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Modeling ventilation in a low-income house in Dhaka, Bangladesh
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Abstract
According to UNICEF, pneumonia is the leading cause of death in children under 5. 70% of worldwide pneumonia deaths occur in only 15 countries, including Bangladesh. Previous research has indicated a potential association between the incidence of pneumonia and the presence of cross-ventilation in slum housing in Dhaka, Bangladesh. The objective of this research is to establish a validated computational framework that can predict ventilation rates in slum homes to support further studies investigating this correlation. To achieve this objective we employ a building thermal model (BTM) in combination with uncertainty quantification (UQ). The BTM solves for the time-evolution of volume-averaged temperatures in a typical home, considering different ventilation configurations. The UQ method propagates uncertainty in model parameters, weather inputs, and physics models to predict mean values and 95% confidence intervals for the quantities of interest, namely temperatures and ventilation rates in terms of air changes per hour (ACH). The model predictions are compared to on-site field measurements of air and thermal mass temperatures, and of ACH. The results indicate that the use of standard cross- or single-sided ventilation models limits the accuracy of the ACH predictions; in contrast, a model based on a similarity relationship informed by the available ACH measurements can produce more accurate predictions with confidence intervals that encompass the measurements for 12 of the 17 available data points.
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Natural ventilation in buildings can improve indoor air quality and thermal comfort and reduce air-conditioning use except for highly polluted area. Traditionally, the inflow boundary conditions for calculating natural ventilation used weather data from a meteorological station that is often in a suburb. The use of hour-by-hour wind velocity from the station may give rise to some errors because of the large time step. This investigation proposed a correlation method for converting the hour-by-hour wind velocity from a meteorological station to minute-by-minute velocity, using data measured on the rooftop of the building of interest. A CFD simulation with the coupled modeling method using the unsteady RNG k-ɛ model was performed to calculate airflow around the building and airflow in an apartment simultaneously. The computed outdoor wind velocity and indoor ventilation rate were compared with the corresponding experimental data. The results showed that the simulations calculated the outdoor wind velocity on the building rooftop and the cross-ventilation rate through the apartment with acceptable accuracy.
Dhaka, the capital of Bangladesh, is among the most polluted cities in the world. This research evaluates seasonal patterns, day-of-week patterns, spatial gradients, and trends in PM2.5 (< 2.5µm in aerodynamic diameter), PM10 (< 10µm in aerodynamic diameter), and gaseous pollutants concentrations (SO2, NO2, CO, and O3) monitored in Dhaka from 2013 to 2017. It expands on past work by considering multiple monitoring sites and air pollutants. Except for ozone, the average concentrations of these pollutants showed strong seasonal variation, with maximum during winter and minimum during monsoon, with the pollution concentration of PM2.5 and PM10 being roughly 5-6 fold higher during winter vs. monsoon. Our comparisons of the pollutant concentrations with Bangladesh-NAAQS and US-NAAQS limits analysis indicate particulate matter (PM2.5 & PM10) as the air pollutants of greatest concern, as they frequently exceeded the Bangladesh-NAAQS and US-NAAQS, especially during non-monsoon time. In contrast, gaseous pollutants reported far fewer exceedances throughout the study period. During the study period, the highest number of exceedances of NAAQS limits in Dhaka city (Darus-Salam site) found for PM2.5 (72% of total study days), followed by PM10 (40% of total study days), O3 (1.7% of total study days), SO2 (0.38% of total study days), and CO (0.25% of total study days). The trend analyses results showed statistically significant positive slopes over time for SO2 [5.6 ppb yr⁻¹, 95% Confidence Interval (CI): 0.7, 10.5] and CO [0.32 ppm yr⁻¹, 95% CI: 0.01, 0.56] which suggest increase in brick kilns operation and high sulfur diesel use. Though the statistically non-significant annual decreasing slopes for PM2.5 [-4.6 µg/m³ yr⁻¹, 95% CI: -12.7, 3.6] & PM10 [-2.7 µg/m³ yr⁻¹, 95% CI: -7.9, 2.5] were observed during this study period, the PM2.5 concentration is still too high (~82.0 µg/m³) and can cause severe impact on human health.
