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Noninvasive aerosol removal processes are sought after in manufacturing industries that utilize powder product recovery. Traditionally, cyclone separators are commonly deployed, however, this method is energy intensive; space demanding and inefficient for particles with a diameter less than 5 μm. While acoustically induced aerosols depositions was proposed as an alternative aerosol removal method which addressed such limitations, the underlying parameters determining the performance of such technique are yet to be explored. This study examines those underlying parameters determining the efficiency of acoustic aerosol removal, such as; the removal efficiencies, varying aerosol inputs, particle initial concentrations and size distributions. Experimental results showed that given the same initial particle number concentrations, aerosols containing two particles sizes, larger (seed) and smaller (test) particles, improved the removal efficiencies for the test particles. A higher seed to test particle concentration ratio further enhanced this effect. For particle number concentrations ranging above 2.0×10 4 #/cm 3 , the increase in concentrations enhanced the removal efficiency of aerosols. Further analysis conducted by numerical simulations demonstrated that the deposition of particles depended on their sizes and their initial positions in the acoustic field. A new parameter, deposition cell ratio was proposed to quantify the deposition probability for particles at a given size and in a given domain. Results suggested that the deposition probability of a group of aerosols may be a function of the sum of the probabilities of each individual particle. [ABSTRACT FROM AUTHOR]

Nanofluid droplet evaporation has attracted great interest due to its applications such as in painting, coating and patterning. In most studies, either the particle size or the concentration of nanofluid is considered as a factor in the formation of the residue pattern. This work aims to investigate the effect of both particle size and concentration on the residue pattern. A comprehensive study was made of the residue patterns of Al 2 O 3 and TiO 2 aqueous nanofluid droplets on different substrates (i.e. glass, stainless steel and Teflon). It was found that a ring-shaped pattern was formed at low concentrations and small particle sizes, while a uniform pattern was formed at high concentrations and large particle sizes for Al 2 O 3 nanofluids. In addition, only ring-shaped residue patterns were observed for all concentrations of TiO 2 nanofluids. In the case of different substrates, on a material with a high contact angle with water, it was difficult to form a ring-shaped pattern. The widths of the ring-shaped pattern were analyzed as well. The results showed that the width of the ring-shaped pattern was larger for small particles. The materials of substrate and nanoparticle also influenced the width. [ABSTRACT FROM AUTHOR]

• A thinner inverted flag starts flapping at a lower wind speed. • The heat dissipation effect is strongly affected by the inverted flag motion. • The flapping mode induces the highest average vorticity compared with other modes. • The flapping mode induces the largest heat transfer enhancement compared with other modes. It has been demonstrated that flexible vortex generators, e.g., flapping flag, can significantly enhance heat transfer inside a heat sink. However, their heat transfer enhancement is only effective when they exhibit flapping behaviors, which require a flow velocity higher than the heat sink working velocity, and thus restraint their application. Minimizing the critical flapping velocity of the flags without sacrificing the heat transfer performance is needed. In this work, we study the cases of inverted flags with different thicknesses in a channel flow. Three flag motion modes are identified by a high-speed camera with increasing flow velocity. In the first mode transition, i.e., the flag starts flapping, the heat dissipation has the highest enhancement. Numerical simulation reveals that compared to the other motion modes, the flapping mode has the strongest average vorticity along the channel wall, leading to the highest heat dissipation among all flag motion modes. Experimental results show that the critical velocity can be as low as 1.5 m/s, at which the heat dissipation enhancement can be as high as 100%. The findings in this work significantly benefit the application of flexible vortex generators in heat sinks, by enabling a decrease in critical velocity and a good enhancement in heat dissipation. [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]

• Trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend with atmospheric window emissivity and solar reflectivity exceeding 97% is developed. • Compared to the baseline radiative cooling materials, trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend enhanced surface temperature suppression by 1.6 ℃ at daytime. • Functionalized glass-PDMS micro-pillar heat exchanger chills water by 2.5 ℃ at an efficiency of 6.3% at nighttime. • Compared to the saturation limits, reduced temperature reduction and efficiency for low Peclet number low is attributed to axial conductive viscous dissipation. Chilled water harvesting is a fundamental application of passive radiative cooling and promotes energy conservation for space cooling in buildings potentially, which relies on well-designed radiative cooling materials and heat transfer interface. This paper reports a scenario leading to viscous dissipative thermo-photonic energy conversion, which takes place in low Peclet number regime of an order of magnitude of 100, where heat transfer is non-Graetzian. Compared to benchmarked glass-polydimethylsiloxane radiative cooler and barium sulphate coating, a newly developed trinary micro-porous 32/4/4 magnesium-oxide/poly(vinylidene-fluoride)/poly(methyl-methacrylate) radiative cooling blend, featuring high atmospheric window emissivity and solar reflectivity, both exceeding 97%, demonstrated a superior cooling performance with additional temperature reduction of 1.6 °C at daytime. Meanwhile, it chilled water at a flow rate of 6.3 µL/s by 1.3 °C upon coating on a glass-polydimethylsiloxane micro-pillar heat exchanger. Quantitative evaluation on the chilled water capacity was carried out at nighttime when the system ran pseudo-steadily. Cooling power measurement on a radiative cooler of same materials recorded a cooling power of 134 W/m2 which is close to the ideal limit. And measured water temperature reduction and cooling efficiency were 2.5 °C and 6.3% respectively. They were significantly lower than the saturation limit. Degraded thermal and energy conversion performances, attributive to extended Graetzian viscous dissipation, were discussed theoretically. [ABSTRACT FROM AUTHOR]

Passive radiative coolers can preserve the surface temperature below ambient by simultaneously reflecting incoming solar radiation and emitting thermal radiation to the sky. Apart from thermo-optical properties of the materials, radiative cooling performance is affected by various environmental factors which determine the atmospheric transmittance. As such, field investigations lack convergence and completion. And energy balance consideration, which aids in interpreting the field investigative results, is a deterministic cogitation on convective and radiative heat transfer by the radiative cooler that ignores the uncertainties abundant in field study. In this work, we examine the cooling performance of radiative cooling materials under different subtropical weather conditions in Hong Kong and approach the problem based on probabilistic regression modelling as an alternative. At nighttime, the response variable of surface temperature reduction can be correlated with a single predictor variable of sky temperature difference, which is a lumped parameter of ambient temperature, relative humidity, and cloud fraction. At daytime, it should be parametrized with an additional variable regarding solar intensity. The regression analysis reveals that, the higher the thermal emissivity, the larger is the temperature reduction at nighttime, especially obvious for large sky temperature difference. And heavy solar heat load is absorbed by the coolers at daytime even they feature reasonably high solar reflectivity. In this regard, further increment in solar reflectivity poses the top priority in improving daytime radiative cooling performance. [ABSTRACT FROM AUTHOR]

• Piezoelectric fan cooling is greatly augmented by geometrical arrangements. • The effect of fan location is much stronger than that of channel configuration on piezoelectric fan cooling. • By balancing the streamwise velocity and spanwise mixing, piezoelectric fan cooling can be maximized. Piezoelectric fan cooling is drawing growing attention for its compact size and high energy efficiency. However, its cooling capacity is not good enough and needs to be maximized to catch up with the industry demand. In this work, we aim to increase heat transfer performance by varying piezoelectric fan locations and channel configurations. The effect of fan location is firstly studied. By tuning the fan position, a higher streamwise velocity is achieved, thus increasing the heat transfer. A fan inserted into the channel achieves a 55% enhancement on the Nusselt number compared with natural convection. On top of this, the effect of the channel configuration is explored. Both expansion and contraction configurations benefit heat transfer, and the optimal one adds a further 10% enhancement on top of the parallel configuration case. That is, the maximum cooling enhancement is 70% compared with natural convection. The fan motion is captured by a high-speed camera. Flow visualization is used to investigate the flow pattern. The channel streamwise fluid velocity is examined. It is found that the contraction configuration boosts the streamwise velocity and generates a jet flow pattern at the channel outlet. The expansion configuration significantly alters the flow motion with more vortices in a longer transmission distance, yielding a stronger spanwise mixing. The findings demonstrate that geometrical arrangements can substantially enhance the heat transfer performance of the piezoelectric fan, thus paving the way for tackling the urgent cooling requirement. [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]

