Quantitative Assessment of the Effect of Surface Deposition and Coagulation on the Dynamics of Submicrometer Particles Indoors

Exposure to airborne particles indoors depends on particle concentration, which is affected by air filtration, ventilation, and particle dynamics. The aim of this work was quantitative assessment of the effects of coagulation, surface deposition, and ventilation on the submicrometer particle concentration indoors. The assessment was obtained from measured particle loss rate and deposition velocity parameters. The experiments were conducted in an experimental chamber for three different types of aerosols: environmental tobacco smoke, petrol smoke, and ambient air aerosols. Particle number concentration and size distribution were measured in the size range between 0.017 and 0.898 w m by SMPS. The average values for the overall deposition loss rates varied from 4.3 2 10 m 5 s m 1 (0.16 h m 1 ) to 1.1 2 10 m 4 s m 1 (0.39 h m 1 ). The overall deposition velocities associated with surface deposition and coagulation ranged from 9.6 2 10 m 4 cm s m 1 to 2.4 2 10 m 3 cm s m 1 , and for surface deposition only from 2.8 2 10 m 4 cm s m 1 to 6.3 2 10 m 4 cm s m 1 . For indoor conditions with an air exchange rate above 1.3 h m 1 , (natural ventilation, no filters) only a reduction in particle number of about 20% is attributed to the surface deposition and coagulation.     

Investigation of particle deposition on a micropatterned surface as an energy-efficient air cleaning technique in ventilation ducting systems

Abstract

Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation system can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This article proposes an air cleaning technique, aimed at submicron particles, by means of installing patterned surfaces on the walls of ventilation ducts, which can be easily cleaned by water and reused. The effect of patterned surfaces on particle deposition was studied numerically. In the numerical simulation, the Reynolds stress turbulent model was correlated at the near-wall regions by turbulent velocity fluctuation at the normal direction. Particle trajectory was solved by using Lagrangian particle tracking. The numerical model was then validated with a particle deposition experiment. A wind tunnel experiment was carried out to quantify the particle deposition on the semicircular micropatterns for a wide range of heights. Based on our numerical results, the semicircular pattern height of 500?µm with a pitch-to-height ratio (p/e) of 10 has 8.58 times enhancement of the energy efficiency compared with a high-efficiency particulate air filter. Our results indicated that adding surface micropatterns to ventilation ducting for submicron particle deposition is a possible energy-efficient air cleaning technique for practical usage.

Copyright © 2020 American Association for Aerosol Research     

NUMERICAL SIMULATION OF VOLATILE ORGANIC COMPOUNDS EVAPORATION IN CLOSED SPACES

Abstract

This paper deals with the simulation of VOCs concentration dispersion, evaporated from flooring materials, with the purpose of understanding VOCs evaporation and dispersion mechanisms. A test chamber is examined whose flooring material emits a number of VOCs. Given the area specific ventilation rate and considering it as boundary conditions, experimental data for the examined compounds concentration, the dispersion of the VOCs concentrations is examined under steady state and transient conditions. The model developed is used in conjunction with a general - purpose CFD code, PHOENFCS, that can provide detailed information on the flow as well as concentration fields. The results of the above two simulation cases are used as a guide for two other cases, where faster restoration of the air indoor quality was investigated by changing the ventilation rate in the chamber. The simulation results were used as a basis for further analysis for VOC evaporation for other flooring materials; this will allow proper material selection as well as proper ventilation system for a more healthy and comfortable environment in a building.     