Implications
This study revealed key insights into air quality challenges across Dhaka, Bangladesh, indicating particulate matter (PM) as Dhaka’s most serious air pollutant threat to human health. The results of these analyses indicate that there is a need for immediate further investigations, and action based on those investigations, including the conduct local epidemiological PM exposure-human health effects studies for this city, in order to determine the most public health effective interventions.
Background:
Improving survival and extending the longevity of life for all populations requires timely, robust evidence on local mortality levels and trends. The Global Burden of Disease 2015 Study (GBD 2015) provides a comprehensive assessment of all-cause and cause-specific mortality for 249 causes in 195 countries and territories from 1980 to 2015. These results informed an in-depth investigation of observed and expected mortality patterns based on sociodemographic measures.
Methods:
We estimated all-cause mortality by age, sex, geography, and year using an improved analytical approach originally developed for GBD 2013 and GBD 2010. Improvements included refinements to the estimation of child and adult mortality and corresponding uncertainty, parameter selection for under-5 mortality synthesis by spatiotemporal Gaussian process regression, and sibling history data processing. We also expanded the database of vital registration, survey, and census data to 14 294 geography-year datapoints. For GBD 2015, eight causes, including Ebola virus disease, were added to the previous GBD cause list for mortality. We used six modelling approaches to assess cause-specific mortality, with the Cause of Death Ensemble Model (CODEm) generating estimates for most causes. We used a series of novel analyses to systematically quantify the drivers of trends in mortality across geographies. First, we assessed observed and expected levels and trends of cause-specific mortality as they relate to the Socio-demographic Index (SDI), a summary indicator derived from measures of income per capita, educational attainment, and fertility. Second, we examined factors affecting total mortality patterns through a series of counterfactual scenarios, testing the magnitude by which population growth, population age structures, and epidemiological changes contributed to shifts in mortality. Finally, we attributed changes in life expectancy to changes in cause of death. We documented each step of the GBD 2015 estimation processes, as well as data sources, in accordance with Guidelines for Accurate and Transparent Health Estimates Reporting (GATHER).
Findings:
Globally, life expectancy from birth increased from 61·7 years (95% uncertainty interval 61·4-61·9) in 1980 to 71·8 years (71·5-72·2) in 2015. Several countries in sub-Saharan Africa had very large gains in life expectancy from 2005 to 2015, rebounding from an era of exceedingly high loss of life due to HIV/AIDS. At the same time, many geographies saw life expectancy stagnate or decline, particularly for men and in countries with rising mortality from war or interpersonal violence. From 2005 to 2015, male life expectancy in Syria dropped by 11·3 years (3·7-17·4), to 62·6 years (56·5-70·2). Total deaths increased by 4·1% (2·6-5·6) from 2005 to 2015, rising to 55·8 million (54·9 million to 56·6 million) in 2015, but age-standardised death rates fell by 17·0% (15·8-18·1) during this time, underscoring changes in population growth and shifts in global age structures. The result was similar for non-communicable diseases (NCDs), with total deaths from these causes increasing by 14·1% (12·6-16·0) to 39·8 million (39·2 million to 40·5 million) in 2015, whereas age-standardised rates decreased by 13·1% (11·9-14·3). Globally, this mortality pattern emerged for several NCDs, including several types of cancer, ischaemic heart disease, cirrhosis, and Alzheimer's disease and other dementias. By contrast, both total deaths and age-standardised death rates due to communicable, maternal, neonatal, and nutritional conditions significantly declined from 2005 to 2015, gains largely attributable to decreases in mortality rates due to HIV/AIDS (42·1%, 39·1-44·6), malaria (43·1%, 34·7-51·8), neonatal preterm birth complications (29·8%, 24·8-34·9), and maternal disorders (29·1%, 19·3-37·1). Progress was slower for several causes, such as lower respiratory infections and nutritional deficiencies, whereas deaths increased for others, including dengue and drug use disorders. Age-standardised death rates due to injuries significantly declined from 2005 to 2015, yet interpersonal violence and war claimed increasingly more lives in some regions, particularly in the Middle East. In 2015, rotaviral enteritis (rotavirus) was the leading cause of under-5 deaths due to diarrhoea (146 000 deaths, 118 000-183 000) and pneumococcal pneumonia was the leading cause of under-5 deaths due to lower respiratory infections (393 000 deaths, 228 000-532 000), although pathogen-specific mortality varied by region. Globally, the effects of population growth, ageing, and changes in age-standardised death rates substantially differed by cause. Our analyses on the expected associations between cause-specific mortality and SDI show the regular shifts in cause of death composition and population age structure with rising SDI. Country patterns of premature mortality (measured as years of life lost [YLLs]) and how they differ from the level expected on the basis of SDI alone revealed distinct but highly heterogeneous patterns by region and country or territory. Ischaemic heart disease, stroke, and diabetes were among the leading causes of YLLs in most regions, but in many cases, intraregional results sharply diverged for ratios of observed and expected YLLs based on SDI. Communicable, maternal, neonatal, and nutritional diseases caused the most YLLs throughout sub-Saharan Africa, with observed YLLs far exceeding expected YLLs for countries in which malaria or HIV/AIDS remained the leading causes of early death.
