Pulsating flows in a Helmholtz pulse combustor tailpipe were numerically simulated by a commercial CFD software package, FLUENT. The effects of ambient temperature on the characteristics of the pulsating tailpipe flows were studied. Two study cases, with high and low levels of ambient temperature, were simulated with compressible flow equations. An additional case, with high ambient temperature, was simulated with incompressible (temperature-dependent density) flow equations. Results showed that the effect of ambient temperature on the mean temperature profile in the tailpipe was limited to the distance where the ambient fluid traveled into the tailpipe during the period of flow reversal. In this region, the amplitude of mass flow rate oscillation significantly increased, due to higher density associated with low ambient temperature. The overall effects of cooler ambient temperature included an increase in mean pressure at the entrance of the tailpipe and a decrease in the magnitude of velocity amplitude profile along the tailpipe. Interestingly, the mean velocities along the tailpipe, even at the tailpipe exit, were not affected by the cooler ambient air. The mean velocity at the exit corresponded to the higher temperature of fresh fluid from upstream, which was not affected by the ambient temperature, driven out of the tailpipe in each oscillation cycle. The linear acoustic theory with appropriate assumptions could be used to calculate the magnitude of the profiles of velocity amplitude along the tailpipe as a fair approximation, at least for the study cases in this thesis.

This research introduces a new application of the three microphones method, which was originally developed to analyse standing waves, to measure the aeroacoustic power of a duct housing a shallow cavity coupled with a longitudinal acoustic mode. In addition, this work provides, for the first time, the spatial distribution of the aeroacoustic sound sources over the cavity region for this type of flow-sound-structure interaction pattern. Furthermore, this research includes a comprehensive study of the effect of cavity geometrical parameters on the characteristics of the cavity aeroacoustic source. An experimental investigation of the aeroacoustic source of an axisymetric cavity in a pipeline is presented. This aeroacoustic source is generated due to the interaction of the cavity shear layer oscillation with the resonant acoustic field in the pipe. The source is determined under high Reynolds number, fully developed turbulent pipe flow. The experimental technique (Sound Wave Method, SWM) employs six microphones distributed upstream and downstream of the cavity to evaluate the fluctuating pressure difference generated by the oscillating cavity shear layer in the presence of externally imposed sound waves. The results of the dimensionless aeroacoustic sources are in good agreement with the concepts of free shear layer instability and the fluid resonant oscillation behavior. A validation study is performed in order to validate the measurement technique and the measured source term from the SWM. The validation methodology consisted of comparing the self-excited resonance response obtained from self-excitation measurements with that estimated from an acoustic model supplemented with the measured source term using the SWM. The comparison depicts a very good agreement for the resonance frequency, lock-in ranges, and the resonance amplitude. Extensive PIV flow measurements are performed to clarify the non-linear behavior of the aeroacoustic source at high levels of the acoustic particle velocity, and to understand the dependence of the flow-sound interaction patterns on the main system parameters such as the Strouhal number and excitation level. The results of a finite element analysis of the resonant sound field are combined with those of the PIV flow measurements into Howe’s aeroacoustic integrand to compute the spatial and temporal distributions of the aeroacoustic sources. The results are also compared with the measured aeroacoustic source strength obtained by means of the SWM. This comparison highlights the superior efficiency of the SWM technique. Identification of the aeroacoustic source distributions as function of the acoustic excitation level showed that the non-linear behaviour of the source strength, which occurs at moderate sound levels, is caused by a gradual transition in the vorticity field oscillation pattern; from a distributed vorticity cloud over the whole cavity length at small excitation amplitudes to a pattern involving rapid formation of (discrete) vortices at the leading edge which becomes dominant at large excitation levels. The spatial distribution of the acoustic power over the cavity length at resonance condition shows sources of sound generation at the first and last thirds of the cavity mouth and an absorption sink in the middle third. This distribution is different from that observed for deep cavities and trapped modes of shallow cavities. Due to these differences in the aeroacoustic source distributions, the effects of cavity geometrical parameters for the present shallow cavity are not necessarily similar to those reported in the literature for deep cavities and trapped mode resonance cases. A comprehensive study of the effect of cavity geometrical parameters (including rounding-off the cavity edges) on the aeroacoustic sound sources is also included. Nine cavity sizes are studied in three different groups of length to depth ratios (L/H) with three different cavity volumes for each group of L/H. The aeroacoustic source strength and the Strouhal number corresponding to its maximum value are found to increase in a systematic manner as the cavity volume is increased for the same L/H ratio. These results indicate that the aeroacoustic sources of shallow cavities are affected not only by the ratio L/H, but also by the cavity volume. The effect of cavity edge curvatures on the resonance response is experimentally investigated by testing different sizes of curvatures at different locations (upstream, downstream or both edges). The results show that rounding-off the cavity edges causes a reduction in the vertical component of the acoustic particle velocity but also an increase in the cavity length. These two consequences have opposite effects on acoustic power generation and therefore, rounding-off the edges has no significant effect on the resonance amplitude in the present case, except for relatively large radius. / Thesis / Doctor of Philosophy (PhD)

