Experimental and numerical investigations of heat transfer and flow characteristics during flow boiling in straight and diverging PDMS microchannel

Recent trends of miniaturization and densely packed circuits have necessitated the development of novel cooling methods, especially for emerging fields like flexible microelectronics, biotechnology, and nanotechnology. A polydimethylsiloxane (PDMS) based microchannel flow boiling heat sink can be used for these applications due to its unique properties as such as unique mechanical properties, biocompatibility, optical transparency, etc. PDMS microchannels are fabricated using soft lithography techniques, which involve using a mould to pattern the PDMS material. The mould is created using photolithography, and the PDMS is poured over the mould and cured to create the microchannels. In this study, an effort is made to compare the flow and heat transfer performance of a PDMS microchannel heat sink with different channel shapes, i.e. straight and diverging microchannel heat sinks for different heat flux and mass flux conditions. The PDMS microchannel was fabricated using the soft lithography technique and a flexible laser-induced graphene heater was fabricated and integrated with the developed PDMS microchannel to act as a heat source. Experimentally, the parameters like bubble pattern, pressure drop, exit vapour quality, and corresponding heat transfer coefficient were determined at varying mass and heat flux ranging from 19 to 114 kg m(-2)s(-1) and from 13.3 to 156.6 kW m(-2) respectively for both the microchannels. It was observed that the flow in the diverging microchannel is more stable and has a 13% higher heat transfer coefficient value compared to the straight microchannel under similar conditions. Further, a 3D numerical study was carried out to corroborate and elaborate on the results. The heat transfer, flow characteristics, and bubble pattern results show an excellent agreement with experimental results. This work has significant relevance in designing flexible microchannel heat sinks.