Interpretation:
At the global scale, age-specific mortality has steadily improved over the past 35 years; this pattern of general progress continued in the past decade. Progress has been faster in most countries than expected on the basis of development measured by the SDI. Against this background of progress, some countries have seen falls in life expectancy, and age-standardised death rates for some causes are increasing. Despite progress in reducing age-standardised death rates, population growth and ageing mean that the number of deaths from most non-communicable causes are increasing in most countries, putting increased demands on health systems.
Funding:
Bill & Melinda Gates Foundation.
This paper presents a validation of airflow network (AFN) and computational fluid dynamics (CFD) simulations for a naturally ventilated office building using wind tunnel measurements as the reference for external pressure coefficients and effective airflow rate prediction. The CFD simulation model is also used to study the effect of partially open windows on the effective flow rate. This study also includes a design exercise for a naturally ventilated office building that analyses the differences in predicted average window open area for a typical weather year. The results obtained show that, for simple isolated buildings, CFD can predict pressure coefficients with less than 20% average error. For cases with interfering surrounding buildings or more complex building geometries the average error is less than 40%. Average errors in bulk flow rates are lower: typically less than 25%. The largest errors occur in effective flow rate predictions for cases where the openings are exposed to recirculations and shear driven flows.
This paper presents the results of a field study concerning indoor conditions in an office space in Grieskirchen, Upper Austria. The office is equipped with a displacement ventilation (DV) as well as personal ventilation (PV) systems. The space can also be naturally ventilated through the manual operation of windows (NV). The main objective of the study was to assess the functionality of the mentioned ventilation systems with regard to indoor air quality and thermal comfort. The configuration of the indoor climate control system in this office allows for the performance evaluation of the ventilation systems through different methods. In fact, the monitored data and short-term user surveys provide a basis to document the indoor conditions and their implications for the occupants. Our study also illustrated the utility of calibrated CFD models toward a better understanding of the airflow phenomena and estimation of the airflow velocity as an input data for calculation of predicted mean vote (PMV) values. The results suggest that the combined operation of the above mentioned systems provides in general acceptable indoor environmental conditions. However, optimal operation of the systems installed requires the continuous and comprehensive monitoring of indoor conditions, systems states, and occupants’ feed-back. © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
This paper examines the hazards that affect slum communities in Dhaka, Bangladesh, and explores the challenges and opportunities for building their resilience. Dhaka is a rapidly urbanising megacity in one of the world's most densely populated and poorest countries. Almost 30% of its more than 14 million population lives in slums characterised by tenure insecurity and evictions, and controlled by ganglords who charge exorbitant rates for basic services. Such a situation deters investments for improving living conditions. Poor quality housing is typical and basic public are non-existent or very limited. A combination of human and natural factors results in various urban hazards with serious impacts on the poor. Flooding and water-logging due to poor drainage are widespread; windstorms and urban fires cause havoc in slums because of the weak construction of houses; unplanned urbanisation and sub-standard building practices pose great risk in the event of a major earthquake; Bangladesh is severely threatened by climate change and erratic weather, increased flooding and temperature rise are evident. Bangladesh has a large number of development agencies, but most do not engage in cities. The Urban Partnerships for Poverty Reduction (UPPR) program is unique and is a very large program targeted for 3 million slum dwellers in 30 cities including Dhaka; WaterAid Bangladesh addresses water, sanitation and hygiene (WaSH) in slums; the Coalition for the Urban Poor (CUP) is a network of more than 40 NGOs and advocates for the rights of slum dwellers, and supports community-based organisations in slum settlements. The challenges of building resilience in slums are many, but there are also opportunities. Particularly recent government policies on urban development and climate change, as well as an emerging interest among agencies to address urban issues, offer potential for advocacy for building resilience in urban slums.
