Ventilation Duct Flow Research Articles

Particle deposition in ventilation ducts: A review

This paper reviewed particle deposition behaviors and mechanisms in turbulent ventilation duct flows. The main theoretical prediction models, experimental techniques and numerical methods used to explore particle deposition were discussed and analyzed. It was observed that turbophoresis, Brownian diffusion, turbulent diffusion and gravitational settling are the main mechanisms of particle deposition in smooth duct flows. The important factors influencing particle deposition behaviors and mechanisms are duct inclination angle, thermophoresis, electrophoresis and surface ribs. Numerical simulation was shown to be the main tool used to investigate complex particle deposition in turbulent duct flows. However, it is necessary to develop accurate experimental measures of particle deposition in complex turbulent duct flows.

Fast and accurate prediction of airflow and drag force for duct ventilation using wall-modeled large-eddy simulation

Rectangular ducts are commonly adopted in ventilation systems to push airflow through pipes, with Reynolds number reaching up to 500000. Computational cost for flows with such a Reynolds number is prohibitive. Moreover, accurate prediction of airflow and drag in duct flows is important for ventilation duct design to remove dust particles and reduce turbulence, which will be beneficial for pipes cleaning and energy consumption. Hence, this work proposed a fast and accurate technique to simulate high-Reynolds-number turbulent flows in a square duct, using improved wall-modeled large-eddy simulation (LES) models. The improved wall-modeled LES approach features an improved wall-stress model and a self-adaptive subgrid-scale (SGS) model, which can dynamically adjust the mixing length to better predict wall-bounded flows. A new filtering strategy (filtering in time and space) is proposed as well. Wall-modeled LES are then conducted for the duct flows at a Reynolds number of 250000 with y+ of the first point (away from the wall) larger than 300. Existing experiments were applied for validations. In the result, the computational time is greatly reduced (over 90%) compared with wall-resolved LES. The improved wall-modeled LES shows a better prediction of wall friction (drag) with acceptable mean velocity profiles. Moreover, the self-adaptive model captures the acceleration of the mean streamwise velocity near the corner but not for the conventional Smagorinsky approach. The methodology developed in this study provides a fast and accurate wall-modeled LES approach for the simulation of high-Reynolds-numbers ventilation duct flows.

Evaluation of cold plasma inactivation efficacy against different airborne bacteria in ventilation duct flow

A full-size, experimental, ventilation ductwork was designed and set up to measure one-pass inactivation efficacies of a cold plasma installation under various practical environmental conditions. Five types of microorganisms, which are commonly associated with nosocomial infection, including Escherichia coli, Pseudomonas alcaligenes, Staphylococcus epidermidis, Micrococcus luteus and Serratia marcescens, were chosen and tests were conducted at four airstream velocities, ranging from 2 to 7 m/s, and two different relative humidity (R.H.) levels. The inactivation efficacies for the first three types of bacteria varied from 20% to 70%. It is interesting to note that the inactivation efficacy increased with velocity. No detectable inactivation effect was found for M. luteus and S. marcescens. The inactivation efficacy at 90% R.H. dropped to 10% of the value measured at 55% R.H. The negative ion intensities measured near the plasma installation were correlated with the airstream velocities. When compared with data from systems using conventional fibrous filters, the pressure drop measured across the plasma unit was very small. Our experiments showed that cold plasma technology has high potential to be used as an energy-efficient method for disinfection. Limitations of application are also discussed.

Study on Motion and Deposition of Particles in a Wind Tunnel

Study on motion and deposition characteristics of particles in a wind tunnel is important for understanding particle transfer in ventilation duct flows. Firstly, polydisperse particles with smaller mean diameter were taken as tracers for airflow velocity test in the wind tunnel under ventilation case and airflow velocities were measured by a Particle Image Velocimetry (PIV) system. Then, the average velocity contours of the particles with the mean (median) diameter of 25μm were determined by the PIV. Next, the relative concentration contours were analyzed for the particle phase at the different test sections. Finally, deposition rates of the particles were determined to the various surfaces in the wind tunnel. The experimental results indicate that PIV can accurately measure the two-dimensional airflow velocities and there is a difference between air and particle phase velocity contours. The smaller the airflow velocity is, the weaker the particles follow the airflow is. During the transfer of the polydisperse particles along the flow, the stratification of relative concentration of the particles along the height seems to take place. The deposition rates of polydisperse particles to the floor are higher than that to the wall and ceiling. For polydisperse particles, the mean diameter, size distribution, and spread parameter should have a comprehensive impact on the floor, wall and ceiling deposition.

