Some questions on dispersion of human exhaled droplets in ventilation room: answers from numerical investigation

Abstract This study employs a numerical model to investigate the dispersion characteristics of human exhaled droplets in ventilation rooms. The numerical model is validated by two different experiments prior to the application for the studied cases. Some typical questions on studying dispersion of human exhaled droplets indoors are reviewed and numerical study using the normalized evaporation time and normalized gravitational sedimentation time was performed to obtain the answers. It was found that modeling the transient process from a droplet to a droplet nucleus due to evaporation can be neglected when the normalized evaporation time is <0.051. When the normalized gravitational sedimentation time is <0.005, the influence of ventilation rate could be neglected. However, the influence of ventilation pattern and initial exhaled velocity on the exhaled droplets dispersion is dominant as the airflow decides the droplets dispersion significantly. Besides, the influence of temperature and relative humidity on the dispersion of droplets can be neglected for the droplet with initial diameter <200 μm; while droplet nuclei size plays an important role only for the droplets with initial diameter within the range of 10 μm–100 μm.

Practical Implications

Dispersion of human exhaled droplets indoor is a key issue when evaluating human exposure to infectious droplets. Results from detailed numerical studies in this study reveal how the evaporation of droplets, ventilation rate, airflow pattern, initial exhaled velocity, and particle component decide the droplet dispersion indoor. The detailed analysis of these main influencing factors on droplet dispersion in ventilation rooms may help to guide (1) the selection of numerical approach, e.g., if the transient process from a droplet to a droplet nucleus due to evaporation should be incorporated to study droplet dispersion, and (2) the selection of ventilation system to minimize the spread of pathogen‐laden droplets in an indoor environment.     

How far droplets can move in indoor environments--revisiting the Wells evaporation-falling curve

A large number of infectious diseases are believed to be transmitted between people via large droplets and by airborne routes. An understanding of evaporation and dispersion of droplets and droplet nuclei is not only significant for developing effective engineering control methods for infectious diseases but also for exploring the basic transmission mechanisms of the infectious diseases. How far droplets can move is related to how far droplet-borne diseases can transmit. A simple physical model is developed and used here to investigate the evaporation and movement of droplets expelled during respiratory activities; in particular, the well-known Wells evaporation-falling curve of droplets is revisited considering the effect of relative humidity, air speed, and respiratory jets. Our simple model considers the movement of exhaled air, as well as the evaporation and movement of a single droplet. Exhaled air is treated as a steady-state non-isothermal (warm) jet horizontally issuing into stagnant surrounding air. A droplet is assumed to evaporate and move in this non-isothermal jet. Calculations are performed for both pure water droplets and droplets of sodium chloride (physiological saline) solution (0.9% w/v). We calculate the droplet lifetimes and how droplet size changes, as well as how far the droplets travel in different relative humidities. Our results indicate that a droplet's size predominately dictates its evaporation and movement after being expelled. The sizes of the largest droplets that would totally evaporate before falling 2 m away are determined under different conditions. The maximum horizontal distances that droplets can reach during different respiratory activities are also obtained. Our study is useful for developing effective prevention measures for controlling infectious diseases in hospitals and in the community at large. PRACTICAL IMPLICATIONS: Our study reveals that for respiratory exhalation flows, the sizes of the largest droplets that would totally evaporate before falling 2 m away are between 60 and 100 microm, and these expelled large droplets are carried more than 6 m away by exhaled air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s (coughing) and less than 1 m away at a velocity of 1 m/s (breathing). These findings are useful for developing effective engineering control methods for infectious diseases, and also for exploring the basic transmission mechanisms of the infectious diseases. There is a need to examine the air distribution systems in hospital wards for controlling both airborne and droplet-borne transmitted diseases.     

