Air Flow Regimes Research Articles

An experimental investigation on the effects of surface gravity waves on the water evaporation rate in different air flow regimes

Estimating rate of evaporation from undisturbed water surfaces to moving and quiet air has been the topic a vast number of research activities. The obvious presence of various shapes of gravity waves on the water body surfaces was the motivation of this experimental investigation. In this investigation experimental measurements have been done to quantify evaporation rate from wavy water surfaces in free, mixed and forced convection regimes. The effects of a wide range of surface gravity waves from low steepness, round shaped crest with slow celerity, to steep and very slight spilling crest waves, on the water evaporation rate have been investigated. A wide range of \({\text{Gr}}/{\text{Re}}^{2} (0.01 \le {\text{Gr}}/{\text{Re}}^{2} \le 100)\) was achieved by applying different air flow velocities on a large heated wave flume equipped with a wind tunnel. Results reveal that wave motion on the water surface increase the rate of evaporation for all air flow regimes. For free convection, due to the effect of wave motion for pumping rotational airflows at the wave troughs and the dominant effect of natural convection for the air flow advection, the maximum evaporation increment percentage from wavy water surface is about 70 %. For mixed and forced convection, water evaporation rate increment is more sensitive to the air flow velocity for the appearance of very slight spilling on the steep wave crests and the leeward air flow structures.

Drag Reduction of Laminar Airflow in Circular Pipe With Magnetic Field

The drag friction of the laminar air flow in a steel pipe with permanent magnets attached was compared to that in a pipe without magnets. The drag reduction rate decreased as a function of the Reynolds number in the laminar flow regime, and it reduced to a low percentage when the Reynolds number was greater than 3000 in the turbulence air flow regime. These results suggest that the velocity of the boundary air flow on the pipe wall surface was accelerated by the magnetic field gradient produced by the permanent magnets attached to the outer pipe surface. The velocity of the boundary air flow, as roughly estimated by calculating the magnetic field distribution, could approximately express the experimental results.

Influence of local air flow regimes on the ozone content of two Pyrenean valleys

The Pyrenees Mountains, the natural border between France and Spain, have experienced a large increase in road traffic in the last decade. Due to this fact, a research program named PAP (Pollution Atmospheric in the Pyrenees) was established in 2004 by several laboratories from Spain and France to address the influence of meteorological regimes on the pollution levels of two adjacent valleys, Aspe valley (France) and Canfranc valley (Spain), situated in the center of the Pyrenean range. Pollution measurements show that mean ozone concentrations increase with height. In Sarrance, the site placed at the bottom of the valleys at 335 m above sea level (ASL), the mean ozone value was 23 ppb, whereas at the Pic Midi observatory (2877 m ASL), the top of the PAP network, the value found for mean ozone values was 52 ppb. A linear trend fits this altitudinal variation with a vertical gradient of 17 ppb km−1. The data demonstrate that the observatories located over 1400 m ASL do not show the classical mean daily ozone cycle, and that mean ozone concentrations throughout the day are nearly constant. By contrast, below 1400 m ASL, the classical mean daily ozone cycle is clear, reaching a maximum around noon. These findings indicate that the photochemical reactions are almost inactive at the elevated observatories and, as a result, it can be concluded that ozone levels at those locations are mostly caused by advection of aged air masses. Consequently, we could find that the gradient inside the valleys follows a linear trend of 29 ppb km−1. Finally, it has been observed that in north Foehn situations, intrusions of polluted air coming from the Free Troposphere (FT) can be detected in the upper part of the Spanish valley of Canfranc, where the mean daily ozone cycle changes significantly and becomes similar to the ones measured at the stations situated above 1400 m ASL. However, the results also pointed out that, except for the Foehn situations, the different local air flow regimes do not strongly affect the mean diurnal cycle of ozone and the mean ozone concentrations.

