Enhanced thermal performance of a thermosyphon for waste heat recovery: Microporous coating at evaporator and hydrophobic coating at condenser

摘要

An enhanced two-phase closed thermosyphon (TPCT) was experimentally investigated for waste heat recovery in heat pipe heat exchanger, which was capable with two different surface coatings: a microporous coating at the evaporator and a hydrophobic coating at the condenser. The TPCT was made of a copper pipe with an inner diameter of 25 mm and was comprised of three sections: evaporator, adiabatic, and condenser sections, which were 300, 300, and 325 mm long, respectively. Distilled water was used as the working fluid and the filling ratio was varied between 0.25 and 1.0, at four inclination angles; 90 degrees (vertical), 60 degrees, 45 degrees, and 15 degrees. The microporous structures of the evaporator section were coated using a sintering process, and the hydrophobic coating at the condenser section was formed by the electrostatic spraying of a perfluoroalkoxy alkane (PFA) resin. From the comparison of the overall thermal resistances of bare and coated TPCTs, it is concluded that the effect of the microporous coating was largest with a low filling ratio and the effect of the hydrophobic coating was largest with a high filling ratio. When both the evaporator and condenser were coated, the TPCT exhibited a maximum enhancement of 100% (filling ratio = 0.25) at a heat flux of 50 kW/m(2) and an average 61% enhancement at a heat flux of 100 kW/m(2), compared with the TPCT with bare surfaces.