One-pass antibacterial efficacy of bipolar air ions against aerosolized Staphylococcus epidermidis in a duct flow

While heating, ventilating, and air-conditioning (HVAC) systems can provide healthy and comfortable indoor environments, virtually any part of an HVAC system can support active microbial growth if sufficient nutrients are present. In this study, we introduced a methodology to enhance the one-pass antibacterial performance of air ions against aerosolized bacteria in a ventilation duct flow. Staphylococcus epidermidis (S. epidermidis) was aerosolized, mixed with the duct flow, and exposed to air ions generated by carbon fiber ionizers installed inside the duct. The S. epidermidis was then sampled at the exit of the duct and incubated to evaluate their cultivability as functions of the ion exposure time, ion concentration, and ion polarity. When the ionizers produced bipolar air ions for 2s, a high antibacterial efficiency of 85% was obtained when four ionizers were positioned both at the top and bottom walls of the duct in a configuration in which there were three changes in ion polarity (one positive ionizer, one negative ionizer, one positive ionizer, and one negative ionizer in series along the flow direction). When the ion exposure time was decreased to 0.2s, an antibacterial efficiency of 50% was realized by applying a configuration with seven changes in ion polarity. By using a scanning electron microscope (SEM), cell contraction of S. epidermidis caused by the bipolar ion treatment was observed.

Prediction of Particle Deposition in Rectangular Ventilation Ducts

The accurate prediction of particle deposition in ventilation ducts is an important step in estimating the exposure risk of building occupants to particulate matter. In this paper a mathematical model for predicting particle deposition in rectangular ventilation duct flow by incorporating existing theoretical and experimental results is presented. The influences of Brownian diffusion, turbulent diffusion and gravity on particle deposition are considered. Model equations are presented for calculating the deposition velocities on vertical surfaces and the floor and ceiling of ducts. Results of calculations are compared with the experimental results of Sippola (2002) and a good agreement of the deposition velocity of particles on the floor is shown. For the deposition velocity of particles on the vertical walls, the results obtained by this model have the same tendency as those observed by Sippola (2002). In the case of ceiling deposition, this model did not match the observation in some cases.

Modeling deposition of particles in vertical square ventilation duct flows

The presence, flow, and distribution of particle in heating, ventilation, and air-conditioning (HVAC) ducts influence the quality of air in buildings and hence the health of building occupants. To shed a better light on the flow of particles in HVAC ducts this a paper has considered the effects of drag, lift force, gravity, Brownian diffusion, and turbulent diffusion on the dimensionless deposition velocity of particles in smooth vertical ventilation ducts using fully developed and developing velocity profiles. Based on the Reynolds stress transport model (RSM) at two different air velocities, 3.0 m/s and 7.0 m/s, the aforementioned effects were predicted using Reynolds-averaged Navier–Stokes (RANS)–Lagrangian simulation on square shaped ducts under vertical flows. Preliminary results suggest that the gravity of particles does not directly change the dimensionless deposition velocity in vertical flows. Nevertheless, the gravity of particles contributes to changing the Saffman lift force. It is thus the Saffman lift force that directly changes the dimensionless deposition velocity of particles in vertical flows. In addition, the difference in the dimensionless deposition velocities between fully developed and developing flows is owing to the turbulent diffusion, turbulent intensity, and needless to say, the Saffman lift force under different dimensionless particle relaxation time.

Deposition rates of unattached and attached radon progeny in room with turbulent airflow and ventilation

In this paper deposition rate coefficients for unattached and attached radon progeny were estimated according to a particle deposition model for turbulent indoor airflow described by Zhao and Wu [2006. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmos. Environ. 40, 457–466]. The parameter which characterizes turbulent indoor airflow in this model is friction velocity, u*. Indoor ventilation changes indoor airflow and friction velocity and influences deposition rate coefficients. Correlation between deposition and ventilation rate coefficients in the room was determined. It was shown that deposition rate coefficient increases with ventilation rate coefficient and that these parameters of the Jacobi room model cannot be assumed to be independent. The values of deposition rate coefficients were presented as functions of friction velocity and ventilation rate coefficient. If ventilation rate coefficient varies from 0.1 up to 1 h −1, deposition rate coefficients for unattached and attached fractions were estimated to be in the range 3–110 h −1 and 0.015–0.35 h −1, respectively.