Despite the existence of low-cost and effective interventions for childhood pneumonia and diarrhoea, these conditions remain two of the leading killers of young children. Based on feedback from health professionals in countries with high child mortality, in 2009, WHO and Unicef began conceptualising an integrated approach for pneumonia and diarrhoea control. As part of this initiative, WHO and Unicef, with support from other partners, conducted a series of five workshops to facilitate the inclusion of coordinated actions for pneumonia and diarrhoea into the national health plans of 36 countries with high child mortality. This paper presents the findings from workshop and postworkshop follow-up activities and discusses the contribution of these findings to the development of the integrated Global Action Plan for the Prevention and Control of Pneumonia and Diarrhoea, which outlines the necessary actions for elimination of preventable child deaths from pneumonia and diarrhoea by 2025. Though this goal is ambitious, it is attainable through concerted efforts. By applying the lessons learned thus far and continuing to build upon them, and by leveraging existing political will and momentum for child survival, national governments and their supporting partners can ensure that preventable child deaths from pneumonia and diarrhoea are eventually eliminated.
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To inform interventions to reduce the high burden of pneumonia in urban settings such as Kamalapur, Bangladesh, we evaluated household air quality risk factors for radiographically confirmed pneumonia in children. In 2009-2010, we recruited children < 5 years of age with pneumonia and controls from a population-based surveillance for respiratory and febrile illnesses. Piped natural gas was used by 85% of 331 case and 91% of 663 control households. Crowding, a tin roof in the living space, low socioeconomic status, and male sex of the child were risk factors for pneumonia. The living space in case households was 28% less likely than in control households to be cross-ventilated. Particulate matter concentrations were not significantly associated with pneumonia. With increasing urbanization and supply of improved cooking fuels to urban areas, the high burden of respiratory illnesses in urban populations such as Kamalapur may be reduced by decreasing crowding and improving ventilation in living spaces.
Designing for wind-driven cross ventilation is challenging due to many factors. While studies have focused on the difficulty of predicting the total flow rate and measuring opening characteristics of cross ventilation, few have investigated the impacts on the distribution of indoor air. This paper provides insights on how local heat sources can generate significant buoyancy driven flow and affect indoor mixing during wind-driven cross ventilation scenarios. Measurements of air distribution were conducted by a tracer gas method for a multi-zone test building located in Austin, Texas, USA, along with cross ventilation flow at the openings. A computational fluid dynamic (CFD) model was also developed for this test building, which utilizes the measured flow properties at the openings as boundary conditions. Resulting air distribution patterns from the CFD model were then compared to the experimental data, validating the model. Further parametric analyses were also conducted to demonstrate the effect of interior heat loads in driving internal air mixing. Key findings of the investigation suggest a local heat source smaller than 35 W/m2 can increase the indoor mixing during cross ventilation from less than 1 air exchange to as high as 8 air exchanges per hour. This result also suggests a typical occupancy scenario (people and electronics) can generate enough heat loads to change the indoor air mixing and alter the effect of cross ventilation.
Building energy analyses are very sensitive to external convective heat transfer coefficients so that some researchers have conducted sensitivity calculations and proved that depending on the choice of those coefficients, energy demands estimation values can vary from 20% to 40%. In this context, computational fluid dynamics calculations have been performed to predict convective heat transfer coefficients at the external surfaces of a simple shape low-rise building. Effects of wind velocity and orientation have been analyzed considering four surface-to-air temperature differences. Results show that the convective heat transfer coefficient value strongly depends on the wind velocity, that the wind direction has a notable effect for vertical walls and for roofs and that the surface-to-air temperature difference has a negligible effect for wind velocity higher than 2 m/s. External convective heat transfer coefficient correlations are provided as a function of the wind free stream velocity and wind-to-surface angle.
