Human Thermal Plume Research Articles

Removal efficiency of restroom ventilation revisited for short-term evaluation

Ventilation efficiency or contaminant removal efficiency is often evaluated using the ratio between the concentrations in the exhaust air and the room air. This ratio does not truly represent the expectation of ventilation in restrooms, where dynamic airflow fields and sources are more typical. This study focuses on a short-term (10 min) pollutant removal percentage in a residential restroom featuring a dynamic airflow field, particularly with the onset of window-induced stack ventilation during toilet uses. Thirteen ventilation scenarios of a residential restroom were studied using the numerical method that was validated by a mock-up experiment. The scenarios differed in the operation of the exhaust fan and window. Results show that the 10-min pollutant removal percentage of a typical exhaust ventilation system at 10 h-1 air change rate (ACH) is only 68.5%. Under exhaust ventilation, opening the window can introduce both adverse short circuit and favorable stack ventilation depending on the difference between the indoor and outdoor temperatures. As the temperature difference increases from 0 to 12.5 °C, the removal percentage increases from below 50%, a drop due to short circuit, to above 98% thanks to a tripled ventilation rate. The human thermal plume has notable effect on the removal percentage, but its effect can be neglected with the presence of stack ventilation. The hybrid ventilation strategy has impact on perceived air quality and thermal comfort. When the outdoor air is colder, opening the window under exhaust ventilation may increase the current sitting user’s exposure to the self-produced pollutants but can reduce the exposure of the next immediate standing user. In addition, opening the window in cold days will make the toilet user thermally uncomfortable with reduced local temperatures and increased airflow velocities. The study highlights the importance of using the short-term removal percentage to evaluate the performance of restroom ventilation.

How Does the Interaction of the Human Thermal Plume and Breathing Affect the Microenvironment and Macroenvironment of an Elevator Cabin?

The details of the interaction of human thermal plume and breathing activities are simulated in the current study of an unsteady turbulent flow field in an elevator cabin. Air velocity and temperature distributions of the circulation flow pattern (i.e., the macroenvironment), the breathing-scale microenvironment’s characteristics, and the thermal plume’s fate are analyzed. The current study is aimed at showing how respiratory activities such as breathing and human thermal plumes affect the flow field and respiratory contaminants dispersion pattern in a nonventilated enclosed environment (the elevator cabin). The results from three cases, i.e., breathing thermal manikins, nonbreathing thermal manikins, and isothermal breathing manikins, are contrasted to unveil better the effects of human thermal plume and breathing on the flow field, including the velocity distribution, dispersion pattern of the exhaled contaminant, the human body’s heat transfer coefficient, and the large-scale flow pattern. Results reveal that breathing inhalation increases the upward velocity of the thermal plume on the one hand, which directly affects the microenvironment and indirectly impacts the macroenvironment due to the more vigorous reflected thermal plume. On the other hand, the upward thermal plume reduces the penetration length of the exhaled jet. Breathing activities create ring vortices that connect the microenvironment and the macroenvironment. The circulation flow features a downward flow in the cabin’s center, affecting the vortex strength and contaminant dispersion pattern. Overall, the human thermal plume and human breathing make comparable contributions to the resulting elevator-cabin flow characteristics.

The penetration phenomenon of the expiratory airflow from thermal plume of human body in the microenvironment around people

Understanding the influence of the human body's thermal plume on exhaled airflow is crucial for accurately assessing indoor infection risks. This study employs numerical simulations and a jet integral modeling method to investigate the airflow dispersion patterns of a single thermal manikin during sustained expiration. It was found that the thermal plume formed a two-stage evolution in exhaled airflow. Initially, the airflow was weakened by the thermal plume in the horizontal dispersion, and subsequently it was bent due to the entrainment of the surrounding air in the upward dispersion of the exhaled airflow. The thermal plume changes the trajectory and lateral dispersion distance of exhaled airflow. When the exhalation velocity is 1 m/s, the airflow is fully deflected towards the head plume region. When the exhalation velocity is between 1.5 and 2 m/s, the airflow is partially deflected, with only part of the upper airflow deflected towards the head plume region. In addition, it is interesting to note that the plume changes the dispersion trajectory of the exhaled airflow, while still adhering to the distributions of buoyant jet flow. This study suggests that the thermal effect of the human body is a significant factor in characterizing the dispersion of exhalation airflow and aerosols. Considering the thermal plume in indoor infection risk assessment is expected to yield more reliable data for theoretical modeling of respiratory airflow transport.

