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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.

In recent years, heat dissipation in high heat flux devices remarkably increased and it is anticipated to reach unprecedented levels in future devices, mainly due to increased power density, compact packaging and high-performance requirements. To address this challenge, in current research, we initially investigate the spray cooling performance and spray residue surface effects of the next generation thermal fluid, called hybrid nanofluid. Subsequently, we investigate the hybrid nanofluid spray cooling potential to address heat dissipation issues in a high heat flux application, that is, the electric vehicle (EV) high power electronics. Our results demonstrate that the critical heat flux (CHF) enhancement up to 126% can be achieved using the hybrid nanofluid spray cooling compared to water spray cooling. The hybrid nanofluid and its spray residue characterization further suggest that high CHF in hybrid nanofluid spray cooling may be due to high latent heat of vaporization and residue wetting and wicking effects. Moreover, the spray cooling efficiency and Nusselt number obtained for hybrid nanofluid spray cooling is more than twice that of water spray cooling. Furthermore, our results indicate that the hybrid nanofluid spray cooling can keep high power electronics of current and future electric vehicles below their failure temperatures, while the same cannot be achieved using water and dielectric fluid spray cooling.

Head orientations directly determine movement directions of exhaled pathogen-laden droplets, while there is a lack of research about the effect of the infected person's head orientations on respiratory disease transmission during close contact. This work experimentally investigated the effect of different head orientations of an infected person (IP) on the bioaerosol deposition on a healthy person (HP) during close contact. Also, the effectiveness of PV flow in reducing bioaerosol deposition on the HP under the IP’s different head orientations was investigated. Bacteriophage T3 was employed to represent viruses inside the cough-generated aerosols. The bioaerosol depositions on different locations of the HP’s upper body (chest, shoulder, and neck) and face (chin, mucous membranes, cheek, and forehead) were characterized by a cultivation method. Results showed that the IP’s different head orientations resulted in significantly different deposition density on the HP. PV flow could reduce the bioaerosol deposition remarkably for most cases investigated. The effectiveness of PV flow in reducing deposition on the HP was significantly affected by the IP’s head orientations. Findings suggest that changing head orientations can be a control measure to reduce the bioaerosol deposition. Personalized ventilation can be a potential method to reduce the bioaerosol deposition on the HP. © 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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 (Ts) in a range of 25 °C ≤ Ts≤ 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 ≤ Ts≤ 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. © 2021

Respiratory bioaerosol deposition in public transport cabins is critical for risk analysis and control of contact transmission. In this work, we built a two-row four-seat setup and an air duct system to simulate a cabin environment. A thermal manikin on the rear left-hand seat was taken as the infected passenger (IP) and “coughed” three times through a cough generator. The deposited viruses and droplets on nearby seats were measured by a cultivation method and microscope, respectively. The effects of seat backrest and overhead gasper jet were studied. Results showed that the number of deposited virus on the front seat was one order of magnitude higher than that on other seats which only contained droplets smaller than 10 µm in diameter. When the backrest was 15 cm higher than the cough, the deposited number of viruses was reduced to 5% of that with the backrest at the same height with the cough. The gasper jet above the IP with a velocity of 1.5 m/s can reduce the deposited viruses to 4% of that with gasper off. It indicates that both the gasper jet and backrest can work as mitigation measures to block the cough jet and protect the nearby passengers. © 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Head orientations directly determine movement directions of exhaled pathogen-laden droplets, while there is a lack of research about the effect of the infected person's head orientations on respiratory disease transmission during close contact. This work experimentally investigated the effect of different head orientations of an infected person (IP) on the bioaerosol deposition on a healthy person (HP) during close contact. Also, the effectiveness of PV flow in reducing bioaerosol deposition on the HP under the IP’s different head orientations was investigated. Bacteriophage T3 was employed to represent viruses inside the cough-generated aerosols. The bioaerosol depositions on different locations of the HP’s upper body (chest, shoulder, and neck) and face (chin, mucous membranes, cheek, and forehead) were characterized by a cultivation method. Results showed that the IP’s different head orientations resulted in significantly different deposition density on the HP. PV flow could reduce the bioaerosol deposition remarkably for most cases investigated. The effectiveness of PV flow in reducing deposition on the HP was significantly affected by the IP’s head orientations. Findings suggest that changing head orientations can be a control measure to reduce the bioaerosol deposition. Personalized ventilation can be a potential method to reduce the bioaerosol deposition on the HP. © 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Droplet evaporation-based cooling techniques, such as the spray cooling, give high heat transfer rates by utilizing latent energy and are usually preferred in thermal applications. However, with the significant rise in heat dissipation levels for high heat flux devices, these devices cannot be thermally managed due to the limited cooling capacity of existing thermal fluids. In this paper, we report the evaporation of the Cu-Al2O3 hybrid nanofluid (HNF) droplet on a copper surface as well as its own residue surface, developed from the evaporation of the first Cu-Al2O3 HNF droplet. As the main novelty, we identify the critical residue size and investigate the residue size effect, above and below the critical residue size, on evaporation rate of the succeeding Cu-Al2O3 HNF droplet resting over a residue surface. We also develop a new analytical model to estimate the Cu-Al2O3 HNF droplet evaporation rate and compare our results with other existing models. The results show that the Cu-Al2O3 HNF droplet gives 17% higher evaporation rate than a water droplet on a copper surface. Also, the evaporation rate of the Cu-Al2O3 HNF droplet on a residue surface sharply increases by 106% with increasing residue size up to the critical residue size. However, further increasing the residue size above its critical value has a negligible effect on the droplet evaporation rate. Moreover, the evaporation rate of the Cu-Al2O3 HNF droplet on its residue surface is enhanced up to 104% when compared to a copper surface. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved.