This thesis investigates the relevance between the acoustic resonance and power harmonics on a metal halide lamp. First, a sinusoidal current ranging from 20 kHz to 400 kHz is used to drive a 70 W metal halide lamp. Second, a hybrid-current test circuit is designed to generate a current waveform consisting of a low-frequency square-wave and a high-frequency sinusoidal wave. Both of the frequency and the amplitude can be adjusted independently. The test lamp is deliberately driven at its acoustic-resonance eigen-frequencies to observe the effect of the power spectrum on the degree of the acoustic resonance. The experimental results indicate that the occurrence of acoustic resonance is indeed affected by the DC level and related power harmonics. The power harmonic spectrum that elucidates the initiation of acoustic resonance is deduced from the observations. It is found that the power harmonics that excites acoustic resonance can be divided into three categories. The first is independent of the average lamp power; it excites acoustic resonance only if the magnitude of its power exceeds a specific level. The thresholds of power harmonics belong to the second category are proportional to their DC powers. One can also find those remaining power harmonics belong to the third category. The power harmonic spectrum of the acoustic resonance is demonstrated by driving the test lamp with quasi-square-wave and triangle-wave currents. This work helps advance the understanding of the phenomena and mechanism of acoustic resonance in a metal halide lamp.

A single-stage high-power-factor electronic ballast with auto frequency searching capability provides a compact and efficient solution for ballasting metal halide lamps. The circuit configuration is originated from the integration of a buck-boost converter and a half-bridge resonant inverter. The buck-boost converter is designed to operate in discontinuous current mode (DCM) to improve the input power factor and at the same time to regulate the output lamp power. The resonant inverter operating at a high frequency is adopted to obtain a high efficiency on the power conversion circuit. The control strategy of auto frequency searching is realized by a microprocessor along with the acoustic resonance detection circuit. To avoid the acoustic resonance, an auto-frequency-searching method is used to search ¡§quite windows¡¨ on operating metal halide lamps with the high-frequency electronic ballast. Provided the acoustic resonance should happen to the lamp on operation, the electronic ballast will automatically change the operating frequency until a stable frequency is located. When the operating frequency has been changed, the duty-ratio of the buck-boost converter is adjusted to regulate the lamp power at the rated value. Experimental tests are carried out on a laboratory with 70-W metal halide lamps to verify the effectiveness of the auto-frequency- searching control.

The acoustic resonance phenomena of metal halide lamps are investigated. A measuring system is set up to examine the effects of the acoustic resonance to the shape of the lamp arc, the lighting energy spectrum, the color temperature, the light output as well as electrical characteristics. Two ballast circuits are built to drive the lamps with the sine-wave current and the square-wave current, respectively. One ballast employs the series resonant inverter to output the sinusoidal lamp current. The other makes use of the full-bridge inverter to drive the lamps with the square-wave current. They are operated over a high-frequency range from 20kHz to 30kHz. For both ballast circuits, the operating frequency and the magnitude of the lamp current can be controlled independently. ¡@¡@The experiments are conducted on the 70-W metal halide lamps. The experimental results show that the spectral energy and the color temperature change more significantly as the acoustic resonance becomes more serious. These effects become less significant when the lamps are driven by the square-wave current. The degree of the acoustic resonance can be identified by detecting the variation of the lamp voltage or the output light. The investigated results of the thesis can provide useful information when a standard of the acoustic resonance is considered.

Abstract The metal halide lamp is one of high-intensity discharge lamps. It has many advantages such as good color rendering, high efficacy and a variety of color presentations. However, the problems of acoustic resonance, a long transition period of cold starting, and an extremely high ignition voltage for hot restarting should be overcome. The investigation of the thesis is focused on the electrical characteristics of the metal halide lamp operating at high-frequency. Included are the acoustic resonance, starting transient and steady state operation. Various ballast circuits incorporating with ignitors are designed to drive several 70-W metal halide lamps. The operating frequency ranges with acoustic resonance, the ignition voltages for both cold starting and hot restarting, the transition period, the dimming performance, and the lamp equivalent resistance during the lamp life are measured and analyzed. Based on the investigated results, a useful guideline can be provided for the operation and design of the electronic ballast for metal halide lamps. Key words : Metal halide lamp, electronic ballast, acoustic resonance, hot restarting.

