Modelling of volatile organic compounds emission from dry building materials

A numerical and an analytical model were developed to predict the volatile organic compound (VOC) emission rate from dry building materials. Both models consider the mass diffusion process within the material and the mass convection and diffusion processes in the boundary layer. All the parameters, the mass diffusion coefficient of the material, the material/air partition coefficient, and the mass transfer coefficient of the air can be either found in the literature or calculated using known principles.

The predictions of the models were validated at two levels: with experimental results from the specially designed test and with predictions made by a CFD model. The results indicated that there was generally good agreement between the model predictions, the experimental results, and the CFD results. The analytical and numerical models then were used to investigate the impact of air velocity on emission rates from dry building materials. Results showed that the impact of air velocity on the VOC emission rate increased as the VOC diffusion coefficient of the material increased. For the material with a diffusion coefficient >10⁻¹⁰ m²/s, the VOC emission rate increased as the velocity increased; air velocity had significant effect on the VOC emission. For the material with a VOC diffusion coefficient <10⁻¹⁰ m²/s, the VOC emission rate increased as the velocity increased only in the short-term; <24 h. In the medium to long-term time range, the VOC emission rate decreased slightly as the air velocity increased; velocity did not have much impact on these materials. Furthermore, the study also found that the VOC concentration distribution within the material; the VOC emission rate and the VOC concentration in the air were linearly proportional to the initial concentration. However, the normalized emitted mass was not a function of the initial concentration: it was a function of the properties of the VOC and the material.     

Gas-Phase Mass Transfer Model for Predicting Volatile Organic Compound (VOC) Emission Rates from Indoor Pollutant Sources

Abstract Analysis of the impact of sources on indoor pollutant concentrations and occupant exposure to indoor pollutants requires knowledge of the emission rates from the sources. Emission rates are often determined by chamber testing and the data from the chamber test are fitted to an empirical model. While the empirical models are useful, they do not provide information necessary to scale the chamber data to buildings nor do they provide information necessary to understand the processes controlling emissions. A mass transfer model for gas-phase-limited mass transfer is developed and described in this paper. Examples of sources with gas-phase-limited emissions are moth cakes, floor wax, stain, and varnish. The mass transfer model expresses the emission rate in terms of a mass transfer coefficient and a driving force. The mass transfer coefficient can be predicted from correlations of the Nusselt number and the Reynolds number. The experiments and data analysis used to develop the correlation are described in the paper. Experiments to verify the assumptions used to describe the driving force are also described. Suggestions for using data from existing empirical emission models to determine parameters for the mass transfer model are provided. The mass transfer model provides a significantly better fit to data from an indoor air quality test house than does the empirical first order decay model.     

Impact of Early Stage Incomplete Mixing on Estimating VOC Emissions in Small Test Chambers

Abstract Most of the existing emission models developed from small-scale chamber tests assume complete mixing in the chamber throughout the test period. This paper examined this assumption using a Computational Fluid Dynamics (CFD) model. The model simulated the three-dimensional air velocity profiles and Volatile Organic Compound (VOC) concentration distributions from wood stain in a well-designed mixing chamber of 1.0X0.8X0.5 m³. The model used measured data to determine the time-dependent VOC surface concentrations of wood stain. The CFD results show that the VOC concentrations in the test chamber were not uniform in the early stage (about 18 minutes). The first-order decay model using the complete mixing assumption will underestimate the Total VOC (TVOC) emission rates by 65% and 59% in the first 3 minutes and next 15 minutes, respectively. Since wood stain emitted about one third of the VOCs in the first 18 minutes, the impact of incomplete mixing in the early period is significant for calculating the material emissions. Furthermore, the mass transfer coefficient of TVOC calculated by CFD is also compared with that calculated by analogue theory and that calculated by experimental correlation.     