This study investigated the effect of airflow distance from a personalized ventilation (PV) system on the personal exposure of a PV user to exhaled droplets from different orientations. Constant and dynamic airflow from the PV system was studied. A PV system was designed to produce both constant and dynamic airflows with different periods. Thermal manikins were used to simulate a healthy person (HP), also the PV user, influenced by a respiratory activity from an infected person (IP) at different orientations. Results show that, with a short distance of 0.2 m between the PV and the HP, both constant and dynamic PV flow can effectively protect the HP at all orientations considered; with a longer distance of 0.5 m between the PV and the HP, at some orientations, neither constant PV flow nor dynamic PV flow could protect the HP from the IP under our testing conditions. PV is a potential method to prevent disease transmission. However, a proper design is essential, and the relative position of the PV terminal, the IP and the HP and the distance of the PV from the HP are important factors to be considered so as to achieve the best performance.

Proper design of personalized ventilation (PV) is an effective way to mitigate the risk of airborne disease transmission. However, there is a lack of research on the effectiveness of PV to reduce bioaerosol deposition on human body surfaces. This work experimentally investigated the performance of PV on prevention of bioaerosol deposition on a healthy person (HP), who was influenced by cough activities from an infected person (IP) over a short distance. Escherichia coli (E. coli) was employed to represent the bacteria from the respiratory droplets. The bioaerosol deposition on the HP's face, shoulder, chest and body back was characterized by a bacterial cultivation method. It is found that the bioaerosol deposition and inhalation can be substantially reduced by turning on the PV. The results show that PV can protect the person effectively and it could be a potential method for infection control in the disease transmission.

Knowledge of respiratory bacteria/virus concentration and distribution on a surface is critical for assessing contact transmission. Here, we investigate the direct bioaerosol deposition and distribution on a near surface from a cough experimentally. A cough generator is used to release bacterium/virus-laden droplets. A solid surface is put in front of the cough generator. Escherichia coli (E-coli) and bacteriophage are used to represent the respiratory bacteria and virus, respectively. Results show that the number concentration of deposited droplets and viable bacteria/viruses on the surface decrease from the impingement point to surface edge. We identify the region of the impingement area on the surface in which the microorganism fraction accounts for around 83% of the overall deposited bacteria/phage along a vertical line. The E. coli-laden droplets have similar distribution with that of the phage-laden droplets, meaning that the microorganisms inside the droplets do not affect the droplet deposition onto near surfaces.

Droplet evaporation-based cooling techniques, such as the spray cooling, give high heat transfer rates by utilizing latent energy and are usually preferred in thermal applications. However, with the significant rise in heat dissipation levels for high heat flux devices, these devices cannot be thermally managed due to the limited cooling capacity of existing thermal fluids. In this paper, we report the evaporation of the Cu-Al2O3 hybrid nanofluid (HNF) droplet on a copper surface as well as its own residue surface, developed from the evaporation of the first Cu-Al2O3 HNF droplet. As the main novelty, we identify the critical residue size and investigate the residue size effect, above and below the critical residue size, on evaporation rate of the succeeding Cu-Al2O3 HNF droplet resting over a residue surface. We also develop a new analytical model to estimate the Cu-Al2O3 HNF droplet evaporation rate and compare our results with other existing models. The results show that the Cu-Al2O3 HNF droplet gives 17% higher evaporation rate than a water droplet on a copper surface. Also, the evaporation rate of the Cu-Al2O3 HNF droplet on a residue surface sharply increases by 106% with increasing residue size up to the critical residue size. However, further increasing the residue size above its critical value has a negligible effect on the droplet evaporation rate. Moreover, the evaporation rate of the Cu-Al2O3 HNF droplet on its residue surface is enhanced up to 104% when compared to a copper surface. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved.

