Thermal buoyancy can drive a flow of ventilation through industrial buildings containing heat sources, which is an energy efficient means of delivering fresh air to the occupants. Previous studies ...contain abundant information on the thermal plume generated from a single heat source. However, the interaction of plumes above multiple heat sources has not been given sufficient attention. In this study, numerical models are constructed and validated to investigate the interaction of thermal plumes generated by two equal heat sources at different spacings, and the air velocity and temperature distribution of different spacing ratios are obtained. In addition, the ratio of velocity difference V* is defined to study the interaction strength of the two plumes in different plume zone. Moreover, the touching height of the two equal plumes was analyzed. The fitting formula of the touching height is obtained and divided into two parts: closely spaced and widely spaced ratios. When the spacing ratios are low, the touching height of the two plumes is pretty low, and increases nonlinearly as the spacing ratios increases.
•The velocity and temperature distribution of the two equal plumes for different spacing ratios were known.•The effect of spacing ratios on the interaction process of the two equal plumes was obtained.•The confluent plume is divided into three interaction zones of different interaction strength.•The touching height of the two equal plumes for different spacing ratios was analyzed.
In collaboration with 16 musicians from the Minnesota Orchestra, we assess the airflow and particle concentration emitted from ten wind instruments under realistic performance conditions. Anemometer ...and schlieren measurement techniques are used to quantify the air flow, and aerodynamic particle sizer, laser sheet, and digital inline holography techniques are used to measure the particle concentration. The regions where the flow speed and particle concentrations are above the measurable background level vary among instruments depending on both air flow generation and particle production, but extend no farther than 30 cm from the instrument outlet for all instruments. Farther away, the upward-moving thermal plume generated by the temperature difference between the human body and ambient air is the dominant source of flow and aerosol transport. Brass instrument air flow increases with music amplitude and particle concentration exhibits an inverse response to note duration. Woodwinds emit more particles when note pitch increases. Covering the trumpet bell with one layer of acoustic fabric reduces the emitted particle concentration by ~60% with little impact on the sound quality. Adding more mask layers blocks more particles, but impedes performance and lowers the sound quality at higher frequencies (>1000 Hz). Computational fluid dynamics simulations initialized with experimental data show that placing an air cleaner above the instrument outlet can reduce the particle concentration by 90% due to the thermal plume driving aerosols upwards. Filtration efficiency further increases considerably (~10%) when lowering the ambient temperature from 25 °C to 20 °C to enhance the temperature difference with the human body.
•We studied airflow and particle emission from wind instruments in an orchestra hall.•Particle concentration hits background level within 30 cm of each instrument outlet.•Human thermal plume suppresses lateral spread of particles at the breathing level.•Air purifier placement and ambient temperature largely impact filtration efficiency.•One-layer mask blocks 60% particles from trumpet with little sound quality impact.
Adjacent near-wall heat sources are widely used in indoor environments. It is important to investigate the particle deposition under the influence of coupled thermal plumes arising from adjacent ...near-wall heat sources to improve indoor air quality and control harmful particle deposition. Thus, this study scrutinizes the behavior of thermal plumes emanating from adjacent near-wall heat sources, focusing on the deposition of particles with diameters of 0.3 μm, 0.5 μm, 1.0 μm and 3.0 μm on the wall behind the heat sources. These findings are juxtaposed with the pattern of particles with varying sizes situated above the single near-wall heat source and away from the heat sources. The study delves into the impact of varying surface temperatures and the distance from the wall behind the heat sources, as well as the top surface of the heat source, on particle deposition in 29 distinct cases. The results indicate that the deposition velocity of particles with the same size is highest above the adjacent near-wall heat sources, followed by that of a single near-wall heat source, and finally, locations away from the near-wall heat source. Also, the decay rate loss coefficient of particles with the same size above the adjacent near-wall heat sources increases with a decrease in the distance of the heat sources from the wall behind them, an increase in the temperature of the heat sources, and a reduction in distance from the top surface of heat sources.
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•The particle deposition under twenty-nine cases with diferent heat sources configurations were studied experimentally.•The adjacent near-wall heat sources accelerated the particle deposition more significant than single near-wall heat source.•The β above adjacent near-wall heat sources decreases as the distance between the heat source and the back wall increases.