Many of the popular building energy simulation programs around the world are reaching maturity — some use simulation methods (and even code) that originated in the 1960s. For more than two decades, the US government supported development of two hourly building energy simulation programs, BLAST and DOE-2. Designed in the days of mainframe computers, expanding their capabilities further has become difficult, time-consuming, and expensive. At the same time, the 30 years have seen significant advances in analysis and computational methods and power — providing an opportunity for significant improvement in these tools.In 1996, a US federal agency began developing a new building energy simulation tool, EnergyPlus, building on development experience with two existing programs: DOE-2 and BLAST. EnergyPlus includes a number of innovative simulation features — such as variable time steps, user-configurable modular systems that are integrated with a heat and mass balance-based zone simulation — and input and output data structures tailored to facilitate third party module and interface development. Other planned simulation capabilities include multizone airflow, and electric power and solar thermal and photovoltaic simulation. Beta testing of EnergyPlus began in late 1999 and the first release is scheduled for early 2001.
Computational fluid dynamics (CFD) results generated by the steady Reynolds-averaged Navier-Stokes equations (SRANS) model and large eddy simulation (LES) are compared with wind tunnel experiments for investigating a cross-ventilation flow in a group of generic buildings. The mean flow structure and turbulence statistics are compared for SRANS based on different two-equation turbulence models with LES based on the Smagorinsky subgrid-scale turbulence model. The LES results show very close agreement with the experimental results in the prediction of the time-averaged velocity, wind surface pressure around and inside the building, and crossing flow through the openings. In contrast, SRANS fails to predict the most important features of cross-ventilation. LES reproduces well the anisotropic turbulence property around and inside the cross-ventilated building, which is closely related to the transient momentum transfer caused in street canyon flows and has a significant influence on the mean flow structure. In contrast, SRANS could not inherently reproduce such transient fluctuations and anisotropic turbulence property, which results in low accurate predictions for the time-averaged velocity components, wind surface pressure distribution and crossing airflow rate up to 100% error.
Natural ventilation can significantly reduce the energy consumption of modern buildings, but robust design is challenging because of the many uncertainties involved. The present work aims to predict the 4-hour nightflush in Stanford’s Y2E2 building during 4 different nights. We employ an uncertainty quantification (UQ) framework that combines an integral model and a computational fluid-dynamics (CFD) model and propagate the uncertainty in the inputs using a polynomial chaos expansion method. The integral model solves a one-dimensional equation for the thermal mass temperature and an equation for the volume-averaged air temperature inside the building. The CFD instead solves for the three-dimensional flow and temperature field in the building, hence enabling direct computation of the heat transfer and discharge coefficients. We first performed a UQ study of the integral model and found that the measured air temperature is inside the 95% confidence interval of the mean prediction ∼80% of the times. Subsequently we employed CFD to improve the accuracy of the probability distributions for the heat transfer and discharge coefficients. In the final step we updated the integral model with these new probability distributions and showed that the standard deviation of the indoor temperature could be reduced up to ∼40%. These results demonstrate that the proposed framework can effectively quantify and reduce uncertainty in models used for the design of natural ventilation systems.
Knowledge of temperature stratification in indoor environments is important for occupant thermal comfort and
indoor air quality and for the design and evaluation of displacement ventilation systems. This paper presents a
detailed and systematic evaluation of the performance of 3D steady Reynolds-Averaged Navier-Stokes (RANS) CFD simulations to predict the temperature stratification within a room with a heat source and two ventilation openings. The indoor air quality within the room is also investigated by assessing the distribution of the age of air. The evaluation is based on validation with full-scale measurements of air temperature. A sensitivity analysis is performed to investigate the impact of computational grid resolution, turbulence model, discretization schemes and iterative convergence on the predicted temperatures and age of air. The results show that steady RANS with the SST k-ω model can accurately predict the temperature stratification in this particular indoor environment. However, of the five commonly used turbulence models, only the SST k-ω model and the standard k-ω model succeed in reproducing the thermal plume structure and the associated thermal stratification, while the three k-ε models clearly fail in doing so. In addition, the iterative convergence criteria have a major impact on the predicted age of air, where it is shown that very stringent criteria in terms of scaled residuals are required to obtain accurate results. These required criteria are much more stringent than typical default settings. This paper is intended to contribute to improved accuracy and reliability of CFD simulations for displacement ventilation assessment.
