Research progress on fractal microchannels for heat transfer process intensification

With the rapid development of microscale/nanoscale manufacturing technology, electronic microchips, microreactors, and microscale fuel cells have attracted considerable attention. The practical applications of miniaturized devices require not only advanced fabrication procedures and materials but also efficient thermal management to maintain their performance. For electronic microchips with high integration and frequency, high heat flux not only significantly limits their performance but also considerably affects their lifetime and reliability. Given that conventional air cooling and single-phase liquid convection cooling methods cannot meet the heat dissipation requirements, microchannel heat transfer technology has become an important alternative to solve the heat transfer problem of miniaturized devices. However, conventional microchannel heat transfer methods usually face two major challenges, namely, microscale dimensions that result in high-pressure drop and high-pump power consumption and temperature increase along the microchannels that considerably affect stability and reliability. The resulting high flow resistance and temperature nonuniformity significantly limit the practical applications of microchannel heat sinks. In recent years, inspired by natural fractals, such as mountain ranges, rivers, leaf venations, plant roots, tree trunks, blood vessels, and lung bronchus, researchers have developed a series of new types of fractal microchannels for heat transfer process intensification. This review provides a comprehensive overview of state-of-the-art research on fractal microchannel heat sinks, such as Y-shaped, H-shaped, T-shaped, Ψ-shaped, Cantor, and Koch fractals. We highlight the principles of heat transfer fractal microchannels, discuss the theoretical and experimental research findings, and identify the current problems and future research directions. Although research on fractal heat sinks has already gained considerable progress, the following challenges should be carefully considered: most studies focus on numerical simulations; meanwhile, experimental studies are relatively limited because of the difficulties in device fabrication. Compared with Y-shaped fractals, the other types of fractal microchannels exhibited a better performance but have received significantly less attention. Both multilayer and hydrogel-assisted fractal microchannels have typically high heat transfer capacity; however, their fabrication process is complicated. There are still a few contradictory results concerning the impact of fractal structures on heat transfer enhancement that need in-depth theoretical modeling and experimental observations. This review can not only provide an in-depth understanding of fractal microchannels but also shed new light on the development of robust fractal heat sinks for intensifying heat transfer applications.