Parameterization of aerosol dry deposition velocities onto smooth and rough surfaces

The velocities of aerosol deposition onto vertical and horizontal surfaces are needed for 2D and 3D calculations of aerosol transport and deposition in the urban and indoor environments. This paper analyzes the experimental results and semiempirical and theoretical models on dry deposition velocities. The features of the solution obtained by Zhao and Wu [(2006a). Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmospheric Environment, 40, 457–466] for the velocity of aerosol deposition onto smooth surfaces are analyzed, the integrals are approximated analytically. To treat the deposition onto rough surfaces, the results of Sehmel [(1973). Particle eddy diffusivities and deposition for isothermal flow and smooth surfaces. Journal of Aerosol Science, 4, 125–138]; Sehmel and Hodgson [(1978). A model for predicting dry deposition of particles and gases to environmental surfaces. PNL-SA-6721. Richland, WA: Battelle, Pacific Northwest Laboratory] are modified and represented in a unified manner for vertical and horizontal surfaces. Cross-comparison with data of Slinn [(1978) Parameterization for resuspension and for Wet and Dry Deposition of Particles and Gases for Use in Radiation Dose Calculations. Nucl.Safety, 19(2), 205–219.] and the model of Lai and Nazaroff [(2000). Modeling indoor particle deposition from turbulent flow onto smooth surfaces. Journal of Aerosol Science, 31, 463–476] allowed reconciliation between the theoretical and semiempirical approaches. Finally, an approximation to the aerosol dry deposition velocities is obtained in the form of convenient parameterization formulas covering a wide range of particle sizes 0.01 μ m ⩽ d ⩽ 1000 μ m and surface roughnesses z 0 ⩽ 10 cm .

Modeling deposition of particles in typical horizontal ventilation duct flows

Particles carried by airflows through ventilation ducts in heating, ventilation and air-conditioning (HVAC) systems have adverse effects on human health. Knowledge of particle deposition rates in ventilation ducts is useful for modeling exposures to particles within buildings and more completely understanding particle fates. To meet these objectives, deposition rates of particles to various internal surfaces in a smooth horizontal square ventilation duct are predicted by Reynolds-Averaged Navier-Stokes (RANS)-Lagrangian simulation at all three air speeds of 3.0 m/s, 7.0 m/s and 9.0 m/s. Drag, lift force, gravity, inertial force and turbulent diffusions are considered in the computation of particle motion. The influences of relaxation time, particle size, air speed, turbulent flow development profiles and surface orientations on predicted deposition rates (dimensionless deposition velocities) of particles is discussed. For particles larger than 10 μm, it is shown that vertical wall deposition rates in a smooth horizontal square ventilation duct where the turbulent flow profiles is fully developed first increase with dimensionless relaxation time increase and then slightly decrease with dimensionless relaxation time increasing at three different air speeds. The maximum of the sidewall deposition rates is 0.12. Floor deposition rates increase with dimensionless relaxing time increase and attain the maximum value of 9.6. Ceiling deposition rates firstly gradually decrease and then rapidly decrease to zero with dimensionless relaxing time increasing at different air speeds. The maximum of the ceiling deposition rates is 0.054. Particle deposition rates to the ceiling, wall and floor in the horizontal duct with a fully developed turbulent flow profile are all lower than that in the horizontal duct with a developing turbulent flow profile. Particle deposition rates are mainly dependent on friction velocity (air speed) and particle size. The differences in deposition rates to the duct floor, wall and ceiling decrease with increasing air speed. The results of Lagrangian simulations in this work are valuable for more accurately predicting and understanding particle deposition in typical ventilation ducts.

Study on particle deposition in vertical square ventilation duct flows by different models

A proper representation of the air flow in a ventilation duct is crucial for adequate prediction of the deposition velocity of particles. In this paper, the mean turbulent air flow fields are predicted by two different numerical models (the Reynolds stress transport model (RSM) and the realizable k– ε model). Contours of mean streamwise velocity deduced from the k– ε model are compared with those obtained from the Reynolds stress transport model. Dimensionless deposition velocities of particles in downward and upward ventilation duct flows are also compared based on the flow fields presented by the two different numerical models. Trajectories of the particles are tracked using a one way coupling Lagrangian eddy-particle interaction model. Thousands of individual particles are released in the represented flow, and dimensionless deposition velocities are evaluated for the vertical walls in fully developed smooth vertical downward and upward square duct flows generated by the RSM and realizable k– ε model. The effects of particle diameter, dimensionless relaxation time, flow direction and air speed in vertical upward and downward square duct flows on the particle deposition velocities are discussed. The effects of lift and gravity on the particle deposition velocities are evaluated in vertical flows presented by the RSM. It is shown that the particle deposition velocities based on the RSM and realizable k– ε model have subtle differences. The flow direction and the lift force significantly affect the particle deposition velocities in vertical duct flows. The simulation results are compared with earlier experimental data and the numerical results for fully developed duct flows. It is shown that the deposition velocities predicted are in agreement with the experimental data and the numerical results.