Evaporation and dispersion of respiratory droplets from coughing

Understanding how respiratory droplets become droplet nuclei and their dispersion is essential for understanding the mechanisms and control of disease transmission via droplet‐borne and airborne routes. A theoretical model was developed to estimate the size of droplet nuclei and their dispersion as a function of the ambient humidity and droplet composition. The model‐predicted dried droplet nuclei size was 32% of the original diameter, which agrees with the maximum residue size in the classic study by Duguid, 1946, Edinburg Med. J., 52 , 335 and the validation experiment in this study, but is smaller than the 50% size predicted by Nicas et al., 2005, J. Occup. Environ. Hyg., 2 , 143. The droplet nuclei size at a relative humidity of 90% (25°C) could be 30% larger than the size of the same droplet at a relative humidity of less than 67.3% (25°C). The trajectories of respiratory droplets in a cough jet are significantly affected by turbulence, which promotes the wide dispersion of droplets. We found that medium‐sized droplets (e.g., 60 μm) are more influenced by humidity than are smaller and larger droplets, while large droplets (≥100 μm), whose travel is less influenced by humidity, quickly settle out of the jet.     

Quantification and molecular characterization of enteric viruses detected in effluents from two hospital wastewater treatment plants

Hospital wastewater has been described as an important source of spreading pathogenic microorganisms in the environment. However, there are few studies reporting the presence and concentrations of gastroenteric viruses and hepatitis A viruses in these environmental matrices. The aim of this study was to assess the contamination by viruses responsible for acute gastroenteritis and hepatitis derived from hospital wastewater treatment plants (WWTPs). Rotavirus A (RV-A), human adenoviruses (HAdV), norovirus genogroup I and II (NoV GI/GII) and hepatitis A viruses (HAV) were detected and quantified in sewage samples from two WWTPs located in Rio de Janeiro (Brazil) that operates different sewage treatments. WWTP-1 uses an Upflow Anaerobic Sludge Blanket (UASB reactor) and three serial anaerobic filters while WWTP-2 uses aerobic processes, activated sludge with extended aeration and final chlorination of the effluents. Viruses’ detection was investigated by using conventional PCR/RT-PCR, quantitative real-time PCR (qPCR) and partial sequencing of the genome of the viruses detected. Rate of viruses detection ranged from 7% (NoV GI in WWTP-1) to 95% (RV-A in WWTP-2) and genome from all viruses were detected. The most prevalent genotypes were RV-A SG I, HAdV species D and F, NoV GII/4 and HAV subgenotype IA. Mean values of viral loads (genome copies (GC)/ml) obtained in filtered effluents from anaerobic process was 1.9 × 10³ (RV-A), 2.8 × 10³ (HAdV) and 2.4 × 10³ (NoV GII). For chlorinated effluents from activated sludge process, the mean values of viral loads (GC/ml) was 1.2 × 10⁵ (RV-A), 1.4 × 10³ (HAdV), 8.1 × 10² (NoV GII) and 2.8 × 10⁴ (HAV). Data on viral detection in treated effluents of hospital WWTPs confirmed the potential for environmental contamination by viruses and could be useful to establish standards for policies on wastewater management.     

Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions

The impact from using cool roof coatings on the cooling and heating loads and the indoor thermal comfort conditions of residential buildings for various climatic conditions is estimated. The energy cooling loads and peak cooling demands are estimated for different values of roof solar reflectance and roof U-value. The results show that increasing the roof solar reflectance reduces cooling loads by 18–93% and peak cooling demand in air-conditioned buildings by 11–27%. The indoor thermal comfort conditions were improved by decreasing the hours of discomfort by 9–100% and the maximum temperatures in non air-conditioned residential buildings by 1.2–3.3 °C. These reductions were found to be more important for poorly or non-insulated buildings. For the locations studied, the heating penalty (0.2–17 kWh/m² year) was less important than the cooling load reduction (9–48 kWh/m² year). The application of cool roof coatings is an effective, minimal cost and easy to use technique that contributes to the energy efficiency and the thermal comfort of buildings.     

