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Effects of Airflow on VOC Emissions from ‘Wet’ Coating Materials: Experimental Measurements and Numerical Simulation,” ASHRAE Transactions, 107(1), 801-811

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Effects of Airflow on VOC Emissions from ‘Wet’ Coating Materials: Experimental Measurements and Numerical Simulation,” ASHRAE Transactions, 107(1), 801-811 Yang, X., Chen, Q., Zeng, J., Zhang, J.S., Nong, G., and Shaw, C.Y. 2001. “Effects of airflow on VOC emissions from ‘wet’ coating materials: experimental measurements and numerical simulation,” ASHRAE Transactions, 107(1), 801-811. EFFECTS OF AIRFLOW ON VOC EMISSIONS FROM “WET” COATING MATERIALS: EXPERIMENTAL MEASUREMENTS AND NUMERICAL SIMULATION Xudong Yang, Ph.D. Member ASHRAE1 Qingyan Chen, Ph.D. Member ASHRAE Jie Zeng, Ph.D. Jianshun Zhang, Ph.D. Member ASHRAE Gang Nong, M.S. Chia-Yu Shaw, Ph.D., P.E. Member ASHRAE ABSTRACT The impact of airflow on volatile organic compound (VOC) emissions from “wet” materials has long been noticed. However, a comprehensive mass transfer model that can predict such impact has not been reported. The objective of this research was to fill that gap. First we measured the VOC emissions of “wet” coating materials (a decane and a wood stain) using a small-scale (0.4m3) and a full-scale (55m3) environmental chamber under different airflow conditions. A numerical model was then developed to numerically simulate the “wet” material emissions. The model considers VOC mass transfer in the air and material-air interface, diffusion in the material film, and also diffusion in the substrate. The results from experimental measurements indicate that local airflow has impacts on “wet” material emissions, especially during the early stage. The numerical model developed can predict emissions under different flow conditions with reasonable accuracy. Numerical simulations have further confirmed that the emissions from “wet” materials applied to an absorptive substrate are dominated by evaporation at the beginning and internal diffusion afterwards, which had been speculated based on previous experimental data. The numerical model has the potential to simulate “wet” material emissions in actual building environments based solely on the small-scale chamber data. Key Words: VOC, Material Emission, Air flow, Measurement, Small-Scale Chamber, Full- Scale Chamber, Numerical Simulation, Modeling INTRODUCTION The impact of local airflow (velocity and turbulence) on volatile organic compound (VOC) emissions from “wet” coating materials has long been noticed and discussed by several researchers (e.g., Zhang et al. 1996a; Haghighat et al. 1994; Sparks et al. 1996; Haghighat and 1 Xudong Yang is an Assistant Professor in the Department of Civil, Architectural, and Environmental Engineering at the University of Miami, Coral Gables, FL. He was formally a research assistant of the Building Technology Program, Massachusetts Institute of Technology, Cambridge, MA. Qingyan Chen is an Associate Professor of the Building Technology Program, Massachusetts Institute of Technology, Cambridge, MA. Jianshun Zhang is an Associate Professor in the Department of Mechanical, Aerospace, and Manufacturing Engineering at Syracuse University, Syracuse, NY. Jie Zeng is a Postdoctoral Fellow, Gang Nong is a Technical Officer, and Chia-Yu Shaw is a Senior Research Officer of the Indoor Environment Program, Institute for Research in Construction, National Research Council of Canada, Ottawa. 1 De Bellis 1998). Understanding such impact allows the scale-up of the emission data measured in an inexpensive small-scale chamber to buildings. The difference of airflow between a small- scale chamber and a building is obvious. A small-scale chamber usually has simple flow boundary conditions and laminar flow patterns, while conditions in a building (e.g., uncontrolled turbulent flow) can be more complicated and significantly different from the test conditions. Two main approaches are available for investigating the impact of airflow on material emissions: experimental measurements and computer simulations. The experimental approach uses environmental test chambers to measure VOC emissions under controlled indoor conditions (ASTM 1990). Previous experimental studies have indicated that the emission process of a “wet” material appears to have three phases (Zhang et al. 1999). The first phase corresponds to a period shortly after the material is applied but still relatively wet. It appears that VOC emissions are related to evaporation at the surface of the material. In the second phase the material dries down. The emissions transit from an evaporation-dominant phase to an internal-diffusion controlled one. Then in the third phase the material becomes relatively dry and the VOC off-gassing rate becomes small. The dominant emission mechanism in this phase is believed to be the internal diffusion of VOCs through the substrate surface (e.g., Chang and Guo 1992; Wilkes et al. 1996; Zhang et al. 1996). Further, Chang et al. (1997) and Zeng et al. (1999) both found that the substrates can significantly affect the emission rates and patterns of “wet” coating materials. Hence, the emissions of “wet” materials are likely to depend on many factors, including environmental conditions (temperature, air