A brief history is given of research into natural ventilation over the first fifty years of the Journal, from the personal perspective of the author. One of my aims is to give an indication of the contribution that the Journal has made to reporting research on this subject. Another aim is to provide a background for researchers who are starting out in research. There are nine areas of research that I have identified, namely:- steady envelope flow models; flow characteristics of openings; unsteady envelope flow models; internal air motion, zonal models and stratification; contaminant transport; age of air; CFD and its applications; scale modelling; full-scale measurements. For reasons given in the text, some of these topics are only very briefly mentioned. For example, the sections on contaminant transport and CFD are relatively brief, partly because they are the subject of other papers in this issue. (Similarly, the feasibility of natural ventilation relies on the adaptability of the occupants to be comfortable, but thermal comfort is not covered at all here, because it too is the subject of other papers.) The paper concludes with a few personal comments on the design process, since the underlying aim of research is to minimise the risks associated with naturally ventilated buildings.
Natural ventilation of buildings has the potential to significantly save cooling and fan electrical energy. However, it is still not widely applied, because only limited information is available on the thermal comfort and indoor air quality of office buildings without air conditioning. Steady state ventilation air flow rates were therefore systematically simulated using both analytical and airflow network methods for different boundary conditions such as temperature and pressure differences across openings for different opening types. Coupled airflow network and dynamic building simulations were carried out to determine the annual thermal comfort and energy savings. The results for the three moderate climate locations Germany, Italy and Turkey show that while indoor air quality can be easily maintained for all opening typologies, the summer cooling potential and thermal comfort strongly depends on the natural ventilation strategy. Control over the openings is crucial for all cases, as otherwise the office rooms tend to cool down too much even in summer conditions.
A 6m cube has been constructed at Silsoe Research Institute in an open country exposed position and detailed measurements have been made of surface pressure on the cube and of the wind velocities in the region around the cube. Detailed measurements have also been made of the approach flow (J. Wind. Eng. Ind. Aerodyn. 87 (2000) 167) but are not reported here. Mean pressure coefficient data from the Silsoe 6m cube are compared with published wind-tunnel data and with the detailed results from the Windtechnologische Gesellschaft comparative wind-tunnel testing program (J. Wind. Eng. Ind. Aerodyn. 74–76 (1998) 599). With the wind perpendicular to one face it is shown that there is general agreement on the windward wall pressures between wind-tunnels and with the full-scale. However, roof and leeward wall pressures appear to be sensitive to approach flow conditions and scale, i.e. velocity profile, turbulence and Reynolds number. The highest suctions occur with high Reynolds number and Jensen number. With the wind at 45° to the faces the strongest roof suctions occur close to the windward edge: in this case there is better agreement between wind-tunnel results and the full-scale.
This handbook presents industry-accepted guidelines for planning and conducting a wind resource measurement program to support a wind energy feasibility initiative. These guidelines, which are detailed and highly technical, emphasize the tasks of selecting, installing, and operating wind measurement equipment, as well as collecting and analyzing the associated data, once one or more measurement sites are located. The handbook's scope encompasses state-of-the-art measurement and analysis techniques at multiple heights on tall towers (e.g., 40 m) for a measurement duration of at least one year. These guidelines do not represent every possible method of conducting a quality wind measurement program, but they address the most important elements based on field-proven experience. The intended audience for this handbook is any organization or individual who desires the planning framework and detailed procedures for conducting a formally structured wind measurement program. Personnel from the management level to field technicians will find this material applicable. The organizational aspects of a measurement program, including the setting of clear program objectives and designing commensurate measurement and quality assurance plans, all of which are essential to ensuring the program's successful outcome, are emphasized. Considerable attention is also given to the details of actually conducting the measurement program in its many aspects, from selecting instrumentation that meets minimum performance standards to analyzing and reporting on the collected data. 5 figs., 15 tabs.