Effect of ventilation duct as a particle filter

This paper discusses the effect of ventilation duct as a particle filter by modeling particle deposition in ventilation ducts, which is the reason that ventilation ducts could “filter” particles. An Eulerian model is employed to predict the particle deposition velocity onto the wall and floor from fully developed turbulent flow in ventilation ducts [Zhao B, Wu J. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmospheric Environment 2006;40:457–66], while an empirical equation is proposed to predict the particle deposition velocity onto the ceiling combined with experimental data and, another empirical equation by McFarland et al. [Aerosol deposition in bends with turbulent flow. Environmental Science and Technology 1997;31:3371–7] is used for predicting the particle penetration through the bends, which are hard to analyze by theoretical method.Losses through ventilation duct are thus evaluated for the particle size range 0.01–100μm. Both straight duct (implies fully developed turbulent flow) and duct bend (implies developing turbulent flow) cases are analyzed. Then particle penetration through the supply duct of a swimming pool, including some straight ducts and bends, is analyzed as an application case. The results show that when particle diameter is larger than 5μm, the filtration of the duct might not be ignored as the obvious loss by deposition.

Modeling particle deposition from fully developed turbulent flow in ventilation duct

This paper proposes an improved Eulerian model to predict particle deposition velocity in fully developed turbulent duct flow. The model is modified based on the three-layer model by Lai and Nazaroff (Journal of Aerosol Science, 31, 463–476, 2000), accounting for turbophoresis as well as Brownian diffusion, turbulent diffusion and gravitational settling. An expression relating the turbophoretic velocity to particle relaxation time, friction velocity and the normal distance to the wall surface is presented to model the turbophoresis. Similar with previous one by Lai and Nazaroff, the model only needs to input friction velocity, which makes it easy to apply. The predicted results agree well with measurement data for floor and vertical walls. And then deposition velocity of airborne particles to smooth walls in straight steel ducts is predicted by the modified model. The results agree with the published measured data, especially for floor and vertical walls of ventilation duct. Thus it is expected to be applied for predicting particle deposition in ventilation duct for indoor air quality control or evaluation. Furthermore, the condition to ignore turbophoresis for particle deposition is discussed by comparing the results of the improved model and that of Lai and Nazaroff.

Particle deposition in ventilation duct onto three-dimensional roughness elements

Gaining insights on particle deposition onto ventilation duct has many important applications. One key pathway by which outdoor polluted air enters the indoor environment is through mechanical ventilation ducts. An experimental system was designed for the study of particle deposition on regular arrays of uniform elements (in the form of discrete protrusions) in a turbulent ventilation duct flow using monodisperse tracer small particles, in the range 0.7– 7.1 μm . The Reynolds number for the test conditions was 44,000 in the 150 mm square duct. Four different types of uniform roughness elements were tested. Compared to earlier measurements in the same duct system involving smooth or ribbed surfaces, a significant increase (up to 74 times) in deposition velocity onto the regular roughness elements is observed.

AEROSOL DEPOSITION IN TURBULENT CHANNEL FLOW ON A REGULAR ARRAY OF THREE-DIMENSIONAL ROUGHNESS ELEMENTS

Understanding particle deposition onto rough surfaces is important for many engineering and environmental applications. An experimental system was designed for the study of aerosol deposition on regular arrays of uniform elements (in the form of discrete protrusions) in a turbulent ventilation duct flow using monodisperse tracer small particles, in the range 0.7– 7.1 μ m . The Reynolds number for the test conditions was 44,000 in the 150 mm square duct. The roughness elements were arranged at two different orientations with respect to the airflow direction and the aerosol deposition velocity and pressure drop were measured for both orientations. Compared to earlier measurements in the same duct system involving smooth or ribbed surfaces, a significant increase in deposition velocity onto the regular roughness elements is observed. In addition, the associated pressure loss penalty is lower than in the presence of the roughness elements than in the presence of the ribbed surfaces. This may be attributable to the small dimensionless roughness height of the elements, which results only in a moderate distortion of the flow structure near the surfaces.