Interaction between water spray and smoke in a fire event in a confined and mechanically ventilated enclosure

This work deals with the interaction between water droplet flows and smoke in a fire event in a confined and ventilated enclosure. The objective is to identify the specific effect of water spray in the specific environment of a confined and ventilated enclosure. The study is based on 17 large-scale fire tests performed in one room of 165 m³ ventilated at a renewal rate of 15.4 h⁻¹. The fire source is a propane gas burner with a heat release rate of between 140 and 290 kW. The water spray system consists of two Deluge nozzles with a nozzle coefficient of 26 l/min/bar^0.5. The test parameters are the fire heat release rate, the water flow rate, from 50 to 124 l/min, and the activation time. The study focuses on three topics, the interaction of the droplets with the smoke, the droplet evaporation process and the energy transferred to the droplets. The water spray significantly modifies the smoke stratification by mixing and cooling the gas phase. The rate of droplet evaporation has been determined from the water mass balance and is of the same order of magnitude as the rate of water vapor production by the combustion reaction. Heat transfer from the smoke to the droplets has been investigated using the energy balance equation. For a fire scenario in a confined and ventilated enclosure, the energy released by the fire is mainly transferred to the walls and extracted by the ventilation network. In the event of water spray activation, a significant share, up to 65%, is transferred to the droplet flows.     

Particulate matter and gaseous pollutants in residences in Antwerp, Belgium

This comprehensive study, a first in Flanders, Belgium, aimed at characterizing the residential indoor air quality of subgroups that took part in the European Community Respiratory Health Survey (ECRHS I—1991 and ECHRS II—1996) questionnaire-based asthma and related illnesses studies. This pilot study aimed at the evaluation of particulate matter and various inorganic gaseous compounds in residences in Antwerp. In addition personal exposure to the gaseous compounds of one individual per residence was assessed. The main objective was to obtain some base-line pollutant levels and compare these with studies performed in other cities, to estimate the indoor air quality in residences in Antwerp. Correlations between the various pollutant levels, indoor:outdoor ratios and the micro-environments of each residence were investigated. This paper presents results on indoor and ambient PM₁, PM_2.5 and PM₁₀ mass concentrations, its elemental composition in terms of K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Pb, Al, Si, S and Cl and the water-soluble ionic concentrations in terms of SO₄²⁻, NO₃²⁻, Cl⁻, NH₄⁺ K⁺, Ca²⁺. In addition, indoor, ambient and personal exposure levels of the gases NO₂, SO₂, and O₃ were determined. Elevated indoor:outdoor ratios were found for NO₂ in residences containing gas stoves. In smoker's houses increased PM concentrations of 58 and 43% were found for the fine and coarse fractions respectively. Contrary to the fact that all I/O ratios of the registered elements in each individual house were significantly correlated to each other, no correlation could be established between the I/O ratios of the different houses, thus indicating a unique micro-environment for each residence. Linear relationships between the particulate matter elemental composition, SO₂ and O₃ levels indoors and outdoors could be established. No linear relationships between indoor and outdoor NO₂ and particulate mass concentrations were found.     

Modeling the evaporation and dispersion of airborne sputum droplets expelled from a human cough

This study contributes a new model to simulate the evaporation and dispersion of sputum droplets from human coughs or sneezes. It is the first time different chemical components have been included in order to estimate the transport of sputum or similar biological droplets. This modified model demonstrates the ability to describe real-world phenomena that the widely used single droplet model lacks. Evaporation and dispersion of airborne sputum droplets originating from a human cough are simulated using this model combined with an initially buoyant turbulent jet. Constituents of sputum droplets such as NaCl, amino acids, carbohydrates, and lipids are included. Effects of these chemical components on evaporation rate, velocity, and temperature of droplets are investigated in detail. The results obtained for sputum droplets will provide a perspective of what conditions the viruses within a droplet might face upon being ejected from the mouth during a cough. Finally, computational fluid dynamics (CFD) and probability density function (PDF) techniques were used to complement the new model with a simulation of a cough jet and the dynamics of droplet nuclei in confined spaces. Numerical results indicate that a 10 microns sputum droplet will evaporate to become a droplet nucleus (3.5 microns) in 0.55 s at 0.8 or 80% RH, in 0.3 s at 0.5 or 50% RH, and in 0.25 s at 0.2 or 20% RH. The droplet temperature decreases rapidly from human body temperature to room temperature, which may affect the viability of any carried virus.     