Natural ventilation is a traditional cost-effective technique to cool and ventilate buildings. Proper simulation tools are highly desired to ensure acceptable thermal performance of naturally ventilated buildings and to assist the design and optimization of such buildings. This study is to advance the use of natural and hybrid ventilation concepts in building design by assessing the accuracy and usability of current thermal-ventilation models. Because of the similarity noted among various models and tools, a prevalent airflow-thermal modeling program, EnergyPlus, was evaluated by simulating three selected real buildings that were supplied with most detailed building and measurement information. The study of these cases reveals that the current model has (1) significant functional limitations for venting and exhaust fan control schemes; (2) need for additional relationships to describe horizontal openings and fully-open connections between zones; and (3) possible over-prediction of buoyancy-driven flows in the case of multi-story buildings. The study further indicates the building and measurement data that are critical for a more accurate validation of thermal-ventilation models, including, on-site measured temperature, wind, and solar conditions; measured volume flow rate data, at critical points throughout the building; and effective area and discharge coefficients for specific vents used in building.Highlights• Assess accuracy and usability of current thermal-airflow models. • Model three real buildings against measurement. • Indicate the deficiencies of the current models. • Highlight the critical measurement data for more accurate validation of models.
Single sided natural ventilation configurations are very frequent especially in buildings in urban environments. Four single sided ventilation experiments were carried out in a full scale outdoor Test Cell facility. Air velocity measurements were taken at various heights in the middle of the opening. The average air flow rate through the opening was derived using the tracer gas decay technique. This work presents the results of an investigation into the relationship between the air velocity at the opening and the bulk air flow rate measurements. The paper presents a description of the experimental set-up as well as a qualitative discussion of the results.
Accurate CFD simulation of coupled outdoor wind flow and indoor air flow is essential for the design and evaluation of natural cross-ventilation strategies for buildings. It is widely recognized that CFD simulations can be very sensitive to the large number of computational parameters that have to be set by the user. Therefore, detailed and generic sensitivity analyses of the impact of these parameters on the simulation results are important to provide guidance for the execution and evaluation of future CFD studies. A detailed review of the literature indicates that there is a lack of extensive generic sensitivity studies for CFD simulation of natural cross-ventilation. In order to provide such a study, this paper presents a series of coupled 3D steady RANS simulations for a generic isolated building. The CFD simulations are validated based on detailed wind tunnel experiments with Particle Image Velocimetry. The impact of a wide range of computational parameters is investigated, including the size of the computational domain, the resolution of the computational grid, the inlet turbulent kinetic energy profile of the atmospheric boundary layer, the turbulence model, the order of the discretization schemes and the iterative convergence criteria. Specific attention is given to the problem of oscillatory convergence that was observed during some of these coupled CFD simulations. Based on this analysis, the paper identifies the most important parameters. The intention is to contribute to improved accuracy, reliability and evaluation of coupled CFD simulations for cross-ventilation assessment.
Traditional methods using steady-state simulations with RANS models may predict adequate flow characteristics of cross ventilation, provided the selected turbulence model, boundary conditions and level of mesh resolution are appropriate. However, these methods are less satisfactory if we are more interested in the statistical description of fluctuating flow rate. This study used a LES method to investigate the fluctuating flow rate induced by wind with a CFD code capable of generating time-dependent and unsteady inflows. Simulations were carried out for two wind directions: normal to the openings (case 1) and parallel to the openings (case 2). The results were in good agreements with experimental data. For case 1, the observed flow rate was relatively steady, whereas for case 2, it exhibited very unsteady phenomena. Observations indicated that the flow pattern around the building significantly affected the ventilation flow rate. For case 1, mean flows were dominant in the air exchange process, whilst for case 2 the fluctuating flows appeared to have more influence on the ventilation mechanism.
Natural ventilation can contribute to the reduction of the air conditioning demand and to the improvement of thermal comfort in buildings. In this paper, the flow field and the air change rate generated by a simple configuration of natural ventilation, namely single-sided ventilation, are examined experimentally. The experiments are realized in a full scale building exposed to outdoor conditions, using several measurement techniques. The main features of the flow generated by stack and wind effect are examined for different outdoor conditions (temperature difference, wind speed and direction). Finally, measured air change rates are compared to those calculated by existing correlations in order to analyze their applicability to the experimental configuration. Results show that the wind generates turbulence diffusion at the opening, counteracting the stack effect. Moreover, in the case of windward opening, there is an additional effect, namely the effect of mixing layer, which tends to increase the airflow rate. Existing correlations give reasonably good results in the case of windward opening, while in the case of leeward opening they overestimate the airflow rate.
