Characterization of Two-Phase Flow in Microchannels

Aluminum multi-port microchannel tubes are currently utilized in automotive air conditioners for refrigerant condensation. Recent research activities are directed toward developing other air conditioning and refrigeration systems with microchannel condensers and evaporators. Three parameters are necessary to analyze a heat exchanger performance: heat transfer, pressure drop, and void fraction. The purpose of this investigation is the experimental investigation of void fraction and frictional pressure drop in microchannels. A flow visualization analysis is another important goal for two-phase flow behavior understanding and experimental analysis. Experiments were performed with a 6-port and a 14-port microchannel with hydraulic diameters of 1.54 mm and 1.02 mm, respectively. Mass fluxes from 50 to 300 kg/s.m2 (range of most typical automotive applications) are operated, with quality ranging from 0% to 100% for two-phase flow experiments. R410A, R134a, and air-water mixtures are used as primary fluids. The results from the flow visualization studies indicate that several flow configurations may exist in multi-port microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Flow mapping of the fluid regimes is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. Experimental analysis and flow observations suggest that pressure drop and void fraction in microchannel is dependent on the most probable flow regime at which the two-phase mixture is flowing. In general, correlations for void fraction and pressure drop predictions are based in a separated flow model and do not predict the experimental results in the range of conditions investigated. A flow regime based model is developed for pressure drop and void fraction predictions in microchannels.Air Conditioning and Refrigeration Project 10

Intermittent maser flare around the high mass young stellar object G353.273+0.641 I: data & overview

We have performed VLBI and single-dish monitoring of 22 GHz H$_{2}$O maser emission from the high mass young stellar object G353.273+0.641 with VERA (VLBI Exploration of Radio Astrometry) and Tomakamai 11-m radio telescope. Two maser flares have been detected, separated almost two years. Frequent VLBI monitoring has revealed that these flare activities have been accompanied by structural change of the prominent shock front traced by H2O maser alignments. We have detected only blue-shifted emissions and all maser features have been distributed within very small area of 200 $\times$ 200 au$^{2}$ in spite of wide velocity range (> 100 km s$^{-1}$). The light curve shows notably intermittent variation and suggests that the H$_{2}$O masers in G353.273+0.641 are excited by episodic radio jet. The time-scale of \sim2 yr and characteristic velocity of \sim500 km s$^{-1}$ also support this interpretation. Two isolated velocity components of C50 (-53 \pm 7 km s$^{-1}$) and C70 (-73 \pm 7 km s$^{-1}$) have shown synchronised linear acceleration of the flux weighted V_{\rmn{LSR}} values (\sim-5 km s$^{-1}$ yr$^{-1}$) during the flare phase. This can be converted to the lower-limit momentum rate of 1.1 \times 10$^{-3}$ M_{\sun} km s$^{-1}$ yr$^{-1}$. Maser properties are quite similar to that of IRAS 20126+4104 especially. This corroborates the previous suggestion that G353.273+0.641 is a candidate of high mass protostellar object. The possible pole-on geometry of disc-jet system can be suitable for direct imaging of the accretion disc in this case.Comment: 13 pages, 5 figures accepted for publication in MNRA

A novel intermittent fault detection algorithm and health monitoring for electronic interconnections

There are various occurrences and root causes that result in no-fault-found (NFF) events but an intermittent fault (IF) is the most frustrating. This paper describes the challenging and most important area of an IF detection and health monitoring that focuses toward NFF situation in electronics interconnections. The experimental work focuses on mechanically-induced intermittent conditions in connectors. This paper illustrates a test regime, which can be used to repeatedly reproduce intermittence in electronic connectors, while subjected to vibration. A novel algorithm is used to detect an IF in interconnection. It sends a sine wave and decodes the received signal for intermittent information from the channel. This algorithm has been simulated to capture an IF signature using PSpice (electronic circuit simulation software). A simulated circuit is implemented for practical verification. However, measurements are presented using an oscilloscope. The results of this experiment provide an insight into the limitations of existing test equipment and requirements for future IF detection techniques. Aside from scheduled maintenance, this paper considers the possibility for in-service intermittent detection to be built into future systems, i.e., can IFs be captured without external test gear

Redundancy Allocation of Partitioned Linear Block Codes

Most memories suffer from both permanent defects and intermittent random errors. The partitioned linear block codes (PLBC) were proposed by Heegard to efficiently mask stuck-at defects and correct random errors. The PLBC have two separate redundancy parts for defects and random errors. In this paper, we investigate the allocation of redundancy between these two parts. The optimal redundancy allocation will be investigated using simulations and the simulation results show that the PLBC can significantly reduce the probability of decoding failure in memory with defects. In addition, we will derive the upper bound on the probability of decoding failure of PLBC and estimate the optimal redundancy allocation using this upper bound. The estimated redundancy allocation matches the optimal redundancy allocation well.Comment: 5 pages, 2 figures, to appear in IEEE International Symposium on Information Theory (ISIT), Jul. 201

