Thermal Performance of Flat Plate Pulsating Heat Pipe Using Aqueous Alcohol Solutions

Pulsating Heat Pipe (PHP) is a passive two-phase heat transfer device based on phase change induced motions of working fluid from evaporator to condenser and, as wicked heat pipes and thermosyphons, exploits both sensible and latent heat transfer modes. Due to high heat transfer capability, simple structure and ability of operating under different gravity levels and different positions, PHPs could become a novel thermal management system. Use of alcohols as additives in water allows increase wettability and reinforce Marangoni effect compared to a pure water.

In this paper, thermal performances of various alcohol solutions for a given Pulsating Heat Pipe are studied; the influence of their respective wettability is also underlined.

Copper closed loop flat plate pulsating heat pipe (CL-FPPHP) with milled 3x3 mm square channels bent into a planar serpentine with eight U-turns curves at the evaporator zone and filled with alcohol aqueous solutions was studied in horizontal and vertical positions. Water, 5%-aqueous solutions of 1-butanol and 2-butanol, and 20% of ethanol were used as working fluids with volumetric ratio of 50%. Measurements were performed via fourteen T-type thermocouples (9 in evaporator zone, 5 in adiabatic zone and 4 for the cooling fluid and ambient air) and one pressure transducer connected to the bottom side of the evaporator. LabVIEW program was used for the system control and data acquisition. All experimental series were completed for the input heat power range of 50 - 200 W, and condenser temperatures of 20 ∘C and 40 ∘C.

For the horizontal mode and condenser temperature of 20 ∘C, stable and regular oscillations were observed for all mixtures for the applied heat power 150 W and higher. Startup was observed for 1-butanol and ethanol solutions at 150 W, and for 2-butanol at 100 W. In case of pure water, any oscillations have not been detected; the device remaining in dry-out mode, until the occurrence of startup with high oscillating peaks starting from 150 W of applied heat power. The same conditions were observed for 1-butanol with a lower temperature amplitude of oscillations. Stable oscillations with low temperature peaks were detected for 2-butanol and ethanol mixtures starting from 50 W of applied heat power up to 200 W. Applied power increase leads to smooth rise of evaporator temperature. These results show the clear improvement of the CL-FPPHP thermal performances with aqueous mixtures with better wettability compared to water. Furthermore, increase of condenser temperature leads to stable operation of FP-PHP and insignificant decrease of evaporator temperatures.

Operating of the FP-PHP in vertical position is characterized by looped thermosyphon mode. Temperature peaks were observed for pure water and water / 1-butanol mixture due to initial liquid collection in the condenser zone. Increase of condenser temperature leads to rise of evaporator temperature, unlike horizontal mode.