Natural ventilation systems, taking advantage of natural driving forces, help to reconcile energy efficient buildings with a good Indoor Air Quality (IAQ). Its efficiency depends on several factors ...including climatic regions, seasons, building configurations, and surrounding characteristics. Building regulations often prescribe minimum airflow, which promotes the implementation of mechanical ventilation systems in new constructions. Their performance is invariant and easily verifiable making them convenient. Increasing incentives towards building energy sobriety encourage the use of natural or hybrid ventilation systems in new buildings. However, the lack of a measurement protocol often prevent from justifying that those systems verify air change requirements. It partially explains their weak implementation in new constructions. The purpose of this paper consists in establishing a literature review concerning the measurement of natural airflows. The literature review allowed identifying tracer gas methods, including also occupant-generated CO2 methods, as being widely used methods to assess natural ventilation. Tracer gas methods allow the measurement of natural airflows, as well as indicators related to air distribution, without interfering with the flow pattern. After describing those methods, this paper discusses their suitability towards the measurement of natural airflows. They either rely on assumptions whose violation impact their accuracy, or practical limitations prevent from in-situ measuring. For instance, some methods, which are widely used in natural ventilation, assume steady airflows during the measurement. Some innovative protocols, also based on tracer gas principles are also reported. Some insights are given towards an increase of the reliability of tracer gas methods.
Although airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been recognized, the condition of ventilation for its occurrence is still being debated. We analyzed ...a coronavirus disease 2019 (COVID-19) outbreak involving three families in a restaurant in Guangzhou, China, assessed the possibility of airborne transmission, and characterized the associated environmental conditions. We collected epidemiological data, obtained a full video recording and seating records from the restaurant, and measured the dispersion of a warm tracer gas as a surrogate for exhaled droplets from the index case. Computer simulations were performed to simulate the spread of fine exhaled droplets. We compared the in-room location of subsequently infected cases and spread of the simulated virus-laden aerosol tracer. The ventilation rate was measured using the tracer gas concentration decay method. This outbreak involved ten infected persons in three families (A, B, C). All ten persons ate lunch at three neighboring tables at the same restaurant on January 24, 2020. None of the restaurant staff or the 68 patrons at the other 15 tables became infected. During this occasion, the measured ventilation rate was 0.9 L/s per person. No close contact or fomite contact was identified, aside from back-to-back sitting in some cases. Analysis of the airflow dynamics indicates that the infection distribution is consistent with a spread pattern representative of long-range transmission of exhaled virus-laden aerosols. Airborne transmission of the SARS-CoV-2 virus is possible in crowded space with a ventilation rate of 1 L/s per person.
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•This outbreak involved ten infected persons in three families.•Full video recording at time of infection allows restoration of the scene.•Time-averaged ventilation rates were only 0.9 L/s per person in the restaurant.•Insufficient ventilation played a role in this outbreak of COVID-19.
In this study, we evaluated several simple natural ventilation models of cross ventilation and single-sided ventilation with data measured in a full-scale field study in London. In the field study, ...the ventilation rate in a naturally ventilated office was measured using a tracer gas technique with CO2. Internal temperatures were measured using a vertical temperature array. The external temperature, wind speed and direction were measured at a nearby weather station. In addition, a 1:200 scale model of the urban area within 300 m of the test room was built in a wind tunnel to measure the pressure coefficients. The ventilation models were evaluated with input data from two sources. Wind data from a nearby airport and pressure coefficients from the literature were used, as is common practice. Alternatively, wind data measured at the local weather station and the pressure coefficients measured from wind tunnel experiments were used. The results showed that, regardless of the input data sources, the cross-ventilation model in general gives reasonable predictions. For single-sided ventilation, several empirical models were evaluated and poor predictions were obtained using the models. We discuss ways in which models of natural ventilation might be improved in the future.
•A full-scale field study in a naturally ventilated office room in London.•Ventilation rates of both cross-ventilation and single-sided ventilation were measured.•Wind-tunnel experiments on a reduced-scale model of the building and surroundings, measuring pressure coefficients.•Measured local wind data and pressure coefficients were compared with commonly used models.•Evaluation of simple analytical ventilation models were conducted.
Interunit dispersion problems have been studied previously mainly through on-site measurements, wind tunnel tests, and CFD simulations. In this study, a scaled outdoor experiment was conducted to ...examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas (CO2) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and reentry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions, it was found that the ventilation performance of the source room varied according to the room location. The air exchange rate distribution of the leeward-side room was more stable than that of the windward side. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest reentry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high probability of experiencing a high reentry ratio based on the maximum reentry ratio of each room. This study provides authentic airflow and pollutant dispersion information in the street canyons in an urban environment. The dataset of this experiment can be used to validate further numerical simulations.
