Research progress on fractal microchannels for heat transfer process intensification
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摘要: 隨著微納制造技術的快速發展,微電子芯片、微反應器和微燃料電池等微型器件受到了研究者越來越多的關注。微型器件的應用不僅對加工工藝和材料具有較高的要求,而且需要高效的熱管理來維持其性能。特別是對于高集成度和高頻化的高性能微電子芯片而言,超高的熱流密度不僅會嚴重制約芯片的性能,而且會顯著影響芯片的壽命和可靠性。鑒于傳統的風冷和液體單相對流換熱冷卻方式無法滿足散熱需求,具有高換熱系數的微通道換熱技術成為解決微型器件散熱問題的重要途徑。然而,常規的微通道換熱技術普遍存在著高流動阻力和非均溫性的難題,限制了該技術的實際規模化應用。近年來,研究者開發出一系列新型的分形微通道技術用于換熱過程強化。本文系統總結了不同類型的分形換熱微通道(包括Y、H、T、Ψ、康托、科赫等分形結構),并對各分形微通道的原理和性能進行了著重介紹,最后對分形微通道換熱的現存挑戰和未來發展方向分別進行了分析和展望,以期為換熱過程強化的發展提供新的研究思路。Abstract: 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.
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Key words:
- microchannel /
- fractal /
- heat transfer /
- process intensification /
- heat sink
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圖 2 Y形分形微通道熱沉結構. (a) Y形分形微通道結構設計示意圖;(b) 典型的通道長度逐漸降低(左)和通道長度逐漸增加(右)的Y形分形微通道網絡[10];(c) 環線連接的Y形分形微通道熱沉[11];(d) 具有凸起(上)和凹陷結構(下)的Y形分形微通道[12];(e) 三維Y形分形微通道換熱翅片[13];(f) 微型針翅陣列形成的Y形分形微通道網絡[14]
Figure 2. Y-shaped fractal microchannel heat sinks: (a) schematic diagram of the structural design of Y-shaped fractal microchannels; (b) typical Y-shaped fractal microchannel network with decreasing (left) and increasing (right) channel lengths[10]; (c) Y-shaped fractal microchannel heat sink with connected loops[11]; (d) Y-shaped fractal microchannels with ribs (top) and cavities (bottom)[12]; (e) 3D Y-shaped fractal microchannel fins[13]; (f) Y-shaped fractal microchannel network formed by specific microscale pin-fins[14]
圖 3 H形分形微通道熱沉結構. (a) H形分形微通道結構設計示意圖[44];(b) 一種典型的H形分形微通道熱沉結構[42];(c) 圓筒狀分布H形分形微通道熱沉[44];(d) 3D打印扭曲H形分形微通道結構[45];(e) 共平面(左)與面外(右)H形分形微通道熱沉[46];(f) 末端為S形通道的H形分形微通道網絡(數字1~8為尺寸不斷遞減的分支通道)[47]
Figure 3. H-shaped fractal microchannel heat sinks: (a) schematic diagram of the structural design of H-shaped fractal microchannels[44]; (b) one kind of typical H-shaped fractal microchannel heat sink[42]; (c) cylindrical H-shaped fractal microchannel heat sink[44]; (d) distorted H-shaped fractal microchannels manufactured by 3D printing[45]; (e) coplanar (top) and out-of-plane (bottom) H-shaped fractal microchannel heat sink[46]; (f) modified H-shaped fractal microchannel network ending with serpentine channels (numbers 1–8 denote the size-reducing branch channels)[47]
圖 4 T形分形微通道熱沉結構. (a) T形分形微通道結構設計示意圖;(b) 銅基底分形結構裝置[58];(c) 硅基底分形結構裝置[59];(d) 多簇T形分形微通道結構設計[61];(e) 基于T形分形結構的矩形微通道[63]
Figure 4. T-shaped fractal microchannel heat sinks: (a) schematic diagram of the structural design of T-shaped fractal microchannels; (b) T-shaped fractal microchannels on the copper substrate[58]; (c) T-shaped fractal microchannels on the silicon substrate[59]; (d) structural design of multiple T-shaped fractal microchannels[61]; (e) rectangular microchannels based on the T-shaped fractal structures[63]
圖 5 Ψ形分形微通道熱沉結構. (a) Ψ形分形微通道結構設計示意圖;(b) 多簇Ψ形分形微通道結構設計[67];(c) Ψ形(左)和直形(右)微通道網絡的溫度分布[68]
Figure 5. Ψ-shaped fractal microchannel heat sinks: (a) schematic diagram of the structural design of Ψ-shaped fractal microchannels; (b) structural design of multiple Ψ-shaped fractal microchannels[67]; (c) temperature distribution of Ψ-shaped fractal microchannels (left) and straight parallel microchannels (right)[68]
圖 6 康托分形微通道熱沉結構. (a) 康托分形微通道結構設計示意圖[71];(b) 康托分形微通道結構近壁區流線圖及溫度分布[72]
Figure 6. Cantor fractal microchannel heat sinks: (a) schematic diagram of the structural design of Cantor fractal microchannels[71]; (b) local streamlines and temperature profiles in the near-wall region of Cantor fractal microchannels[72]
圖 7 科赫分形微通道熱沉結構. (a) 方形科赫分形結構設計示意圖[74];(b) 方形科赫分形微通道結構流速場(左)及溫度場(右)分布圖[75]
Figure 7. Koch fractal microchannel heat sinks: (a) schematic diagram of the structural design of quadratic Koch fractal microchannels[74]; (b) contour color plot of the velocity (left) and temperature (right) fields of quadratic Koch fractal microchannels[75]
圖 8 其他類型分形微通道熱沉結構. (a) 羊毛纖維狀分形通道網絡[76];(b) 六邊形分形微通道換熱結構[77];(c) 菱形分形微通道熱沉[78];(d)分形骨架結構模擬孔狀鋁換熱過程[79]
Figure 8. Other types of fractal microchannel heat sinks: (a) fractal channel network of wool fiber [76]; (b) heat transfer platform with hexagonal fractal microchannels[77]; (c) microchannel heat sink with rhombus fractal structures[78]; (d) fractal skeleton structures to simulate the heat transfer process of porous aluminum[79]
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