Absorbent paper–stainless-steel composite wet stack for a phase change thermoacoustic engine

Low-grade heat is widely available from solar energy, industrial waste heat, and geothermal resources, yet conventional thermoacoustic engines generally require relatively large temperature differences for onset. Phase change can enhance thermoacoustic conversion through periodic evaporation, condensation, and mass transfer. This study proposes a composite wet stack comprising absorbent paper and stainless-steel sheets. The absorbent paper provides liquid absorption and storage and forms a potential capillary transport pathway, whereas the stainless-steel sheets and slotted base provide mechanical support and maintain the initial parallel-channel geometry. The selected absorbent paper exhibited a capillary rise of 15.0 ± 0.5 mm within 1 s and a water absorption ratio of 296 ± 5 %. Compared with the dry composite stack with identical geometry and material composition, which did not achieve sustained self-excited oscillation even when the temperature difference approached 250 K, the wet composite stack achieved self-excited oscillation at an onset temperature difference of 60 ± 3 K. An atmospheric-pressure air–water standing-wave phase-change thermoacoustic engine equipped with the wet stack maintained sustained oscillations at an input heating power of 55 W. These findings demonstrate the important role of water availability and associated phase-change effects in enabling sustained thermoacoustic oscillation under low-temperature-difference conditions.