Experimental study of ventilation performance and contaminant distribution of underfloor ventilation systems vs. traditional ceiling-based ventilation system

Field measurement of ventilation rates

Ventilation rates have significant impacts on building energy use and indoor contaminant concentrations, making them key parameters in building performance. Ventilation rates have been measured in buildings for many decades, and there are mature measurement approaches available to researchers and others who need to know actual ventilation rates in buildings. Despite the fact that ventilation rates are critical in interpreting indoor concentration measurements, it is disconcerting how few Indoor Air Quality field studies measure ventilation rates or otherwise characterize the ventilation design of the study building(s). This paper summarizes parameters of interest in characterizing building ventilation, available methods for quantifying these parameters, and challenges in applying these methods to different types of buildings and ventilation systems. These parameters include whole-building air change rates, system outdoor air intake rates, and building infiltration rates. Tracer gas methods are reviewed as well as system airflow rate measurements using, for example, duct traverses. Several field studies of ventilation rates conducted over the past 75 years are described to highlight the approaches employed and the findings obtained.

Transport characteristics of expiratory droplets and droplet nuclei in indoor environments with different ventilation airflow patterns

Expiratory droplets and droplet nuclei can be pathogen carriers for airborne diseases. Their transport characteristics were studied in detail in two idealized floor-supply-type ventilation flow patterns: Unidirectional-upward and single-side-floor, using a multiphase numerical model. The model was validated by running interferometric Mie imaging experiments using test droplets with nonvolatile content, which formed droplet nuclei, ultimately, in a class-100 clean-room chamber. By comparing the droplet dispersion and removal characteristics with data of two other ceiling-supply ventilation systems collected from a previous work, deviations from the perfectly mixed ventilation condition were found to exist in various cases to different extent. The unidirectional-upward system was found to be more efficient in removing the smallest droplet nuclei (formed from 1.5 mum droplets) by air extraction, but it became less effective for larger droplets and droplet nuclei. Instead, the single-side-floor system was shown to be more favorable in removing these large droplets and droplet nuclei. In the single-side-floor system, the lateral overall dispersion coefficients for the small droplets and nuclei (initial size </=45 mum) were about an order of magnitude higher than those in the unidirectional-upward system. It indicated that bulk lateral airflow transport in the single-side-floor system was much stronger than the lateral dispersion mechanism induced mainly by air turbulence in the unidirectional-upward system. The time required for the droplets and droplet nuclei to be transported to the exhaust vent or deposition surfaces for removal varied with different ventilation flow patterns. Possible underestimation of exposure level existed if the perfectly mixed condition was assumed. For example, the weak lateral dispersion in the unidirectional ventilation systems made expiratory droplets and droplet nuclei stay at close distance to the source leading to highly nonuniform spatial distributions. The distance between the source and susceptible patients became an additional concern in exposure analysis. Relative significance of the air-extraction removal mechanism was studied. This can have impact to the performance evaluation of filtration and disinfection systems installed in the indoor environment. These findings revealed the need for further development in a risk-assessment model incorporating the effect of different ventilation systems on distributing expiratory droplets and droplet nuclei nonuniformly in various indoor spaces, such as buildings, aircraft cabins, trains, etc.