Natural convection and heat transfer in triangular cavities are widely used in the thermal engineering of buildings and heat dissipation in electronic devices. In this study, we analyze dimensionally ...the development of thermal flow for Pr > 1 in a triangular cavity under top-cooling conditions. The formation and development of the boundary layer under a sloping wall are given in terms of instantaneous flow and heat transfer characteristics. The dimensional dependence of the thickness of the thermal boundary layer on time, as well as the timescale of the balance between convection and heat conduction, are also presented. We obtain the volume flow rate relation for the bottom horizontal intrusion flow. The variation of the flow dynamics and heat transfer control mechanisms over time is presented, the thickness and velocity dimensions of the horizontal intrusion flow at different stages are obtained, and the horizontal distance travelled by each mechanism is predicted. For the development of the initial plume, the virial relation between the initial plume thickness and the velocity is presented for different mechanisms. The dynamical and heat transfer control mechanisms of horizontal intrusions and plumes are summarized, the timescales of convective and heat conduction equilibria are given for different dynamical mechanisms, and the flow laws of intrusions and plumes in transient processes are discussed. It provides a reference for natural convection in triangular cavities.
Thermal discharge from nuclear power plants poses a threat to the received natural water bodies, but the long-term extent and intensity of their surface thermal plumes remain unclear. In this study, ...we proposed a method to determine the background area for each drainage outlet and delineate the mixed surface thermal plumes based on 7,172 Landsat thermal infrared images. We further used a deep convolutional neural network integrated with prior location knowledge to extract core surface thermal plumes for 74 drainage outlets of 66 nuclear power plants worldwide. Our final model achieved a mean Intersection over Union (mIoU) of 0.8998 and an F1 score of 0.8886. We found that the mean maximal water surface temperature (WST) increment of the studied plants globally was 4.80 K. The Tianwan plant in China experienced the highest WST increase (8.51 K), followed by the Gravelines plant in France and the Ohi plant in Japan (7.91 K and 7.71 K, respectively). The Bruce plant in Canada had the largest thermal-polluted surface area (7.22 km2). We also provided the dataset, Global Coastal Nuclear power plant Thermal Plume (GCNT-Plume), to describe the long-term occurrence of water surface thermal plumes. Three influencing factors of the water surface thermal plume were further analyzed in this study, including total capacity, drainage type, and location type, which were associated with operating power, drainage method, and geographical features, respectively. Total capacity was more statistically related to the maximum of WST increment under shallow drainage condition. The mean WST increment of shallow drainage was 1.22 K higher than that of deep drainage. Surface plumes larger than 4 km2 frequently occurred in the Great Lakes, while small surface thermal plumes (< 1 km2) were primarily found in estuaries. The proposed method provides an important framework for future operational water surface thermal plume detection using remotely sensed observations and deep learning.
•A practical method was developed to capture mixed thermal plumes from nuclear power plants using Landsat images.•A deep learning model integrated with prior location knowledge was applied to extract core areas of mixed thermal plumes.•The first map of the thermal pollution footprint of 66 global nuclear power plants was provided.•The intensity and extent of thermal pollution detected are related to total capacity, drainage type, and location type.
Buoyancy-driven ventilation of an enclosure containing a convective area heat source with low surface emissivity is studied in this paper. Based on the dynamics of turbulent plumes generated from an ...area heat source, the mathematical models for dimensionless thermal stratification height are deduced and integrated into a unified formula in this study. And a mathematical model governing the transition zone between these two zones is derived and developed. In order to test the validity of the theoretical model, a series of simulated models were conducted to compare theoretical results with simulated results. It was found that the transition zone will disappear directly when virtual origin distance zv/Ds≥−1.14, which results in that the thermal stratification height can only be obtained in the zones close to the area source and far away from the area source. And the thermal stratification height varies widely when the A∗/H2 values are from 0 to 0.1. In addition, thermal stratification height is negatively correlated with the dimensionless source diameter ξs. This study can be used as a basis for follow-up study of buoyancy-driven flows by area heat source in naturally ventilated building.
•The theoretical models were derived to study the buoyancy-driven flows by area heat sources.•A new law of thermal stratified flow was obtained for the transition zone.•The approximate theoretical models were verified by numerical simulations.•The thermal stratification height is negatively correlated with the dimensionless source diameter.
•Non-uniform formation of convective cells at higher Grashof number.•Flow is found to be oscillating and bidirectional through the horizontal opening.•Significant change in flow behavior with ...increase in Grashof number.•Heat source location has strong influence on thermal mixing and vent flow rates.•Higher Grashof number enhances the mixing rates inside the enclosures.
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The buoyancy driven thermal plume behavior inside a partial square enclosure with protruding isothermal heat source block is numerically investigated. The turbulent flow is modeled by using unsteady Favre-averaged Navier-Stokes (UFANS) equation with buoyancy modified Lam Bremhorst low Reynolds number k−ϵ model. A Simplified Marker and Cell algorithm (SMAC) is used to solve the governing equations and the computations are performed using an in-house code based on Finite difference method. The heat transfer characteristics and the bidirectional exchange across the vent are analyzed for different Grashof number 108 ≤ Gr ≤ 1011. Further, the thermal plume patterns are investigated by mounting the heat source block at five different locations δ = 0.25 H, 0.35 H, 0.5 H, 0.65 H and 0.75 H. A significant enhancement in the net mass flow rate and average Nusselt number is observed by increasing the Grashof number. It is found that the heat source location has strong influence on the vent bidirectional exchange rate, thermal mixing and heat transfer characteristics inside the enclosure. The present model is validated with the experimental and numerical benchmark results available in literature.
