Thermoacoustic engines convert heat energy into high amplitude acoustic waves and subsequently into electric power. This article provides a review of the four main methods to convert the (thermo)acoustic power into electricity. First, loudspeakers and linear alternators are discussed in a section on electromagnetic devices. This is followed by sections on piezoelectric transducers, magnetohydrodynamic generators, and bidirectional turbines. Each segment provides a literature review of the given technology for the field of thermoacoustics, focusing on possible configurations, operating characteristics, output performance, and analytical and numerical methods to study the devices. This information is used as an input to discuss the performance and feasibility of each method, and to identify challenges that should be overcome for a more successful implementation in thermoacoustic engines. The work is concluded by a comparison of the four technologies, concentrating on the possible areas of application, the conversion efficiency, maximum electrical power output and more generally the suggested focus for future work in the field.
A bidirectional impulse turbine to convert thermoacoustic power into electricity is investigated. Experimental measurements are done with a loudspeaker for varying acoustic conditions and turbine loads. The results are used to characterize the turbine performance and compare it to steady flow turbomachinery and turbines in oscillating water columns. A dimensional analysis is done to identify the variables that influence the turbine performance, after which a scaling is determined that uniquely determines the efficiency of the turbine. The work is finished by providing the impedance of the bidirectional turbine such that it can be implemented in a thermoacoustic engine.
Determination of propagation model matrix in generalized cross-correlation based inverse model for broadband acoustic source localization
Abstract-Despite the end of the Internet bubble, operators continue to increase the capacity of their networks. The question now rises whether these improvements still result in faster communications, or whether most flows are limited by other aspects. In order to answer this question, actual network traffic needs to be analyzed. Therefore, in this paper methods are proposed to identify the factors that limit the speed of TCP flows. Three main categories will be distinguished: the network, the TCP buffers and the application layer. Our proposed methods have been tested on real traces; in many cases it turned out that the network was not the limiting factor. I. INTRODUCTIONIn the mid eighties of the previous century the first version of the Dutch research network, called SURFnet, allowed end users to communicate at speeds up to 9.6 Kbps. Now, with the introduction of the sixth version of SURFnet, users get the ability to communicate at Gigabit speed. In twenty years network capacity thus grew with a factor of a hundred thousand, which is more than the increase of CPU power or computer memory. Whereas in the past bandwidth shortage has always been a problem, the question rises if this still holds. Should bandwidth still be considered a potential performance bottleneck, or is communication speed limited by other factors? Research on extensions of TCP for long-delay paths already indicated that the size of the receive window can limit the speed of a TCP flow too [1]. An interesting question is therefore how to detect if a given flow is limited by the network, or by the receive window. Or, if a flow is not limited by these factors, are there other factors that limit the speed?The main problem addressed in this paper is how to identify the factors that limit the speed of a TCP flow. Our goal is to find methods that determine these factors in an algorithmic manner, without human intervention. We will also apply these methods to traffic traces previously collected from SURFnet, to get an idea of whether all factors play a role in practice, or only some of them.The structure of this paper is as follows. Section II identifies the factors that, in theory, could limit the speed of a TCP flow. Then, Section III discusses related work. Section IV develops methods that, for a given flow, detect which of these factors limits the speed in practice. Section V focuses on one of these methods and shows, in detail, how it works. Due to space constraints it is impossible to discuss all methods in detail; the interested reader is referred to [2]. To give an impression of which factors play a role in the current
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