Effects Of Thermal Plumes Research Articles

Numerical Simulation of the Distribution Patterns of Particle Deposition on the Vertical Wall Behind Near-Wall Heat Source

Near-wall heat sources have a crucial part to play in the process of particle deposition. Thus, this study investigates the impact of the near-wall heat source on the distribution patterns of particle deposition on the vertical wall behind the heat source, taking into account the variability in heat source temperatures and distances from the vertical wall. A model based on the Eulerian–Lagrangian method was established for tracking the motion trajectories of 1000 particles with a density of 1400 kg/m3 and a particle size range of 0.01–10.0 μm. The temperature field, airflow field, and particle deposition distribution in six cases were analyzed. It was shown that the heat source temperature significantly affectis the temperature field, airflow field, and particle deposition distribution on the vertical wall behind the heat source. This study demonstrated that as the temperature rises, the quantity of particles deposited in the upper-right region of the vertical wall decreases more noticeably. The quantity of particles deposited onto the vertical wall is inversely related to the distance between the near-wall heat source and the vertical wall. On one hand, the deposition distribution law serves as a foundation for advancing the technology aimed at removing suspended particles via thermal plumes. On the other hand, it provides critical insights for addressing the challenges associated with harmful particle deposition linked to the attachment effects of thermal plumes.

Self-correction of the optical distortion effect of thermal plumes in particle image velocimetry

Optical distortion caused by changes in the refractive index of fluid flow is a common issue in flow visualization using techniques, such as particle image velocimetry (PIV). In thermally driven convection, this distortion can severely interfere with PIV results due to the ubiquitous density and, therefore, refractive index heterogeneity in the fluid. The distortion also varies spatially and temporally, adding to the challenge. We propose a composite filter, the shadow-affected PIV region filter, which combines a series of conventional image filters to address this issue, focusing on optical distortion of thermal plumes in laminar flow. We verify the effectiveness of the filter using both synthetic particle images created from ray tracing and real particle images from the laboratory. For the first time, we effectively mitigate the optical distortion from plumes while preserving the in-plane plume velocity and overall flow pattern, with the PIV data alone. Our filter is efficient and does not require additional measurements, expensive ray tracing, or a large dataset to begin with. It can be extended to separate the flow field and the effect of optical distortion in other fluid experiments when the two components are visually distinct. Additionally, this filter can serve as a baseline algorithm for comparison when developing more advanced methods like neural networks.

Thermofluid dynamics and droplets transport inside a large university classroom: Effects of occupancy rate and volumetric airflow

The airborne droplets released by humans while speaking or coughing are the main sources of human-to-human contagion for airborne transmissible diseases. Due to the recent SARS-Cov2 pandemic, indoor transmission in classrooms and other environments is an interesting topic investigated by the scientific community. In this work, the thermofluid dynamic conditions and the droplet transport was analysed in a large University Classroom (UC), as a function of the inlet airflow and the occupancy rate. These parameters assume a relevant role in the design stage. However, their effects on droplet transport were not deeply investigated in the literature. Therefore, different conditions were analysed, such as: full occupancy, which represents the design condition where no social distancing is enforced; half occupancy, which is representative of social distancing; and empty UC, to be considered as a term of comparison. The main novelty of this work is related to the analysis of the combined effect of thermal plume and airflow in a large UC. The numerical model for the thermofluid dynamics simulation was validated in a previous work and the droplet emission model was retrieved from the literature. High volumetric airflow is not always beneficial for the reduction of the airborne transmission risk. An important aspect is related to the combined effect of the inlet airflow and the thermal plume, which cannot be ignored if the occupancy rate assumes a significant value. Moreover, the evacuation rate and the age of the droplets are relevant indicators to assess the cleaning time.

Numerical Prediction of the Effect of Thermal Plume of a Standing Human on the Airborne Aerosol Flow in a Room: Assessment of the Social Distancing Rule

The purpose of the study is to investigate the dispersion of droplet nuclei/aerosol which are produced during coughing and continuous talking to quantify the risk of infection due to airborne disease transmission. A three-dimensional modelling of aerosol transport due to human respiratory activities such as coughing and talking within a room environment has been simulated using CFD technique. An inert scalar transport equation was used to represent aerosol cloud, while turbulence was modelled with the k-epsilon turbulence model. A modified Wells–Riley equation was used to calculate the risk of infection based on quanta emission concept. The spatial and temporal distribution of aerosol cloud within the room is initially driven by the upward flowing thermal plume surrounding the human, but later driven by the flow field constrained by the walls and cooler air movement. While the cough generated aerosols are concentrated in a smaller space within the room, the continuous talk generated aerosols are distributed throughout the room. Within an indoor environment, 2 m distancing will not be enough to protect healthy people from aerosols coming from an infected person due to continuous talking with prolonged exposure.

