A vapor-liquid phase-change thermoacoustic engine driven by near room-temperature heat source

Utilization of low-grade heat has attracted considerable attention as an inspiring pathway towards sustainable and low-carbon energy future. However. low-grade heat, either in the form of natural source or waste heat, is abundantly available but not yet effectively exploited. With the outstanding merits of free maintenance and high reliability, thermoacoustic engine is conceived as one of cost-effective and reliable alternative technologies for low-grade heat recovery. Most recently, two-phase thermoacoustic engine induced increasing interest, thanks to its capability of running across a small temperature difference between heat source and heat sink. In this work. a vapor-liquid phase-change thermoacoustic engine, where the working fluid undergoes periodic condensation and evaporation, is constructed to realize low temperature-differential operation. The engine consists of a looped thermoacoustic unit and a branch resonator, where the former contains a cold heat exchanger. a regenerator. a hot heat exchanger and segments of tubes, while the latter contains a gas reservoir and a load tube. R134a is adopted as the working fluid in the preliminary study of this thermoacoustic engine, for its boling point near ambient temperature at the operating pressure around 0.5 MPa. Besides, R1234ze(E) is also used as the working fluid. thanks to its zero ozone depletion potential and low global warming potential. The experiments focus on onset temperature and pressure ratio, since the former determines the lowest grade of usable heat source, while the latter is an essential parameter for evaluating the intensity of pressure oscillation. The results show that an onset temperature of 18.1 degrees C. lowest ever reported, can be achieved with R134a as working fluid when the regenerator is packed with copper mesh screens. The highest pressure ratio of 1.094 is achieved with R1234ze(E) as working fluid at a hot temperature of 183 degrees C and a heating power of 200 W. which is comparable to the single-stage looped travelling wave thermoacoustic engine with helium of 1 MPa as working fluid at a similar hot temperature but a much lower heating power. Besides, the onset temperature firstly drops and then rises with the decreasing mesh number of regenerator's mesh screen, while the pressure ratio always rises. It is also found that regenerator material has marked effect on the engine performance. The engine with copper mesh screen achieves the lowest onset temperature, then stainless steel, and finally nylon. Besides, the cases of pressure ratio are in the increasing order of nylon_ stainless steel and copper. The low temperature-differential onset enables the phase-change thermoacoustic engine to harvest the near roomtemperature heat source. Further work on performance optimization of this novel thermoacoustic engine is still necessary and will be our near future work.