A novel ultra-thin flexible loop-type heat pipe for application in wearable electronic devices cooling

As electronic devices become more compact, flexible, and integrated, such as foldable smartphones, wearables, and VR glasses, traditional cooling solutions, including graphite thermal pads and conventional heat pipes, are no longer adequate. These solutions struggle to cope with complex geometries, confined spaces, and dynamic environments due to their rigid structures. Moreover, the increasing power density in electronic devices calls for more efficient, stable, and lightweight thermal management strategies. This work presents a novel ultra-thin flexible loop-type heat pipe (UFLHP) with an evaporator thickness of only 0.7 mm. The primary wick employed a trapezoidal serrated nickel felt structure, whereas the secondary wick was fabricated from an oxidized spiral-woven copper mesh (SWM). Experiments were conducted using both single- and double-layer SWM arrangements to assess their influence on performance. A systematic investigation was carried out to evaluate the thermal performance of the UFLHP across different operational parameters. The UFLHP with a double-layer secondary wick demonstrated excellent thermal performance, capable of handling heat fluxes up to 7 W/cm², with a thermal resistance as low as 0.987 K/W and an equivalent thermal conductivity of 24,204.4 W/(m·K). It remained stable even under a 90° bend, effectively managing up to 5 W of heat input with a thermal resistance of 2.22 K/W, exhibiting strong flexibility and adaptability. A theoretical model was developed to estimate steady-state parameters, including temperature and thermal resistance, and to analyze the influence of pipe diameter on system performance. At high power levels, the model predicted the evaporator temperature with an error of less than 15 %.