Comparative analysis of realistic CT-scan and simplified human airway models in airflow simulation

Efforts to model the human upper respiratory system have undergone many phases. Geometrical proximity to the realistic shape has been the subject of many research projects. In this study, three different geometries of the trachea and main bronchus were modelled, which were reconstructed from computed tomography (CT) scan images. The geometrical variations were named realistic, simplified and oversimplified. Realistic refers to the lifelike image taken from digital imaging and communications in medicine format CT scan images, simplified refers to the reconstructed image based on natural images without realistic details pertaining to the rough surfaces, and oversimplified describes the straight wall geometry of the airway. The characteristics of steady state flows with different flow rates were investigated, simulating three varied physical activities and passing through each model. The results agree with previous studies where simplified models are sufficient for providing comparable results for airflow in human airways. This work further suggests that, under most exercise conditions, the idealised oversimplified model is not favourable for simulating either airflow regimes or airflow with particle depositions. However, in terms of immediate analysis for the prediction of abnormalities of various dimensions of human airways, the oversimplified techniques may be used.

Natural Ventilation as a Factor Controlling the Dispersal of Airborne Toxins in Subway Systems in a Disaster Situation

In the event of a terrorist attack or an accidental release of hazardous substances in a subway system, an understanding of the dispersal of a toxic agent across the system with and without train movements is key information to aid rescue and escape procedures. This information is only available with knowledge of the airflow conditions inside the system. Apart from the train movements or active ventilation, the natural airflow regime determines which parts of the system will be contaminated. Tests with the tracer gas sulfur hexafluoride (SF6) and climatic studies have clearly confirmed the expected complex airflow system inside subway systems. The assumption that climatic and topographic factors of track routing as well as architectural characteristics have an influence on the air flow regime have been verified. The results show that in many instances designated escape routes are not safe. The emergency response could be improved and lives could be saved by implementing dynamic escape routing systems based on knowledge of the natural air flow.

Measurement of the convective moisture transfer coefficient from porous building material surfaces applying a wind tunnel method

This article presents results for the average convective moisture transfer coefficients of several porous building material samples exposed to airflow. The experimental measurements explore the effect of the various air velocities, air temperatures and local positions on the average convective moisture transfer coefficients. Selected building materials were soaked in distilled water at least 2 weeks before the measurements. A thin building specimen with moisture content close to the saturation point was mounted in level with the bottom wind tunnel surface. A stable airflow regime was measured over the thin samples placed in the specimen holder. Water from the sample holder was absorbed from the bottom side of the building materials and evaporated from the upper side of the specimen during the airflow exposure. Two different membranes were fixed over the water cup as reference materials for comparison. The measurements were carried out at a relative humidity of 50% ± 3%, air temperatures of 23.6°C ± 0.5°C, 26.5°C ± 0.5°C and 30.0°C ± 0.5°C, and air velocities of 1.1, 3.0 and 5.5 m/s. The experimental data show that the convective moisture transfer coefficient is a function of velocity, temperature difference between the ambient air and material surface, local position as well as of the material type. The experimental results from water surfaces were compared to the expressions for the convective moisture transfer coefficients from the literature.

Surface geophysics and borehole inspection as an aid to characterizing karst voids and vadose ventilation patterns (Nerja research site, S. Spain)

A combined approach using two surface geophysical methods (capacity-coupled geoelectrics and ground-penetrating radar) and careful inspection by optical televiewer probes in a network of boreholes (< 30 m) has been undertaken at an experimental karst site located in the immediate vicinity of the Nerja cave (S. Spain), an important show cave developed in Triassic dolomitic marbles. Our approach has proved helpful in characterizing geometrically and speleologically the vadose zone of these highly heterogeneous environments in that we have been able to identify a considerable number of caves and arrive at an idea of their approximate volume and length. Televiewer logging in the borehole network allowed us to identify a number of karst features: horizontal and vertical openings and cavities, as well as the relative degree of speleothem development. We had previously made a systematic record of the CO2 content of the air inside these boreholes and the results of this earlier study together with the present one have allowed us to further our knowledge of the ventilation patterns in the vadose zone of the research site, which suggest that high CO2 contents (up to 60,000 ppm) are directly linked to the degree of karstification. The general geometrical model of karstification in the area suggests a common genesis for the Nerja cave and the numerous cavities revealed during this study. The flows of CO2-rich air and the ventilation patterns in the vadose zone in general seem to be different in the Nerja cave from those in the network of small cavities surrounding it, a circumstance that most likely derives from the fact that the main cave communicates both naturally and artificially with the outside atmosphere. The air-flow regime in the network of boreholes has still to be studied in detail however.