Collective effect of thermal plumes on temperature fluctuations in a closed Rayleigh–Bénard convection cell

We report a systematic study of the collective effect of thermal plumes on the probability density function (p.d.f.) $P(\delta T)$ of temperature fluctuations $\delta T(t)$ in turbulent Rayleigh–Bénard convection. By decomposing $\delta T(t)$ into four basic fluctuation modes associated with single and multiple warm and cold plumes and a turbulent background, we derive an analytic form of $P(\delta T)$ based on the convolutions of the five independent modes. To test the derived form of $P(\delta T)$ in the multiple-plume regions, where the thermal plumes are heavily populated, we conduct time series measurements of temperature fluctuations in two convection cells; one is a vertical thin disk and the other is an upright cylinder of aspect ratio unity. For a given normalized position in most regions of the convection cell, all of the measured p.d.f.s $P(\delta T)$ for different Rayleigh numbers fall onto a single master curve, once $\delta T$ is normalized by its root-mean-square (r.m.s.) value $\sigma _T$ . It is found that the measured $P(\delta T/\sigma _T)$ at different locations along the symmetric horizontal and vertical axes of the convection cells can all be well described by the derived form of $P(\delta T/\sigma _T)$ . The fitted values of the parameters associated with the number of plumes in multiple plume clusters and their relative strengths and degrees of intermittency are closely linked to the spatial distribution of thermal plumes and local dynamics of the large-scale circulation in a closed convection cell. Our work thus provides a unified theoretical approach for understanding scalar p.d.f.s in a turbulent field, which is very useful not only for the present study but also for the study of many turbulent mixing problems of practical interest.

Study on ventilation performance of inclined solar chimney coupled with indoor heat source

The study of ventilation air distribution of the building with built-in heat source and set inclined solar chimney is carried out through 3D numerical simulation. The synergistic effect of thermal plume formed by internal heat source and solar chimney effect is analyzed. The effects of different solar radiation intensity I, heat flow density W at the surface of the internal heat source and structural parameters (including the height of the internal heat source from the ground h and the mezzanine inlet spacing b) on the indoor natural ventilation performance of the building are investigated.

Effect of human thermal plume and ventilation interaction on bacteria-carrying particles diffusion in operating room microenvironment

In the operating room (OR), ventilation and thermal plumes can affect the distribution of bacteria-carrying particles (BCPs). Especially in the operating microenvironment, the development of human thermal plumes cannot be ignored. Therefore, in this study, six different air supply velocities were used to study human thermal plumes and the diffusion of BCPs. The numerical simulation based on computational fluid dynamics was verified by experiments, and the realizable k-ε turbulence model and Lagrangian particle tracking model were used. The results show that the interaction of the thermal plumes and ventilation significantly affected the diffusion of BCPs in the operating area, which cannot be ignored. The maximum thermal plumes in the ORs are markedly higher than those in other rooms. The strong thermal plumes caused BCPs to reach the operating area and infect the wound of the patient. For most ORs with a vertical unidirectional air supply, ventilation and thermal plumes achieve a good offset when the air supply velocity reaches 0.25 m/s. In addition, increasing the air supply velocity alone cannot effectively control the effect of thermal plumes on BCPs. This study can provide a practical guideline for controlling the diffusion and distribution of BCPs in ORs.

Heat Convective Effects on Turbulence and Airflow inside an B767 Aircraft Cabin

Thermal plumes generated by human bodies can affect the temperature and humidity of the surrounding environment. An experimental study investigated the effects of thermal plumes formed by aircraft passengers on airflow and turbulence characteristics inside aircraft-cabins. An 11-row, wide-body B767 cabin mockup was used with actual seats, air diffusers and cabin profile. Thermal manikins were used simulating passengers in the cabin. Tracer gas and air speed inside the cabin were measured while the heat from the manikins was turned on and off to help understand the effects of the thermal heat released by the manikins. Results showed that tracer gas distribution were more uniformly and equally distributed around the release source and the air speed fluctuation were lower under cooler environments when the thermal manikins were turned off. Heated environments increased the values of turbulence kinetic energy and the turbulence intensity levels. However, the effects on the turbulence intensity were less significant compared to the turbulence kinetic energy. On the other hand, the dissipation rates were higher for unheated cases in the front and back sections of the mockup cabin. The relative uncertainty for tracer gas sampling ranged between ±5–14% for heated manikins versus ±8–17% for unheated manikins. Higher uncertainty levels accompanied the turbulence measurements due to the highly chaotic nature of the flow inside the cabin.