Determination of the emission indices for NO,NO2, HONO,HCHO, CO,and particles emitted from candles

In the present study, emission indices for NO, NO₂, HONO, HCHO, CO, particle mass, and particle numbers including particle size distributions for three different offering candles were determined. The candles investigated showed similar emission characteristics with emission indices (g/kg) in good agreement with former candle emission studies. An average HONO/NO_x emission ratio of 6.6 ± 1.1% was obtained, which is much higher compared to most other combustion sources, indicating that candles may be a significant indoor source of this important trace gas. The particle size distributions indicate that the majority of the emitted particles are in the size range 7 - 15 nm. Three modes were observed during burning the candles with very different emission profiles: a “normal burning” mode characterized by low particle number emission rates and small particles; an initial “sooting” behavior after ignition, and a final “smoldering” phase upon candle extinction with higher particle number emission rates and larger particles. The particle emission upon extinction is dependent on the extinction method. The NO_x emission indices were applied in a simple box model to calculate typical indoor NO_x concentration levels from candle emissions, which were in excellent agreement with direct measurements in a typical indoor environment.     

Screening of formaldehyde indoor sources and quantification of their emission using a passive sampler

The formaldehyde emission rates from building and furniture materials in 24 student rooms were measured using a passive sampling method parallel to a monitoring of indoor and outdoor concentrations. This passive tool represents an interesting alternative to standard dynamic methods as it is easier to implement for field investigation. Although the indoor formaldehyde concentrations (21.3 μg m⁻³ on average) are at a medium level, consistent with earlier published results, the recorded emission rates are globally low (from 1 to 15 μg m⁻² h⁻¹) except for the high emission of beds identified in one building (87.3 μg m⁻² h⁻¹ on average). Data analysis revealed that the emissions released from furniture and building materials are the main contributions to the indoor formaldehyde concentrations with 45 and 43% on average. The high formaldehyde levels in rooms are mainly explained by the rise of formaldehyde emissions from indoor materials with temperature although the buildings and the furniture were older than 7 years. Basing on the data of emission rates, outdoor concentrations and air exchange rates, a one compartment mass balance model was used to calculate indoor concentrations. A good agreement was found between the predictions of the model and the measured indoor concentrations. This methodology could lead to the definition of arrangements for the efficient reduction of indoor formaldehyde levels.     

Effects of Fine Water Mist on a Confined Blast

A computational, multi-phase, model has been developed to study the interactions between water droplets and radial expansion of a gas cloud in a spherical chamber. Initial conditions for the gas cloud are specified based on chemical equilibrium calculations for the detonation of a high explosive (RDX). Mono-dispersed water droplets are injected at uniform concentration into the chamber prior to the expansion. A Lagrangian model is used to track the breakup of the parent drops near the shock front to form child drops, which are extremely small. The Navier–Stokes solutions show that the child droplets accumulate near the shock front and evaporate at 100 times higher rate than the parent droplets. Latent heat absorption is the dominant mechanism followed by the sensible heat absorption by the water vapor (and droplets), and momentum absorption from the high velocity gases by the child droplets. The simulations also show that the water vapor formed by the evaporation increases the gas density at the shock front. The increased density and reduced gas temperature (cooling) have opposite effects on the pressure at the shock front. This leads to only a modest suppression in the pressure. At realistic droplet concentrations (0.08 kg droplets/m³ of air), the water mist is shown to evaporate completely in a short time (2.42 ms) prior to shock reflection at the chamber wall mainly due to the breakup at the shock front. High concentrations of mist may be desirable, but are difficult to achieve in practice at the total flooding conditions.     