Reactor cavity cooling system (Rccs) experimental characterization

The development of a new concept for a high temperature gas cooled reactor is strictly correlated to the Reactor Cavity Cooling System (RCCS) design. This is the latest heat sink designed to ensure the cooling down of the vessel and other structural materials during an accident scenario. An experimental facility was built at Texas A&M to quantitatively analyze heat transfer phenomena and the air flow regime inside the reactor cavity. The thermal measurements were performed using 18 thermocouples mounted on the vessel surface, 8 on the external surface of one of the cooling pipes and 24 on a movable rack that captures the axial temperature profile inside the cavity. Flow regime measurements were performed with Particle Tracking Velocimetry techniques (PTV), using a high speed camera and spraying a special dust in the cavity for the tracking. The results demonstrate that the main heat transfer mode inside the cavity is the radiation (about 80%); also, they show the complexity of the flow regime inside the cavity due to natural circulation. The experimental conditions were a vessel surface temperature of about 300 °C heated at fixed power and different flow rates for the cooling pipes.

A procedure to quantify the impact of mitigation techniques on the urban ventilation

Urban heat island (UHI) and urban pollution island (UPI) are recent phenomena caused by human interference in an environment. This interference has an impact on the pedestrian comfort and pollution exposure (PCE). Even though studies have revealed a mutual relation between the UHI and UPI, their interaction is rarely considered in the development of mitigation techniques. That means UHI mitigation technique does not necessarily have similar impact on the pollution exposure and vice versa. For example, the effect of tree planting in a high-rise street canyon can be different from a low-rise one since solar radiation absorption and airflow regime is remarkably different in these street canyons. This implies that street canyon-scale treatment is necessary in order to first clarify the significance of each interaction and, then to apply a proper mitigation strategy to improve the PCE. The existing urban planning guidelines are mainly qualitative and not quantitative, and no procedure has been developed to evaluate the impact of different mitigation technologies on the PCE. This paper proposes a systematic approach to quantify the level of environmental condition inside a street canyon. This approach is also capable of evaluating the possible advantages of passive and active mitigation strategies using a frequency of occurrence concept. For this purpose, a computational fluid dynamics model is developed to be used to investigate the impact of contributing parameters on the PCE. A case study of a street canyon, located in Montreal, is also considered to investigate the performance of the proposed approach.

CFD simulation of airborne pathogen transport due to human activities

Computational Fluid Dynamics (CFD) is an increasingly popular tool for studying the impact of design interventions on the transport of infectious microorganisms. While much of the focus is on respiratory infections, there is substantial evidence that certain pathogens, such as those which colonise the skin, can be released into, and transported through the air through routine activities. In these situations the bacteria is released over a volume of space, with different intensities and locations varying in time rather than being released at a single point. This paper considers the application of CFD modelling to the evaluation of risk from this type of bioaerosol generation. An experimental validation study provides a direct comparison between CFD simulations and bioaerosol distribution, showing that passive scalar and particle tracking approaches are both appropriate for small particle bioaerosols. The study introduces a zonal source, which aims to represent the time averaged release of bacteria from an activity within a zone around the entire location the release takes place. This approach is shown to perform well when validated numerically though comparison with the time averaged dispersion patterns from a transient source. However, the ability of a point source to represent such dispersion is dependent on airflow regime. The applicability of the model is demonstrated using a simulation of an isolation room representing the release of bacteria from bedmaking.

Evaporation of pure liquid droplets: Comparison of droplet evaporation in an acoustic field versus glass-filament

The rate of heat and mass transfer to droplets in sprays is a critical issue in the design of many industrial spray systems. Applications like fuel injection in internal combustion engines or spray drying stimulate interest in studying the processes related to the evaporation of droplets. In this study an acoustic levitator and the glass filament method are used to observe single droplets during evaporation. The introduction of a droplet into the acoustic field leads to the formation of two steady toroidal vortices close to the droplet surface, known as outer acoustic streaming. The results of this study illustrate how this “outer acoustic streaming” affects the heat and mass transfer. The elimination of these vortices has been achieved through applying a ventilating air flow. Particle Image Velocimetry has been used to characterize the flow field and the interfacial transfer. Furthermore, the resulting evaporation rates have been verified by comparing them with existing model predictions. A comparison of these results to those obtained with the glass-filament method revealed good agreement when the air flow was increased to a limit where the inner acoustic streaming is eliminated, i.e. when forced convection was the primary mechanism in determining the evaporation rate of the liquid droplet. For other air-flow regimes no direct comparisons between results obtained with the two techniques were permissable, as confirmed by the differing Sherwood numbers obtained in the experiments.