Study on natural ventilation driven by a restricted turbulent buoyant plume in an enclosure

Vertical thermal stratification will be produced by buoyant plumes in a naturally ventilated enclosure. The buoyant plume located near the wall or corner of a ventilated enclosure will be restricted by the wall or corner during the rising process. Theoretical models are deduced and validated to quantitatively predict the thermal stratification height of an enclosure containing a buoyant plume restricted by a wall or corner. The theoretical models can describe the relationship between the thermal stratification height and the height difference between the top and bottom openings of the enclosure, the effective area of the top and bottom openings of the enclosure and the distance between the plume and the wall or the corner. Thresholds for the distance between the plume and the side boundary are defined to determine whether the plume is restricted by the side boundary. Simultaneous equations containing the enclosure dimensions are proposed to predict the thresholds. These results could be used as a reference on natural ventilation design for buildings containing restricted heat sources to make full use of the effect of thermal plumes and create a suitable indoor environment.

Statistics of kinetic and thermal energy dissipation rates in two-dimensional turbulent Rayleigh–Bénard convection

We investigate the statistical properties of the kinetic$\unicode[STIX]{x1D700}_{u}$and thermal$\unicode[STIX]{x1D700}_{\unicode[STIX]{x1D703}}$energy dissipation rates in two-dimensional (2-D) turbulent Rayleigh–Bénard (RB) convection. Direct numerical simulations were carried out in a box with unit aspect ratio in the Rayleigh number range$10^{6}\leqslant Ra\leqslant 10^{10}$for Prandtl numbers$Pr=0.7$and 5.3. The probability density functions (PDFs) of both dissipation rates are found to deviate significantly from a log-normal distribution. The PDF tails can be well described by a stretched exponential function, and become broader for higher Rayleigh number and lower Prandtl number, indicating an increasing degree of small-scale intermittency with increasing Reynolds number. Our results show that the ensemble averages$\langle \unicode[STIX]{x1D700}_{u}\rangle _{V,t}$and$\langle \unicode[STIX]{x1D700}_{\unicode[STIX]{x1D703}}\rangle _{V,t}$scale as$Ra^{-0.18\sim -0.20}$, which is in excellent agreement with the scaling estimated from the two global exact relations for the dissipation rates. By separating the bulk and boundary-layer contributions to the total dissipations, our results further reveal that$\langle \unicode[STIX]{x1D700}_{u}\rangle _{V,t}$and$\langle \unicode[STIX]{x1D700}_{\unicode[STIX]{x1D703}}\rangle _{V,t}$are both dominated by the boundary layers, corresponding to regimes$I_{l}$and$I_{u}$in the Grossmann–Lohse (GL) theory (J. Fluid Mech., vol. 407, 2000, pp. 27–56). To include the effects of thermal plumes, the plume–background partition is also considered and$\langle \unicode[STIX]{x1D700}_{\unicode[STIX]{x1D703}}\rangle _{V,t}$is found to be plume dominated. Moreover, the boundary-layer/plume contributions scale as those predicted by the GL theory, while the deviations from the GL predictions are observed for the bulk/background contributions. The possible reasons for the deviations are discussed.

Numerical Modeling of Water Thermal Plumes Emitted by Thermal Power Plants

This work focuses on the study of thermal dispersion of plumes emitted by power plants into the sea. Wastewater discharge from power stations causes impacts that require investigation or monitoring. A study to characterize the physical effects of thermal plumes into the sea is carried out here by numerical modeling and field measurements. The case study is the thermal discharges of the Presidente Adolfo López Mateos Power Plant, located in Veracruz, on the coast of the Gulf of Mexico. This plant is managed by the Federal Electricity Commission of Mexico. The physical effects of such plumes are related to the increase of seawater temperature caused by the hot water discharge of the plant. We focus on the implementation, calibration, and validation of the Delft3D-FLOW model, which solves the shallow-water equations. The numerical simulations consider a critical scenario where meteorological and oceanographic parameters are taken into account to reproduce the proper physical conditions of the environment. The results show a local physical effect of the thermal plumes within the study zone, given the predominant strong winds conditions of the scenario under study.

