International Journal of Heat and Mass Transfer
Visualization observation of onset and damping behaviors in a traveling-wave thermoacoustic engine by infrared imaging
Introduction
Thermoacoustic engine (TE) is a new type of long-life, non-polluting engine that can utilize thermal energy, even low grade thermal energy, with feature of no moving parts [1]. It has promising applications in the fields of electronic device cooling, natural gas liquefaction, pulse tube refrigeration, and power generation. In recent years, researchers have been making efforts to increase the output acoustic power and pressure ratio of a TE by improving the resonator [2], applying cascade or new coupling configurations [3], [4].
Onset indicates the transition of a thermoacoustic engine from the stationary to periodic oscillating state, when the temperature gradient along the regenerator is larger than the critical value (the corresponding heating temperature is so-called onset temperature). As the onset temperature determines the heat grade that a TE can utilize, the onset mechanism attracts more and more academic attentions. In fact, theoretical study including linear [5], [6], nonlinear [7], and computational fluid dynamics (CFD) study [8], [9] needs more detailed experimental verification. In experimental study, temperature distribution of regenerator is commonly used to analyze the onset process. Unfortunately, only discrete temperature distribution instead of temperature field can be obtained [10], [11] due to limited temperature sensors, which is insufficient for theoretical verification, especially for CFD study. Up to now, there is no visual method reported to obtain the full-scale images of the temperature distributions in TE study.
Gedeon streaming appears in the oscillating flow when there is a loop [12]. Former research shows that the Gedeon streaming significantly changes the temperature distribution of the regenerator and may seriously deteriorate the performance of a TE [1], [13], [14], [15]. In order to improve the performance of thermoacoustic engine, some methods have been proposed to suppress [1] or even eliminate (by using an elastic membrane) the Gedeon streaming [13]. However, the onset and damping processes with the influence of Gedeon streaming have not been fully investigated [16], [17] because of theoretical complexity and measurement difficulty.
Infrared (IR) imaging camera can conveniently obtain the visual images of the radiated heat of the objects in the absence of light. The visual temperature distribution has been successfully demonstrated as an alternative means in TE study by present authors [18]. More visual information has been obtained, which is nearly impossible with traditional methods. This inspired us to use an IR imaging camera to visually measure the temperature distribution and evolution of a thermoacoustic regenerator during the onset and damping processes.
Section snippets
Experimental apparatus
Fig. 1 illustrates the schematic of a traveling-wave TE with an IR imaging camera. The traveling-wave TE is detailed elsewhere [19], which consists of a loop and a resonator tube. The loop is composed of a heater, cooler, regenerator, feedback tube, and compliance. An elastic membrane is used to eliminate the Gedeon streaming in the TE, which is placed between the compliance and main cooler [13]. Without the membrane, the Gedeon streaming appears in the loop of the TE. The configuration when
Influence of Gedeon streaming on the onset process
Fig. 3 shows the IR temperature distribution images of the thermoacoustic regenerator in the onset process under two conditions: without and with Gedeon streaming. The temperature of all the images is adjusted in the range of 15–200 °C without streaming, and 30–200 °C with streaming. The black region represents the lowest temperature, while the white is the highest. All the images display obvious color separation in the axial direction of the regenerator, indicating the existence of temperature
Discussion
IR imaging is an indirect measurement method for characterizing the regenerator temperature distribution. It measures the regenerator wall which is not insulated. The traditional method using thermocouples also measures the outside wall of regenerator, which is covered by asbestos. They are used to estimate the internal distribution of regenerator based on the hypothesis: (1) the thickness of the tube wall is small enough that the heat conduction along the tube wall from the heater to the
Conclusion
Using an IR imaging camera as a means of characterizing temperature distributions of thermoacoustic regenerators, the visual IR images have been obtained for the first time in TE study. The visual IR images enhanced the understanding of the dynamic processes of onset and damping without and with Gedeon streaming in the loop of the TE. When the outside wall of the regenerator is not insulated, without Gedeon streaming, the axial temperature distribution is nonlinear before onset and linear after
Acknowledgements
The project is supported by National Funds for Distinguished Young Scientists of China under contract No. 50825601 and the Major State Basic Research Development Program of China under contract No. 2010CB227303.
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