Thermal optimization of flat plate closed-loop oscillating heat pipes using microencapsulated phase change materials suspensions in hollow evaporator sections: an experimental study

• The performance of a novel FP-CLOHP structure explored with various working fluids. • 2%vol ME\PCM slurry in FP-CLOHP shows higher thermal performance than other fluids. • A 50% filling ratio is the optimal for FP-CLOHPs, balancing liquid and vapor plugs. • Lower inclination angles lead to thermal resistance rise, the vertical is the best. • 2 %vol ME\PCM slurry reaches critical temperature in 30.4 h, being the longest period. Flat plate closed-loop oscillating heat pipes (FP-CLOHPs) are efficient devices that use oscillatory fluid motion for heat transfer between evaporator and condenser sections. This study presents a novel approach to enhance thermal performance by modifying the working fluid’s properties instead of expanding the FP-CLOHP structure. It investigates the effects of microencapsulated phase change materials (ME\PCMs) combined with water and polyethylene glycol, specifically focusing on 2% and 5% volume suspensions. The study analyzes varying filling ratios (25%, 50%, and 75%) and inclination angles (0°, 30°, 60°, and 90°) on thermal resistance, temperature differences between the evaporator and condenser, and the time to reach a critical temperature of 80°C. Results reveal that the 2%vol ME\PCM suspension outperforms both deionized water and the 5%vol ME\PCM suspension, showing lower thermal resistance and longer time to reach critical temperature. An optimal filling ratio of 50% is established, providing a 6% and 32% thermal resistance improvement over the 25% and 75% ratios, respectively. The vertical orientation (90° inclination) achieves the lowest thermal resistance, aided by gravity-driven fluid circulation. Overall, this innovative design significantly enhances heat transfer efficiency compared to conventional FP-CLOHP configurations.