Here we report on a single-pulse search for dispersed radio
bursts over a wide range of Galactic latitudes (|b| < ) in data
previously searched for periodic sources by Burgay et al. We re-detected 20 of
the 42 pulsars reported by Burgay et al. and one rotating radio transient
reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into
account this result, and other recent surveys at Parkes, we corrected for
detection sensitivities based on the search software used in the analyses and
the different backends used in these surveys and find that the all-sky FRB
event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be FRBs day sky, where the
uncertainties represent a confidence interval. While this rate is lower
than inferred from previous studies, as we demonstrate, this combined event
rate is consistent with the results of all systematic FRB searches at Parkes to
date and does not require the need to postulate a dearth of FRBs at
intermediate latitudes.Comment: Accepted, 10 pages, 6 figure
Esterification of Glycerol with Acetic Acid over Highly Active and Stable Alumina-based Catalysts: A Reaction Kinetics Study
The catalytic activity of Cu- or Ni monometallic and Cu-Ni bimetallic (Cu/Ni ratio = 3, 1, 0.33) catalysts supported on γ-Al2O3 and SO42–/γ-Al2O3 catalysts were evaluated for esterification of glycerol. The reactions were performed in a batch reactor under reflux at standard reaction conditions: temperature 110 °C, atmospheric pressure, glycerol to acetic acid molar ratio 1:9, and catalyst loading 0.25 g. The best catalytic activity was observed over 2 M SO42–/γ-Al2O3 catalyst, which showed the glycerol conversion of 97 % within 2 hours of reaction. At this condition, the selectivity to glyceryl monoacetate (MAG), glyceryl diacetate (DAG), and glyceryl triacetate (TAG) were 27.0 %, 49.9 % and 23.1 %, respectively, after 5 h of reaction. After three consecutive runs, the 2 M SO42–/γ-Al2O3 catalysts showed superior performance and no loss in activity was observed. The reaction kinetics results over 2 M SO42–/γ-Al2O3 catalyst showed that the dependence on the reaction rate to glycerol concentration was of pseudo-second order, while the activation energy was found to be 106 kJ mol–1
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