Experimental Investigation of Exhaled Aerosol Transport Under Two Ventilation Systems

Although the importance and implications of studying expiratory droplet concentration distribution in indoor environments are obvious, experimental measurements are very scarce and incomplete. In the work described herein, spatial and temporal aerosol concentrations generated by a modeled expiratory process were studied experimentally. Two heated manikins were placed inside a chamber with either a displacement ventilation system or a ceiling supply and ceiling return ventilation system. One of the manikins emitted 0.05 micrometer monodisperse polystyrene microspheres mimicking the generation of expiratory droplets. Flow characteristics were measured by a hot-film anemometry system before and after the momentary emission of aerosols at three locations. The temporal concentration profiles at eight locations were also measured. The results show that a high-speed air jet easily penetrates the boundary layer of the manikin regardless of the ventilation system used. Transient ventilation effectiveness was also evaluated and compared for both ventilation systems. The results show that the ventilation effectiveness of displacement ventilation is always higher than that of ceiling-type ventilation. Experimental observation also suggests that the presumption of complete mixing must be applied cautiously.     

NUCLEAR REACTOR SAFETY AEROSOLS

The deposition rate constants of the different unattached decay products of radon (²²²Rn) are evaluated from the measured radon and decay product concentrations in a 1-m³ chamber as a function of the degree of turbulence. The turbulence is induced by ventilation and/or generating heat. The coefficient of eddy diffusivity, k_e, in the theoretical formula of Crump and Seinfeld for calculating the deposition rate is examined by fitting the Crump and Seinfeld formula to our experimental results. The expression for k_e thus obtained is proportional to λ³ _v (ventilation) and W ^3/2 (generated heat). The deposition rate constant of ²¹⁸Po is found to be about three times that of ²¹⁴Pb, which means that the associated diameter of the ²¹⁴Pb particle is about twice as large as the diameter of the ²¹⁸Po particle. This difference could be due to the physical and chemical properties of the two elements     

Design optimization of twin-fluid atomizers with an internal mixing chamber for heavy fuel oils

The present work is devoted to determine the magnitude of the main parameters that yield the optimum results for twin-fluid nozzles with an internal mixing chamber. The focus is placed on the study of the interaction of both air and liquid flows at the internal chamber and its effects on the resulting spray. To this end, some experiments have been performed for different air central channel diameters and liquid ports, as well as for several experimental conditions (air and liquid mass flow rates), in order to understand the influence of the flow conditions at the mixing chamber on the size of the droplets produced. It has been demonstrated that under certain experimental conditions the atomizing fluid discharged to the internal chamber is choked. The sonic condition is achieved for different air and liquid mass flow rates as a function of the air central channel diameter. It has also been obtained that to achieve the best results with moderate atomizing fluid flow rates, it is convenient to operate in choked conditions. This is an important result that will help in the optimum design of this type of nozzles.     

Methane Transport Capacity of Rice Plants. I. Influence of Methane Concentration and Growth Stage Analyzed with an Automated Measuring System

A major portion (60–90%) of the methane (CH₄) emitted from rice fields to the atmosphere is transported through the aerenchyma of the rice plants. However, a rapid and accurate method to study the CH₄ transport capacity (MTC) of rice plants is not available. We developed a gas sampling and analytical device based on a closed two-compartment chamber technique and analyzed the enrichment of the CH₄ mixing ratio inside the shoot compartment of cylindrical cuvettes enclosing individual rice plants under ambient conditions. The computer-controlled analytical system consists of a gas chromatograph (GC) and a pressure-controlled autosampler for eight cuvettes (seven for plants and one for CH₄-calibration gas). The system automates closure and opening of plant cuvettes using pneumatic pressure, air sample collection and injection into the GC, and CH₄ analysis. It minimizes sources of error during air sampling by continuously mixing headspace air of each cuvette, maintaining pressure and composition of the headspace inside the cuvettes, purging the dead volumes between the sampler induction tube and GC, and running a reference CH₄-calibration gas sample in each cycle. Tests showed that the automated system is a useful tool for accurate sampling of headspace air of cylindrical cuvettes enclosing individual rice plants and enables rapid and accurate fully automated analysis of CH₄ in the headspace air samples. A linear relationship was obtained between CH₄ transported by rice plants of two cultivars (IR72, a high-yielding dwarf, and Dular, a traditional tall cultivar) and concentration of CH₄ up to 7,500 ppm used for purging the nutrient culture solution surrounding the roots in the root compartment of the chamber. Further increase in CH₄ emission by shoots was not observed at 10,000 ppm CH₄ concentration in the root compartment of the chamber. The MTC of IR72 was measured at six development stages; it was lowest at seedling stage, increasing gradually until panicle initiation. There was no further change at flowering, but a marked decrease at maturity was noted. These results suggest that the plants have 45–246% greater potential to transport CH₄ than the highest CH₄ emission rates reported under field conditions, and plants would not emit CH₄ at early growth and at a reduced rate close to ripening.     

