Bulletin of the American Physical Society
2011 Annual Meeting of the Four Corners Section of the APS
Volume 56, Number 11
Friday–Saturday, October 21–22, 2011; Tuscon, Arizona
Session M7: Acoustics |
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Chair: Koen Visscher, University of Arizona Room: UA Student Union Tubac |
Saturday, October 22, 2011 11:10AM - 11:22AM |
M7.00001: Determining the Order of Importance of Six Geometric Parameters Related to Heat Exchangers for Thermoacoustic Prime Movers Michael Jorgensen, Bonnie Andersen Thermoacoustic devices can be used to convert heat energy into sound energy. Heat exchangers (HEXs) deliver to and remove heat from the device. HEX elements have thicknesses and spacing between elements to allow heat to flow to the working fluid. A significant portion of thermal and viscous losses can, therefore, result from non-optimal HEXs. For high frequency engines, using pre-fabricated copper-woven screens for HEXs is an attractive alternative to more traditional structures at the small scales required. However, with set wire sizes and spacing, the ideal geometry is not easily achieved. Principal component analysis is employed to determine which factors contribute the most variability to the performance of the device. The parameters explored are the relative sizes of the thermal penetration depth of both the working fluid and the screen to the wire spacing and wire radius of the screens and the relative size of the oscillating displacement of the working fluid to the diameter of the wires of the screens. The relative size between the oscillating displacement and the wire diameter is found to have the largest impact. [Preview Abstract] |
Saturday, October 22, 2011 11:22AM - 11:34AM |
M7.00002: Effect of Varying Resonator Length for Thermoacoustic Prime Movers Cory Heward, Bonnie Andersen Improvement of performance of thermoacoustic engines is critical to advancing the technology. Bottle-shaped themoacoustic prime movers have a quarter-wave resonator neck and a large cavity. The cylindrical neck region consists of a cold side and a hot side with the heat exchangers and the ``stack'' between the two. The geometric parameters of these parts determine the resonant frequency and affect the acoustic pressure of the device. To achieve an optimum of acoustic power, the stack should be positioned at the center of the neck where contributions of the pressure and velocity waves are a maximu,, giving an ideal ratio of hot to cold side length of 50{\%}. However, radiation loss is directly proportional to the length of the hot side. Thus, the optimum ratio will be less than 50{\%}. Studies show that increasing the length of the neck decreases the resonant frequency according to a transcendental equation that is independent of this ratio. Results confirm that decreasing the ratio increases the acoustic pressure within the device. Three cold side lengths were tested with up to eight hot side lengths. For the longest cold side tested, the ratio of 36{\%} is optimum. [Preview Abstract] |
Saturday, October 22, 2011 11:34AM - 11:46AM |
M7.00003: Ultrasound-induced nanofragmentation of bubbles Debra Cox, James Thomas Micron-sized bubbles are of considerable interest for use in biomedical imaging and drug delivery. Lipid-coated bubbles have been reported to rapidly shrink in the presence of short (3 $\mu$s) ultrasound pulses, purportedly by shedding of the coat during compression. Loss of coat would increase the internal pressure, enhancing diffusive gas loss long after the pulse. Note that during such a short pulse, diffusive gas loss is insignificant. If lipid-shedding is the mechanism for bubble shrinkage, a coated bubble in ultrasound may shrink no faster than a quiescent uncoated bubble; the shrinkage rate is entirely dominated by diffusive loss between pulses. Remarkably, we find that most insonated lipid-coated bubbles do shrink faster than quiescent uncoated bubbles. If bubbles cannot shrink by diffusive gas loss, they must fragment, though no fragmentation was observed. The results are consistent with ``nanofragmentation,'' where sub-micron fragments (which entrap gas) are lost from the bubble. Entrapment of gas in fragments may have important consequences for their efficacy in ultrasound-mediated drug delivery, and could affect their ability to transfer drugs to cells. [Preview Abstract] |
Saturday, October 22, 2011 11:46AM - 11:58AM |
M7.00004: Optimizing the Stack Length of Thermoacoustic Prime Movers David Pease, Bonnie Andersen Thermoacoustics involves the fusion of thermodynamic and acoustic phenomena. Common applications of thermoacoustics include air conditioning and electricity generation. There are many components within a thermoacoustic device to optimize. The quarter-wave resonator used contains the heat exchangers and the stack. The temperature gradient between heat exchangers across the stack must exceed a critical temperature gradient for acoustic oscillations to be produced. The power of the device is also proportion the ratio of the two gradients minus one. The region between stack elements is the productive volume where the thermoacoustic effect takes place. Decreasing the length between heat exchangers will increase the temperature gradient, but will reduce the productive volume. This research tested four different stack lengths from 0.014'' to 0.060'' to find the optimum stack length for a temperature difference of 150 K. In order to do this, an optimum amount of stack, given the local ambient environmental conditions, operating frequency, and stack length had to be found first. With the optimum stack amount, the optimal stack length was found to be 0.030'', giving a temperature gradient of 1970 K/cm. [Preview Abstract] |
Saturday, October 22, 2011 11:58AM - 12:10PM |
M7.00005: The Role of Higher Harmonics In Musical Interval Perception Richard Krantz, Jack Douthett Using an alternative parameterization of the roughness curve we make direct use of critical band results to investigate the role of higher harmonics on the perception of tonal consonance. We scale the spectral amplitudes in the complex home tone and complex interval tone to simulate acoustic signals of constant energy. Our analysis reveals that even with a relatively small addition of higher harmonics the perfect fifth emerges as a consonant interval with more, musically important, just intervals emerging as consonant as more and more energy is shifted into higher frequencies. [Preview Abstract] |
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