Quantifying the size‐resolved dynamics of indoor bioaerosol transport and control

Understanding the bioaerosol dynamics of droplets and droplet nuclei emitted during respiratory activities is important for understanding how infectious diseases are transmitted and potentially controlled. To this end, we conducted experiments to quantify the size‐resolved dynamics of indoor bioaerosol transport and control in an unoccupied apartment unit operating under four different HVAC particle filtration conditions. Two model organisms (Escherichia coli K12 and bacteriophage T4) were aerosolized under alternating low and high flow rates to roughly represent constant breathing and periodic coughing. Size‐resolved aerosol sampling and settle plate swabbing were conducted in multiple locations. Samples were analyzed by DNA extraction and quantitative polymerase chain reaction (qPCR). DNA from both organisms was detected during all test conditions in all air samples up to 7 m away from the source, but decreased in magnitude with the distance from the source. A greater fraction of T4 DNA was recovered from the aerosol size fractions smaller than 1 μm than E. coli K12 at all air sampling locations. Higher efficiency HVAC filtration also reduced the amount of DNA recovered in air samples and on settle plates located 3‐7 m from the source.     

Performance characteristics of a fan filter unit (FFU) in mitigating particulate matter levels in a naturally ventilated classroom during haze conditions

The performance of a low-cost fan filter unit (FFU) in mitigating hazardous particulate matter (PM) levels in a naturally ventilated school classroom is presented. The FFU can be considered as a simplified mechanical ventilation and air-conditioning system without heating and cooling functions. The FFU improves indoor air quality through introduction of cleaned outdoor air to flush out internally generated heat and moisture and reducing infiltration by maintaining indoor pressurization. Indoor particle number concentrations were reduced between 85% and 95%. The particle removal performance (PRF_FFU) of the FFU is determined and incorporated into the augmented façade penetration factor (P_aug). A case-specific recursive dynamic mass balance model is used to characterize the infiltration factor (F_INF), deposition rate (K), and the penetration efficiency (P_aug) from continuously monitored indoor and outdoor mass concentration levels. Computed “P_aug” (0.07, 0.09, and 0.13) and “F_INF” (0.06, 0.08, and 0.11), respectively, for PM10, PM2.5, and PM1 suggest that exposure to PM was significantly reduced indoors. The effectiveness of the FFU for reduced “F_INF” and “P_aug” may be attributed to its superior filtration, dilution, and exfiltration mechanisms. In comparison with alternative PM mitigation solutions, the FFU is effective, affordable, and sustainable.     

Quantitative in situ monitoring of organohalogen compounds in domestic sewage resulting from the use of hypochlorite bleach

A field monitoring program was carried out on the sewage of five large apartment blocks in the city of Parma, Italy, to verify under realistic conditions existing laboratory data and model calculations on organohalogen formation by domestic hypochlorite (NaOCl) bleach usage. The average adsorbable organic halogens (AOX) level was determined for three distinct experimental phases: an “undisturbed period,” a “no bleach period,” and a “controlled bleach usage period.” The study involved participation of the site inhabitants in the second and third phases.In line with what could be predicted from the hypochlorite chemistry, an effect of the use of hypochlorite bleach on the AOX concentration in domestic sewage was detected. In a laboratory simulation conducted in parallel with the field study, the degree of NaOCl-to-AOX conversion ranged from 0.75 to 2.25% (w/w). The degree of conversion in the field study itself was of the order of 1.5% (or 0.075% when expressed for a typical bleach product containing 5% NaOCl). Under the specific conditions of the site, the difference in average AOX concentration in sewage between the “no bleach” and the “undisturbed” bleach use period was of the order of 37 μg·1⁻¹ (P < 0.05). The average bleach-related AOX emission at the study site was around 7 mg·(inhabitant·day)⁻¹. Overall, these figures correspond well with existing literature data. Linking the measured amounts of AOX with available ecotoxicity data for treated domestic effluents and their organohalogen constituents, it can be considered unlikely that the organohalogens formed by domestic bleaching will exert or trigger any adverse environmental effect.     