Analysis of the influence of large-scale room circulation on the human microenvironment

In this study, the effects of indoor large-scale circulation on the human microenvironment under natural convection and air-conditioning conditions were investigated by experiments and numerical simulations in a climate chamber. The effect of human thermal plumes on airflow was studied by large-scale circulation period characteristics. The main influencing factors of the vortex structure distribution are analysed under two conditions. The results show that the thermal plume under natural convection condition is the main factor affecting the large-scale circulation, and the indoor large-scale circulation has a significant effect on the microenvironment around the human body. The supply air jet can significantly reduce the large-scale circulation period. Moreover, the air supply jet and the human thermal plume are coupled to form multiple small-scale circulations.

Coughed droplets dispersion pattern in two-bed ward under different heating strategies

This study aims to compare the dispersion of coughed droplets with different diameters under two heating strategies of hospital ward in winter. An experimentally verified computational fluid dynamics (CFD) method has been used to simulate the evaporation and diffusion of droplets with five different diameters in a two-bed ward with air conditioning strategy (MV) and floor radiant heating strategy (FH). Results show that in the boundary of the human body, indoor air flow, human expiratory airflow and thermal plume play an important role in the dispersion of coughed droplets. Small and medium-sized droplets suspension in the ward for a longer time, while large droplets deposit rapidly. The removal ratio of 50 μm droplets under MV strategies is the highest. The 100 μm droplets in the two heating strategies are completely removed within 30 s. There is an obvious difference of droplets deposition ratio on each surface in the ward.

Experimental assessment of an air curtain-sealed personal protective equipment for medical care: Influence of breathing and thermal plume

This paper presents the performance assessment of a novel air-sealed visor for medical care based on experiments. This Personal Protective Equipment (PPE) provides effective aerodynamic sealing of the breathing zone, thus reducing the risk of inhaling droplets and aerosols carrying a potential viral load. The air curtain supply system was designed after computational fluid dynamics exploratory simulations to determine suitable airflow conditions. The main parameters under study are the air curtain velocities and the fraction of contaminated air in the breathing zone. The impact of different airflow rates, human breathing, thermal plume and lateral barriers to reduce the risk of leakage, was investigated. For the designed airflow rate and considering the worst-case scenario with a contaminated environment, results indicate that this PPE can reduce the contaminated air in the breathing zone by 31% and 82%, without and with lateral physical barriers, respectively. Although the human body thermal plume has a high impact on the air curtain velocity profile, it just affects the device efficiency by 2%. The breathing process increases the air curtain velocities improving the PPE efficiency by 32%. This study shows the potential of using a visor with aerodynamic sealing as an infection control barrier for healthcare workers.

Effect of Spatial Proximity and Human Thermal Plume on the Design of a DIY Human-Centered Thermohygrometric Monitoring System

Wearable devices have been introduced for research purposes and especially for environmental monitoring, with the aim of collecting large amounts of data. In a previous study, we addressed the measurement reliability of low-cost thermohygrometers. In this study, we aim to find out how human thermal plume could affect the measurement performance of thermohygrometers. For this purpose, we used a Do-It-Yourself device that can be easily replicated. It consists of 10 iButtons with 3D-printed brackets to position them at different distances from the body. The device was attached to the user’s belt in a seated position. We considered two scenarios: a summer scenario with an air temperature of 28 °C and a clothing thermal resistance of 0.5 clo and an autumn scenario with an air temperature of 21 °C and a clothing thermal resistance of 1.0 clo. The results show that the proximity of the measurement station to the body significantly affects the accuracy of the measurements and should be considered when developing new wearable devices to assess thermal comfort. Therefore, we recommend that at least two thermohygrometers be considered in the development of a new wearable device if it is to be worn on a belt, with one positioned as close to the body as possible and the other at least 8 cm away, to determine if and how the standard thermal comfort assessment differs from the user’s personal perception and whether spatial proximity might also play a role.

