Effect of dual diameter length on the performance characteristics of a single loop pulsating heat pipe

This study investigates the combined effects of geometry, filling ratio, inclination angle, and heating power on the heat transfer characteristics of single loop dual diameter pulsating heat pipes (DPHPs). Three DPHPs with varying dual-diameter length ratios were constructed, each featuring a larger internal diameter (ID) of 2.5 mm and a smaller ID of 1.8 mm. The smaller diameter section lengths were 88 mm for DPHP1, 118 mm for DPHP2, and 150 mm for DPHP3. DPHP1 and DPHP2 confined the smaller diameter to the adiabatic section, while DPHP3 extended it 5 mm into the evaporator. Constructed from borosilicate glass, these PHPs were evaluated using visualization techniques and thermal resistance calculations. The study compared these with a uniform diameter PHP (UPHP) with a consistent ID of 2.5 mm. Results showed that heat transfer mechanisms evolve with heat load, with film evaporation becoming significant at higher loads. DPHP1 had the best start-up performance, while DPHP2 exhibited the lowest thermal resistance, indicating superior thermal performance. DPHP2's optimal performance was at a 60% filling ratio and 90 degrees inclination, operating effectively at 30 degrees. Its maximum heat input capacity increased by 20% compared to UPHP and DPHP1, and by 50% compared to DPHP3 in vertical orientation.