Correlating indoor and outdoor temperature and humidity in a sample of buildings in tropical climates

The incidence of several respiratory viral infections has been shown to be related to climate. Because humans spend most of their time indoors, measures of indoor climate, rather than outdoor climate, may be better predictors of disease incidence and transmission. Therefore, understanding the relationship between indoor and outdoor climate will help illuminate their influence on the seasonality of diseases caused by respiratory viruses. Indoor-outdoor relationships between temperature and humidity have been documented in temperate regions, but little information is available for tropical regions, where seasonal patterns of respiratory viral diseases differ. We have examined indoor-outdoor correlations of temperature, relative humidity (RH), and absolute humidity (AH) over a 1-year period in each of seven tropical cities. Across all cities, the average monthly indoor temperature was 25 ± 3°C (mean ± standard deviation) with a range of 20–30°C. The average monthly indoor RH was 66 ± 9% with a range of 50–78%, and the average monthly indoor AH was 15 ± 3 g/m³ with a range of 10–23 g/m³. Indoor AH and RH were linearly correlated with outdoor AH when the air conditioning (AC) was off, suggesting that outdoor AH may be a good proxy of indoor humidity in the absence of AC. All indoor measurements were more strongly correlated with outdoor measurements as distance from the equator increased. Such correlations were weaker during the wet season, especially when AC was in operation. These correlations will provide insight for assessing the seasonality of respiratory viral infections using outdoor climate data, which is more widely available than indoor data, even though transmission of these diseases mainly occurs indoors.     

Potential impacts of ultra-high-pressure (UHP) technology on NFPA Standard 403

Ultra-high-pressure (UHP) technology as well as compressed air foam (CAF) and combined agent firefighting systems (CAFFS) have proven to enhance the performance of firefighting equipment using water and aqueous film forming foam (AFFF). UHP systems are capable of producing small water droplets at high velocity. As droplet size is reduced, surface area relative to mass increases, improving heat transfer. Smaller droplets however, experience greater drag, reducing throw distance. Findings indicate that on average, 150 m/s exit plane velocities result in maximum throw distances of between 4600 and 5600 orifice diameters. UHP prototype and full-scale testing conducted from 2004 to 2006 found that exit plane velocities of 150 m/s were found to produce 90–100 μm droplets, sizes considered optimal for fire extinguishment. In addition, UHP systems were able to extinguish two-dimensional fuel fires ranging in area from 81.6 to 613.8 m² using one-third the agent when compared to baseline AFFF tests, and one-tenth the NFPA 403 standard.     

Significant OH production under surface cleaning and air cleaning conditions: Impact on indoor air quality

We report measurements of hydroxyl (OH) and hydroperoxy (HO₂) radicals made by laser‐induced fluorescence spectroscopy in a computer classroom (i) in the absence of indoor activities (ii) during desk cleaning with a limonene‐containing cleaner (iii) during operation of a commercially available “air cleaning” device. In the unmanipulated environment, the one‐minute averaged OH concentration remained close to or below the limit of detection (6.5×10⁵ molecule cm^?3), whilst that of HO₂ was 1.3×10⁷ molecule cm^?3. These concentrations increased to ~4×10⁶ and 4×10⁸ molecule cm^?3, respectively during desk cleaning. During operation of the air cleaning device, OH and HO₂ concentrations reached ~2×10⁷ and ~6×10⁸ molecule cm^?3 respectively. The potential of these OH concentrations to initiate chemical processing is explored using a detailed chemical model for indoor air (the INDCM). The model can reproduce the measured OH and HO₂ concentrations to within 50% and often within a few % and demonstrates that the resulting secondary chemistry varies with the cleaning activity. Whilst terpene reaction products dominate the product composition following surface cleaning, those from aromatics and other VOCs are much more important during the use of the air cleaning device.     