•A scaled outdoor experiment was conducted to study pollutant dispersion in 2D streets.•The tracer gas method using CO2 was adopted to simulate interunit transmission routes.•Air exchange rate/reentry ratio were used to assess ventilation/pollutant dispersion.•Ventilation performance in street canyon varied with the room location.•Highest reentry ratio appeared at room nearest to the source room along transportation route.
Current IoT applications in indoor air focus mainly on general monitoring. This study proposed a novel IoT application to evaluate airflow patterns and ventilation performance using tracer gas. The ...tracer gas is a surrogate for small-size particles and bioaerosols and is used in dispersion and ventilation studies. Prevalent commercial tracer-gas-measuring instruments, although highly accurate, are relatively expensive, have a long sampling cycle, and are limited in the number of sampling points. To enhance the spatial and temporal understanding of tracer gas dispersion under the influence of ventilation, a novel application of an IoT-enabled, wireless R134a sensing network using commercially available small sensors was proposed. The system has a detection range of 5-100 ppm and a sampling cycle of 10 s. Using Wi-Fi communication, the measurement data are transmitted to and stored in a cloud database for remote, real-time analysis. The novel system provides a quick response, detailed spatial and temporal profiles of the tracer gas level, and a comparable air change rate analysis. With multiple units deployed as a wireless sensing network, the system can be applied as an affordable alternative to traditional tracer gas systems to identify the dispersion pathway of the tracer gas and the general airflow direction.
Air infiltration has a significant impact on building energy performance and the indoor environment. An accurate estimate of air infiltration rate informs envelope retrofit decisions to improve ...airtightness. However, air mobility and other environmental factors, such as wind or indoor-outdoor temperature differences, often make the accurate measurement of air infiltration challenging. Further, conventional air infiltration testing approaches such as fan pressurization and tracer gas tests possess certain drawbacks limiting their applicability in commercial buildings. To address the limitations of air infiltration tests, this study proposes a low-cost inverse model-based approach for estimating air infiltration rates by extracting the naturally occurring indoor CO2 and relative humidity (RH) data from a building automation system (BAS). These data were used to develop a linear regression model, and a tracer gas experiment was also used to verify the applicability of the proposed approach. The results indicated that the proposed method could conveniently lend itself to estimate air infiltration rates at a reasonable accuracy using existing CO2 data; however, humidity ratio data (converted from RH and temperature sensor measurements) may only help to track building infiltration characteristics over time due to inaccuracies caused by adsorptive and desorptive properties of various building materials.
•Infiltration rates in a private office are analyzed.•An inverse model-based approach is developed for infiltration estimates.•CO2 and humidity ratio data extracted from a building automation system are used.•Tracer gas tests are conducted to verify the application of the inverse model-based approach.•Pre-requisites of using the proposed approach are discussed.
•Dispersion of vehicle emissions was experimentally studied in a real-world street canyon.•Low-cost air sensors network and portable emission measurement system were combined.•Exhaust of test vehicle ...itself had unmeasurable effect on roadside CO2 concentrations.•Both line and point tracer gas systems produced distinct CO2 elevations at roadsides.•CO2 is a suitable tracer gas for studying vehicle emission dispersion in street canyons.
High-rise buildings form deep urban street canyons and restrict the dispersion of vehicle emissions, posing severe health risks to the public by aggravating roadside air quality. Field measurements are important for understanding the dispersion process of tailpipe emissions in street canyons, while a major challenge is the lack of a suitable tracer gas. Carbon dioxide (CO2), which is safe to the public and inexpensive to obtain, can be reliably measured by existing gas analysers. This study investigated the suitability of using CO2 as a tracer gas for characterising vehicle emission dispersion in a real-world street canyon. The tracer gas was released via a line or point source, whose dispersion was characterised by a sensors network comprising low-cost air quality sensors. The results showed that the CO2 contained in the exhaust gas of a test vehicle itself had unmeasurable effect at roadsides. Both the line and point sources produced obvious CO2 level elevations at approximately 30 s after the test vehicle passed by. In addition, for both line and point sources, the CO2 elevations were much more distinct at the roadside next to tailpipe exit than the opposite side, and were higher at 0.8 m than 1.6 m above the ground. The present study demonstrated that using CO2 as a tracer gas is feasible for investigating vehicle emission dispersion in real-world street canyons. Future studies are needed to improve the gas release rate of the developed tracer gas systems for more reliable measurements and larger street canyons.