Influence of office furniture on exposure risk to respiratory infection under mixing and displacement air distribution systems

The furniture layout between two people complicates the interaction between indoor airflow, human thermal plume, and exhalation flow in the breathing microenvironment. It thus increases the uncertainty of exposure risk assessment. This study aims to investigate the effects of office furniture under two different air distribution systems on the transmission of respiratory contaminants and airborne cross-infection risk between two people using a computational fluid dynamics method. Two thermal manikins models with realistic breathing functions were simulated, sitting face to face at two separation distances between their mouths in a single-office room served by a mixing ventilation system (MV) and displacement ventilation system (DV). Tracer gas (N2O) was used to simulate small droplet nuclei pathogen-laden exhaled by the infected person, and the exposure risk was assessed using the intake fraction (IF) index. The results show that placing a desk in the microenvironment between two occupants increased the exposure risk by about 50% under DV conditions and 22% under MV conditions compared with the cases without a desk. The exposure risk was reduced remarkably by increasing the separating distance. The results show that the furniture layout significantly affects the airborne transmission risk and cross-infection, and this effect varies with the ventilation conditions and separation distances between the occupants.

Numerical Study of Air Distribution and Evolution Characteristics in Airliner Cabin

The distribution and evolution of air in airliner cabins is an important basis for the study of cabin thermal environment and pollutant propagation. Due to the complex heat and mass transfer problems caused by forced and natural convection in a large-scale space, the accurate prediction of air distribution in airliner cabins still faces huge challenges. This study takes the cabin of the Airbus A320 as the research object. The accurate numerical simulation of the flow and heat transfer process in an airliner cabin under mixing ventilation mode was carried out using the Hybrid Thermal Lattice Boltzmann Method (HTLBM) combined with GPU (Graphics Processing Unit) acceleration technology, and the influence of human thermal plumes on air distribution and evolution characteristics in an airliner cabin was analyzed. The research shows that the human thermal plume changes the air distribution in the passenger cabin. The thermal plume slows down the jet attenuation, increases the energy exchange in the area near the passengers, and offsets the jet trajectory. The airflow self-locking effect is easier to form in the passenger breathing area, which increases the time for the flow field to develop into the steady state. The influence of human thermal plumes on the airflow distribution in the passenger cabin cannot be ignored.

Error analysis of human inhalation exposure simulation in industrial workshop

The evaluation of inhalation exposure of workers in industrial production environments is significant. Current studies on inhalation exposure in industrial workshops are quite limited and lack experimental data validation. This study established a scaled experimental cabin and performed inhalation exposure experiments. Subsequently, the validated numerical model was used to study the influences of different factors on human inhalation exposure prediction. The factors emphasize the meshing strategy, the manikin geometry and the thermal plume of the manikin. The results show that the polyhedron grid offers a greater advantage and that the required grids and computing time represent only 20% and 33% of the tetrahedral grid. CFD results are more consistent with human inhalation exposure experimental results when a 5% mean computational error is used as a criterion to verify grid independence. The inhalation exposure computational results of different manikin models differed by only about 2%. And compared with the 3D scanned model, the humanlike multi-box and single cuboid models are easier to build and consume fewer resources. Therefore, using humanlike multi-box and single cuboid models to compute human inhalation exposure in the industrial workshop is more recommended. The Ri number evaluates the intensity of the human thermal plume. The computational errors between the isothermal and thermal manikin models are 1.76%, 5.14%, 9.18%, and 14.95% for inhalation exposures with 20, 15, 10 and 5 ACH (air changes per hour), respectively, and the corresponding Ri numbers are 0.09, 0.16, 0.34, and 1.41.The computational error between the isothermal and thermal manikin models is less than 10% when the ACH is larger than 10. This study can provide a reference for the prediction of inhalation exposure of workers in industrial workshops.