A high-frequency electronic ballast with auto-tracking control was proposed to operate the metal halide lamps at a specific frequency free from acoustic resonance. In case the acoustic resonance should happen, the operating frequency is changed step by step with the auto-tracking control, until the lamp is operated at a frequency with stable operation. The electrical characteristics of the lamps are first investigated. Based on the investigated results, a detection circuit is designed to identify the occurrence of acoustic resonance. With the auto-tracking control, the Class-D half-bridge series-resonant inverter can be adopted for the high-frequency electronic ballast to achieve high efficiency and high power density. The control strategy of auto-tracking is practically realized by a single-chip microprocessor. The proposed approach is implemented on a 70 W test lamp with an operating frequency range from 20 kHz to 30 kHz. To regulate the lamp power at its rated value, a buck-boost converter is used as a pre-regulator, which serves also as a power-factor-corrector to achieve a high power factor at the input line.

Acoustic methods can often be used with limited or no sample preparations making them ideal for rapid process analytical technologies (PATs). This dissertation focuses on the possible use of acoustic resonance spectroscopy as a PAT in the pharmaceutical industry. Current good manufacturing processes (cGMP) need new technologies that have the ability to perform quality assurance testing on all products. ARS is a rapid and non destructive method that has been used to perform qualitative studies but has a major drawback when it comes to quantitative studies. Acoustic methods create highly non linear correlations which usually results in high level computations and chemometrics. Quantification studies including powder contamination levels, hydration amounts and active pharmaceutical ingredient (API) concentrations have been used to test the hypothesis that bootstrap enhanced n-dimensional deformation of space (BENDS) could be used to overcome the highly non linear correlations that occur with acoustic resonance spectroscopy (ARS) eliminating a major drawback with ARS to further promote the device as a possible process analytical technology (PAT) in the pharmaceutical industry. BENDS is an algorithm that has been created to calculate a reduced linear calibration model from highly non linear relationships with ARS spectra. ARS has been shown to correctly identify pharmaceutical tablets and with the incorporation of BENDS, determine the hydration amount of aspirin tablets, D-galactose contamination levels of Dtagatose powders and the D-tagatose concentrations in resveratrol/D-tagatose combinatory tablets.

The operating characteristics of metal halide lamps are investigated, including acoustic resonance, spectral energy, and luminous efficacy. To operate metal halide lamps at intended conditions, two test sophisticated ballast circuits are built to drive the lamps with sine-wave current and square-wave current, respectively. One ballast employs the series resonant inverter to output sinusoidal lamp current over a high-frequency range from 20 kHz to 300 kHz. The other makes use of the full-bridge inverter to drive the lamps with square-wave current from 50 Hz up to 300 kHz. For both test circuits, the operating frequency and the magnitude of the lamp current can be controlled independently. On the other hand, the lamp power is adjusted by regulating the DC-link power. Several conclusions are drawn from experimental results: (1) Little difference is found between the lighting spectra of a lamp when driven by sinusoidal current and square-wave current. (2) Luminous efficiency deteriorates as the operating frequency increases. The deterioration is more significant at lower frequencies. (3) Luminous efficiency decreases considerably as the lamp power is reduced. (4) Arc instability from acoustic resonance is highly related to the waveform of the lamp current. The investigated results give better understanding on the steady state operation of metal halide lamps and provide useful information for the design of the electronic ballasts.

The metal halide lamp has become an attractive lighting source because of its compact size, good color rendering, long lamp life, and high luminous efficacy. As a member of high-intensity discharge lamps, it has a negative incremental resistance, which claims the necessity of a ballast circuitry. Similar to other gas discharge lamps, the operating performance can be further improved when driven by a high-frequency electronic ballast. However, there are some obstacles in ballasting the metal halide lamp with the high-frequency inverter. For a cold lamp, an ignition voltage up to several kVs is required for breaking down the electrodes during starting period. The breakdown voltage and the equivalent lamp resistance may vary from time to time and lamp to lamp, and is sensitive to the used time. Furthermore, the ignition voltage for restarting a hot lamp can be ten times that for a cold lamp. On the other hand, the lamp driven by a high-frequency electronic ballast may suffer from acoustic resonance. All these make it difficult in the design of an electronic ballast, especially for the applications with hot restarting. In this dissertation, the operating characteristics for both starting transient and steady-state of the metal halide lamp are first investigated. Then, a simple method by measuring the lamp voltage is proposed to detect the happening of acoustic resonance. Based on the investigated results, several electronic ballasts are designed for driving metal halide lamps with capabilities of wide input voltage range, high input power factor, hot restarting, fast transition. In addition, an inverter circuit is configured for ballasting multiple lamps. A buck-boost power-factor-correction circuit is integrated into the load resonant inverter to achieve a high power factor, fast transition, and constant power operation. The extremely high ignition voltage for hot restarting is generated by an auxiliary ignitor. The electronic ballast is precisely operated at the specific frequency at which acoustic resonance will not occur. In addition to these features, a protection circuit is included to prevent from high voltage and/or current stresses on circuit components in case that the lamp fails to be started up or comes to the end of its life-time. For the ballast with multiple lamps, the load circuits with abnormal lamps can be isolated from the others which are under normal operation. Prototypes of the proposed circuits are built and tested. Experimental results present the satisfactory performances.