Monitoring and optimizing the co-composting of dewatered sludge: A mixture experimental design approach

The management of dewatered wastewater sludge is a major issue worldwide. Sludge disposal to landfills is not sustainable and thus alternative treatment techniques are being sought. The objective of this work was to determine optimal mixing ratios of dewatered sludge with other organic amendments in order to maximize the degradability of the mixtures during composting. This objective was achieved using mixture experimental design principles. An additional objective was to study the impact of the initial C/N ratio and moisture contents on the co-composting process of dewatered sludge. The composting process was monitored through measurements of O 2 uptake rates, CO 2 evolution, temperature profile and solids reduction. Eight (8) runs were performed in 100 L insulated air-tight bioreactors under a dynamic air flow regime. The initial mixtures were prepared using dewatered wastewater sludge, mixed paper wastes, food wastes, tree branches and sawdust at various initial C/N ratios and moisture contents. According to empirical modeling, mixtures of sludge and food waste mixtures at 1:1 ratio (ww, wet weight) maximize degradability. Structural amendments should be maintained below 30% to reach thermophilic temperatures. The initial C/N ratio and initial moisture content of the mixture were not found to influence the decomposition process. The bio C/bio N ratio started from around 10, for all runs, decreased during the middle of the process and increased to up to 20 at the end of the process. The solid carbon reduction of the mixtures without the branches ranged from 28% to 62%, whilst solid N reductions ranged from 30% to 63%. Respiratory quotients had a decreasing trend throughout the composting process.

Simplified Convective Heat Transfer in Vertical Airflow Regimes with Shading Device

The design of ventilated glass-glass facades in large buildings seems a promising technology to enhance energy efficient building design. In order to describe the optical and thermal heat transfer mechanism in glass-glass configurations with vertical airflow regimes radiation and conduction models have been developed and validated. The convective heat transfer can be calculated depending on air temperatures and air velocity. But obstructions in the vertical airflow regime (i.e. shading device) with reflective and absorptive properties demand the set-up of a new heat transfer balance. Radiative, conductive, and convective heat transfer coefficients for the heat transfer through glass-glass configuration with internal obstructions were developed. The simulation results were validated with measured data of air temperature regimes and different solar radiation in an existing ventilated double-skin façade. The comparison show very good agreement and give confidence in using the model for further energy analysis of this type of façade.

The influence of spent household batteries to the organic fraction of municipal solid wastes during composting

The objective of this work was to investigate the potential transfer of 9 heavy metals from spent household batteries (zinc–carbon and alkaline–manganese batteries) to the organic fraction of municipal solid wastes during active composting. Six runs were performed including one control and 2 replications. Eleven types of alkaline and non-alkaline batteries were added at 3 different levels to the organic fraction of municipal solid wastes, namely at percentages equal to 0.98% w/w (low), 5.2% w/w (medium) and 10.6% w/w (high). Experiments were performed in 230 l insulated plastic aerobic bioreactors under a dynamic air flow regime for up to 60 days. Iron, copper and nickel masses contained in the organic fraction of the wastes were found significantly higher in the high level runs compared to the corresponding masses in the control. No metal transfer was obtained in the low and medium level runs. Metal mass balance closures ranged from 51% to 176%. Metals' concentrations in the leachates were below 10 mg l −1 for most metals, except iron, while an increasing concentration trend versus time was measured in the leachates of the high level runs. In all cases, the contents of 5 regulated heavy metals in all end products were below the Hellenic limits.

