Estimation of sensible heat, water vapor, and CO2 fluxes using the flux-variance method.

This study investigated the flux-variance relationships of temperature, humidity, and CO(2), and examined the performance of using this method for predicting sensible heat (H), water vapor (LE), and CO(2) fluxes (F(CO2)) with eddy-covariance measured flux data at three different ecosystems: grassland, paddy rice field, and forest. The H and LE estimations were found to be in good agreement with the measurements over the three fields. The prediction accuracy of LE could be improved by around 15% if the predictions were obtained by the flux-variance method in conjunction with measured sensible heat fluxes. Moreover, the paddy rice field was found to be a special case where water vapor follows flux-variance relation better than heat does. However, the CO(2) flux predictions were found to vary from poor to fair among the three sites. This is attributed to the complicated CO(2) sources and sinks distribution. Our results also showed that heat and water vapor were transported with the same efficiency above the grassland and rice paddy. For the forest, heat was transported 20% more efficiently than evapotranspiration.

Numerical study of the effect of ventilation pattern on coarse, fine, and very fine particulate matter removal in partitioned indoor environment.

An indoor size-dependent particulate matter (PM) transport approach is developed to investigate coarse PM (PM10), fine PM (PM2.5), and very fine PM (PM1) removal behaviors in a ventilated partitioned indoor environment. The approach adopts the Eulerian large eddy simulation of turbulent flow and the Lagrangian particle trajectory tracking to solve the continuous airflow phase and the discrete particle phase, respectively. Model verification, including sensitivity tests of grid resolution and particle numbers, is conducted by comparison with the full-size experiments conducted previously. Good agreement with the measured mass concentrations is found. Numerical scenario simulations of the effect of ventilation patterns on PM removal are performed by using three common ventilation patterns (piston displacement, mixing, and cross-flow displacement ventilation) with a measured indoor PM10 profile in the Taipei metropolis as the initial condition. The temporal variations of suspended PM10, PM2.5, and PM1 mass concentrations and particle removal mechanisms are discussed. The simulated results show that for all the of the three ventilation patterns, PM2.5 and PM1 are much more difficult to remove than PM10. From the purpose of health protection for indoor occupants, it is not enough to only use the PM10 level as the indoor PM index. Indoor PM2.5 and PM1 levels should be also considered. Cross-flow displacement ventilation is more effective to remove all PM10, PM2.5, and PM1 than the other ventilation patterns. Displacement ventilation would result in more escaped particles and less deposited particles than mixing ventilation.

Quantitative prediction of traffic pollutant transmission into buildings.

An integrated air quality model that combines a CFD model and multi-room pollutant transport model has been developed to study the effect of traffic pollution on indoor air quality of a multi-room building located in close proximity to busy roads. The CFD model conducts the large eddy simulation of the three-dimensional turbulent flows and pollutant transport processes in outdoor, whereas the multi-room pollutant transport model performs zonal airflow and pollutant transport in indoor. The integrated model is verified with available field measurement of traffic-induced CO concentrations. Twelve scenarios of numerical experiments for various configurations of window openness are carried out to study the effects of the air change rate and the outdoor pollutant dispersion on indoor air quality. It is concluded that the windward side opening is a significant factor contributing to indoor air quality. Using air inlets on the sideward and leeward envelopes simultaneously can effectively lower the daily mean and peak indoor levels of traffic pollutants and maintain a desirable air change rate.

Numerical evaluation of the effect of traffic pollution on indoor air quality of a naturally ventilated building.

A computational fluid dynamics technique was used to evaluate the effect of traffic pollution on indoor air quality of a naturally ventilated building for various ventilation control strategies. The transport of street-level nonreactive pollutants emitted from motor vehicles through the indoor environment was simulated using the large eddy simulation (LES) of the turbulent flows and the pollutant transport equations. The numerical model developed herein was verified by available wind-tunnel measurements. Good agreement with the measured velocity and concentration data was found. Twelve sets of numerical scenario simulations for various roof- and side-vent openness and outdoor wind speeds were carried out. The effects of the air change rate, the indoor airflow pattern, and the external pollutant dispersion on indoor air quality were investigated. The control strategies of ventilation rates and paths for reducing incoming vehicle pollutants and maintaining a desirable air change rate are proposed to reduce the impact of outdoor traffic pollution during traffic rush hours. It was concluded that the windward side vent is a significant factor contributing to air change rate and indoor air quality. Air intakes on the leeward side of the building can effectively reduce the peak and average indoor concentration of traffic pollutants, but the corresponding air change rate is relatively low. Using the leeward cross-flow ventilation with the windward roof vent can effectively lower incoming vehicle pollutants and maintain a desirable air change rate during traffic rush hours.

Removal dynamics of airborne road dust in a ventilated airspace.

We derive a simple linear dynamic equation to describe the removal mechanisms of airborne road dust from a ventilated airspace. The dynamic equation is sufficiently to take into account the simultaneous removal effects of turbulent coagulation, turbulent diffusive deposition, gravitational sedimentation, and airflow pattern within a ventilated airspace. Three dimensionless parameters TC, TD, and GS that characterize the relative effects of turbulent coagulation, turbulent diffusive deposition and gravitational settling, respectively, in a ventilated airspace were introduced to generalize the removal dynamics of airborne road dust. An environmental chamber test was carried out not only to determine the particle size distributions but also to verify the removal dynamics of airborne road dust in a ventilated airspace. Our results demonstrate that there is no significant variation for particle size distributions of road dust obtained from urban and suburban areas in north Taiwan region and both followed a lognormal distribution with average geometric mean diameter of 1.08 +/- 0.02 microm and geometric standard deviation of 2.59+/-0.03. Measured values match the simulated values with an r2 value of 0.93, whereas the overall RMSE value of 2.36 +/- 1.05 mg m(-3) is low, indicating that the ability to predict the removal dynamics of airborne road dust within a ventilated airspace using an average particle size based linear equation. Effects of TC, TD, GS, and various ventilation systems on the time-dependent road dust concentrations are also justified.