Design and performance evaluation of triboelectrostatic separation system for the separation of PVC and PET materials using a fluidized bed tribocharger

A triboelectrostatic separation system using a fluidized-bed tribocharger for the removal of PVC material in the mixture of PVC/PET plastics was designed and evaluated as a function of tribocharger material, air flow rate, electric field strength, and the mixing ratio of two-component mixed plastics. The test system consists of the fluidized-bed tribocharger, a separation chamber, a collection chamber and a controller. PVC and PET particles can be imparted negative and positive surface charges, respectively, due to the difference in the work function values of plastics suspended in the fluidized-bed tribocharger, and can be separated by passing them through an external electric field. Experimental results show that separation efficiency is strongly dependent on the tribocharger material, electric field strength and particles mixing ratio. In the optimum conditions of 150l/m air flow rate and 2.6 kV/cm electric field strength, highly concentrated PVC (99.1%) can be recovered with a yield of more than 95% from the mixture of PVC and PET materials for a single stage of processing.     

Expiratory Aerosol Transport in a Scaled Chamber under a Variety of Emission Characteristics: An Experimental Study

Pollutant concentration is a major factor affecting exposure. Expiratory process generates high-velocity droplets for a very short period of time that leads to highly transient temporal and nonuniform concentration profiles. In this work, solid aerosol generated by an expiratory process was measured spatially and temporally inside a scaled ventilated chamber. Conventional ceiling-supply, ceiling-return, and displacement ventilation systems were studied. Two sizes of monodisperse microspheres, 0.05 and 10 μm, were generated by a modeled expiratory process. Two heated, scaled manikins were used to mimic a simple yet typical exposure scenario with an aerosol source and a receiver while buoyancy flow was presented. First, the influence of relative orientation of the source to the inlet diffuser on spatial concentration was investigated. The source was located in such a position that the bulk airflow was against to the expiratory process. Second, the relative orientation of the source-to-receiver was also investigated to study how the concentration distribution was being affected. Two orientations were tested as one of the source faced to the receiver and the other source faced to a sidewall. The dimensionless temporal concentration results were presented at six locations. Transient ventilation effectiveness was also evaluated and compared for both ventilation systems. With the “counter flow” scenario, the emitted particles tended to suspend in the mechanical ventilated chamber for a long period; this indicates a high exposure will be resulted. For the displacement scheme of the “face-to-wall” orientation, due to the low y-component of air velocity, poor lateral dispersion was observed.     

The role of nozzle convergence in diesel combustion

An experimental study has been performed for identifying the role of injector nozzle hole convergence and cavitation in diesel engine combustion and pollutant emissions. For doing so, five nozzles were tested under different operating and experimental conditions. The critical cavitation number of each nozzle was analyzed. With this value, an estimation of the mixing process at different conditions obtained. This data is used to explain the combustion results which are analyzed in terms of the apparent combustion time, rate of heat release, in-cylinder pressures, adiabatic temperatures and soot and NO_x emissions.Special emphasis is put in developing an expression to explicitly link the mixing process and the injection rate with the rate of heat release.The results show that the fuel–air mixing process can be improved by the use of both convergent and cavitating nozzles, thus lowering the soot emissions. The NO_x production, being dependent of the injection rate and the mixing process, does not necessarily increase with the use of more convergent nozzles.     

