Thermal performance comparison of flat plate pulsating heat pipes of different material thermal conductivity using ethanol-water mixtures

A flat plate pulsating heat pipe (FPPHP) consists of mini/micro capillary channels machined on a flat plate in a serpentine manner. The working fluid oscillates in the form of liquid slugs and vapor plugs transferring heat. The slug-plug oscillations, internal flow regimes, startup heat requirement, thermal resistance, and evaporator dryout depend on the thermal conductivity of the flat plate. High thermal conductivity material has larger heat spreading through it with a possibility of pressure homogenizing in the system, low working fluid oscillations, and early dry-out of the working fluid. No study has compared the thermal conductivity of the different plate materials suitable with various working fluids in an FPPHP, and the present work fills this gap in the literature. In this work, the thermal characteristics of a high thermal conductivity copper and a comparatively low thermal conductivity steel FPPHPs are experimentally investigated and compared using different ethanol-water mixtures as the working fluid. Results are reported for pure water and ethanol and mixtures of ethanol:water in the ratio 1:1, 3:1, 5:1, 9:1, and 30:1 as the working fluids, filled at 60%. The power inputs are given from 40 to 200 W. The ethanol-water mixed in an ideal ratio compared to pure water or ethanol as the working fluid gives smaller thermal resistances, larger slug-plug oscillations, and lesser evaporator drying-out occurrences. The working fluid oscillations mainly influence the total thermal resistance in the steel FPPHP, and the fluid resistance is the minimum at all power inputs. In copper FPPHP, the spreading (dry) resistance through the plate primarily influences the total thermal resistance. At the optimum ethanol-water mixture ratio of 9:1, steel FPPHP has lower thermal resistance and larger slug velocities than copper FPPHP in the wide range of power inputs of 60 to 140 W. The present work on the effect of thermal conductivity and binary mixtures on the thermal performance of the FPPHP will help to design suitable FPPHP for different applications.