The impact of surface air movement on material emissions

The effects of surface air movement on material emissions were investigated experimentally. A field study was carried out to understand the characteristics of surface air movement in real rooms, and a velocity-controlled test chamber was designed and built, based on the field study results, to provide a uniform mean airflow and boundary layer condition over the test area. An extensive experimental study on the effects of air movement on material emissions was carried out, under different mean flow velocities and turbulence fluctuations, by using the small velocity-controlled test chamber. It was found that material emission rates are a function of the surface air flow conditions: as surface air velocity increases, contaminants from materials deplete faster, the turbulent fluctuation has a lesser effect on material emissions.     

确定建筑材料有害物散发量的三种传质模型

室内空气污染源散发量的确定是建立室内空气质量(IAQ)模型的重要步骤之一。目前研究建筑装饰材料和建筑涂料散发污染物的模型主要有经验模型和理论模型两类。经验模型简洁，但应用受到测试条件的限制，不具有普适性。基于传质理论而提出的理论模型目前研究较多，本文对研究建筑材料散发有害物的三个传质模型进行了评述。VB模型是一个简单的溶剂基室内涂料散发的总挥发性有机物(TVOC)的传质模型，低估了污染物的长期散发量。对流传质模型是基于界面平衡所导出的稳态模型，适用于固体和液体等材料的散发过程。Yang等提出的四层传质模型通常需与数值求解结合，计算较麻烦，但能较好地确定建筑材料散发挥发性有机物的散发量。     

Fundamental Mass Transfer Model for Indoor Air Emissions from Surface Coatings

Emissions from freshly applied paints and other coatings can cause elevated indoor concentrations of vapor-phase organics. Methods are needed to determine the emission rates over time for these products. Some success has been achieved using simple first-order decay models to evaluate data from small dynamic test chambers. While such empirical approaches may be useful for assessing the emission potmial of indoor sources, a more fundamental approach is needed to fully elucidate the relevant mass transfer processes. As a first step, a simple model based on boundary layer theory has been developed. In this model, the mass transfer rate is assumed to be controlled by the boundary layer mass transfer coefficient, the saturation vapor pressure of the material being emitted, and the mass of volatile material remaining in the source at any point in time. Static and dynamic chamber tests and test house experiments were conducted to obtain model validation data, Preliminary validaion results indicated that the model can be applied to different products with similar solvents. The model provides a better fit to chamber-derived emissions data than the empirical first-order decay model, especially over the decaying portion of the concentration vs. time curve     

Volatile organic compound emissions from latex paint--Part 1. Chamber experiments and source model development

Latex paints are widely used in residential and commercial indoor environments. The surface areas covered by the paints in these environments are relatively large. Thus, latex paints have the potential for having a major impact on indoor air quality (IAQ). A study was undertaken to develop methods for evaluating the impact of latex paint emission on IAQ. Small chamber experiments using stainless steel and painted and unpainted gypsum board substrates were conducted to determine the emission characteristics of latex paint. The emissions from the stainless steel were relatively short lived (3 to 4 days), whereas the emissions from gypsum board lasted for over 200 days. Because gypsum board is a common substrate for latex paint, all emission models were developed for the gypsum board substrates. The data from the small chamber tests led to the development of two empirical and two mass-transfer-based source emission models. Approximately 100 to 200 days of data were required to estimate the parameters required for the empirical models. Only 8 days of data were required to estimate the parameters for the mass-transfer-based models. The final models use paint formulation and mass transfer correlations to predict the emissions of the major individual volatile organic compounds emitted by latex paint.     

多孔材料污染物散发外部影响因素作用分析

基于所建的污染物散发模型,分析了室内温度、传质系数、换气次数对多孔材料污染物散发的影响。结果显示,室内温度对材料内污染物散发的影响非常明显,温度越高,污染物散发时间越短;提高传质系数不能显著加速污染物散发,而降低该值却可以抑制污染物的释放;换气次数对污染物散发影响很小,但可以通过改变换气次数来控制室内污染物浓度。     