Measurement of 220Rn/222Rn progeny deposition velocities on surfaces and their comparison with theoretical models

The deposition velocities of ²²²Rn (radon) and ²²⁰Rn (thoron) progeny species have been measured in a chamber, in a test house, and in dwellings by relating the atom deposition fluxes of these species to their atom concentrations in air. These measurements were carried out using absorber-mounted nuclear track detectors (LR-115) which selectively register the tracks due to alpha emissions from ²¹²Po and ²¹⁴Po from the deposited atoms of ²²⁰Rn and ²²²Rn progeny species, respectively. These are termed as DRPS (direct radon progeny sensor) and DTPS (direct thoron progeny sensor). Measurement of parameters such as ventilation rate, particle size distribution and unattached fractions were also carried out along with deposition velocity. The experimental data on deposition velocity in test house and chamber were compared with the predictions based on the indoor progeny dynamics model and particle deposition models. These showed excellent agreement with experimental values although the data on radon progeny showed slightly higher dispersion. The progeny deposition velocities were also measured in living rooms of dwellings in Mumbai and were found to be close to the model results which in turn imply that in the long term, the average environmental conditions are similar to that in the test house. These results point at a plausible constancy of long time averaged indoor deposition velocities. From these studies, we are inclined to assign summary values of deposition velocities of 0.075 m h^?1 for ²²⁰Rn progeny and 0.132 m h^?1 for ²²²Rn progeny, for indoor conditions.     

NUMERICAL SIMULATION OF VOLATILE ORGANIC COMPOUNDS EVAPORATION IN CLOSED SPACES

This paper deals with the simulation of VOCs concentration dispersion, evaporated from flooring materials, with the purpose of understanding VOCs evaporation and dispersion mechanisms. A test chamber is examined whose flooring material emits a number of VOCs. Given the area specific ventilation rate and considering it as boundary conditions, experimental data for the examined compounds concentration, the dispersion of the VOCs concentrations is examined under steady state and transient conditions. The model developed is used in conjunction with a general - purpose CFD code, PHOENFCS, that can provide detailed information on the flow as well as concentration fields. The results of the above two simulation cases are used as a guide for two other cases, where faster restoration of the air indoor quality was investigated by changing the ventilation rate in the chamber. The simulation results were used as a basis for further analysis for VOC evaporation for other flooring materials; this will allow proper material selection as well as proper ventilation system for a more healthy and comfortable environment in a building.     

Measurement of constant drying rate of wet material placed in a fluidized bed of inert particles under reduced pressure

The objective of this study is to estimate the drying characteristics of a relatively large material immersed in a fluidized bed under reduced pressure by measuring the constant drying rate. The constant drying-rate period in a fluidized bed under reduced pressure is difficult to measure because it is extremely short. To maintain the constant drying-rate period, distilled water is directly supplied to the drying material. Through our experiment, the heat transfer coefficient of the material surface was also determined. The results were compared with data on hot air drying. The constant drying rate is higher for fluidized bed drying than for hot air drying. It suggests that the heat transfer coefficient on the surface of the drying material is much larger for fluidized bed drying than for hot air drying. For fluidized bed drying, the effect of pressure in the drying chamber on the heat transfer coefficient is slight at the same normalized mass velocity of dry air (G/G_mf). The temperature difference between the inside of the drying chamber and the drying material is much smaller for fluidized bed drying than for hot air drying. The constant drying rate increases as the pressure in the drying chamber decreases.     