Coastal foredune topography as a control on secondary airflow regimes under offshore winds

AbstractAeolian sediment transport under offshore winds is usually discounted from sediment budget considerations of coastal dunes. Results presented here indicate however, that depending on wind orientation and foredune morphology, positive contributions to the sediment budget can arise from offshore winds. Lee‐side airflow patterns, seaward of the foredune crest, under offshore winds were studied at three sites of varying foredune geometry along Magilligan Strand, Northern Ireland. Ultrasonic anemometers recorded 1 Hz wind velocity data at 1° intervals. Descriptive statistics and wind rose diagrams were used in the analysis of these data. Under perpendicularly offshore winds each site exhibited a different type of lee‐side airflow pattern. A tall (11·4 m), sharp‐crested foredune produced flow separation with a recirculation cell. A lower (6·6 m), more rounded, foredune crest exhibited lee‐side flow that remained attached but was deflected parallel to the crest, while flow over a low incipient foredune crest resulted in attached flow with no deflection in the lee. The morphology of the lee slopes best explained these airflow adjustments. More obliquely offshore winds were also found to affect the secondary airflow patterns, with a smooth transition from flow separation to flow attachment as the incident angle decreased. At higher wind velocities this smooth transition was replaced with a more abrupt switch between the secondary airflow types. This threshold was located at similar approach angles to those recorded for desert dunes (90° +/− 20°). These findings have significant implications for the post‐storm recovery and long‐term evolution of beach‐dune systems where the predominant winds are offshore, and offer an explanation for the presence of many beach‐dune systems in lee‐coast locations. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Observations of speciated atmospheric mercury at three sites in Nevada: Evidence for a free tropospheric source of reactive gaseous mercury

Air mercury (Hg) speciation was measured for 11 weeks (June–August 2007) at three sites simultaneously in Nevada, USA. Mean reactive gaseous Hg (RGM) concentrations were elevated at all sites relative to those reported for locations not directly influenced by known point sources. RGM concentrations at all sites displayed a regular diel pattern and were positively correlated with ozone (O3) and negatively correlated with elemental Hg (Hg0) and dew point temperature (Tdp). Superimposed on the diel changes were 2‐ to 7‐day periods when RGM concentrations increased across all three sites, producing significant intersite correlations of RGM daily means (r = 0.53–0.76, p &lt; 0.0001). During these periods, enhanced O3 concentrations and lower Tdp were also observed. Back trajectories were applied to develop gridded frequency distribution (GFD) plots and determine trajectory residence times (TRT) in specific source boxes. The GFD for the upper‐quartile RGM daily means at one site showed a contributing airflow regime from the high‐altitude subtropics with little precipitation, while that developed for the lower‐quartile RGM concentrations indicated predominantly lower‐altitude westerly flow and precipitation. Daily mean TRT in a subtropical high‐altitude source box (&gt;2 km and &lt;35°N) explained a component of the daily mean RGM at two sites (r2 = 0.37 and 0.27, p &lt; 0.05). These observations indicate that long‐range transport of RGM from the free troposphere is a potentially important component of Hg input to rural areas of the western United States.

Shifts in early spring wind regime in North-East Europe (1955–2007)

Abstract. Changes to the winter-to-spring switch-time of the upper air flow regime at the 850 and 500 hPa levels over the north-eastern Baltic Sea are analyzed based on a data set extending from 1955 to 2007. The long-term variation of the air flow in early spring (March) exhibits multiple regime shifts. The shifts are extracted by means of a vector analysis of the monthly mean air flow and using statistical shift detection technology. In the middle of the 1960s the average air flow turned from NW (WNW) to W (WSW) at the 500 (850) hPa level. The original regime was restored in the mid-1990s. The regime shifts in the average air flow in March can be interpreted as changes in the transition time from winter to summer circulation type.

