Experimental investigation of flow boiling heat transfer in novel oblique-finned microchannels

Oblique fins are modified from its conventional straight fins counterpart via the introduction of oblique cuts at specific locations, leaving obliquely-shaped fins with secondary microchannels. The benefits of having these secondary microchannels have been conclusively demonstrated in single-phase mode, however, they have yet to be tested in two-phase mode, which forms the motivation behind this study. Flow boiling experiments are conducted in the oblique-finned microchannel heat sink, made from a copper block with a footprint area of 25 × 25 mm2. 40 parallel microchannels, each with a nominal width of 0.3 mm are machined with wire-cut electro-discharge machining process. The nominal width of the oblique cuts, on the other hand, is half of that of the parallel microchannels. Tests are performed with FC-72 dielectric fluid with an inlet temperature of 29.5 °C, with a range of mass fluxes from 175 kg/m2 s to 350 kg/m2 s and heat fluxes from 6 W/cm2 to 120 W/cm2. Flow boiling results and discussion on its heat transfer performance and pressure drop characteristics are presented in this paper. It is found that nucleate boiling is the dominant heat transfer mechanism at low heat fluxes, with a transition to convective boiling-dominant via thin-film evaporation occurring at medium heat fluxes. A full convective boiling mechanism is observed thereafter prior to CHF. These observations agreed well with high-speed flow visualisation images. Inlet pressure fluctuations within the oblique-finned microchannels are extremely small, indicating that the oblique fins are able to considerably stabilise the flow boiling process.