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• Edge effect on droplet condensation and frozen characteristics are presented. • High air velocity notably shortens the duration of droplet condensation and frozen. • High air velocity weakens freezing wave propagation velocity caused by edge effect. • Turning point for frost roughness at 1.5 m/s surpassing that at 0.5 m/s is 390 s. To retard the negative effects of frosting, it is necessary to better understand the frosting mechanism on cold surfaces. In this study, a systematical study on the dynamic frosting characteristics on the edge zone of a horizontal copper plate under forced convection was conducted. The results showed that the air velocity effect and edge effect on droplet condensation, frozen and frost layer growth characteristics were significant. The duration of droplet condensation stage decreased from 365 to 113 s with a decrease of 69.0% when air velocity increased from 0.5 to 2.5 m/s. Besides, the average freezing wave propagation velocity for Region I (an area where a row of water droplets closest to plate edge) was significantly larger than that for Region II (the remained area). As a result, the duration of droplet frozen stage for Region I was much shorter than that for Region II. Accordingly, frost layer amount for Region I was notably larger than that for Region II. Besides, the frost surface roughness for Region I increased as air velocity increased at the early frosting stage, but decreased as air velocity increased at the later frosting stage. The general turning point for frost surface roughness at 1.5 m/s surpassed the value at 0.5 m/s was around 21.4 × 10−6 m at 390 s, and that between 2.5 m/s and 1.5 m/s was around 30.3 × 10−6 m at 450 s. This study can help to establish a better relationship among different frosting stages, and better understand the plate edge effect on frosting. [ABSTRACT FROM AUTHOR]

• Analytical solution of non-boiling heat transfer during droplet impact is obtained. • The analytical solutions are applicable to low velocity and low wall temperature. • Wall thermal effusivity can decide heat flux alone, but not the temperature drop. • Droplet property shows great and similar effect on heat flux and temperature drop. • Thermal effusivity is a decisive parameter and high thermal effusivity is preferred. Droplet impact process exhibits excellent heat and mass transfer characteristics. Analytical solution with simple explicit form and certain accuracy can clearly reflect the correlation of physical parameters, however the analytical study on droplet impact heat transfer is quite insufficient and limited to flow field analysis. In this paper, the analytical solutions of heat flux and temperature variation of droplet impact on heated wall in non-boiling regime were obtained based on boundary layer asymptotic matching method. Parameter analysis of thermophysical properties was conducted to explore the decisive properties of droplet and wall materials. The results showed that the analytical solutions are appropriate to predict non-boiling heat transfer at low impact velocity and low initial wall temperature. Parameter analysis showed that thermal effusivity is the decisive property rather than the thermal diffusivity. Wall thermal effusivity could independently determine heat flux, but should determine temperature variation together with wall thermal conductivity. Droplet thermal effusivity and Prandtl number were the decisive properties, however the effect of Prandtl number in reality can be ignored. Wall materials and droplet liquids with higher thermal effusivity showed higher heat transfer capacity. The importance of thermal effusivity was demonstrated and high thermal effusivity was preferred in future thermal management. [ABSTRACT FROM AUTHOR]

• Multiscale wicks were high-efficiently prepared by a multi-beam femtosecond laser. • Capillary performance and thickness of wicks outperforms recently reported data. • Modified wicking dynamics reduced the prediction error from 72.9 % to 4.5 %. • Model-guided design improves 56.7 % in wicking compared to the conventional. • The modified model proposes a universal approach for quantitative wick design. Ultrathin titanium vapor chambers with wicks fabricated by ultrafast laser direct writing provide a promising solution for future electronics' lightweight thermal management. However, despite being a powerful tool for wick manufacturing, the application of ultrafast laser is restricted by its low fabrication efficiency and imperfect model for wick design. Herein, a multiscale titanium wick with stable superhydrophilic nano ripples and microgrooves was high-efficiently fabricated by a multi-beam femtosecond laser processing technology. A modified wicking dynamic model was proposed for ultrafast laser manufactured microgroove wick design. It considered the extra capillary force by laser induced nanoripple and viscous resistance of the narrow V-shape grooves, which is ignored in the classic model. The prediction error of wicking performance reduced from 72.9 % to 4.5 %. Compared to the conventional V-shape design the optimized microgrooves wick improved by 56.7 % and exhibited capillary performance parameter (K/R eff) of 1.88 μm at a thickness of 80 μm, which outperformed most of the recently reported data. The modified model, multiscale high-performance wick, and its highly efficient fabrication method propose a universal and handy approach for accurate and quantitative wick design and manufacturing. [ABSTRACT FROM AUTHOR]