Highlights • 0.5:0.5 is the optimum mixing ratio with enhanced overall hydrothermal properties. • Hybrid nanofluid shows three distinct concentration stratified zones. • Mixing ratio above 0.5:0.5 results in rapid settling due to high settling velocity. • Stability for mixing ratio below 0.5:0.5 is less dependent on zeta potential. Abstract The dispersion stability and thermophysical properties of metal–metal oxide hybrid nanofluid for various mixing ratios are investigated in this paper. Cu nanoparticles having high thermal conductivity and poor dispersion stability are dispersed in water with Al 2 O 3 nanoparticles that have a high dispersion stability and low thermal conductivity, in mixing ratios 0.3:0.7 (MR-1), 0.5:0.5 (MR-2) and 0.7:0.3 (MR-3) to achieve a Cu-Al 2 O 3 hybrid nanofluid with improved hydrothermal properties. Dispersion stability and thermophysical properties of the hybrid nanofluid were studied for 240 h using various experimental techniques such as zeta/particle size analyser, UV–Vis spectroscopy, transmission electron microscope, sedimentation, thermal analyser and viscometer. The results show that the hybrid nanofluid transforms into low, medium and high concentration stratified zones over time. Also, stability in Cu/Al 2 O 3 single particle nanofluids and MR-3 hybrid nanofluid is related to both sedimentation velocity and zeta potential while sedimentation velocity has the dominating effect on stability of MR-1 and MR-2 hybrid nanofluids. Mixing ratio above MR-2 is identified for rapid settling due to high sedimentation velocity. Also, MR-2 is determined as an optimum mixing ratio to achieve enhanced overall hydrothermal properties for the hybrid nanofluid due to its improved thermal conductivity and relatively better stability. [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]

Droplet phase change is the key phenomenon for high heat transfer rates in spray or drop-wise cooling applications. Despite high cooling efficiency of the spray cooling technology, conventional fluids, such as water, cannot be used for thermal management of modern high heat flux devices due to their immense power density, resulting in early device failures. To address this issue, in this research, we experimentally study the evaporation performance for various volumes of the copper-alumina hybrid nanofluid (CAHF) droplet on a plain copper substrate and compare it with water (H 2 O) droplet in sub-boiling and boiling regimes (i.e., for substrate temperatures of 25–170 °C). We also numerically investigate and compare the internal velocity and thermal fields of CAHF and H 2 O droplets on a heated plain copper substrate. Besides the plain copper surface, we examine the phase change behaviour of the subsequent CAHF droplet over a heated residue surface that was obtained from the phase transition of the first CAHF droplet on a heated plain copper substrate. Our results demonstrate that the evaporation rate of CAHF droplets on a plain copper surface is up to 24% and an order of magnitude higher than water droplets in sub-boiling and nucleate boiling regimes, respectively. Moreover, the evaporation rate of the CAHF droplet on a residue surface increases up to 141% and 800% compared to that on a plain copper surface in sub-boiling and nucleate boiling regimes, respectively. Furthermore, the latent heat flux up to 10 times can be achieved using the CAHF droplet compared to H 2 O droplet on a plain copper substrate in the nucleate boiling region, making the CAHF a potential fluid for high heat flux cooling applications. • Evaporation rate of CAHF droplet is up to 24% that of water droplet for T s ≤ 100 °C. • CAHF droplet evaporation rate is up to 1300% that of water droplet for T s > 100 °C. • Latent heat flux of CAHF droplet is up to 1000% that of water droplet at T s > 100 °C. • Evaporation rate of CAHF droplet on its residue is up to 800% than on plain copper. • CAHF droplets exhibit higher thermal Marangoni convection than H 2 O droplets. [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]

Understanding of the droplet detachment behavior from rough hydrophilic surfaces is important in many biological and industrial applications such as biomedicine, surface coating, and pathogen-laden aerosol resuspension. Due to the partial detachment of the droplet on hydrophilic surfaces, leaving behind some droplet residues, the adhesion behavior is extremely complicated. Here we reveal a new adhesion mechanism between a droplet and a rough hydrophilic surface. The adhesion behavior is controlled by a liquid film stuck in the surface microstructure. We establish a model to describe the contributions of liquid film and naked solid peaks, to the work of adhesion and verify the model experimentally. We also find that the normal adhesion force is about 3.35 ± 0.25 times of the lateral adhesion force for different surface roughness, meaning that the separation direction is an important factor affecting adhesion due to the different separation mechanisms. The results of this work shed new insights on the understanding of droplet detachment and adhesion to a rough surface. • Rough surface attaches larger liquid aerosol than smooth surface, while rough surface holds smaller solid aerosol. • A model is proposed and experimentally verified to reveal adhesion mechanism of a liquid aerosol to a rough surface. • Normal adhesion force is about 3.35±0.25 times of lateral adhesion force, which is independent of surface roughness. [ABSTRACT FROM AUTHOR]