Heat loss through the piping of a large solar collector field

The effect of the heat loss from the piping system of a solar collector field of a large size is measured and the effect of this loss on the effective collector efficiency is evaluated. The data are used to develop a model that can be used to evaluate the effect of piping loss on collector efficiency for solar field of different areas. The collector field used in this study consists of 1064 evacuated tube collectors; each one has an absorber area of 1.75 m². They are connected in a series–parallel arrangement and designed in a U-shape. Measurements of the heat loss in the piping system indicate that a drop in the collector efficiency occurs due to this heat loss. The effect of the heat loss on the collector efficiency was found to depend on the collector x-parameter with higher x corresponding to larger efficiency drop. The standard Hottel–Whillier–Bliss (HWB) equation for the collector efficiency was modified so that the form of the equation is maintained while the parameters change due to the inclusion of piping loss in the equation. The modified parameters were estimated for the collector field based on measurement and the data were used in the model to generalize the results for field with different piping to collector area ratios.     

Irreversibility Analysis of MHD Flow over an Exponentially Stretching Sheet

This laminar fluid study investigates the effects of a magnetic field on the entropy generation during fluid flow and heat transfer due to an exponentially stretching sheet. Using the suitable transformations we have obtained the analytical solutions for momentum and energy equation in terms of Kummer's function. The velocity and temperature profiles are obtained for various physical parameters which are utilized to find the entropy generation number N_s and the Bejan number Be. The effects of various parameters on entropy production number and the Bejan number are studied through graphs using velocity and temperature profiles. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21112     

Dynamic bandwidth allocation for quality-of-service over Ethernet PONs

Ethernet-based passive optical network (EPON) technology is being considered as a promising solution for next-generation broadband access networks due to the convergence of low-cost Ethernet equipment and low-cost fiber infrastructures. A major feature for this new architecture is the use of a shared transmission media between all users; hence, medium access control arbitration mechanisms are essential for the successful implementation of EPON: i.e., to ensure a contention-free transmission and provide end users with equal access to the shared media. We propose to use the multipoint control protocol defined by the IEEE 802.3ah task force to arbitrate the transmission of different users, and we present different dynamic bandwidth allocation (DBA) algorithms to allocate bandwidths effectively and fairly between end users. These DBA algorithms are also augmented to support differentiated services, a crucial requirement for a converged broadband access network with heterogeneous traffic. We show that queueing delays under strict bandwidth allocation algorithms result in an unexpected behavior for certain traffic classes, and we suggest the use of DBA with appropriate local queue management to alleviate this inappropriate behavior. We conduct detailed simulation experiments to study the performance and validate the effectiveness of the proposed protocols.     

Resolution Improvement of Scanning Acoustic Microscopy Using Sparse Signal Representation

Scanning acoustic microscopy is an imaging method in which the focused high frequency ultrasound is used to visualize the micro structures. The morphology and acoustic properties of the biological tissues can be evaluated using scanning acoustic microscope system. To determine thin tissues having micrometer thickness, the high acoustic frequency is required for conventional SAM. In practice the acoustic frequency is restricted by the penetration depth through the material. Characterization of thin sliced tissue is difficult, as the reflected signals from top and bottom are superimposed. In order to improve the axial resolution of conventional SAM, a technique based on sparse signal representation in overcomplete time–frequency dictionaries is investigated and among the great number of algorithms for finding sparse representation, we first apply matching pursuit (MP) and basis pursuit (BP) and then propose the orthogonal matching pursuit (OMP) and stagewise orthogonal matching pursuit (StOMP) algorithms to decompose the A-scan signal to an overcomplete Gabor dictionary. Different criteria are used for measuring the performance of these algorithms in C-scan imaging. The proposed method can separate closely space overlapping echoes beyond the resolution of conventional SAM systems and also the final results show that StOMP performs best overall in extracting the specific echo, since this algorithm is precise and fast.     

The dual-edge alignment technique with improved spur reduction effects in ring oscillators

In this paper, the technique of dual-edge phase-alignment and its effects on ring oscillators are presented. This technique provides resetting of the built-up jitter at both rising and falling edges of a clean aligning signal named as the reference. Using signals and systems theory, the effect of the technique on the spurs due to the aligning reference signal is completely analyzed. We show that adding properly the dual-edge aligning feature to phase-aligned ring oscillators cause the spurs in the neighborhood of the carrier to disappear. First, the validity of the results obtained from the derived analytical equations is verified through simulations using MATLAB. Then a complete circuit considering realistic models for its components is simulated in TSMC RF CMOS 0.18um process using ADS for further validation.     

Genetic Algorithm for the Multiple-Query Optimization Problem

Producing answers to a set of queries with common tasks efficiently is known as the multiple-query optimization (MQO) problem. Each query can have several alternative evaluation plans, each with a different set of tasks. Therefore, the goal of MQO is to choose the right set of plans for queries which minimizes the total execution time by performing common tasks only once. Since MQO is an NP-hard problem, several, mostly heuristics based, solutions have been proposed for solving it. To the best of our knowledge, this correspondence is the first attempt to solve MQO using an evolutionary technique, genetic algorithms     

An 18-mW 2.5-GHz/900-MHz BiCMOS dual frequency synthesizer with<10-Hz RF carrier resolution

A 2.5-GHz/900-MHz dual fractional-N/integer-N frequency synthesizer is implemented in 0.35-μm 25-GHz BiCMOS. A ΔΣ fractional-N synthesizer is employed for RF channels to have agile switching, low in-band noise, and fine frequency resolution. Implementing two synthesizers with an on-chip ΔΣ modulator in a small package is challenging since the modulator induces substantial digital noise. In this work, several design aspects regarding noise coupling are considered. The fractional-N synthesizer offers less than 10-Hz frequency resolution having the in-band noise contribution of -88 dBc/Hz for 2.47-GHz output frequency and -98 dBc/Hz for 1.15-GHz output frequency, both measured at 20-kHz offset frequency. The prototype dual synthesizer consumes 18 mW with 2.6-V supply     

Low-loss high power RF switching using multifinger AlGaN/GaN MOSHFETs

We demonstrate a novel RF switch based on a multifinger AlGaN/GaN MOSHFET. Record high saturation current and breakdown voltage, extremely low gate leakage current and low gate capacitance of the III-N MOSHFETs make them excellent active elements for RF switching. Using a single element test circuit with 1-mm wide multifinger MOSHFET we achieved 0.27 dB insertion loss and more than 40 dB isolation. These parameters can be further improved by impedance matching and by using submicron gate devices. The maximum switching power extrapolated from the results for 1A/mm 100 /spl mu/m wide device exceeds 40 W for a 1-mm wide 2-A/mm MOSHFET.     

Compact WLAN disc antennas

A novel conical beam patch antenna design, suitable for local area network applications, uses a central cylindrical connection from the disc to ground to achieve a large reduction in resonant size. Results on efficiency, bandwidth, and pattern are reported for several prototypes.