Gas dispersion near a cubical model building. Part I. Mean concentration measurements

The dispersion of effluent plumes emitted on or in the near-wake region ($\text{x/H ? 5.0}$) of a cubical model building has been examined. The model study was performed in a wind tunnel with a simulated neutrally stratified shear layer. Mean concentration measurements were made on the model building for three different roof vent locations and three different building orientations. A full-scale measurement was conducted in the near-wake region for central roof vent release.The concentration level on the lee face of a model building is greatly reduced by the presence of a sharp edge on the model. The optimum location for the intake vent on the building, for equal vent exhaust to vent intake distance, is a position away from the downwind direction and where it cannot “see” the exhaust vent. Orientation of the building at an angle of 45° results in a secondary peak concentration on the building and in the near-wake region.     

Experimental Study on Gas Temperature During Fire in a Compartment with a Sloped Roof Vent

Twenty-two experiments have been conducted, with six different sloped roof vent areas, three fire source areas, and five door sizes, in a model-scale compartment with geometric dimensions of 150 cm length by 80 cm width by 150 cm height, to explore fire growth behavior and the temperature distribution in a compartment with a sloped roof vent (natural ventilation). It has been found in the experiments that, with increase of the sloped roof vent area, the maximum mass burning rate decreases gradually and the total combustion duration increases, in the condition of equal weight of total fuel in each test. This phenomenon demonstrates that the influence of the weakening effect owing to more energy loss via the roof vent is stronger than the strengthening effect due to combustion acceleration by enlarging the roof vent area, in the current experimental configuration. Moreover, the relationships between the average gas temperature rise and the roof vent area, as well as between the average gas temperature rise and the ratio of the roof vent area to total vent area, in the steady fire stage are summarized and quantified. In addition, it is found that, even when the roof vent area reaches 6.7 % of the floor area, the smoke layer height descends quickly, which poses a challenge to human evacuation.     

Effect of varying wind direction on exhaust gas dilution

The influence of changing wind direction on the dilution of exhaust gases around buildings was investigated in a simulated, neutrally stable atmospheric boundary layer using a low speed, open circuit wind tunnel. Mean concentration measurements were made on six flat-roofed model buildings at 2–5 wind incidence angles of between 0 and 45°.When vertical exhaust momentum was low, dilution levels 2–8 times lower (depending on building shapes and vent location) were observed for wind incidence angles >30°. However, at high exhaust momentum, the minimum dilution lost its sensitivity to wind direction. A semi-empirical model was devised to quantify, within a factor of 2, the dependence of minimum dilution on wind incidence angle and exhaust momentum.     

Performance evaluation of green roof and shading for thermal protection of buildings

The present paper describes a mathematical model for evaluating cooling potential of green roof and solar thermal shading in buildings. A control volume approach based on finite difference methods is used to analyze the components of green roof, viz. green canopy, soil and support layer. Further, these individual decoupled models are integrated using Newton's iterative algorithm until the convergence for continuity of interface state variables is achieved. The green roof model is incorporated in the building simulation code using fast Fourier transform (FFT) techniques in Matlab. The model is validated against the experimental data from a similar green roof-top garden in Yamuna Nagar (India), and is then used to predict variations in canopy air temperature, entering heat flux through roof and indoor air temperature. The model is found to be very accurate in predicting green canopy-air temperature and indoor-air temperature variations (error range ±3.3%, ±6.1%, respectively). These results are further used to study thermal performance of green roof combined with solar shading. Cooling potential of green roof is found adequate (3.02 kWh per day for LAI of 4.5) to maintain an average room air temperature of 25.7 °C. The present model can be easily coupled to different greenhouse and building simulation codes.     