Risk management approach for monitoring UF membrane integrity and experimental validation using MS2-phages

Low-pressure membrane filtration systems, such as microfiltration (MF) and ultrafiltration (UF), have received a great deal of attention in the past 15 years due to their ability to remove microbial pathogens, especially Cryptosporidium and Giardia. The major concern for the application of membrane technology is, however, how to ensure integrity of these barriers, since small defects in membranes could result in a significant reduction in pathogen removal efficiency. In order to ensure safe drinking water treatment, a number of environmental agencies request the membrane operators to conduct regular direct integrity tests to control the microbial log removal values (LRV) of the plants. Typically, test conditions must be selected to provide information on defects larger than 3 μm to ensure Cryptosporidium removal. In that context, the objective of this project was to develop and validate, both at bench-scale and full-scale, a model based on the equations proposed by USEPA and ASTM that uses the air flow rate throughout a defect during the air pressure test for predicting the microbial LRV. The project was conducted on a pressurised low-pressure membrane module. MS2-phages were used at bench-scale to validate the model and the selected assumptions with various calibrated defects carried out on the membrane fibres. The validity of the model was then evaluated at full-scale. A user-friendly tool using the Hagen Poiseuille Model proposed by the ASTM was developed to assist membrane operators in the integrity monitoring. The calibration of the model with full-scale tests resulted in adjusting some key-parameters representing air diffusion, flow regime and particles deposition. The numerical applications provide a very reliable result in predicting the pathogen removal efficiency and the equivalent number of broken fibres. This model could detect one complete broken fibre out of more than 700,000 fibres, which guaranteed more than 4 log of microorganism removal efficiency.

Numerical analysis of the street canyon thermal conductance to improve urban design and climate

Built environment is increasingly dependent on the scientific knowledge which integrates urban design and climate. In the work presented here, the canyon thermal conductance which quantifies the heat transported outside of a canyon street, is analyzed to improve on understanding of how to accomplish this integration. A two-dimensional, steady, k – ε turbulence model is used to study the influence of a windward heated wall on the air flow circulation in a street canyon with building height-to-street width ratio (aspect ratio) from 0.7 to 1.5. The numerical results presented here suggest that the air flow regime is strongly affected by buoyancy and three configurations are predicted: (I) and (II) with high Froude numbers ( ≈ 10 1 ) result in one or two stable counter-rotating vortices, with an intenser upper vortex; air flow regime (III), with low Froude numbers ( ≈ 10 - 1 ), is dominated by the lower vortex whose intensity is enhanced by a strong upward current close to the heated surface confining the upper vortex to a strict leeward zone of the canyon. Transitional Froude numbers are found as a function of canyon aspect ratio for transitions between regimes. The relevance of the results for urban design are quantified and analyzed in terms of canyon thermal conductance. The main conclusion is that, for one vortex skimming air flow regime, the canyon thermal conductance linearly increases with wind intensity, being larger streets more exposed to thermal losses. Multiple vortices in the air flow regime significantly decrease the canyon thermal conductance and, therefore, narrow streets provide protection from heat losses on windy and cloudy days and nights.

Impact of Occupant Modelling on the Prediction of Airflow around Occupants in a Ventilated Room

Localized ventilation systems typically create highly asymmetric or non-isothermal environments around occupants with significant vertical temperature gradient and highly non-uniform airflow regimes that could be directed toward a segment of the body. These effects may have pronounced impact on occupant’s thermal comfort. The airflow field and temperature distribution near the occupant can be determined either by performing full-scale measurements or by simulation methods. Usually, human subjects or manikins are used in field studies involving measurement techniques. However, as an alternative to full-scale measurement, Computational Fluid Dynamics (CFD) has been proven to be a practical and valuable tool for predicting the airflow field. At the same time, the accuracy of the predictions of the local airflow within the microclimate of the occupant is highly dependent on the proper modelling of the occupant itself. The human body not only has a complicated physical shape, but also has complex thermo-physiological properties. Modelling of all these aspects is a formidable challenge and an extremely time-consuming task. Therefore, various simplifications have been made in order to decrease the level of complexity so that the computation may be performed with the available computer resources.This paper reports the results of a detail numerical simulation to study the impact of occupant modelling on the airflow and temperature distribution and their influences on the occupant’s thermal comfort. First, the predictions made by the CFD model were compared with experimental data that were measured in a specially designed experimental chamber. Good agreement was observed. Four type of configuration were used to model the occupant: a conventional block form, three-node, six-node and finally eight-node configurations. Further simulations were carried out to investigate the assumption of uniform heat distribution. An assessment of uniform and non-uniform heat distribution scenarios for various occupant configurations and ventilation systems showed that the assumption of uniform heat distribution is valid for a wide range of operating conditions.