Adjustable frequency dielectric resonator antenna

An easy method of tuning the resonant frequency of cylindrical and ring dielectric resonator (DR) antennas using different diameters of conducting plates is presented. This technique can tune a DR antenna to operate at the design frequency without changing antenna performance. The maximum frequency tuning range can reach up to 300-500 MHz     

Parasitic coplanar three-element dielectric resonator antennasubarray

A slot-coupled rectangular dielectric resonator antenna in the parasitic coplanar configuration is investigated experimentally. The measured results for impedance and radiation patterns are presented for the three-element case. It is found that by using suitable choice of the dimensions and offset distance of each dielectric resonator, the impedance bandwidth can be as high as three times that of the single-element structure     

Investigation of optical losses in photoelastic and ridge waveguides in GaAs-AlGaAs heterostructures

Two approaches have been used to fabricate stable photoelastic waveguides with planarized surfaces on GaAs-AlGaAs heterostructures. The first approach uses tensile Ni/sub 3/GaAs stressors formed by metal-semiconductor reactions. The second approach uses inert, refractory and compressive stressors, such as RF sputtered W and RF co-sputtered WNi films. For comparison purposes, ridge waveguides have also been fabricated using the same heterostructure by a dry etching technique. Optical losses of photoelastic waveguides, measured by Fabry-Perot (FP) method at a wavelength of 1.53 /spl mu/m, are comparable to or better than those of the ridge waveguides. Material loss appears to be the primary loss mechanism in both photoelastic and ridge waveguides. These results indicate that the photoelastic waveguide processing technique reported in this study is a promising alternative to commonly used dry etching techniques for planarization.     

Very low surface recombination velocities on p-type silicon wafers passivated with a dielectric with fixed negative charge

Surface recombination velocities as low as 10 cm/s have been obtained by treated atomic layer deposition (ALD) of Al₂O₃ layers on p-type CZ silicon wafers. Low surface recombination is achieved by means of field induced surface passivation due to a high density of negative charges stored at the interface. In comparison to a diffused back surface field, an external field source allows for higher band bending, that is, a better performance. While this process yields state of the art results, it is not suited for large-scale production. Preliminary results on an industrially viable, alternative process based on a pseudo-binary system containing Al₂O₃ are presented, too. With this process, surface recombination velocities of 500–1000 cm/s have been attained on mc-Si wafers.     

Integration of biohydrogen fermentation and gas separation processes to recover and enrich hydrogen

An integrated system for biohydrogen production and separation was designed, constructed and operated where biohydrogen was fermented by Thermococcus litoralis, a heterotrophic archaebacterium, and a two-step gas separation process was coupled to recover and concentrate hydrogen. A special liquid seal system was built to deliver, compress and collect the laboratory scale, low volume gas mixtures consisting of hydrogen, nitrogen and carbon dioxide. As a result, gas mixture with 73% high hydrogen content was produced by a combination of a porous and a non-porous gas separation membrane.     

Combined cooling, heating and power: A review

Combined cooling, heating and power (CCHP) systems, including various technologies, provide an alternative for the world to meet and solve energy-related problems, such as energy shortages, energy supply security, emission control, the economy and conservation of energy, etc. In the first part of this paper, the definition and benefits of CCHP systems are clarified; then the characteristics of CCHP technologies—especially technical performances—are presented, as well as the status of utilization and developments. In the third part, diverse CCHP configurations of existing technologies are presented, particularly four typical systems of various size ranges. The worldwide status quo of CCHP development is briefly introduced by dividing the world into four main sections: the US, Europe, Asia and the Pacific and rest of the world. It is concluded that, within decades, promising CCHP technologies can flourish with the cooperative efforts of governments, energy-related enterprises and professional associations.     

Impacts of a jet''s exit flow pattern on mixing and combustion performance

The influence of modifying a jet's exit flow pattern on both the near and far-field turbulent mixing processes and on the resulting combustion performance, is explored. This reveals that, in contradiction to some common assumptions, increasing the coherence of large-scale motions can decrease molecular mixing rates, and yet can still be beneficial in some applications.

Even relatively minor changes to the exit flow pattern of a non-reacting round jet, through changes to the nozzle profile are found to propagate downstream into the far field, apparently through the underlying turbulent structure. Importantly, while a jet from a smoothly contracting nozzle is found to have higher rates of entrainment, mean spread and mean decay of the scalar field than does a long pipe jet, it has a lower rate of molecular mixing. That is, increased large-scale mixing does not necessarily result in increased fine-scale mixing. A range of devices are reviewed which enhance, or stimulate the large-scale, coherent motions in an emerging jet using acoustic, mechanical or fluidic methods. The available evidence suggests that those methods which induce instantaneously asymmetric flow structure are more effective at increasing the near-field spreading than are those which induce instantaneously axisymmetric flow structure. Only limited data are available of the effects of such near-field changes on the far-field properties. Nevertheless, the available data reveal a clear trend that this near-field flow undergoes a transition to a far-field state whose spread and decay is comparable with that of a steady jet, albeit being indelibly altered by the near-field excitation. It also suggests that “self-exciting” devices (i.e. that are not externally forced), cause a net reduction in the total entrainment relative to the unexcited jet, due to the losses induced by the device itself. Nevertheless, the changes which they can impart to the flow, such as redistributing the turbulent energy from the fine to the larger scales, can be beneficial for combustion in applications where high radiant heat transfer is desirable.

Precessing and flapping jets are found to cause an increase in flame volume relative to an equivalent simple jet (SJ), implying lower molecular mixing rates. However, importantly, this decrease in mixing is achieved with no increase in the flame length. Rather the width to length ratio of these flames is increased significantly. This is of practical significance because the length of a flame is often the limiting dimension in industrial systems. The reduced strain-rates lead to an increased presence of soot within the flame, while not, in general, significantly influencing the emission of soot from the flame. The increased volume of soot leads to increased radiation, which in turn acts to reduce flame temperature, so lowering thermal NO_x emissions through a global residence time–temperature reduction. For example, in full-scale cement kilns these burner nozzles are found to reduce NO_x emissions by around 40–60% and increase fuel efficiency (or output) by around 5–10%.     

Probabilistic capacity of a grid connected wind farm based on optimization method

This paper proposes a new method to find the capacity of a future wind farm regarding several limits of the grid system: voltage stability limits, thermal limits, voltage limits, load tap changing limitation and generator power output limitation. The method combines the optimization method and a probabilistic analysis to maximize the number of the wind turbines subject to those limits. Two types of wind farms are modeled in this paper: fixed speed wind farms and variable speed wind farms. It is concluded that the proposed method is a fast and accurate method to deal with the uncertainty of wind energy in the planning of wind farm capacity.     

Stability analysis of thermo-bioconvection in suspensions of gravitactic microorganisms in a fluid layer

In this paper, we investigate the effect of heating or cooling from below on the stability of a suspension of motile gravitactic microorganisms in a shallow fluid layer. The linear perturbation theory is used to obtain the stability diagram and the critical conditions for the onset of convection. It is found that the thermo-effects may either stabilize or destabilize the suspension, and decrease or increase the wavelength of the bioconvective pattern.