The effect of surface roughness and emissivity on radiator output

The effect of altering the emissivity and the roughness of a wall behind a radiator on the radiator heat output has been studied experimentally and by using computational fluid dynamics.The results of a 3D RNG k–? turbulent model agree well with, and have the same trend as, the experimental results. The results indicate that the presence of large scale surface roughness and a high emissivity surface increases both the heat flow rate and the air velocity behind the radiator compared to a smooth shiny surface. The former increases the wall surface emissivity which causes the surface temperature of the wall to increase, effectively creating additional convective heat transfer surface. The surface roughness will increase both the surface area for heat transfer and the turbulent intensity which increase the mass transfer and free convective heat flux through the air gap.The results indicate that the heat transfer can be increased by about 26% through the use of a high emissivity saw-tooth surface compared to a smooth shiny one. This means that using a wall surface with high roughness and emissivity behind the radiator will increase the heat output from the radiator.     

P-dichlorobenzene emission rates from moth repellents and leakage rates from cloth storage cases

Emission rates of p-dichlorobenzene (pDCB) from five kinds of commercially available moth repellents and leakage rates from six kinds of commercially available cloth storage cases were measured. The emission rates from moth repellents immediately after purchase (E(max)), which were obtained as the slope of the total emission amount vs. time elapsed, were 0.0033 to 0.035 g/h at 25 degrees C. Moth repellents from different manufacturers differed greatly in emission rates at 0.0033, 0.0060, and 0.011 g/h, even if the moth repellent pieces were of similar shape and initial weight. This is thought to be because emission rates depended on the diffusion resistance of the repellent packing material used by each manufacturer. Therefore, the manufacturer-specific apparent mass transfer coefficients (K) were calculated for each moth repellent from the measured emission rates and a calculated saturated concentration. The apparent mass transfer coefficient K and the concentration gradient between the in-case (inside a cloth storage case) bulk air and the air at the surface of a moth repellent (pDCB tablet) were used for the calculation of the general emission rates. The leakage rates, defined as the air-exchange rate for a cloth storage case, from six cloth storage cases ranged from 0.41 to 78/h. Despite the large difference (more than 100 times) in the leakage rates of the cloth storage cases, the differences in emission rates of pDCB from these cloth storage cases to the indoor environment were small (<30%). This is because the concentration gradients between the in-case bulk air and the air at the surface of a repellent piece were almost the same among the storage cases. This suggests that pDCB emissions from cloth storage cases into the indoor environment should be reduced by making the storage cases more tightly sealable so that the leakage rate is <0.01/h. The results of adsorption tests of pDCB onto woollen clothes showed that the adsorption of pDCB on the clothes was assumed to be very strong and the transfer between the gas phase and the cloth phase in the cloth storage case was assumed to be small. PRACTICAL IMPLICATIONS: The use of pDCB provides benefits such as moth control and deodorizing, but on the other hand, poses potential risks to human health. There is a risk-benefit tradeoff, i.e. a tradeoff between risks of pDCB to human health and benefit of moth control effects. The pDCB concentration in the cloth storage case and in the indoor air can be estimated by using emission rates and the leakage rates obtained in this study. By comparing the concentration to the deterrent efficiency of pDCB, the risk for the use of pDCB moth repellents can be managed.     

Experimental study on the frost characteristics of fin tube heat exchanger

The heat and mass transfer characteristics under frosting on surface of heat exchanger were experimentally investigated in different conditions of air temperature, relative humidity, and face velocity. The heat transfer and heat transfer coefficient decreased faster with the high relative humidity, low air temperature and initial face velocity. The air pressure drop rose faster with the high relative humidity and low air velocity.     

