Thermoacoustic Phenomena in Small-Scale Systems

Sungmin Jung

Thermoacoustic phenomena in small-scale systems are investigated, and results are presented on the following topics: the acoustic properties of porous and fibrous materials, the modeling of thermoacoustic resonators with nonuniform medium and boundary conditions, and the harvesting of energy from tonal sound excited by heat addition and vortex shedding. The transfer function measurement system is used to find the acoustic properties of porous and fibrous materials. The complex wave numbers and characteristic impedances of reticulated vitreous carbon (RVC) and plastic mesh are determined using a variation of the three-microphone and four-microphone methods with the transfer function technique. The wave numbers and characteristic impedances of RVC and plastic mesh can be estimated from the obtained results. To find the effect of temperature difference, relative acoustic power changes across RVC, stacked plastic screens and stacked steel screens at f´T=0,,aC and at f´T,,l100,,aC are experimentally determined and compared. It can be concluded that these porous stacks generate acoustic power under a temperature gradient, partly compensating for the acoustic losses when sound energy propagates through the stack. The numerical modeling of thermoacoustic resonators with nonuniform media and boundary conditions is carried out. Sparse numerical grids are used in the bulk of resonators, and in boundary layers near solid surfaces analytical solutions impose proper boundary conditions. The main advantage of this method is computational efficiency. Since it can quickly estimate the effects of all parameters of geometry and material properties, the present model is suitable for optimizing thermoacoustic systems. A small-scale, low-aspect-ratio thermoacoustic engine with a flexing wall oscillator is modeled. If the natural frequency of the flexing wall oscillator is selected to be much lower than the natural frequency of the acoustic resonator, the engine operates at satisfactory efficiencies and requires a relatively low temperature difference threshold. Heat transfer calculations, consideration of large-amplitude acoustic effects, and analysis of electroacoustic transducers are needed for further developments. Three resonator-type acoustic energy harvesters are tested and demonstrated. In the resonator, tonal sound is excited by heat addition or vortex shedding in the presence of mean flow. A PZT disk with a brass back plate is used as an electroacoustic transducer. The first system with baffles in the mean flow generated more than 0.5 mW of electric power at a resistance of 10 k£[ and a mean flow velocity around 2.6 m/s. The second system has one side open and generated a maximum electric power of 0.446 mW at a resistance of 14.8 k£[. In the third experiment, the closed resonator produced a maximum harvested electric power of 7.02 mW at a resistance of 14.8 k£[. The experimental results correlate reasonably well with those of previous studies by other researchers. To increase power output, optimization of the piezoelement and system geometry is required.

Thermoacoustic Phenomena in Small-Scale Systems

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