Numerical model of the three-dimensional isothermal flow patterns and mass fluxes in a pitched-roof greenhouse

The three-dimensional isothermal flow patterns and mass fluxes in a full-scale, pitched-roof, single-span greenhouse were numerically resolved, and data from tests on a full scale were used to validate the code, the inlet boundary conditions and the greenhouse design grid method. For numerical solution of turbulent flow, a high-Reynolds-number k-ε model is suitable. Computational domain sizes were selected so as to fulfil the requirements of free-stream conditions whilst ensuring that grid geometrical characteristics satisfy the physical limitations of the standard k-ε model. A special feature of a case of a wind blowing parallel to a ridge (0°) is that the flow in the leeward half of the greenhouse comprises two vortexes with opposite senses of rotation, which bring in air mass through the vents and deliver it to the windward half. A spiral type of flow was found for winds blowing at 15-75° to the ridge direction: part of the air enters via the windward wall vent near the leeward gable-wall and emerges through the leeward roof vent near the windward gable-wall.Mass fluxes and flow patterns on wind direction, and on the opening angles of the windward and leeward vents. Thus, the ventilation rate induced by a wind directed perpendicularly to the greenhouse ridge is 4-4.9 times as great as that induced by a wind parallel to the ridge. A ventilation rate of a simulated greenhouse type was found to be significantly less responsive to a change in wind direction from 45° to 90° than to one from 0° to 45°. Present numerical results are in good agreement with those of other experiments and observations.     

LES and RANS comparison of flow and pollutant dispersion in urban environment

This study investigates air pollution dispersion in urban areas by means of Computational Fluid Dynamics (CFD). The commercial CFD software FLUENT was used to implement two different turbulence simulation methods (RANS and LES), in domains similar to complex urban environments. Particularly, different combinations of roof shapes were studied and simulation results of pollutant (ethane) concentrations were compared against experimental data. The building height (H) to the neighbour building distance (B) ratio was also taken into consideration. Previous studies showed that both RANS and LES models are accurate enough to predict pollutant concentrations fields in B/H = 1. In the present study the incapability of RANS models to predict accurately pollutant concentration in B/H = 0.5 for all roof shapes configurations is revealed.     

Carbon monoxide transport in an enclosed room with sources from a water heater in the adjacent balcony

This paper has presented a computational analysis of carbon monoxide (CO) concentration inside a typical enclosed room of a residential building in Taiwan. CO is produced from a house-used natural gas water heater installed in the balcony. It is then diffused into the adjacent bedroom, which often causes serious poisoning accidences. A general-purpose computational fluid dynamics (CFD) code is employed to predict the CO concentration and airflow fields inside a three-dimensional (3D) modeled house. The variation of CO concentration was simulated under different scenarios of vent air flow rates and exit openings. It was found that under the ventilation conditions of V>0.0003 m/s, the levels of CO concentration in the bedroom is significantly decreased due to the entrainment of fresh air into the bedroom from the inside door. The present results could be used as a base for ventilation design for enclosed rooms, aiming at a proper ventilation system selection for avoiding the CO poisoning.     

Assessment of pollutant dispersion from rooftop stacks: ASHRAE,ADMS and wind tunnel simulation

The prediction of downwind concentration of effluents from stack located on top of buildings is important. Most current dispersion models assess the pollutant concentration at distances away from the building. It is important to study pollutant dispersion within the recirculation zone of the building, since studies have shown that effluents released from rooftop stacks have a tendency to re-enter the building through intakes located on the roof. These effects get more pronounced with the influence of RoofTop Structures (RTS). This paper presents a comparative study of the Atmospheric Dispersion Modelling System (ADMS), American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE 2003 and 2007 versions) and wind tunnel results. Four different cases involving a low-rise and high-rise building for stack heights (h_s) ranging from 1 m to 7 m, exhaust momentum ratios (M) ranging from 1 to 5 and wind direction (θ) of 0° and 45°, have been studied for neutral atmospheric stability conditions. In this regard the effect of RTS has also been examined by using wind tunnel, ADMS and ASHRAE models. ADMS yields higher dilutions near the stack at θ = 0° and cannot model the effect of RTS. Wind tunnel data compare well with ASHRAE 2003 at M = 5 for the low-rise building, but generally predict higher dilutions for the high-rise building. ASHRAE 2003 predicts lower dilutions than ADMS for the high-rise building, while ASHRAE 2007 yields very low dilutions for all cases, suggesting a need to reassess its suitability for practical design.     