Effects of Intermittent Emission: Noise Inventory for Scintillating Pulsar B0834+06

We compare signal and noise for observations of the scintillating pulsar B0834+06, using very-long baseline interferometry and a single-dish spectrometer. Comparisons between instruments and with models suggest that amplitude variations of the pulsar strongly affect the amount and distribution of self-noise. We show that noise follows a quadratic polynomial with flux density, in spectral observations. Constant coefficients, indicative of background noise, agree well with expectation; whereas second-order coefficients, indicative of self-noise, are about 3 times values expected for a pulsar with constant on-pulse flux density. We show that variations in flux density during the 10-sec integration account for the discrepancy. In the secondary spectrum, about 97% of spectral power lies within the pulsar's typical scintillation bandwidth and timescale; an extended scintillation arc contains about 3%. For a pulsar with constant on-pulse flux density, noise in the dynamic spectrum will appear as a uniformly-distributed background in the secondary spectrum. We find that this uniform noise background contains 95% of noise in the dynamic spectrum for interferometric observations; but only 35% of noise in the dynamic spectrum for single-dish observations. Receiver and sky dominate noise for our interferometric observations, whereas self-noise dominates for single-dish. We suggest that intermittent emission by the pulsar, on timescales < 300 microseconds, concentrates self-noise near the origin in the secondary spectrum, by correlating noise over the dynamic spectrum. We suggest that intermittency sets fundamental limits on pulsar astrometry or timing. Accounting of noise may provide means for detection of intermittent sources, when effects of propagation are unknown or impractical to invert.Comment: 38 pages, 10 figure

Advanced information processing system: Local system services

The Advanced Information Processing System (AIPS) is a multi-computer architecture composed of hardware and software building blocks that can be configured to meet a broad range of application requirements. The hardware building blocks are fault-tolerant, general-purpose computers, fault-and damage-tolerant networks (both computer and input/output), and interfaces between the networks and the computers. The software building blocks are the major software functions: local system services, input/output, system services, inter-computer system services, and the system manager. The foundation of the local system services is an operating system with the functions required for a traditional real-time multi-tasking computer, such as task scheduling, inter-task communication, memory management, interrupt handling, and time maintenance. Resting on this foundation are the redundancy management functions necessary in a redundant computer and the status reporting functions required for an operator interface. The functional requirements, functional design and detailed specifications for all the local system services are documented

Random Access Protocol for Massive MIMO: Strongest-User Collision Resolution (SUCR)

Wireless networks with many antennas at the base stations and multiplexing of many users, known as Massive MIMO systems, are key to handle the rapid growth of data traffic. As the number of users increases, the random access in contemporary networks will be flooded by user collisions. In this paper, we propose a reengineered random access protocol, coined strongest-user collision resolution (SUCR). It exploits the channel hardening feature of Massive MIMO channels to enable each user to detect collisions, determine how strong the contenders' channels are, and only keep transmitting if it has the strongest channel gain. The proposed SUCR protocol can quickly and distributively resolve the vast majority of all pilot collisions.Comment: Published at the IEEE International Conference on Communications (ICC), 2016, 6 pages, 6 figures. (c) 2016 IEEE. Personal use of this material is permitte

A Survey on Wireless Security: Technical Challenges, Recent Advances and Future Trends

This paper examines the security vulnerabilities and threats imposed by the inherent open nature of wireless communications and to devise efficient defense mechanisms for improving the wireless network security. We first summarize the security requirements of wireless networks, including their authenticity, confidentiality, integrity and availability issues. Next, a comprehensive overview of security attacks encountered in wireless networks is presented in view of the network protocol architecture, where the potential security threats are discussed at each protocol layer. We also provide a survey of the existing security protocols and algorithms that are adopted in the existing wireless network standards, such as the Bluetooth, Wi-Fi, WiMAX, and the long-term evolution (LTE) systems. Then, we discuss the state-of-the-art in physical-layer security, which is an emerging technique of securing the open communications environment against eavesdropping attacks at the physical layer. We also introduce the family of various jamming attacks and their counter-measures, including the constant jammer, intermittent jammer, reactive jammer, adaptive jammer and intelligent jammer. Additionally, we discuss the integration of physical-layer security into existing authentication and cryptography mechanisms for further securing wireless networks. Finally, some technical challenges which remain unresolved at the time of writing are summarized and the future trends in wireless security are discussed.Comment: 36 pages. Accepted to Appear in Proceedings of the IEEE, 201