Droplet evaporation is an efficient process as it removes a large amount of heat by using the latent energy, making it suitable for heat transfer applications. In this research, evaporation of the silver-graphene hybrid nanofluid (SGHF) droplet, because of its synergistic thermal conductivity, is investigated for substrate temperature in a range of 25-100 °C. The experiments for droplet evaporation were performed in an environmental facility for two droplet sizes, 3 µL and 30 µL volume, on a copper plate. A 100 W silicone heater mat was used to heat the copper plate from the underside, while two T-type thermocouples were used to monitor its surface temperature. As droplet evaporation ended, a porous residue was formed on the copper surface. Subsequently, a 3 µL volume of the SGHF droplet was dispensed on the porous residue surface. The results showed a tremendous rise in the evaporation rate (up to 160%) for the subsequent SGHF droplet sitting on the porous residue as compared to the non-wetted copper surface. Moreover, the evaporation rate of the SGHF droplet on the copper surface increased up to 56% as compared to the water droplet for a substrate temperature range of 25-100 °C. © 2020 ASME

This study investigated the effect of airflow distance from a personalized ventilation (PV) system on the personal exposure of a PV user to exhaled droplets from different orientations. Constant and dynamic airflow from the PV system was studied. A PV system was designed to produce both constant and dynamic airflows with different periods. Thermal manikins were used to simulate a healthy person (HP), also the PV user, influenced by a respiratory activity from an infected person (IP) at different orientations. Results show that, with a short distance of 0.2 m between the PV and the HP, both constant and dynamic PV flow can effectively protect the HP at all orientations considered; with a longer distance of 0.5 m between the PV and the HP, at some orientations, neither constant PV flow nor dynamic PV flow could protect the HP from the IP under our testing conditions. PV is a potential method to prevent disease transmission. However, a proper design is essential, and the relative position of the PV terminal, the IP and the HP and the distance of the PV from the HP are important factors to be considered so as to achieve the best performance. © The Author(s) 2020.

Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation system can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This paper proposes an air cleaning technique, aimed at submicron particles, by means of installing patterned surfaces on the walls of ventilation ducts, which can be easily cleaned by water and reused. The effect of patterned surfaces on particle deposition was studied numerically. In the numerical simulation, the Reynolds stress turbulent model was correlated at the near-wall regions by turbulent velocity fluctuation at the normal direction. Particle trajectory was solved by using Lagrangian particle tracking. The numerical model was then validated with a particle deposition experiment. A wind tunnel experiment was carried out to quantify the particle deposition on the semi-circular micropatterns for a wide range of heights. Based on our numerical results, the semi-circular pattern height of 500 µm with a pitch-to-height ratio (p/e) of 10 has 8.58 times enhancement of the energy efficiency compared with a high-efficiency particulate air filter (HEPA). Our results indicated that adding surface micropatterns to ventilation ducting for submicron particle deposition is a possible energy-efficient air cleaning technique for practical usage.

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. © 2020

Droplet evaporation is a complex phase change process with a wide range of cooling applications, such as spray cooling and dropwise hotspot cooling in microelectronics, to name a few. The hybrid nanofluid droplet evaporation and its residue effects on evaporation of the subsequent hybrid nanofluid droplet is investigated in this research. Silver-graphene (Ag-GNP) hybrid nanofluid exhibiting synergistic thermal properties is investigated and prepared by dispersing silver nanoparticles along with graphene nanoplatelets in water at 0.1% volume fraction and with different mixing ratios, followed by ultra-sonication. The evaporation rate and wetting characteristics of a 3 µl volume of Ag-GNP hybrid nanofluid droplet on a copper surface were studied using an optical tensiometer. Once dried, the nanoporous structure of the residue surface was examined using a scanning electron microscope, while the surface roughness was measured using an optical profiler. Experiments were continued to further investigate the evaporation rate and wetting effects of the subsequent Ag-GNP hybrid nanofluid droplet over the residue surface. The results showed improved wetting characteristics, with 88% reduction in initial static contact angle and 163-196% enhancement in evaporation rate of the subsequent Ag-GNP hybrid nanofluid droplets over the residue surfaces as compared to the copper surface. Copyright © 2019 ASME.

