Search

Expiratory droplets cause high infection risk to nearby passengers via airborne route. Methods : We built a two-row four-seat setup to simulate a public transport cabin. A cough generator and a nebulizer were used to simulate the cough and talk processes respectively. Exposure and infection risk of nearby passengers was studied. The effect of gasper jet and backrest on risk mitigation was investigated. For the activity of coughing, the front passenger has much higher infection risk, which was around four times of that of other passengers, because of the concentration surge in the inhalation zone. For talking, the nearby passengers have similar infection risk because nearby passengers were all exposed to concentration surges with similar peak value. Gasper jet of the infected passenger and higher backrest can extinguish or reduce the concentration surge of front passengers and reduce the infection risk due to coughing and talking droplets. The passengers near the infected passenger have very high infection risk. The overhead gasper and a higher backrest can reduce the exposure and mitigate the risk of infection. It is believed that the control measures to protect nearby passengers are urgently needed in public transport cabins. • A four-seat setup and airduct system were built to simulate public transport cabin. • Airborne concentration near passengers' noses from both cough and talk was counted. • Airborne infection risk of each passenger near infected passenger were studied. • Effects of overhead gasper and backrest height on mitigating risk were evaluated. • Nearby passengers have high risk, and gasper jet or higher backrest was suggested. [ABSTRACT FROM AUTHOR]

• Inserting a fluttering flag into a channel enhances its heat dissipation. • Heat dissipation effect by a split flag is superior to that by a full flag. • Hydraulic loss of a split flag is lower than that of a full flag. • A split flag starts flutter at a lower wind speed than a full flag. • The fluid-structure interaction of a split flag is stronger than that of a full flag. The use of flags as vortex generators inside heat sinks has been successfully demonstrated as a heat transfer enhancement technique. However, their thermal-hydraulic performance is usually diminished by the blocking effect induced by the fluttering phenomenon. To tackle this problem, with the expectation to maximize the fluid mixing while minimizing the pressure drop, we report a simple and direct design by splitting a flag into multiple strips. Flags with different strip widths were compared with a full flag on the performance of pressure drop and heat transfer. A high-speed camera is used to investigate their fluttering motion. A piezoelectric plate is attached to the wall to measure the flag's flutter frequency. The results show that the performance of a flag with multiple strips outperforms that of the full flag for its lower pressure drop and higher heat dissipation effect. The performance of split flag is not linear with the strip number and the optimal way in our study is to split the full flag into 4 strips. The maximum thermal-hydraulic performance factor of the split flag is 1.91, which is 26% higher than that of the full flag. Besides, the split flag starts fluttering at a lower wind velocity. All these results demonstrate that the split strategy of a flag acting as a vortex generator is of great potential for improving the heat sink performance. [ABSTRACT FROM AUTHOR]

• Bio-inspiration is a promising approach for energy efficiency. • Bio-inspired cooling technologies focusing on buildings were reviewed. • Review according to the classification of building elements and application scale. • Heat transfer mechanisms behind each technology were also discussed. In response to the growing demand for indoor environmental quality (IEQ) and energy efficiency, abundant innovative bio-inspired cooling technologies have been proposed and their applications in buildings have been greatly demonstrated in the previous decades to enhance the benefits of building occupants. IEQ is associated with human health and productivity but maintaining good IEQ requires continuous air-conditioning resulting in a high energy consumption, especially space cooling. Bio-inspired cooling technologies focus on the fundamental mechanisms of heat transfer used by animals or plants which are considered as the keys to create a harmony between buildings and the nature, whereby IEQ can be enhanced while achieving energy efficiency. This review provides a comprehensive summary on the current bio-inspired cooling technologies, including the concepts in the research stage and the well-developed products applied in buildings, and discusses some promising designs that have the most potential for future applications. This paper is structured according to building elements, in which technologies regarding HVAC system, building materials, opaque building envelope and transparent building envelope are reviewed. The heat transfer mechanisms behind each technology including conduction, convection, evaporation or phase change and radiation are discussed. Yet successful green buildings involve a smart thermal management system for which a section is dedicated to discussing various approaches in design optimization. In the last section, a case study simulation of implementation of bio-inspired cooling technologies in a house and its energy efficient performance are analyzed. The authors attempt to motivate the future research and development in energy efficient buildings. [ABSTRACT FROM AUTHOR]