Diffusion‐controlled reference material for VOC emissions testing: effect of temperature and humidity

A polymethylpentene film loaded with toluene is being developed as a reference material to support the reliable measurement of volatile organic compound emissions from building materials using environmental chambers. Earlier studies included the measurement of the material‐phase diffusion coefficient (D) and material/air partition coefficient (K) at 23°C. A fundamental mass‐transfer model can then be used to predict toluene emissions from the reference material at 23°C, serving as a reference for validating chamber‐measured emission profiles. In this study, the effect of temperature and humidity on performance of the reference material was investigated. Reference material emissions were measured at 10, 23, and 30°C and at different relative humidity (RH) levels. D and K at different temperatures and RH were determined using an independent method. Results showed that RH does not significantly affect D and K and had no effect on emissions. However, emissions increased substantially at elevated temperatures due to the relationship between D and temperature. A statistical analysis shows good agreement between model‐predicted and measured gas‐phase concentrations, indicating that the model can accurately predict emission profiles as a function of temperature. The reference material can therefore be applied to a wide range of emission chamber testing conditions.     

风致内外压下圆弧形落地大跨屋盖结构风荷载特性

对某墙面开洞的圆弧形落地大跨钢屋盖机场航站楼风荷载特性进行了风洞试验研究;基于计算流体力学软件FIUENT 6.3,采用RNG ?κ-ε?湍流模型对墙面开洞屋盖结构的内外表面平均风压系数分布、分区净体型系数、风速矢量以及风场流迹线等风荷载特性进行了系统研究,并将数值模拟结果与风洞试验结果进行比较分析。结果表明:数值模拟的净体型系数和平均风压系数分布规律与试验结果吻合良好;墙面洞口全开的情况下,由于迎风洞口与背风洞口处压力差的作用,屋盖内表面风压均表现为风吸力,风压分布亦受到洞口的影响;墙面洞口对屋盖上表面平均风压系数分布影响较小;屋盖迎风挑檐区域受到风荷载下顶上吸的叠加作用,最大净体型系数达-2.83。     

Influence of temperature on styrene emission from a vinyl ester resin thermoset composite material

Composite materials made with vinyl ester resins are lighter, stronger and corrosion resistant compared to most metals, and are increasingly being used as building materials and in public transportation. Styrene monomer is used as both a diluent and strengthener in the production of vinyl ester resin (VER) composites. Some researchers contend that free styrene in VER composites is available to diffuse out of the material into air, perhaps leading to adverse health effects via inhalation exposures in humans, yet there is no known data on styrene emissions from these materials in the literature. In this study, a typical VER composite made with resin containing 38% by weight styrene, reinforced with E-glass fiber and formed using a vacuum assisted resin transfer method was characterized for styrene emissions by environmental test chamber (ETC) methodology. Styrene concentrations in the ETC were measured over a temperature range of 10 to 50 °C. Initial evaporative styrene emissions increase with increasing temperature. There is a nearly linear relationship in the total mass of styrene emitted and emission factor as emissions increase with increasing temperature. Styrene emission factors appear to vary for different materials, which could indicate more complex processes or the influence of material physical properties on emission rates. These results can be used to validate and improve mass transfer emission models for the prediction of volatile organic compound concentrations in indoor environments.     

3D-CFD analysis of diffusion and emission of VOCs in a FLEC cavity

This study is performed as a part of research that examines the emission and diffusion characteristics of volatile organic compounds (VOCs) from indoor building materials. In this paper, the flow field and the emission field of VOCs from the surface of building materials in a Field and Laboratory Emission Cell (FLEC) cavity are examined by 3D Computational Fluid Dynamics (CFD) analysis. The flow field within the FLEC cavity is laminar. With a total flow of 250 ml/min, the air velocity near the test material surface ranges from 0.1 to 4.5 cm/s. Three types of emission from building materials are studied here: (i) emission phenomena controlled by internal diffusion, (ii) emission phenomena controlled by external diffusion, and (iii) emission phenomena controlled by mixed diffusion (internal + external diffusion). In the case of internal diffusion material, with respect to the concentration distribution in the cavity, the local VOC emission rate becomes uniform and the FLEC works well. However, in the case of evaporation type (external diffusion) material, or mixed type materials (internal + external diffusion) when the resistance to transporting VOCs in the material is small, the FLEC is not suitable for emission testing because of the thin FLEC cavity. In this case, the mean emission rate is restricted to a small value, since the VOC concentration in the cavity rises to the same value as the surface concentration through molecular diffusion within the thin cavity, and the concentration gradient normal to the surface becomes small. The diffusion field and emission rate depend on the cavity concentration and on the Loading Factor. That is, when the testing material surface in the cavity is partially sealed to decrease the Loading Factor, the emission rate become higher with the decrease in the exposed area of the testing material. PRACTICAL IMPLICATIONS: The flow field and diffusion field within the FLEC cavity are investigated by CFD method. After presenting a summary of the velocity distributed over the surface of test material and the emission properties of different type materials in FLEC, the paper pointed out that there is a bias in the airflow inside the FLEC cavity but do not influence the result of test emission rate, and the FLEC method is unsuitable for evaporation type materials in which the mass transfer of the surface controls the emission rate.     

