Experimental demonstration of a liquid piston Stirling engine with a wet regenerator

• A liquid piston Stirling engine (LPSE) with a wet regenerator was designed and built. • A water-absorbing wick was used to continuously supply liquid to the regenerator. • Critical temperature and power output between LPSE(dry) and LPSE(wet) were compared. • LPSE (wet) operates at a temperature of 35 °C lower than that of LPSE(dry). • LPSE (wet) has >10 times the acoustic power of LPSE(dry) at low temperatures. In this study, a liquid piston Stirling engine with a water-absorbing wick to keep the regenerator wet (LPSE(wet)) was designed and built. The feasibility of operating the LPSE (wet) at lower temperatures to increase power output compared with that of a liquid piston Stirling engine with a dry regenerator (LPSE (dry)) was experimentally verified. First, the critical temperatures of LPSE (wet) and LPSE (dry) were measured. The experimental results revealed that LPSE (dry) operated at 100 °C, whereas LPSE (wet) operated at 65 °C. Thus, in this study, LPSE (wet) can be operated at a critical temperature 35 °C lower than that of LPSE (dry). The LPSE (wet) could be operated continuously for 900 min, demonstrating the effectiveness of the water-absorbing wick mechanism for long-duration operation. Next, the specific acoustic impedance distribution, the phase difference between the pressure and the cross-sectional mean velocity distribution, and the acoustic power distribution inside the device were measured for LPSE (dry) and LPSE (wet) during each operation. The results confirmed that both LPSE (dry) and LPSE (wet) realized a traveling wave field in which the phase difference between the pressure and cross-sectional mean velocity was close to zero and the specific acoustic impedance more than 30 times the characteristic impedance of a sound wave propagating in free space, which are required to obtain high thermal efficiency, near the regenerator position. The ratio of acoustic power before and after the regenerator was 1.18 (at a hot end temperature of 130 °C) for LPSE (dry) and 1.54 (at a hot end temperature of 90 °C) for LPSE (wet). Under the same conditions, the acoustic power of LPSE (wet) was 10 times or above that of LPSE (dry). These results confirmed that LPSE (wet) designed in this study could operate at lower temperatures and exhibit higher-power output than LPSE(dry).