Wind effects of parapets on low buildings: Part 1. Basic aerodynamics and local loads

This is the first paper in a series on the effects of parapets on the wind-induced loads on low buildings. Part 1 focuses on the basic aerodynamic effects of parapets and the local (components and cladding) loads. Wind tunnel data were obtained from about 700 pressure taps in the area of a corner panel of 3.7 m×7.6 m (equivalent full-scale dimensions) for several parapet heights and configurations. Significant downward loads were observed which exceed code values for all parapet heights. This may be significant when combined with other loads (such as snow or water). It was also found that parapets alter the suction loads on the roof by changing the location of the corner vortex relative to the roof, for continuous perimetric parapets, and the type of vortex formed, for isolated (single wall) parapets. In the ASCE-defined interior region, the measured coefficients for component and cladding loads exceed those in the code for all parapets and areas examined. For the edge zone, the experimental coefficients for areas less than 1 m² exceed the code values (except for tall perimetric parapets). However, it was found that the component and cladding loads in the ASCE 7 adequately envelope the uplift caused by perimetric parapets in the corner zone for H=4.6 m, but not for isolated parapets, in particular for areas less than 1 m². It was also discussed that the ASCE 7 will be unconservative for larger eaves heights since H² is the correct normalizing factor for roof areas beneath the separated flow. Furthermore, the use of edge zone coefficients in the corner zone for h ?0.9 m should be changed to h/(H+h)?0.23 in the ASCE 7.     

Experimental investigation of averaging time effects on building influenced atmospheric dispersion under different meteorological stability conditions

Field experiments were carried out to measure concentration and concentration fluctuation distributions on the surfaces of a complex shaped building under different stability conditions and to investigate the dependence of these distributions on the averaging time. Meteorological conditions varied from neutral to unstable. The measurements were conducted for two different building orientations to the mean wind direction. Periods of 30 min were selected based on the mean wind direction being normal to either the long or the short building wall within ±10 degrees. For shorter averaging times, the selected concentrations corresponded to the 1, 3, 5, 10 or 15 min periods in which each detector was indicating the highest average within this 30 min period. Averaging time seems to influence more the values of these maximum mean concentrations under unstable conditions than under neutral conditions on all the building walls, while the influence of averaging time is weaker in the case of the wind impinging on the short building wall under neutral conditions. Averaging time seems to have a stronger influence on concentration fluctuation intensity under unstable conditions, while the influence of averaging time on concentration fluctuation intensity is similar for both building orientations investigated. A power law function was found to describe well the dependence of the concentration on averaging time for all examined cases. The absolute values of the power law exponent p measured on the building surface are often higher than the values presented in the literature for open terrain.     

A ventilated slab thermal storage system model

A simplified dynamic thermal model of a hollow core concrete slab thermal storage system and associated room is described. The model is based on a thermal network that can address the heat exchange between the slab cores and the ventilation air, the thermal storage in the building fabric, and the effect of the heat disturbances on the room. The increase in convective heat transfer at the corners of the ventilation cores is also discussed. For normal cyclic operation, the simulated mass and zone temperatures are both in phase with measured performance data. The model root mean square error between the simulated and measured performance is no more than 0.5 °C for the average slab mass temperature and 1.0 °C for the zone air temperature.     

The effects of roof albedo modification on cooling loads of scale model residences in Tucson,Arizona

Data supporting reductions in cooling load and related demand for electric power possible from increasing building surface albedo are limited. Electrical use of wall-mounted air conditioners, roof temperatures, and related environmental factors were monitored during the summer of 1990 on three initially identical 1/4-scale model buildings situated in rock mulch landscapes in Tucson, Arizona. Model thermodynamic properties were scaled to approximate thermodynamic similarity with full-size buildings. With ceiling insulation of R value 5.28 m² K W⁻¹ (R-30) installed, increasing roof albedo of the gray composition shingles (0.30 albedo, 0.94 emissivity) by painting one roof silver and another white (0.49 and 0.75 albedos, 0.70 and 0.98 emissivities, respectively) reduced daily total and hourly peak electrical use for air conditioning approximately 5% for the house with white-colored roof compared to either gray or silver-colored roofs. Larger differences were found without ceiling insulation, with daily total and peak hourly demand for houses with white compared to dark brown roofing (0.9 albedo, 0.98 emissivity) reduced 28 and 18%, respectively. Computer simulations of daily total energy use confirmed comparable savings for similar full-sized buildings. White roofs were 20 to 30°C cooler than either silver or dark-colored roofs on hot, sunny days, indicating that expected cooling due to an increase in albedo may not be realized if it is accompanied by a decrease in emissivity. Light colored roofs, by maintaining cooler attic temperatures, may provide savings in addition to those presented here by reducing heat gain to air distribution systems located in the attic space.     

