Arrays of surface ribs have been reported to significantly enhance particle collection efficiency in particle removal devices. However, the surface ribs also cause a higher pressure drop. Therefore, the overall performance needs to take into consideration the above factors. In this study, different forms of surface ribs inspired by nature were designed and parametric studies were performed to enhance deposition efficiency. Our parametric studies comprised three different aspects: geometry of the patterned surface, pitch-to-height ratio, and particle size. The flow field around patterned surfaces was simulated in a two-dimensional channel flow by using the Reynolds stress model, corrected by turbulence velocity fluctuation in the wall-normal direction. The particle trajectory was solved by using Lagrangian particle tracking. When the overall efficiency ratio was considered, a semi-circular pattern had the best overall efficiency with 1137 times increase when compared to the case without patterns. Although the open-circular pattern has the minimum particle deposition enhancement, the overall efficiency of the open-circular pattern has 862 times increase compared to the case without patterns. Surface ribs (semi-circular, triangular and rectangular) can achieve a higher particle deposition velocity, but a higher flow resistance is generated compared with applying the open-circular surface ribs. The deposition location was then investigated for different surface ribs at different pitch-to-height ratios (p/e). This study shows that the semi-circular surface pattern should be recommended to enhance the overall performance of particle removal devices, especially for submicron particles.

A passive daytime radiative cooler is made of a sky facing surface which can preserve the indoor air temperature below ambient without energy consumption by simultaneously reflecting solar radiation and emitting thermal radiation to the universe through the atmospheric window located between 8–13 µm of the electromagnetic spectrum. After the first demonstration of radiative cooling under direct sunlight, a solar mirror coated with a mid-infrared (MIR) emissive thin film has become the standard device architecture. This study firstly reviews recent developments in daytime passive radiative cooling, followed by describing the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m² passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.