Infrasonic ray tracing applied to small-scale atmospheric structures: thermal plumes and updrafts/downdrafts.

A ray-tracing program is used to estimate the refraction of infrasound by the vertical structure of the atmosphere in thermal plumes, showing only weak effects, as well as in updrafts and downdrafts, which can act as vertical wave guides. Thermal plumes are ubiquitous features of the daytime atmospheric boundary layer. The effects of thermal plumes on lower frequency sound propagation are minor with the exception of major events, such as volcanoes, forest fires, or industrial explosions where quite strong temperature gradients are involved. On the other hand, when strong, organized vertical flows occur (e.g., in mature thunderstorms and microbursts), there are significant effects. For example, a downdraft surrounded by an updraft focuses sound as it travels upward, and defocuses sound as it travels downward. Such propagation asymmetry may help explain observations that balloonists can hear people on the ground; but conversely, people on the ground cannot hear balloonists aloft. These results are pertinent for those making surface measurements from acoustic sources aloft, as well as for measurements of surface sound sources using elevated receivers.

Effect of Thermal Plume on Personal Thermal Comfort in Displacement Ventilation at One Side of the Room

The Indoor Zero-Equation Turbulence Model in software Airpak was adopted to simulate the personal thermal comfort in the condition of displacement ventilation at the left side. The parameters of the simulated environment were 25°C and 40%. The simulated results show that over the person, there was a clear phenomenon of thermal plume which entrains the around air resulting in the decrease of the temperature with the increasing of the height. As for the predicted mean vote (PMV) and the percent person's dissatisfied (PPD), their gradient were depended on the temperature distribution formed by thermal plume, and the average value was 0.249 for PMV and 7.25% for PPD. All these show that the effect of thermal plume can not be neglected in the design of HVAC system.

Desk fans for the control of the convection flow around occupants using ceiling mounted personalized ventilation

This study aims at studying by modeling and experimentation the performance enhancement of ceiling-mounted personalized ventilation (PV) nozzle when assisted by small desk-mounted fans to reduce the effect of thermal plume generated by the occupant. Detailed computational fluid dynamics CFD simulations were performed to study the flow, thermal, and CO2 concentration fields around an occupant using a single jet ceiling-mounted PV nozzle in the conditioned space while the fans are on. The CFD model was integrated with a bioheat model to determine the corresponding human body segmental heat fluxes and convective plumes for predicting the occupant segmental and overall comfort. The CFD model was validated with experiments using a cylindrical heat source and comparing measured velocity, temperature and CO2 concentrations in the vicinity of the nozzle and the cylinder. Good agreement is found between the measured and the numerically predicted values. The performance of the PV nozzle assisted with desk fans is assessed by evaluation of ventilation effectiveness in the occupant microclimate and comfort conditions and results are compared with single jet PV and co-axial jet PV, both without the use of desk fans. The desk-mounted fans were able to reduce the convection plumes around the occupant and improved the performance of the single jet PV nozzle by doubling the ventilation effectiveness and improving comfort. They permitted also to achieve a reduced energy saving by up to 13% when compared with conventional mixing ventilation systems.

Thermal plume effects: A multi-disciplinary approach for assessing effects of thermal pollution on estuaries using benthic diatoms and satellite imagery

Rapid, reliable and cost-effective techniques for assessing and monitoring pollution are required because of increased development pressures associated with continued population growth. An innovative multi-disciplinary approach was applied to a power station discharge in Lake Macquarie, Australia, using benthic diatoms, water quality, satellite imagery and temperature loggers. Triplicate sediment samples at five sites across a thermal gradient in one plume affected and two control bays were analysed for benthic diatoms. Multivariate analysis indicated that diatom assemblages and environmental gradients in the receiving water embayment were significantly different to control bays. The plume affected benthic assemblages to greater depths (∼4.7 m) than observed by previous studies and this is likely to have implications for estimates of estuarine productivity and nutrient cycling. Of the 244 diatom taxa identified, Navicula rhaphoneis appeared to best identify areas of the lake bed exposed to temperatures 3–4 °C above ambient (ΔT). Tryblionella lanceola, Tryblionella littoralis, Grammatophora spp. and Psammodictyon panduriformis also contributed to gradients and might be used as plume indicator species. Temperature, ammonia, oxidised nitrogen and selenium significantly explained gradients in the species data (p = 0.02). Satellite imagery indicated that receiving bay temperature gradients (<7 °C) were greatest in winter, whereas loggers showed ΔT was greatest in autumn then winter. These analyses highlighted that seasonality is an important factor when considering the effects of thermal plumes on receiving environment ecology. Analyses of imagery and logger data are effective techniques for managers to routinely assess plume intensity and extent. This study demonstrates that both benthic diatoms and satellite imagery are valuable tools for the monitoring and assessment of thermal pollution in coastal environments.