Buoyancy and inertial force on oscillations of thermal-induced convective flow across a vent

Natural vents are commonly installed in buildings for smoke control. Air motion is induced by buoyancy of the thermal sources inside the building. Hot smoke is expected to be exhausted out of the vent. However, directions of air flowing across the vent might be oscillating under some conditions. The ratio B of buoyancy to inertial force defined by the Grashof number over the square of the Reynolds number is the key parameter in determining airflow oscillations.     

Pedestrian-level wind environment near a super-tall building with unconventional configurations in a regular urban area

Unconventional configurations of tall buildings are noticeably different from their counterpart of traditional building designs but nevertheless, the unconventional configurations have often been adopted for tall buildings without their impact on the pedestrian-level wind environment (PLWE) fully understood. To fill the existing knowledge gap, this study investigates the PLWE near a 400 m super-tall building with various conventional and unconventional configurations in a regular urban area. Computational fluid dynamics (CFD) simulations were conducted for three incident wind directions (θ = 0°, 22.5°, and 45°) to investigate mean wind speed at the pedestrian level using the three-dimensional (3D), steady-state, Reynolds-averaged Navier-Stokes (RANS) technique. The results reveal 1.5- to 2.5-fold increase in maximum wind speed in the urban area after the construction of a super-tall building. The magnitude of the maximum wind speed and areas with high and low wind speeds in the PLWE, however, significantly vary with building design and incident wind direction. The configurations with sharp corners, large plan aspect ratios and frontal areas and the orientation consistently show a strong dependency on incident wind direction except the one with rounded plan shapes. The location of building openings and direction of building inclination are two other factors that modify the PLWE in an urban area. Moreover, the projected width of the super-tall building at a height slightly above the roof level of surrounding buildings is critical for estimating the areas of high and low wind speed at the pedestrian level.

     

Experimental and numerical studies of flows through and within high-rise building arrays and their link to ventilation strategy

Urban ventilation implies that wind from rural areas may supply relatively clean air into urban canopies and distribute rural air within them to help air exchange and pollutant dilution. This paper experimentally and numerically studied such flows through high-rise square building arrays as the approaching rural wind is parallel to the main streets. The street aspect ratio (building height/street width, H/W) is from 2 to 5.3 and the building area (or packing) density (λ_p) is 0.25 or 0.4. Wind speed is found to decrease quickly through high-rise building arrays. For neighbourhood-scale building arrays (1-2 km at full scale), the velocity may stop decreasing near leeward street entries due to vertical downward mixing induced by the wake. Strong shear layer exists near canopy roof levels producing three-dimensional (3D) vortexes in the secondary streets and considerable air exchanges across the boundaries with their surroundings. Building height variations may destroy or deviate 3D canyon vortexes and induced downward mean flow in front of taller buildings and upward flow behind taller buildings. With a power-law approaching wind profile, taller building arrays capture more rural air and experience a stronger wind within the urban canopy if the total street length is effectively limited. Wider streets (or smaller λ_p), and suitable arrangements of building height variations may be good choices to improve the ventilation in high-rise urban areas.     

Wind effects of parapets on low buildings: Part 3. Parapet loads

As part of the study on the effects of parapets on wind-induced loads on low buildings, measurements of the pressures on parapet surfaces have been carried out. Pressures were measured on both the exterior and interior for several parapet heights, h=0.46, 0.9, 1.8, 2.7 m, and building heights, H=4.6, 9.1, 18 m, for both a uniform perimetric parapet and an isolated parapet on one wall. These data were used to quantify the local (component and cladding) and structural wind loads on the parapets. It was found that the worst structural load coefficients over all wind angles are approximately constant with h and H because of opposing trends of the pressures on the interior and exterior parapet surfaces. That is, the loads increase on the interior surface with H (as they do for roof loads), while decreasing on the exterior surface. The current structural load coefficients prescribed by the ASCE 7-02 capture this well for the building configurations considered. However, the suction component and cladding loads on the interior surface of isolated parapets are not well captured by the code.     

