Heat driven thermoacoustic cooler based on traveling–standing wave
Introduction
The thermoacoustic engine (TAE) converts heat to acoustic energy by the thermoacoustic principles. The acoustic energy produced by the TAE can be used to drive the thermoacoustic cooler (TAC). The system combining the TAE and TAC is called a heat driven thermoacoustic cooler system [1], [2]. In the heat driven thermoacoustic cooler system, when a steep temperature gradient is set up along the regenerator, the acoustic wave with the oscillating pressure and the oscillating velocity is excited, where ω is the angular frequency and φz is the leading phase of p1 to u1. When the acoustic wave passes through the TAC’s regenerator, a temperature gradient along the TAC’s regenerator with a certain coefficient of performance (COP) is set up. The acoustic wave drives gas parcels in the thermoacoustic regenerators to experience a certain thermodynamic cycle. Then, the conversion from heat to acoustic energy and the heat pumping occur without any moving parts in the system. Compared with traditional heat devices, the heat driven thermoacoustic cooler system has three main advantages: (1) it has simple structure, no moving parts, low cost of manufacture, and high reliability; (2) by using inert gases as working fluid, this kind of machines is environmentally friendly; (3) the thermoacoustic devices can be driven by low quality energy source such as exhausting thermal energy and solar energy, so it is significant for remote rural areas where electricity is scarce.
The research on the heat driven thermoacoustic cooler system has progressed steadily over decades, and profited greatly from the publication of thermoacoustic theory. In the early time, the heat driven thermoacoustic cooler is heat driven orifice pulse tube coolers [3], [4] which used the TAE instead of the compressor. These inventions are effective cryocooler apparatus without any moving part. However, the orifice pulse tube cooler is efficient only in the cryogenic temperature region and is inherently inefficient for small temperature spans. For the orifice pulse tube cooler, the cooling power is comparable in magnitude to the mechanical power dissipated as heat due to the orifice. For cryogenic coolers, the Carnot COP is very small, so the energy wasted by the orifice is almost negligible and less important than other factors. For a small temperature span cooler (e.g. an air conditioner), the orifice pulse tube cooler would have a COP less than unity. And a heat driven cooler would have a total efficiency defined as the ratio of the cooling power to the heating power very much less than unity.
Subsequently, research on heat driven thermoacoustic cooler with small temperature span has drawn many attentions [5]. Because the match between varied TAE and TAC has great influence on the efficiency, many researchers have paid particular attention to the cascade thermoacoustic device with compact structure. In 1989 [1], a heat driven thermoacoustic cooler was designed by Wheatley et. al. Although the cooler was simple and had no moving parts, it had a very poor total efficiency in the range of 0.05–0.1 for a modest cooling temperature span. The reason of the low efficiency is that most of the standing wave component (TWC) propagates in opposite direction of the temperature gradient in the TAE’s regenerator, which results in that the TWC consumes acoustic energy [6]. This substantially decreases the efficiency of the engine. In 1997, Hofler [7] enhanced the efficiency of the engine by adjusting the contribution of the TWC in this type of device. However, the regenerators of that device mainly worked in the standing wave mode whose thermoacoustic conversion was based on an intrinsically irreversible thermodynamic cycle. In 2002, Ueda et al. [8] studied a system of two regenerators in one loop tube and pointed out that the reversible functions of both the TAE and the TAC was realized. In such heat driven thermoacoustic cooler system, the TAC was located above the TAE. The cold end of the TAC’s regenerator was close to the pressure antinode (PAN), and the ambient end of the TAE’s regenerator was close to the PAN. As the description of Ref. [8], the arrangement of the regenerators can be obtained as shown in Fig. 1. For the cooler’s regenerator, the TWC pumps heat from cold end to hot end [6], while the standing wave component (SWC) transports heat from hot end to cold end [9]. This degrades the efficiency of the TAC. For the TAE’s regenerator, the TWC propagates from cold end to hot end to amplify the acoustic power [6], while the SWC consumes acoustic energy [9]. This substantially decreases the efficiency of the TAE.
For improving the thermoacoustic efficiency, this paper will introduce a heat driven thermoacoustic cooler by adjusting the contributions of the TWC and the SWC. Although the two regenerators of the system introduced in this paper are also located in one loop tube coupled with a resonator tube, the TAC is located below the TAE, as shown in Fig. 2. By designing these thermodynamic components carefully, it is possible to make the PAN in the center of the thermal buffer tube (TBT) between the TAE and the TAC, and the direction of the acoustic power flow in the loop tube is anti-clockwise. This makes the arrangement of the regenerators be same as Fig. 3. For the cooler’s regenerator, both the TWC and the SWC pump heat from the cold end, which improves the COP of the TAC [6], [9]. For the engine’s regenerator, both the SWC and the TWC realize the conversion from heat to acoustic energy [6], [9], which improves the efficiency of the TAE.
Section snippets
Simulation of the thermoacoustic device
Although there are many computation software of thermoacoustics (e.g. Design Simulation for ThermoAcoustic Research (DSTAR) [10], REGEN3 [11], and Design Environment for Low-amplitude ThermoAcoustic Energy Conversion (DeltaEC) [12]), just DeltaEC can be used to calculate a variety of complicated thermoacoustic device with low-amplitude pressure. The calculation capability and precision of the thermoacoustic devices has been validated by many research [13], [14]. DeltaEC written by Los Alamos
Experimental system
The thermoacoustic device studied here is a heat driven thermoacoustic cooler as shown in Fig. 2. It is composed of a torus-shaped section and a resonator tube. A description of the each parts of the system is given in the following.
The resonator tube includes a straight pipe with an inner diameter of 22 mm, a taper tuber, and a cavity with an inner diameter of 80 mm. The lengths L1, L2 and L3 of the resonator tube are 0.46 m, 0.28 m and 0.14 m respectively.
The torus-shaped section contains a
Experimental results and discussion
To test practical performances of the whole system, a series of experiments have been done. During the preliminary experiments, the hot end temperature of the TAE’s regenerator was mostly kept around 650 °C or below. The heat was input into the HHE of the TAE and an acoustic oscillation starts spontaneously when the temperature inside the HHE rises above a critical value.
We present firstly the agreement between the measured and calculated in order to yield confidence in the calculation, and
Conclusions
This paper proposed a new configuration of the heat driven thermoacoustic cooler. Based on new configuration idea and the analysis results, a miniature scale heat driven thermoacoustic cooler has been designed, manufactured and tested. Compared with the existed types of the heat driven thermoacoustic coolers, this device has the merits: (1) it utilizes synthetically the thermoacoustic performance contributed by both of the traveling wave component and the standing wave component; (2) the
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 50890181).
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