Measurements of pollen grain dispersal in still air and stationary, near homogeneous, isotropic turbulence

The dispersal of ragweed, pine and corn pollen as well as polystyrene spheres in still air and stationary, near homogeneous, isotropic turbulence (HIT) was investigated using high-speed, digital inline holographic cinematography enabling Lagrangian tracking of the particles. Mean still air settling velocities were similar as reported literature values. Small discrepancies were most likely related to species/size differences and water content of the grains. Near-HIT was generated by loudspeakers mounted on the corners of a 40 cm³ chamber and the turbulent flow field at the center of the chamber was validated using stereoscopic Particle Image Velocimetry (PIV). Results showed near homogeneity and near isotropy with mean velocities 5–10 times smaller than the corresponding rms values of velocity fluctuations. The turbulent kinetic energy dissipation rate was determined from the PIV data sets and used to calculate the Kolmogorov scales and Taylor microscales. Experiments were carried out for two different loudspeaker amplifications corresponding to Taylor microscale Reynolds numbers, R_λ=144 and 162, respectively. The mean settling velocity in turbulent conditions was in all cases higher than the corresponding still air value, the difference becoming smaller as particle Stokes numbers increased. For the present conditions, the still air particle settling velocity was lower than the rms values of air fluctuating velocities. As a result, dispersion was dominated by inertia and for a given R_λ, particle fluctuating velocity autocorrelations fell more rapidly as the particle Stokes number decreased; corresponding particle diffusion coefficients also decreased. Transverse particle diffusion coefficients were lower than those in the direction of gravity in agreement with the continuity effect. Under the present range of experimental parameters, results showed that inertial particles (0.6<St<11) in highly turbulent conditions disperse more effectively than the air.     

Experimental Study and CFD Modeling of Wall Deposition in a Spray Dryer

The wall deposition phenomenon in a pilot-scale spray dryer was investigated based on mathematical modeling and experimental trials. For this purpose, the governing equations were obtained and solved numerically by applying a mathematical modeling technique and an open-source computational fluid dynamics (CFD) software. The wall deposition, velocity distribution of the existing phases, and droplet trajectory in the drying chamber were determined. The effect of the operating parameters including the feed flow rate, inlet concentration of dissolved solid, and initial droplet diameter on the air flow pattern, droplet trajectory, and wall deposition was investigated. Through the experiments, the wall deposition of powder product in different positions of the drying chamber was measured. In modeling part of this study, we attempted to determine the effect of particle diameter on the percentage of wall deposition and the position where it occurred.

The model results obtained for wall deposition were compared with collected experimental data and good agreement was observed.     

Numerical modelling for rotational moulding with non-isothermal heating

Abstract

In the rotational moulding process, the internal air temperature has been widely recognised as a tool to predict an optimum cycle time. This paper presents a new numerical approach to predict the internal air temperature in a two-dimensional (2-D) static model without requiring the consideration of the tumbling motion of polymer powder. The initial non-isothermal heating of the static model is actually formed by two changeable plastic beds (stagnant and mixing beds), which represent the actual stagnant and mixing pools inside a rotating mould respectively. In the numerical approach, the lumped-parameter system and coincident node technique are proposed to incorporate with the Galerkin Finite Element Method in order to account for the complex thermal interaction of the internal air. It helps to overcome the difficulty of multidimensional static models in predicting an accurate internal air temperature during the heating stage of rotationally powdery plastic. Importantly, the predicted temperature profiles of the internal air, oven times for different part thicknesses and process conditions accord with the available experimental results.     

Modeling of rotational molding process: Multi‐layer slip‐flow model,phase‐change,and warpage

A new multilayer slip‐flow model has been developed to simplify and to overcome current numerical difficulties of two‐dimensional model in predicting the internal air temperature inside a mold during a rotational molding process. The proposed methodology considers a macroscopic “layer‐by‐layer” deposition of a heating polymer bed onto the inner mold surface. A semi‐implicit approach is introduced and applied to compute the complex thermal interactions between the internal air and its surroundings. In the model, the lumped‐parameter system and the coincident node technique are incorporated with the Galerkin finite element model to address the internal air and the deposition of molten polymer beds, respectively. The simple phase‐change algorithm has been proposed to improve the computational cost, numerical nonlinearity, and predicted results. The thermal aspects of the inherent warpage are explored to study its correlation to the weak apparent crystallization‐induced plateau in the temperature profile of the internal air, as in practice. The overall predicted results are in favor with the available experimental data for rotomolded parts of cross‐sectional thicknesses up to 12 mm. POLYM. ENG. SCI. 46:960–969, 2006. © 2006 Society of Plastics Engineers     