Transient free convection in passive buildings using 2D air-filled parallelogram-shaped enclosures with discrete isothermal heat sources

This work evaluates the transient convective exchanges taking place in a building wall made up of air-filled inclined cells. Each cell is formed by two vertical active walls connected by a channel of insulating material. The active hot wall is composed by alternated isothermal and adiabatic bands and is opposite to the active cold wall. Both walls are vertical and separated by a distance equal to their height. The channel connecting these walls is inclined at an angle α with respect to the horizontal, being the values considered in the present work 0° (square cell), ±15°, ±30°, ±45° and ±60°. Two-dimensional temperature fields and streamlines are presented at some representative instants. The temporal evolution of the average Nusselt number at each band of the hot wall is determined for all the treated configurations. Numerical results are validated by comparison with other experimental and numerical studies for cavities with isothermal hot wall in steady state. The maximum deviation found is about 9% for the Nusselt number. This can be considered as very satisfactory for this type of studies characterized by high Rayleigh numbers varying between 1 × 10⁵ and 3 × 10⁸, representative of real building installations.     

Passive cooling for air-conditioning energy savings with new radiative low-cost coatings

Passive cooling is considered as an alternative technology to avoid unwanted heat gains, to reduce urban heat islands and to generate cooling potential for buildings (limiting air-conditioning energy). According to materials and surface treatments, the roof can represent to be a major heat gain source from opaque elements of the building fabric, heating up the outer surface and increasing heat flow by conduction. This paper presents low-cost new radiative materials (1 ∉/m²) allowing to limit heat gains during diurnal cycle for hot seasons. To evaluate the relevance of these new substrates, their reflective UV-VIS-IR behavior are studied and compared to classical roofed materials available in industrial and developing countries. A 48 m² experimental roof having different surfaces (plate steel sheets, fiber cement, terra cotta tiles and corrugated sheets) allows to determine the temperature ratio δ between uncoated and coated materials. Up to 34% surface temperature gains are obtained for white coated CS, 25% for FC and ∼18% for TCT and PSS. According to uncoated materials for a surface temperature T₀ = 60 °C, simulations showed that the low-cost white opaque reflective roofs (50 m²) presented in this study would reduce cooling energy consumption by 26-49%.     

A field validation of a roof-top dispersion formula in an urban centre

A tracer experiment of gaseous dispersion for a flush roof-top vent was carried out for ten months. The concentration of the tracer gas in front of the fresh air-intake duct, installed on the same roof, was monitored.The medium-rise building is among a group of buildings of similar height. By using Wilson's semi-empirical formula, with one modification to suit the field measurement problem of representative wind velocity within a city centre, it was shown that a conservative estimate of dilution factors was obtained.     

A modified method for removal and stabilization of cesium metal in the vitrified matrix

Laboratory experiments were designed to investigate the separation and stabilization of cesium metal. Cesium was removed from simulated waste through sorption under certain physicochemical conditions. Silica sand (locally purchased) was used to remove cesium from simulated liquid waste. The range of pH and temperature was optimized and maximum removal (94–98%) of cesium was achieved with pH 10 at the temperature 36°C. Under optimized conditions with a temperature range of 301–315K ΔH, ΔSand ΔG _{309 K} for 150 ppm solution are ?27.22 ± 0.18 KJ/mol, ?74.1 ± 0.96 J/mol and ?3071 ± 2.1 KJ/mol respectively, and for 200 ppm solution thermodynamic entities are ΔH = ?20.2 ± 0.20 KJ/mol, ΔS = ?47.86 ± 0.66 J/mol and ΔG _{301 K} = ?4344 ± 3.7 KJ/mol. The sorbed metal ion has chances of desorption under changed physicochemical conditions in final disposal. To overcome this problem the final “secondary waste (metals on sorbents)” was stabilized by converting it into a stable vitreous borosilicate matrix through the vitrification process to prevent leaching. It was found that the sorbed cesium was evaporated during heating at 1250°C. The evaporation of cesium during vitrification was overcome by modifying the process. This modified vitrification process is found excellent to immobilize the sorbed cesium. Stability was tested by desorption attempts at different pH.