Analysis of indoor PM2.5 exposure in Asian countries using time use survey

Most household fuels used in Asian countries are solid fuels such as coal and biomass (firewood, crop residue and animal dung). The particulate matter (PM), CO, NOx and SOx produced through the combustion of these fuels inside the residence for cooking and heating has an adverse impact on people's health. PM 2.5 in particular, consisting of particles with an aerodynamic diameter of 2.5 μm or less, penetrates deep into the lungs and causes respiratory system and circulatory system diseases and so on. As a result, the World Health Organization (WHO) established guideline values for this type of particulate matter in 2005. In this study, the authors focused on PM 2.5 and estimated indoor exposure concentrations for PM 2.5 in 15 Asian countries. For each environment used for cooking, eating, heating and illumination in which people are present temporarily (microenvironment), exposure concentrations were estimated for individual cohorts categorized according to sex, age and occupation status. To establish the residence time in each microenvironment for each of the cohorts, data from time use surveys conducted in individual countries were used. China had the highest estimate for average exposure concentration in microenvironment used for cooking at 427.5 μg/m³ , followed by Nepal, Laos and India at 285.2 μg/m³, 266.3 μg/m³ and 205.7 μg/m³ , respectively. The study found that, in each country, the PM2.5 exposure concentration was highest for children and unemployed women between the ages of 35 and 64. The study also found that the exposure concentration for individual cohorts in each country was greatly affected by people's use of time indoors. Because differences in individual daily life activities were reflected in the use of time and linked to an assessment of exposure to indoor air-polluting substances, the study enabled detailed assessment of the impact of exposure.     

Data-based mechanistic modelling of indoor temperature distributions based on energy input

Energy used for building heating, ventilating and air conditioning contributes to a great share in the total energy consumption worldwide. Better understanding and management of energy distribution in those processes is essential for the improvement of process quality and efficiency of energy use. This paper presents the data-based mechanistic modelling approach which has been developed to model the dynamic indoor temperature distribution in an imperfectly mixed ventilated airspace based on energy input to the system. The combination of classical heat balance differential equations and the data-based modelling techniques for continuous-time system has brought a robust dynamic model suitable for model-based controlling and yet providing a profound insight at the energy and temperature distributions in ventilated systems. The effect of changing heat input on the temperature distribution inside a ventilated structure was studied. Dynamic response of indoor temperature to varying energy input could be explained by a second order transfer function model with a high coefficient of determination (R² > 0.99), a low Young Identification Criterion (YIC < −2.3) and a low model standard error (SE < 0.028 °C). The physically meaningful model parameters as local heat load fraction γ and the coefficient of local temperature change h (°C J⁻¹) were revealed. This modelling approach is very useful for future design of model-based predictive controller for zonal control of indoor temperature by the direct adjustment of heat load into ventilated structures.     

Dispersion and settling characteristics of evaporating droplets in ventilated room

Movement and evaporation of small droplets in the room air are investigated in this paper through CFD simulations. A modified drift-flux model is presented with the droplet evaporation rate and the drift velocity expressed as simple algebra functions of droplet diameter, which is integrated in the transport equations of droplet number density and droplet bulk density. Evaporating droplets are treated as a continuum phase with one way coupling with the carrier phase, i.e. air. Our numerical simulations reveal that the distribution of the large evaporating droplets in the ventilated room air is characterized by a combination of the settling feature when droplets are first generated and released and the dispersion feature after the droplets are evaporated to be either very fine or become droplet nuclei. For droplets less than 50 μm in diameter, the dispersion feature is dominant in the test room that we simulated, while for droplets larger than 100 μm in diameter, the settling feature dominates. For evaporating droplets between these two sizes, the spatial distribution of droplets tends to be located at the lower part of the test room than that of small neutral aerosol particles. Within this size range, a lower initial position of the droplets in the room results in a higher deposition rate of the droplets on the floor.