Experimental and numerical study of the flow dynamics and thermal behavior inside a car cabin: Innovative air diffusers and human body plumes interactions

In confined spaces, such as vehicle cabins, airflow and temperature distribution are the most critical factors affecting thermal comfort and pollutant dispersion. To develop innovative and energy-efficient HVAC systems, a deep understanding of the interaction of the jet flow from the air diffusers on the development process of the human thermal plume became essential to improve the knowledge of airflow patterns for optimizing ventilation system design, thermal comfort, and, indirectly, the energy efficiency. The human thermal plume is one of the most challenging phenomena to capture with optical measurement methods and validate with complex numerical models, given its unsteady nature governed by the buoyancy forces. This study aims to validate the interaction of jet flows from classical and innovative air diffusers on the thermal plume by comparing measured data and numerical simulation results. This validation was made with a specific approach by comparing PIV fields from measurements and CFD results with boundary conditions of air velocity distribution from LDV data. The PIV measurements for flow distributions at the air vents and the thermal plume of an advanced thermal manikin in the driver’s seat were performed in a 1:1 scale mock-up of a Renault Megane car cabin in a climatic chamber. The results obtained with both methods showed good agreement regarding air velocity ranges and distributions. Despite the confined cabin space restricting the development of the thermal plume, a difference in maximum velocities of 0.08 m/s was observed when the distance between the top of the head and the ceiling was doubled. With innovative air diffusers, air velocity distribution showed a more uniform flow than classical diffusers.

Stereoscopic PIV Measurements Of The Human Thermal Plume

A human thermal plume contributes significantly towards indoor fluid flows. Such flows impact the ventilation system designs like mixing and displacement ventilation. Further, the human thermal plume influences the dispersion of air constituents to and away from the breathing zone. This is also relevant in the transport of respiratory droplets which results in spread of respiratory diseases including COVID-19. One of the general scenarios in many ventilated spaces is the sitting condition which is the focus here. Experiments were performed with a thermal manikin and comparisons were made with a real human. The thermal plume is visualised by Stereoscopic PIV with the visualisation plane located 30 cm above the head parallel to ground. The results presented here show that the thermal plume from a manikin without breathing function have many similarities in the average flow field above the head with the field induced by a real person. The peak velocity observed with the real person is slightly lower than the one of the thermal manikin and the structures of the thermal plumes resemble to some extent in both cases. The real person with breathing or talking revealed a bi-lobe like structure with the secondary lobe due to the breath. The thermal manikin also showed a second lobe which possibly is due to the thermal plume from the legs. Furthermore, the effect due to breathing, speaking, and movement of the real person are analysed. The integral values do not vary with respect to the different situations except for the movement case. The characterizations of these phenomena are important because they cannot be simulated by the manikin, while they affect the extent and strength of the thermal convection flow.

Bacteria-carrying particles diffusion in the operating room due to the interaction between human thermal plume and ventilation systems: An experimental-numerical simulation study

During surgery, the release of bacterial-carrying particles (BCPs) by surgeons is one of the major pollution sources in operating rooms (ORs), which may cause surgical site infection (SSIs) or health problems. The human thermal plume emitted by the human body may affect the airflow in the surgical micro-environment, leading to the further spread of BCPs. Therefore, this paper aimed to evaluate and compare the diffusion of human thermal plume and BCPs under the design conditions of four ventilation systems (vertical laminar airflow ventilation (VLAF1), horizontal laminar airflow ventilation (HLAF), vertical laminar airflow ventilation (VLAF2), and temperature-controlled airflow ventilation (TAF)). The experiments of airspeed measurements and biological particle emission were carried out in a standard vertical laminar ventilation operating room. Computational fluid dynamics (CFD) was used to extend the investigation of the different ventilation systems. In summary, both the VLAF and TAF systems were able to interact effectively with the human thermal plume, and the TAF ventilation system was superior to the other three systems in reducing BCPs levels in the air of the operating room. The TAF system was able to control the BCPs concentration in the surgical area to 20 CFU/m3. The interaction between human thermal plume and ventilation plays an important role in the clearance of BCPs in the operating room, so it is significant to systematically compare the performance of different ventilation systems.