One of the main disturbances caused by coastal nuclear power plants is the discharge of thermal effluents capable of affecting a number of marine systems, including macroalgal forests that support ...key ecosystem services such as carbon uptake, fisheries increment and coastal protection. This study aimed at describing the long-term trend (1992–2022) in the abundance of Sargassum forests from sites located inside and outside areas affected by the thermal effluent discharged by the Brazilian Nuclear Power Station (BNPS) and at evaluating the relationship between Sargassum cover and seawater temperature. This information is interesting to provide insights on whether and how Sargassum populations would likely be affected by increasing temperature due to climate change. We detected a long-term decline in Sargassum cover inside, but not outside the area affected by the BNPS thermal plume. Mean summer surface seawater temperature above 30 °C was identified as an important factor driving the decline of Sargassum abundance. This study highlights the impact caused by decades of discharge of the BNPS thermal effluent on Sargassum forests, which leads to predict the likely disappearance of marine forests under a climate change scenario in other sites situated in warm temperate regions.
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•Decline of marine forests inside an area affected by nuclear power thermal effluent.•Temperatures above 30 °C are an important driver of Sargassum loss.•Risk of disappearance of marine forests in the Ilha Grande Bay under global warming.
Indoor air stability is a general term that refers to the indoor air's tendency to encourage or discourage its initial inertia motion. In stable conditions where vertical temperature gradients are ...positive, would prevent the vertical motion of fluid flows, thus the initial inertia motions are maintained, whereas in unstable conditions where vertical temperature gradients are negative, fluids experience intensive convection, thus the initial inertia motions are more easily to get disturbed. This study experimentally and numerically investigated the dispersion of exhaled contaminants and body thermal plume in a displacement-ventilated room integrated with unstable and stable conditions. Two real-human subjects participated in the full-scale experiments. The effects of participants' relative position with reference to the ventilation vents and of ventilation rates on the flow field were examined. Results show that the thermal stratification associated with the displacement ventilation system was disturbed by unstable conditions, resulting in a uniformly distributed contaminants and thermal fields, therefore a reduced exposure level; whereas the thermal stratification was enhanced by stable conditions so that the transport of contaminants and thermal more inclined to follow their initially released direction and led to a relatively high exposure level in the breathing microenvironment. Air distribution in unstable conditions, when compared with stable conditions, was more sensitive to the relative position between the participants and the vents. Increasing the ventilation rate could greatly reduce the contaminant level of unstable conditions but may not achieve as good results in stable conditions. This paper has practical significance in controlling the transmission of exhaled contaminants and preventing the spread of infectious diseases.
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•Stable air with positive temperature gradients exerts directional effect on fluid.•Unstable air with negative temperature gradients develops turbulent instability.•Thermal stratification gets enhanced in stable air but disturbed in unstable air.•The relative position of people with vents matters more in unstable air.•Increasing ventilation rates is not an optimal way to reduce cross-infection.
This study proposed a method for reconstructing three-dimensional (3D) non-axisymmetric temperature fields in water formed by mixed convection surrounding a small heating sphere. The proposed method ...is based on the temperature dependence of the absorption coefficient of water at a wavelength of 1150 nm in the near-infrared (NIR) region. The absorbance images of a 10-mm-thick water cell containing a 1-mm diameter steel sphere heated via electromagnetic induction were acquired using a two-orthogonal-direction NIR imaging system. The 3D reconstruction was implemented by applying a non-axisymmetric inverse Abel transform to the absorbance images. In addition, particle tracking velocimetry (PTV) was applied to the same images to visualize and quantify the flow field. The results indicated that the temperature distribution within the plume was distorted compared to an axisymmetric one, and the plume shape, length, and angle were dependent on the forced flow and heating power. Further, employing a thermal balance model with temperature distributions, the heat production rates were determined and discussed based on the plume characteristics. The PTV results revealed a mixed convection field inconsistent with the plume shape, which was also verified by comparison with numerical simulations.
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•Simultaneous measurement of temperature and flow fields employing two-orthogonal-direction near-infrared imaging method.•Reconstruction of three-dimensional non-axisymmetric laminar thermal plumes.•Determination of heat production rate in the sphere.•Particle tracking velocimetry using near-infrared absorbance images.
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