Owing to excellent solar reflectivity and sky window emissivity, disordered heterogenous materials, including filler-abundant matrices, paints, and coatings, as well as foam-like, fiber-stacked and composite porous structures, form a major class for efficient passive radiative cooling. Contrary to well-established empirical understanding, this work offers a generalized analytical overview of their macroscopic thermo-optical properties from the microscopic electromagnetic perspective of Maxwell-Garnett effective medium theory. With the family of micro-porous poly(vinylidene-fluoride)/poly(methyl-methacrylate) blends as a representative example, procedures for tailoring mid-infrared spectral emissivity via effective permittivity are outlined. Theoretical framework and design scheme are validated by finite difference time domain simulation and Fourier transform infrared spectrometry. It is shown that poly(vinylidene-fluoride) and poly(methyl-methacrylate) form a pair of complementary constitutive materials for near unity thermal emission through the atmospheric window. Optimized binary polymeric blend, prepared by spray-coating method, features a window emissivity of 98% and realizes nocturnal radiative cooling with a temperature reduction of 6.8 °C and a cooling power of 94 W/m2 in an outdoor field investigation. It can serve as a promising bifunctional material for simultaneous radiative heat dissipation and capacitive energy storage, which meets the demand for nocturnal, radiative cooling aided thermoelectricity generation and storage potential. • Maxwell-Garnett effective medium theory is employed for disordered heterogeneous radiative cooling materials design. • Thermal-radiative properties of PVDF/PMMA blends are optimized. • Temperature reduction of 6.8 °C and cooling power of 94 W/m2 are realized in nocturnal field investigation. [ABSTRACT FROM AUTHOR]

• The MR-2 SGHF exhibits the highest synergistic thermal conductivity. • The low mixing ratio droplets give highest evaporation rates for 25 °C≤ T s ≤ 100 °C. • The high mixing ratio droplets give highest evaporation rates for 103 °C≤ T s ≤ 125 °C. • The SGHF droplet evaporation rate on residue is up to 10 times that on copper. • The latent heat flux removal up to 890 W/cm2 is obtained using SGHF droplets. Thermal management of many high heat flux devices depends on droplet based cooling, such as the spray cooling or electro-wetting for hotspot cooling. Recently, heat dissipation in these devices increased to unprecedented levels, pressing a need for advanced thermal fluids in droplet based cooling systems. In this paper, we address this challenge by investigating the evaporation and boiling performance of the silver-graphene hybrid nanofluid (SGHF) droplet for its various mixing ratios and droplet sizes on a heated copper and a residue surface, obtained from the evaporation of the first SGHF droplet. The results show that low mixing ratio (MR ≤ 0.1) SGHF droplets exhibit highest evaporation rates for substrate temperature (T s) in a range of 25 °C ≤ T s ≤ 100 °C. However, this trend is reversed in the nucleate boiling regime, where high mixing ratio (MR ≥ 0.9) droplets give highest evaporation rates. Moreover, all SGHF droplets, irrespective of their mixing ratio, exhibit similar evaporation rates in the film-boiling regime. Furthermore, the SGHF droplet evaporation rate on its porous residue surface increases up to 173% for 25 °C ≤ T s ≤ 100 °C and by an order of magnitude in the nucleate boiling regime as compared to a plain copper surface. We also show that besides the synergistic thermal effect, the thermal Marangoni convection also affects the SGHF droplet evaporation rate. Moreover, we develop a diffusion-convection evaporation model that can predict the evaporation rate for different mixing ratios of the SGHF droplet on heated copper and residue surfaces. Moreover, we demonstrate that the latent heat flux up to 890 W/cm2 and 850 W/cm2 can be achieved using a SGHF droplet on heated copper and residue surfaces, respectively, suggesting its potential application in high heat flux device cooling. Finally, we discuss the effects of spray hydrodynamic parameters on critical heat flux of the SGHF spray cooling. [ABSTRACT FROM AUTHOR]

• MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%). • The evaporation rate increases by 240% as V fd /V sd increases from 1 to 20. • The evaporation rate drops moving from MR-2 to MR-5 hybrid nanofluid droplets. • High droplet spreading is achieved in GNP/MR-1 nanofluid/hybrid nanofluid droplets. • The pore size and areal porosity increase with increasing residue size. Droplet evaporation offers high heat rejection rates and is widely used in the form of spray cooling or dropwise cooling of various heat dissipating devices. However, due to the limiting heat flux removal capacity of conventional fluids, such as water, these cannot be used in thermal management of high heat flux devices. In this research, the evaporation of silver (Ag)-graphene (GNP) hybrid nanofluid droplet and its residue effects on the evaporation of following Ag-GNP hybrid nanofluid droplet, due to its synergistic thermal properties, is experimentally investigated for various mixing ratios, from MR-1 (0.1(Ag):0.9(GNP)) to MR-5 (0.9(Ag):0.1(GNP)), and different residue sizes. A theoretical model is also proposed for hybrid nanofluid droplet evaporation and semi-empirical relations are developed to estimate the hybrid nanofluid droplet spreading over its residue surface. The results show a substantial increase in the droplet evaporation rate with increasing residue size and decreasing mixing ratio. MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%) on its highly wetted residue surface, while the evaporation rate significantly drops moving from MR-2 to MR-5 hybrid nanofluid droplets on their partially wetted residue surfaces. Moreover, the evaporation rate substantially increases (up to 240%) with increasing residue size for MR-1 hybrid nanofluid droplet resting on its residue surface, however, the effect of residue size on droplet evaporation rate considerably diminishes moving from MR-2 to MR-5 hybrid nanofluid droplets resting on their respective residues. Image, graphical abstract [ABSTRACT FROM AUTHOR]

It has been shown that the exposure to airborne particulate matter is one of the most significant environmental risks people face. Since indoor environment is where people spend the majority of time, in order to protect against this risk, the origin of the particles needs to be understood: do they come from indoor, outdoor sources or both? Further, this question needs to be answered separately for each of the PM mass/number size fractions, as they originate from different sources. Numerous studies have been conducted for specific indoor environments or under specific setting. Here our aim was to go beyond the specifics of individual studies, and to explore, based on pooled data from the literature, whether there are generalizable trends in routes of exposure at homes, schools and day cares, offices and aged care facilities. To do this, we quantified the overall 24 h and occupancy weighted means of PM 10 , PM 2.5 and PN - particle number concentration. Based on this, we developed a summary of the indoor versus outdoor origin of indoor particles and compared the means to the WHO guidelines (for PM 10 and PM 2.5 ) and to the typical levels reported for urban environments (PN). We showed that the main origins of particle metrics differ from one type of indoor environment to another. For homes, outdoor air is the main origin of PM 10 and PM 2.5 but PN originate from indoor sources; for schools and day cares, outdoor air is the source of PN while PM 10 and PM 2.5 have indoor sources; and for offices, outdoor air is the source of all three particle size fractions. While each individual building is different, leading to differences in exposure and ideally necessitating its own assessment (which is very rarely done), our findings point to the existence of generalizable trends for the main types of indoor environments where people spend time, and therefore to the type of prevention measures which need to be considered in general for these environments. [ABSTRACT FROM AUTHOR]

During the COVID-19 pandemic, WHO and CDC suggest people stay 1 m and 1.8 m away from others, respectively. Keeping social distance can avoid close contact and mitigate infection spread. Many researchers suspect that suggested distances are not enough because aerosols can spread up to 7–8 m away. Despite the debate on social distance, these social distances rely on unobstructed respiratory activities such as coughing and sneezing. Differently, in this work, we focused on the most common but less studied aerosol spread from an obstructed cough. The flow dynamics of a cough jet blocked by the backrest and gasper jet in a cabin environment was characterized by the particle image velocimetry (PIV) technique. It was proved that the backrest and the gasper jet can prevent the front passenger from droplet spray in public transportation where maintaining social distance was difficult. A model was developed to describe the cough jet trajectory due to the gasper jet, which matched well with PIV results. It was found that buoyancy and inside droplets almost do not affect the short-range cough jet trajectory. Infection control measures were suggested for public transportation, including using backrest/gasper jet, installing localized exhaust, and surface cleaning of the backrest. • Keeping social distance in public transportation is difficult, meaning high risk. • Flow dynamics of cough jet in a cabin environment is studied by PIV technique. • Backrest and gasper jet can be compensation for not keeping social distance. • Localized exhaust under each seat is suggested to timely remove bulk aerosols. • A universal model is proposed to predict the obstructed cough jet trajectory. [ABSTRACT FROM AUTHOR]