Development of a kinetic model for the moderate temperature chemical vapor deposition of SiO2 films from tetraethyl orthosilicate and oxygen

An apparent kinetic model for the chemical vapor deposition of SiO₂ from tetraethyl orthosilicate (TEOS) and O₂ was developed in a poorly investigated range of operating conditions, that is, at atmospheric pressure and between 350 and 500°C, based on literature survey and experimental results obtained in a hot wall tubular reactor. The kinetic model was implemented into the computational fluid dynamics code FLUENT and validated both in shape and value by comparison with experimental deposition rate profiles. It reveals that for the conditions tested, a possible mechanism of SiO ₂ deposition involves two intermediate species formed from TEOS homogeneous decomposition, the first one being active from 300°C and the second one contributing to deposition for temperatures higher than 370°C. The calculated local profiles of gas flow, gas temperature, species mass fraction, and silica deposition rate indicate that the first intermediate species leads to marked film thickness gradients, the second one being more stable as producing uniform thicknesses. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3958–3966, 2018     

Experimental and modeling assessment of a novel automotive cabin PM2.5 removal system

Poor air quality inside vehicles and its impact on human health is an issue requiring attention, with drivers and passengers facing levels of air pollution potentially greater than street-side outdoor air. This paper assesses the potential effectiveness of a car cabin filtration system to remove fine particulate matter PM_2.5 and improve air quality for car passengers. The study was conducted as a practical evaluation coupled to a model implementation. First, the effectiveness of PM_2.5 filter material was investigated in a chamber experiment under a range of environmental and loading conditions using a realistic automotive auxiliary scrubber. Second, implementation of such a system was evaluated in a full air flow 3D computational fluid dynamical model configured for a realistic cabin and ventilation system, and related to the chamber results through a simple decay model. Additionally, performance of low-cost dust sensors was evaluated as potential cabin monitoring devices. The experiment and modeling support the feasibility of a robust system which could be integrated into automotive designs in a straightforward manner. Results suggest that an auxiliary scrubber in the rear of the cabin alone would provide suboptimal performance, but that by incorporating a PM_2.5 filter into the main air handling system, cabin PM_2.5 concentrations could be reduced from 100?µg m^?3 to less than 25?µg m^?3 in 100?s and to 5?µg m^?3 in 250?s. A health impact assessment for hypothetical occupational driver populations using such technology long term showed considerable reductions in indicative PM_2.5 attributable mortality.

Copyright © 2018 The Authors. Published with license by Taylor & Francis Group, LLC     

Flow Characteristics in a Vortex Chamber

In this article we examine confined swirling flows using the integral equations of continuity and energy, along with the minimum pressure criterion. The pressure drop and the core size have been studied in the swirling confined vortex chamber. Both the n = 2 vortex model, with reverse and non‐reverse flow, and the free vortex model have been used at the vortex chamber exit plane. The influence of vortex chamber geometry, such as contraction ratio, inlet angle, area ratio, aspect ratio, and Reynolds number, on the flow field has been analyzed and compared with the present experimental data. The pressure drop across the vortex chamber differs from that in pipe flow, due to the mechanism of swirl flow that depends mainly on the intensity of tangential velocity. If the chamber length is increased, the vortex decays producing a weaker tangential velocity (less centrifugal force) that leads to less pressure drop. Based on the present theory, a new approach to determine the tangential velocity and radial pressure profiles inside the vortex chamber is developed and compared with the available experimental data. It shown that the n = 2 vortex model with reverse flow gives better results for strongly swirling flow.