Experimental study on heat transfer performance of pulsating heat pipes with hybrid working fluids

Pulsating heat pipe (PHP), a passive two-phase heat transfer device, has excellent application prospects in electronic cooling, renewable energy systems, heat recovery devices, etc. An experimental study was conducted to investigate the heat transfer performance of PHP with hybrid working fluids, i.e., ethanol-water mixtures (20, 30, 40, 50 and 75 vol%), graphene nanofluids (0.1, 0.3, 0.5, 0.75 and 1.0 mg/ml) and surfactant aqueous solutions (25, 50, 100, 200 and 500 ppm). For this purpose, liquid-phase exfoliated graphene (LPEG) was dispersed in 40 vol% ethanol-water mixture to prepare graphene nanofluids with excellent stability. The thermophysical properties of hybrid working fluids were then measured. Experimental results indicate that non-ionic surfactant, Pluronic® F-127, can cost-effectively exfoliate graphene from graphite using high-shear mixer. It provides a favorable condition for the large-scale application of graphene in heat transfer devices. Among ethanol-water mixtures, 30 vol% ethanol-water mixture exhibits the best thermal performance, i.e., E_p=16.01%. Graphene nanofluids as working fluids can decrease thermal resistance up to 25.16%, and the higher thermal resistance for 1.0 mg/ml graphene nanofluids is attributed to the higher dynamic viscosity and nano sheets accumulations. The heat transfer performance of PHP with surfactant aqueous solution is related to both type and concentration of surfactant. Compared with other surfactants (SDS, CTAC, Pluronic® F-127, PVP), Triton X-100 surfactant aqueous solution appears to be more effective in the improvement of thermal performance with the average enhancement ratio of 13.56%. These experimental results may help to choose the more appropriate working fluids for PHP.