Performance alteration of standing-wave thermoacoustically-driven engine through resonator length adjustment

Abstract: The production of sound-wave in thermo-acoustic device is necessary to induce cooling or generate electricity. The magnitude of the sound-wave is normally proportional to the amount of heat provided to the device. The possibility to use waste heat in any locations can be enough to justify the use of thermo-acoustic technology for sustainable electricity generation or refrigeration. In this work, an adjustable thermoacoustically-driven engine has been developed using the Design Environment for Low-amplitude ThermoAcoustic Energy Conversion (DELTAEC). Many studies have highlighted the relationship between the geometry of the stack and the performance of the device. Unlike previous studies, the resonator of this thermoacoustically-driven device, made of two portions, was adjusted. The performance of the device has been analysed in order to evaluate the influence of the alteration of the resonator on the heat-to-sound conversion. Performance indicators like the acoustic power, the temperature difference across the stack and the frequency of the sound-wave have been studied. This work point out the possibility to regulate the performance of thermo-acoustic engine by adjusting the geometry of the resonator

Design and construction of a thermoacoustically driven thermoacoustic refrigerator

Abstract: This work describes the design and construction of a standing wave Thermoacoustically Driven Thermoacoustic Refrigerator (TADTAR). The thermoacoustic cooler is proposed in this study as an alternative sustainable solution to current issues with vapor compression refrigerators, due to its environmentally friendlier attributes and its solar energy driven capabilities. However, one of the main hindrances to the expansion of this technology is its current lack of efficiency and performance closely related to the designing of the device. Hence, a model has been designed and constructed to perform an experimental investigation of the device’s performance at the University of Johannesburg. This model will mainly be used to investigate the dynamics of the TADTAR arrangement. The TADTAR consists of two thermoacoustic systems namely a thermoacoustic engine coupled to a thermoacoustic refrigerator. The thermoacoustic engine consists of a heat source and a cordierite honeycomb stack which converts heat into acoustic energy. The heat pumping takes place within a thermoacoustic refrigerator. Guidance on the material selection, constraints and calculation of the geometrical configuration describing the device constitute the main contribution of this wor

The MACHO Project 2nd Year LMC Microlensing Results and Dark Matter Implications

The MACHO Project is searching for galactic dark matter in the form of massive compact halo objects (Machos). Millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are photometrically monitored in an attempt to detect rare gravitational microlensing events caused by otherwise invisible Machos. Analysis of two years of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing events, far more than the $\sim1$ event expected from lensing by low-mass stars in known galactic populations. From these eight events we estimate the optical depth towards the LMC from events with 2 < \that < 200 days to be \tau_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}. This exceeds the optical depth of 0.5\ten{-7} expected from known stars and is to be compared with an optical depth of 4.7\ten{-7} predicted for a ``standard'' halo composed entirely of Machos. The total mass in this lensing population is \approx 2^{+1.2}_{-0.7} \ten{11} \msun (within 50 kpc from the Galactic center). Event timescales yield a most probable Macho mass of 0.5^{+0.3}_{-0.2}\msun, although this value is quite model dependent.Comment: 10 pages, 6 epsf figures and style file included, 451k, also at http://wwwmacho.mcmaster.ca/Pubs/Pubs.html; To appear in the Proceedings of "Sources and Detection of Dark Matter in the Universe", Santa Monica, CA, Feb., 199

Single-mode neodymium fibre lasers

A laser medium in the form of a single mode optical fibre offers a number of attractive features. For example, using a laser to end pump such a fibre provided with feedback mirrors, allows a very low oscillation threshold to be attained. In the case of Nd doped fibre a GaAs diode laser can be used as the pump, thus providing a very simple and compact laser device. The low threshold performance also suggests that other, weaker transitions may be capable of laser action eg. the 1.3µm 4F3/2 to 4I13/2 transition in Nd or transitions in other dopants which have not previously shown laser action in a glass host. Other possible applications include the use of fibres as amplifiers e.g. as an in-line amplifier in an optical communication system where it could play the role of a repeater. As a power amplifier the fibre device could have advantages over the bulk glass systems by reducing the problems of thermal distortion and thermal fracture. In this paper we report some results obtained with neodymium-doped single mode fibres

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

The Kuiper Belt and Other Debris Disks

We discuss the current knowledge of the Solar system, focusing on bodies in the outer regions, on the information they provide concerning Solar system formation, and on the possible relationships that may exist between our system and the debris disks of other stars. Beyond the domains of the Terrestrial and giant planets, the comets in the Kuiper belt and the Oort cloud preserve some of our most pristine materials. The Kuiper belt, in particular, is a collisional dust source and a scientific bridge to the dusty "debris disks" observed around many nearby main-sequence stars. Study of the Solar system provides a level of detail that we cannot discern in the distant disks while observations of the disks may help to set the Solar system in proper context.Comment: 50 pages, 25 Figures. To appear in conference proceedings book "Astrophysics in the Next Decade

Q-switching and modelocking of neodymium-doped monomode fibre laser at 1.08µm and 906nm

There has recently been great interest in the use of rare earth-doped monomode optical fibre in high gain laser systems. The monomode fibre geometry confers a low threshold for laser operation, so that compact and highly efficient devices using semiconductor diode laser pumping can be constructed. Moreover the strong inhomogeneous broadening of the rare earth transitions in the glass fibre offers the possibility of broadband tunable operation

Monomode neodymium-doped fibre laser: tunable continuous-wave oscillation at 0.9µm

Recent experiments have shown that monomode silica fibre which has rare earth impurity ions incorporated into the core region can exhibit laser action with low threshold and high efficiency when end pumped by an external laser source. Initial studies featuring the 4F3/2 to 4 I 11/2 transition around 1.08 µm of Nd3+ ions in monomode silica fibre investigated this system under conditions of Q-switching and active modelocking and demonstrated tuning over the range 1.07 µm - 1.14 µm. A number of different sources have been used to pump fibre lasers including a cw dye laser, and GaAs diode laser, and an Ar+ ion laser

Q-switched operation of a neodymium-doped monomode fibre laser

Q-switched operation of a neodymium-doped monomode fibre laser at 1.08 µm has been demonstrated. An intracavity acousto-optic modulator was used to switch pulses of 200 ns duration and 8.8 W peak power at a repetition rate of 100 Hz

Mode-locking of a neodymium-doped monomode fibre laser

A neodymium-doped monomode fibre laser operating at 1.08 µm has been actively mode-locked using intracavity acousto-optic loss modulation. The mode-locked laser output consisted of a train of pulses of less than 1 ns FWHM with an energy of ~17 pJ at a repetition rate of 41.45 MHz. When the laser was simultaneously Q-switched the peak power of the mode-locked pulses inside the 690 ns-wide envelope was enhanced by more than three orders of magnitude