The flow around a circular cylinder with porous material coating (PMC) is numerically investigated based on unsteady Reynolds-averaged Navier–Stokes (URANS) method at subcritical Reynolds number. The results are compared with some available results in the open literature. The interaction of PMC with the near wake of a circular cylinder such as streamline, vorticity field, and shear stress are studied in detail. Subsequently, the fluctuation forces and velocity distribution in the boundary layer are analyzed and the effect of various thicknesses of PMC is investigated. The numerical results reveal that PMC has prominently modified the flow characteristic of the near wake of circular cylinder and significantly mitigated the fluctuations of aerodynamic forces from two aspects of frequency and amplitude. It means that the vortex shedding from the bluff body is suppressed. It also is found that the thickness of the PMC is a sensitive parameter to the aerodynamic forces and velocity distribution in the boundary layer. Furthermore, the mean drag can be reduced to a certain extent when the thickness is appropriate. It is expected that the modification of flow characteristic and aerodynamic forces is closely related to the flow-induced noise reduction. Those results will be helpful to understand the mechanism of flow control on bluff body flow by using porous material coating and accumulate meaningful information for further industrial application.

Boiling heat transfer intensification is of significant relevance to energy conversion and various cooling processes. This study aimed to enhance the saturated pool boiling of FC-72 (a dielectric liquid) by surface modifications and explore mechanisms of the enhancement. Specifically, circular and square micro pin fins were fabricated on silicon surfaces by dry etching and then copper nanoparticles were deposited on the micro-pin-fin surfaces by electrostatic deposition. Experimental results indicated that compared with a smooth surface, the micro pin fins increased the heat transfer coefficient and the critical heat flux by more than 200 and 65–83%, respectively, which were further enhanced by the nanoparticles up to 24% and more than 20%, respectively. Correspondingly, the enhancement mechanism was carefully explored by high-speed bubble visualizations, surface wickability measurements, and model analysis. It was quantitatively found that small bubble departure diameters with high bubble departure frequencies promoted high heat transfer coefficients. The wickability, which characterizes the ability of a liquid to rewet a surface, played an important role in determining the critical heat flux, but further analyses indicated that evaporation beneath bubbles was also essential and competition between the wicking and the evaporation finally triggered the critical heat flux.

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.