velocity, turbulence, humidity), properties of the material itself, and the substrate. Since the emission characteristics of wet materials are affected by many external and internal factors, emission data from environmental chambers may not be used directly for buildings. It would also be too difficult to investigate the emissions under actual building conditions by experiments. A feasible way would be to develop computer models to simulate the emission processes and interpret the data from inexpensive small-scale chamber tests for applications in buildings. Currently, there is a lack of a complete emission model that considers both the internal diffusion and the interaction between the material and its environment. Much progress has been made in developing mass transfer models for the evaporation period. Guo and Tichenor (1992) developed an evaporative mass transfer model, the Vapor pressure and Boundary layer (VB) model for VOC emissions from interior architectural coatings. The VB model has been widely used for simulating the early stage emissions of “wet” materials. However, it may not apply to the entire emission process as the model neglects the internal diffusion in the materials film and the effect of substrate. Further, the VB model requires the gas phase mass transfer coefficient as an input. Gas phase mass transfer coefficient is usually determined by the flow field, such as air velocity, flow direction, and turbulence. Sparks et al. (1996) developed a correlation of the gas phase mass transfer coefficient with the flow Reynolds number (Re) and the VOC Schmidt number (Sc). However, the correlation may not be applicable to buildings in which the flow conditions usually cannot be represented by a single Reynolds number. Yang et al. (1997, 1998a) and Topp et al. (1997) numerically simulated the dynamic mass transfer coefficient under a small or full-scale environmental chamber. Despite such progresses, not much work has been done on the material side. Limited work other than the VB model either assumes a constant surface concentration (Topp et al. 1997), or uses curve fitting to obtain the time-dependent surface concentration from the measured chamber concentration data (Yang et al. 1998a). These models are either not accurate or not practical in simulating the emission process. 2 The objective of this research was to study quantitatively the effects of an important environmental factor – airflow, on VOC emissions from “wet” coating materials. First, we conducted experiments using a 0.4 m3 test chamber and a 55 m3 full-scale chamber that simulates a real room. The data were further used to develop and validate a numerical model. This numerical model applies to the entire emission phases of “wet” materials. The use of this model requires the thermodynamic properties of compounds or materials. These properties can be inferred from inexpensive headspace analysis and small-scale chamber data. Hence, the model has the potential to simulate “wet” material emissions in actual building environments based solely on the small-scale chamber data. EXPERIMENTAL The purposes of our experiments to measure material emissions were two folds: (1) to investigate the effects of air velocity, turbulence, and chamber scale on emission characteristics of "wet" materials directly and (2) to provide emission data for developing and validating material emission models. Testing Materials Two “wet” coating materials, a commercial oil-based wood stain and a single compound (decane) were tested for emissions. Wood stain is a common architectural coating material with well-known emissions characteristics (Chang and Guo 1992; Sparks et al. 1996; Yang et al. 1998a; Zhang et al. 1999), although different wood stains may have different compositions and exhibit different emission characteristics. A headspace analysis was conducted on the wood stain to determine the compounds emitted. In this procedure, a sample of wood stain was placed in a small airtight glass container. Samples of the air inside the container (i.e., headspace) were then analyzed by gas chromatography and mass spectrometry (GC/MS). The headspace analysis indicated that the wood stain contains five major compounds: nonane, decane, undecane, dodecane, and octane. The single compound decane was also tested in that its emission characteristics can be compared with those of the wood stain as reference data. To test material emissions under a real scenario, the materials were applied on an oak substrate of 0.06 m2 (0.24m × 0.25m). Testing Facility To order to generate different types of aiflow patterns, both a small-scale and a full-scale environmental chamber were used. The small-scale chamber (Zhang et al. 1996a) consists of an inner and an outer chamber, both made of stainless steel (Figure 1). The outer chamber is 1.0 m long × 0.8 m wide × 0.5 m high (0.4 m3). The material was placed in the inner chamber, where the local velocity was controlled by a stainless tube-axial fan. The full-scale chamber (Zhang et al. 1996b) is 5 m long × 4 m wide × 2.75 m high (55 m3) as shown in Figure 2. A square air diffuser was located at the center of the ceiling and an exhaust opening was at the corner of the ceiling. The air supply from the diffuser flowed along the ceiling and then mixed with the chamber air.
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