Particle resuspension has been recognized as a secondary source of indoor air pollution by many field studies. However, some laboratory studies showed that the air velocities or force fields required to resuspend aerosol particles are very high that rarely occurred in indoor environments. In fact, the surfaces used in these studies were treated to ensure cleanliness, but in reality, dusty surfaces are ubiquitous in our daily life. This work aims to investigate the effect of dust on a surface on resuspension of a coarse particle (polyethylene) by a centrifugal method. Dusty surfaces with different loadings were made by gravitational settling of Arizona test dust on a clean poly(methyl methacrylate) substrate inside a deposition chamber. The resuspension of dust particles was first investigated, and it was found that dust particles were resuspended by two stages with different rates of resuspension. For the resuspension of the particles on the dusty surface, the remaining fraction of the polyethylene particles decreased with increasing force field and dust loading. Dust could greatly reduce the adhesion of the particles from one to two orders of magnitude depending on loadings. This gives an explanation to the discrepancy between the field and the laboratory studies.

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 Al2O3 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-Al2O3 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/Al2O3 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. © 2018

Exposure to inhaling airborne particles in indoor and outdoor environments is a threat to public health. Indoor air passes through ventilation ducts and continuously circulates with the air outdoors, resulting in a higher concentration of suspended particles in indoor environments when compared to outdoor environments. Thus removing airborne particles in ventilation ducts becomes essential, especially in large buildings. Repeated surface ribs have been reported to greatly enhance the particle collection efficiency while it also causes a significant pressure drop, leading to higher energy consumption. In this study, an overall efficiency ratio is defined, taking into consideration the particle removal rate and the associated pressure drop, to evaluate the overall performance of different surface patterns in a ventilation duct. After the design and optimization processes, the semi-circular patterns are shown to have the best overall efficiency, i.e. a 1137 times increase when compared with having no patterned surface. The deposition velocity on the semi-circular surface found in simulation results were validated with a fully-developed wind tunnel experiment. This study shows that semi-circular surface patterns with a pitch-To-height ratio of 4 are recommended for the overall enhancement in the ventilation duct, especially for capturing submicron particles. © Published under licence by IOP Publishing Ltd.

The impact of sinusoidal airflow with periods of 16s, 8s and 3s produced by a personalized ventilation (PV) system on the personal exposure and the concentration of airborne droplets in the inhalation zone of a seated thermal manikin in an office is investigated in this study. A droplet generator is used to produce airborne droplets to simulate expiratory activities. And the real-time concentration of airborne droplets is measured by an aerosol spectrometer. This study reveals that sinusoidal airflow with periods of 8s and 3s from PV can effectively prevent airborne droplets entering into the inhalation zone and plays an important protection role. It can be seen that PV with proper dynamic airflow can be a potential method to improve the indoor air quality. © 2018 15th Conference of the International Society of Indoor Air Quality and Climate, INDOOR AIR 2018. All rights reserved.

Resuspension of droplets from surfaces is an important indoor phenomenon. If the droplets are infectious, they would become a new source of airborne disease transmission. Since different kinds of materials that have different surface roughness are used in an indoor environment, investigation of the effect of surface roughness on droplet resuspension behavior is essential. Experiments were carried out by using a centrifuge. Unlike solid particles which detach as a whole, droplets detach by portion when they are under an increasing force field. The adhesive force between droplets and surfaces is found to be proportional to the contact diameter, and the adhesion coefficient is dependent on the roughness. Different from solid particles, the larger the roughness, the harder it is for droplets to be resuspended. This work reveals how roughness affects droplet resuspension and how droplet resuspension behavior differs from that of solid particles. This will be useful for risk assessment on disease transmission. © 2018 15th Conference of the International Society of Indoor Air Quality and Climate, INDOOR AIR 2018. All rights reserved.

Resuspension is considered as a source of indoor air pollutants. These airborne pollutants can be in the form of liquid or solid. It has been previously found that the detachment mechanism of liquid droplets is different from the solid particles on the poly(methyl methacrylate) (PMMA) surface. Liquid droplets detach by portion when they are under an increasing normal force field while droplets detach completely when under a tangential force field. In this research, droplet detachment experiments are extended to different substrate materials, which are PMMA, glass, and stainless steel by the means of centrifuge. Also, the differences in detachment between pure glycerol-water (pure fluid) and a glycerol solution with the addition of nanoparticles (nanofluid) are investigated under different substrate materials. It is found that liquid droplets, again, detach by portion under normal force for all the substrate materials. For tangential force, the droplets detach completely if the exerted force was sufficiently large and the threshold values are material dependent, which is further elaborated by retention theory. After the addition of nanoparticles, a higher removal force was required compared to the droplets of pure fluid within the same size range. Also, solid residues with a negligible amount of fluid were found on the substrate after each removal of droplets under both normal and tangential force. The involvement of nanoparticles could be the pioneer work for future studies on commonly found liquid pollutants, which are prone to be contaminated by solid particles, such as in salivary excretion. Copyright © 2018 American Association for Aerosol Research. © 2018 American Association for Aerosol Research.