Even though opening a window for ventilation of a room seems very simple, the flow that occurs in this situation is rather complicated. The amount of air going through the window opening will depend on the wind speed near the building, the temperatures inside and outside the room, the wind direction, the turbulence characteristics in the wind and the pressure variations caused by e.g. wind gusts. Finally, it also depends on the size, type and location of the opening. Many of these parameters are unsteady which makes the calculation of air-change rates even more complicated. In this work, full-scale wind tunnel experiments have been made with the aim of making a new expression for calculation of the airflow rate in single-sided natural ventilation. During the wind tunnel experiments it was found that the dominating driving force differs between wind speed and temperature difference depending on the ratio between the forces and the wind direction. This change is also found in the velocity profiles measured in the opening, which might change from wind dominated to temperature dominated under the same wind direction but with increasing temperature difference.
The design of natural ventilation in buildings is often performed by means of computational fluid dynamics (CFD) techniques, whose application is gaining popularity. In the present study, Reynolds averaged Navier–Stokes equation (RANS) approach is applied to wind driven natural ventilation in a cubic building. Two different models are considered, namely the two-equation k–ɛ model and the Renormalization Group (RNG) theory. The velocity and pressure distribution inside and around the building are determined, as well as the ventilation rate, for three different configurations: cross ventilation, single-sided ventilation with an opening on the windward wall and single-sided ventilation with an opening on the leeward wall. The numerical results are compared with experimental data, showing a good agreement, particularly when using RNG. The discrepancy in the determination of the ventilation rate is reasonable and the flow distribution inside the building is properly described when RNG model is used. However, the k–ɛ model fails to determine the correct velocity components near the horizontal surfaces. According to these results, the RNG model can be considered a useful tool for the study of wind driven natural ventilation, especially for the assessment of the ventilation rate and of the air distribution inside a room.
This paper describes the fluid mechanics of natural ventilation by the combined effects of buoyancy and wind. Attention is restricted to transient draining flows in a space containing buoyant fluid, when the wind and buoyancy forces reinforce one another. The flows have been studied theoretically and the results compared with small-scale laboratory experiments. Connections between the enclosure and the surrounding fluid are with high-level and low-level openings on both windward and leeward faces. Dense fluid enters through windward openings at low levels and displaces the lighter fluid within the enclosure through high-level, leeward openings. A strong, stable stratification develops in this case and a displacement flow is maintained for a range of Froude numbers. The rate at which the enclosure drains increases as the wind-induced pressure drop between the inlet and outlet is increased and as the density difference between the exterior and interior environment is increased. A major result of this work is the identification of the form of the nonlinear relationship between the buoyancy and wind effects. It is shown that there is a Pythagorean relationship between the combined buoyancy and wind-driven velocity and the velocities which are produced by buoyancy and wind forces acting in isolation. This study has particular relevance to understanding and predicting the air flow in a building which is night cooled by natural ventilation, and to the flushing of gas from a building after a leak.
Natural ventilation in buildings can create a comfortable and healthy indoor environment, and can save energy used in the mechanical ventilation systems. Two subgrid-scale models of large eddy simulation (LES), a Smagorinsky subgrid-scale (SS) model and a Filtered dynamic subgrid-scale (FDS) model, have been used to study airflow in buildings with natural ventilation. It was found that, for fully developed turbulence flow with a high Reynolds number, both the SS and FDS models provide good results. However, if the flow has both turbulent and laminar characteristics, or the wall effect is significant, the SS model performs poorly due to its constant model coefficient. The FDS model can still predict this flow correctly because its model coefficient varies over space and time according to flow types. Furthermore, for a single-sided ventilation, it is important to obtain instantaneous flow information in order to correctly predict ventilation rate and air change effectiveness. Reynolds Averaged Navier Stokes (RANS) modeling cannot correctly calculate the ventilation rate in this case.
Handbook of energy efficiency in buildings: a life cycle approach
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