A review of human thermal plume and its influence on the inhalation exposure to particulate matter

This paper reviews studies on the human thermal plume and its influence on the inhalation exposure to particulate matter in the breathing zone under different conditions. The human thermal plume transports particle pollutants from the floor to the breathing zone, increasing the inhaled particulate matter concentration. The concentration can be four times higher than that in the ambient environment. Studies have reported that the human thermal plume may prevent particulate matter from entering the breathing zone under specific conditions. Indoor airflow patterns significantly affect the dispersion of pollutants, especially in rooms equipped with displacement ventilation at low airflow velocities. It has been shown that the particle concentration is two times lower in the breathing zone of a rotating manikin than a static manikin. Understanding the characteristics and influencing factors of the human thermal plume is crucial to formulate measures to mitigate the inhalation exposure to particulate matter, achieve independent and personalized control of the human microenvironment, and create a healthy, intelligent and energy-saving indoor environment.

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.

Airborne transmission of COVID-19 and mitigation using box fan air cleaners in a poorly ventilated classroom.

Many indoor places, including aged classrooms and offices, prisons, homeless shelters, etc., are poorly ventilated but resource-limited to afford expensive ventilation upgrades or commercial air purification systems, raising concerns on the safety of opening activities in these places in the era of the COVID-19 pandemic. To address this challenge, using computational fluid dynamics, we conducted a systematic investigation of airborne transmission in a classroom equipped with a single horizontal unit ventilator (HUV) and evaluate the performance of a low-cost box fan air cleaner for risk mitigation. Our study shows that placing box fan air cleaners in the classroom results in a substantial reduction of airborne transmission risk across the entire space. The air cleaner can achieve optimal performance when placed near the asymptomatic patient. However, without knowing the location of the patient, the performance of the cleaner is optimal near the HUV with the air flowing downwards. In addition, we find that it is more efficient in reducing aerosol concentration and spread in the classroom by adding air cleaners in comparison with raising the flow rate of HUV alone. The number and placement of air cleaners need to be adjusted to maintain their efficacy for larger classrooms and to account for the thermal gradient associated with a human thermal plume and hot ventilation air during cold seasons. Overall, our study shows that box fan air cleaners can serve as an effective low-cost alternative for mitigating airborne transmission risks in poorly ventilated spaces.

What dominates personal exposure? Ambient airflow pattern or local human thermal plume

The interaction between ambient airflow and human thermal plume may affect airflow distribution in human micro-environment and then personal exposure. This study aims to investigate this interaction and to determine what dominates personal exposure: ambient airflow pattern or local human thermal plume. A computational fluid dynamics (CFD) method was employed in a downward ventilated room with a standing person below the supply grill. The research has taken into consideration the effects of room height, supply air opening area on the interaction. Results showed that the airflow was dominated by the downward jet from supply grill when the downward jet velocity was higher than high turning point of 0.275 m/s, but was dominated by the upward human thermal plume when the downward jet velocity was lower than low turning point of 0.075 m/s in the Case where room height is 2.7 m and supply air opening size is 0.6 m × 0.6 m. Dual steady flows were found when the downward jet velocity was between low and high turning point in the same case, indicating that the airflow pattern was determined by initialized airflow field with same boundary conditions. Besides, a higher downward jet velocity, larger air supply opening area and lower height were more likely to produce downward airflow pattern. This study may help to develop appropriate ventilation strategies to reduce personal exposure and save energy.

How human thermal plume influences near-human transport of respiratory droplets and airborne particles: a review.