流体流速对钛板板式换热器总换热系数的影响

利用实验数据对钛板板式换热器总换热系数与流速之间的关系式进行拟合,计算分析流速对总换热系数的影响。一级侧(热流体)或二次侧(冷流体)流速增大时,换热器总换热系数随之增大。设定一级侧流体流速不变,二级侧流体流速越高,通过增大流速强化传热的作用越弱。     

Predicting oxygen transfer of fine bubble diffused aeration systems--model issued from dimensional analysis

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.     

低气压条件下电加热器自然对流换热性能测试

从理论上分析了表面传热系数与气压的关系,实验测试了电加热器在低压与常压下的换热性能,得出以下结论:低压与常压下表面传热系数的比值与气压比值的0.2次幂成正比;电加热器在低压与常压下散热功率基本相等;表面温度随气压的降低而升高。     

Gas transfer from air diffusers

The bubble and surface volumetric mass transfer coefficients for oxygen, k(L)a(b) and k(L)a(s), are separately determined for 179 aeration tests, with diffuser depths ranging from 2.25 to 32 m, using the DeMoyer et al. 12003. Impact of bubble and free surface oxygen transfer on diffused aeration systems. Water Res 37, 1890-1904] mass transfer model. Two empirical characterization equations are developed for k(L)a(b) and k(L)a(s), correlating the coefficients to air flow, Qa, diffuser depth, hd, cross-sectional area, Acs, and volume, V. The characterization equations indicate that the bubble transfer coefficient, k(L)a(b), increases with increasing gas flow rate and depth, and decreases with increasing water volume. For fine bubble diffusers, k(L)a(b) is approximately six times greater than k(L)a(b) for coarse bubble diffusers. The surface transfer coefficient, k(L)A(s), increases with increasing gas flow rate and diffuser depth. The characterization equations make it possible to predict the gas transfer that will occur across bubble interfaces and across the free surface with a bubble plume at depths up to 32 m and with variable air discharge in deep tanks and reservoirs.     

Increase in Perceived Odor Emissions with Loading of Ventilation Filters

The development of odor emission rates from EU6 classified glass fiber bag filters was studied in four air-handling units (AHU), and emissions from the same kind of filters with EU3 classified polyester prefilters were studied in two units. The filters were loaded in six AHU in downtown Helsinki. The pressure drop was measured, and the odors of the filters were evaluated by a trained panel under laboratory conditions (T = 20°C, face velocity 1.0 m/s) every sixth week. The odor emissions of simultaneous atmospheric dust samples were also studied. The odor emissions of the filters rose during the first three months to a level where every third person would be dissatisfied. The emissions from coarse prefilters were similar to those from the more efficient filters without prefilters, and the emissions of the main filters were significantly lower if used with prefilters. This result indicates that the prefilters effectively protected the fine filters from odor-causing particles. The results of tests made with atmospheric samples agree with this result. Relative odor emissions were the highest in coarse fractions (> 10.0 μm). The pressure drop increased with the particle mass collected on the ventilation filter, but it did not correlate well with the odor emission of the filter. Thus, pressure drop alone is not an adequate criterion for changing supply air filters when hygienic aspects are a concern.