Over the past four years, five major variants of the SARS-CoV-2 virus have circulated globally, causing seven million deaths. Meanwhile, specific preventative ventilation measures were recommended to ...minimize exposure to the aerosol infection risk, such as opening doors, and disabling ventilation systems. Given more attention to high-rise buildings, where quite a few outbreaks have been reported, there is a notable lack of reported studies assessing preventive ventilation measures. Therefore, this study focused on practicing several ventilation measures in a 16-story high-rise building located in Montreal, Canada. A variety of inter-zonal tracer measurements, including room-to-floor and inter-floor tests, were carried out considering controlled airflow/pressure across zones. An automated data acquisition system was developed for real-time monitoring of tracer/CO2 spatial concentrations. Findings show that enabled ventilation with opened doors reduces CO2 concentrations by up to 82 % in nearby source locations and positively pressurized zones; however, negatively pressurized zones experience a 40 % increase in tracer exposure. With ventilation active, a corridor source leads to a higher risk of tracer exposure (up to 86 %) than the in-room source. Disabled ventilation with closed doors is a recommended ventilation measure, for corridors with infection sources, to minimize tracer transport across zones. Zones with infection sources follow the latter ventilation measure; however, ventilation with open doors offers a conditional ventilation measure if careful airflow is designed particularly at return and exhaust points within the ventilation system. This study has implications for practicing preventative ventilation measures for post-COVID-19 building practices and design considerations.
•Field study conducted in 16-story building, assessing preventive ventilation measures.•Inter-zonal ventilation performance across floors was assessed by tracer measurements.•Automated real-time monitoring system measured tracer/CO2 concentrations.•Corridor source with ventilation poses 86 % higher tracer transport than in-room source.•Ventilation affects tracer transport based on air flow pressurization across zones.
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•Gas leakage from riser to bubbling fluidized bed via a loop seal is investigated.•Detailed description of the pressure situation in a CLC facility.•Tracer gas measurements with CO2 ...at hot conditions.•Evaluation of gas distributions from loop seals.•Operation and plant design proposals for minimization of gas leakage.
Chemical Looping Combustion (CLC) is a technology to generate a pure stream of CO2 during the combustion of fossil or renewable fuels for power generation. This is carried out in a system of interconnected fluidized bed reactors, at least one so-called air reactor and a fuel reactor. Gas leakages are a major problem in those CLC systems as undesired gases in the exhaust can make the later sequestration more expensive and might lead to further gas cleaning. Therefore, the off-gas of the fuel reactor, where the fuel is mainly converted, should be as pure as possible. In those interconnected fluidized beds, the loop seals or so-called siphons should prevent the gas leakage from one side to the other. The present work focuses on the pressure conditions inside an experimental 25 kWth pilot scale fluidized bed reactor system for CLC to minimize gas leakage between the single reactors. The system consists of a riser air reactor and a two-stage bubbling fluidized bed fuel reactor system. By using CO2 as tracer gas at different positions, the gas distribution from the two loop seals through the system as well as the leakage flow from the air reactor via the cyclone and loop seal to the fuel reactor is analyzed. From a detailed experimental and sensitivity analysis of the system behavior in different operation conditions, measures to minimize gas leakage are developed. A key factor for the understanding of the gas leakages in the system was derived from pressure drop analysis of 23 pressure ports. Each process unit is examined and design criteria for such systems of interconnected fluidized beds are derived.
The measure of air change rate (ACR) in building is a difficult and usually expensive task. The tracer gas method is the reference technique but its implementation is difficult and the interpretation ...of results is not straightforward. In the present work, the concentration decay method by multiple CO2 transmitters is experimentally validated in the case of cross-ventilation. It is observed that in-situ CO2 transmitters lead to ACR values in good agreement with reference measurements obtained from mechanically controlled values. Whereas multiple transmitters in different sampling positions show the imperfect mixing, a sensor located at the outlet or an averaged value of all sensors can provide an accurate measure of the ACR. Moreover, the spatial variation of CO2 concentration can be used to assess the ventilation efficiency in the test chamber. Different measures and calculation methods are discussed, and the uncertainty analysis of each method is carried out.
•In-situ CO2 sensors are tested to measure air change rate by tracer gas technique.•Measured ACR by in-situ CO sensors are in good agreement with reference values.•Multiple sensors in different positions allow to assess the ventilation efficiency.•Multi-points calculation method gives more accurate results than two-points method.
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