Transport and Removal of Expiratory Droplets in Hospital Ward Environment

The transport and removal characteristics of expiratory droplets at different supply airflow rates and “coughing” orientations were investigated both numerically and experimentally in a three-bed hospital ward setting. A Lagrangian-based particle-tracking model with near-wall correction functions for turbulence was employed to simulate the fate of the expiratory droplets. The model was tested against experimental droplet dispersion data obtained in an experimental hospital ward using Interferometric Mie Imaging and a light-scattering aerosol spectrometer. The change in airflow supply rate had insignificant effect on the transport and deposition of very large droplets (initial sizes ≥ 87.5 μm) due to the dominance of gravitational settling on these behaviors. Smaller droplets (initial sizes ≤ 45 μm) exhibited certain airborne behaviors. The effect of thermal plumes from heat sources was observed only when the supply airflow was low and when the droplet size was small, as observed in the vertical mixing patterns of the droplets of various sizes. Larger droplets tended to settle lower and lateral dispersion of the droplets became weak at the low supply airflow rate. The deposition characteristics for different surfaces in the room are described. The heat plumes seemed to obstruct small droplets from being deposited onto heated surfaces. More deposition was predicted in the lateral injection case compared with the vertical injection case. Adopting near-wall correction for turbulence in the model reduced the predicted deposition removal fraction by 25% for 1.5 μm droplets. This reduction became less significant for larger droplets due to the smaller dependence on turbulent diffusion in their deposition.

Analysis of Poultry House Ventilation Using Computational Fluid Dynamics

The airflow in and around poultry houses was studied numerically with the goal of determining the disease spread characteristics and comparing two ventilation schemes. A typical manure-belt laying hen egg production facility was considered. The continuity, momentum, and energy equations were solved for flow both inside and outside poultry houses using the commercial computational fluid dynamics (CFD) code FLUENT. The geometry was constructed by making some simplifying assumptions, such as two-dimensionality. The spread of virus particles was considered analogous to diffusion of a tracer contaminant gas, in this case ammonia (NH3). The effect of thermal plumes produced by the hens in the poultry house was also taken into consideration. Two ventilation schemes with opposite flow directions were compared. Contours of temperature and contaminant mass fraction for both cases were obtained and compared. The analysis shows that ventilation and air quality characteristics were much better for the case in which the airflow was from bottom to top instead of from top to bottom (top to bottom is how most current poultry houses are configured). This has implications for air quality control in the event of epidemic outbreaks. Decreased contaminant spread to downwind poultry houses was observed in the bottom-to-top airflow scheme.

Effect of Thermal Discharges on the Fish Assemblages of a Nuclear Power Plant in Northern Taiwan

The purpose of this paper is to study whether the thermal plume can affect the fish assemblages in the waters around the outlet area of the Second Nuclear Power Plant located at Kuosheng. a coastline between Yehliu and Chinshan village, northern coast of Taiwan. Both experimental and control stations of underwater census for reef fishes and of drift net sampling for pelagic or demersal fishes above sandy bottom were monitored four times per year from March 2001 to September 2004. The results show that no significant differences were found between the fish assemblages of the thermal waters and normal ambient waters for both coral reef fishes and pelagic or demersal fishes. It was probably due to microhabitat difference rather than water difference because the above fishes are mostly living near bottom where the water temperature are similar to each others for these two stations. On the contrary, the seasonal effect due to low water temperature in the winter had greater influence on the fish assemblages than the effect of thermal plume.

Potential Weather Modification Caused by Waste Heat Release from Large Dry Cooling Towers

A numerical model of a cooling tower plume is employed to study the possible atmospheric effects of thermal plumes from natural draft dry cooling towers. Calculations are performed for both single and multiple towers, each of which can dissipate the waste heat from a nominal 1000 MWe power generating unit, and the results are compared with those for wet cooling towers associated with plants of the same generating capacity. Dry cooling tower plumes are found to have a higher potential for inducing convective clouds than wet cooling tower plumes, under most summertime meteorological conditions. This is due to the fact that both the sensible heat and momentum fluxes from a dry tower in summer are approximately one order of magnitude larger than those from a wet cooling tower.