With mounting evidence and notable cases of large clustered infections, airborne transmission via droplets and particles has been recently acknowledged as an effective mode of transmission for COVID-19. How droplets and aerosol particles disperse are being transported into the human breathing zone—the last few inches for airborne transmission to effectuate—remains a key question which has been widely overlooked. Human thermal plume refers to the constantly rising airflows around the boundary layer of human body due to persisting temperature gradients between the body surfaces and the ambient air. Ample evidence indicated that the thermal plume controls the dispersion and transport of aerosols in the human microenvironment. Given that in calm indoor environments most air inhaled by human comes from the boundary layer where thermal plume flows through constantly, the role of thermal plume needs to be scrutinized to predict the diffusion of droplets, aerosols and other airborne carriers of the novel coronavirus around the human body for prioritizing infection control strategies. Here, we assessed the potential influences of the thermal plume on the transmission of COVID-19 and other airborne pathogens by reviewing the most pertinent evidence and analyzing key variables in the formation of thermal plume in indoor environments, e.g., ambient temperature, human posture and type of clothing. Our reviewed evidence and data indicate that the human thermal plume should facilitate the airborne transmission of COVID-19 in enclosed spaces by elevating small droplets and airborne particles into the breathing zone from lower regions and ascending respiratory droplets from the sources into the upper atmosphere. By drawing attention to aerosol transport dynamics in the human microenvironment, these insights may be useful for understanding COVID-19 transmission in enclosed spaces, especially those intended for public use.

The Last Inches: Human Thermal Plume As An Overlooked Factor in Airborne Transmission of COVID-19 in the Human Microenvironment

With mounting evidence and notable cases of large clustered infections, airborne transmission via small droplets and particles has been recently acknowledged as an effective mode of transmission for COVID-19. How those droplets and particles (‘aerosols’) disperse and being transported into the human breathing zone – the last few inches for airborne transmission to effectuate – remains a key question which has been widely overlooked. As an intrinsic part of the human microenvironment, human thermal plume (HTP) refers to the constantly rising airflows around the boundary layer of the human body due to persisting temperature gradients between the body surfaces and the ambient air. Ample evidence indicated that HTP has dominant influences on the dispersion and transport of aerosols in the human microenvironment, by carrying aerosols into the human breathing zone while intercepting and ascending respiratory droplets along the body’s boundary layer into the atmosphere. Given that in calm indoor environments most air inhaled by human comes from the boundary layer where HTP flows through constantly, the role of HTP needs to be scrutinized to predict the diffusion of droplets, aerosols, and other airborne carriers of the novel coronavirus around the human body for prioritizing infection control strategies in these environments. Here we assess the potential influences of HTP on the transmission of COVID-19 and other airborne pathogens by reviewing the most pertinent and up-to-date evidence and analyzing the key variables in the formation of HTP in indoor environments (e.g., ambient temperature, human posture, and type of clothing). By drawing attention to the aerosol transport dynamics in the human microenvironment, these insights may be useful for understanding COVID-19 transmission in enclosed spaces, especially those intended for public use.

The effect of the thermal plume generated by body heat dissipation on the containment of fume hood

The impact of human body heat dissipation on the containment of a fume hood was conducted via experiments and numerical model. The experiments evaluated hood face velocity and the temperature around the mannequin; the results validated the simulation. The numerical model was based on the governing equations of fluid flow via the finite volume method. The face velocities (0.3–0.9 m·s− 1 ) and temperature differences (11°C, 8°C and 5°C) between the surface of the mannequin and its surroundings were used as variables. The numerical results show that in addition to the blockage effect of the worker standing in front of the fume hood, there is a more important thermal effect on the containment of fume hood. The thermal plume carries pollutants leaking out of the hood face to the breathing zone. The face velocity and dimensionless value (Gr/Re 2 ) are recommended to be 0.4–0.6 m·s− 1 and 20–35 respectively, to reduce the influence of human thermal plume on the containment of fume hood and energy waste. The formula related to the rising distance of thermal plume, Grashof and Reynolds numbers (Gr/Re 2 ) was determined.