Review of Fractal Heat Exchangers

Nature has inspired many scientists and engineers to solve problems through observation and mimicry. One such example is heat transfer enhancement. The enormous natural heat and mass transfer phenomena have led engineers to seek solutions to heat transfer enhancement problems from nature. Fractal geometries are found in respiratory and vascular systems of plants and animals, such as blood vessels, human lungs, leaves, coastlines, etc. Inspired by this, fractal heat exchangers have been developed and were found to have intrinsic advantage of minimized flow resistance and strong heat transfer capability. In current study, a comprehensive literature review was carried out to investigate the thermal and hydraulic performance of fractal heat exchangers, with a focus on fractal channels. Fractal theory, model development, performance comparison with traditional designs, heat transfer and fluid mechanisms and design methods are discussed, separately. Fractal theory was proposed in 1926 and developed in the past decades. Model developed from 1-D to 2-D and 3-D with the assumptions being more complex and realistic, leading to more accurate results. Fractal channels’ thermal and hydraulic performances were found to surpass that of traditional parallel channels and serpentine channels due to flow mixing and pressure drop recovery caused by bifurcation. Design methods include traditional scaling laws and a newly proposed topology optimization methodology. At last, the research gaps for future researches are discussed. Main research gaps are insufficient experimental data, lacking of study on optimal design, no research on liquid-to-gas heat exchangers and need of research on topology optimization.