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Computational Fluid Dynamics (CFD) Modeling Indoor Chemical Reactions under Varied Lighting and HVAC Operation Conditions.

INTRODUCTION

People living in urban areas tend to spend about 90% of their time indoors (Klepeis et al. 2001). While living indoors, occupants can be exposed to multiple air pollutants. The contaminants are not only transported from outdoors, but also emitted from indoor sources such as consumer products, indoor coatings, adhesives, and sealants on indoor materials. Indoor pollutants are also produced by human activities such as cleaning, smoking, and cooking (Weschler 2016). Many gaseous contaminants such as VOCs emitted in buildings undergo chemical reactions with reactive gases such as ozone and radicals and generate reaction products. Some products can affect human health adversely. For example, VOCs react with hydroxyl radicals (OH radicals) generating products such as aldehyde which is a carcinogen (Finlayson-Pitts and Pitts. 1997). Also, alkene from chemical cleaner reacts with ozone producing skin and eye irritants (Colome et al. 1994). Although indoor chemical reactions play important roles in toxicity and distribution of air pollutants, there are few studies that examined formations of reactive gases and distributions of products and VOCs under varied building operating conditions.

OH radical is a strong reactive gas which influences in chemical reactions in buildings. OH radicals react with most of VOCs indoors and some products from the reactions can be toxic and carcinogenic (Atkinson and Arey. 2003). The main source of OH radicals is the photochemical reaction of HONO under sunlight outdoors. Indoor photochemical reaction associated with HONO has received far less attention because the intensity of solar radiation is smaller than outdoors. However, the recent study has revealed that when sunlight is incident indoors through windows, the OH radicals can be measured up to 0.07 ppt in a school classroom (Alvarez et al. 2013). Although the altitude of sunlight influences the OH radical formation, the concentration is on the same order of magnitude of outdoor OH levels.

The indoor chemical reactions are very sensitive to building operating conditions (i.e., ventilation and lighting). For example, the air flow rate and patterns are one of the main factors determining the pollutant distributions in the space. The solar radiation angle influences the generation of the reactive gas such as OH radical, and accordingly the concentrations of reaction products. Based on the backgrounds, the objective of this study is to (1) examine generations of OH radicals due to a HONO photochemical reaction under different light conditions and (2) examine spatial distributions of products from the chemical reaction between OH radicals and VOCs under varied conditions.

METHOD

A new CFD model framework is developed by using Star-CCM+ multi-component chemical reaction model to study photolysis of HONO as well as spatial distributions of VOCs and reaction byproducts under varied indoor environmental conditions.

The CFD model geometry and chemical reaction

The 30 [m.sup.3] (1,059 [ft.sup.3]) volume room is designed for the CFD simulation. The supply air is injected through the inlet diffuser located on a wall, the air is exhausted through the outlet on the ceiling. The room temperature and supply air temperature are 25 [degrees]C (77 [degrees]F). The model is iso-thermal model, so the solar radiation does not affect room temperature. Air exchange rate is 1 [h.sup.-1]. The source of HONO is the surface of the sphere. VOCs are emitted from wall, floor, and ceiling. Previous measurement campaigns show that the average concentration of HONO ranges from 4.3ppb to 19.5ppb (Zhou et al., 2018). In this study, average room concentrations of HONO and VOCs at the steady state condition without any chemical reactions are about 10 ppb and 100 ppb, respectively. The size of the window is 2 m x 4 m (6.6 ft x 13.1ft) and the solar radiation is incident on the room through the window. The zone under the dashed line on Figure 1 indicates solar radiation zone where HONO molecules are decomposed to OH radical and NO. The produced OH radicals react with VOCs emitted from indoor surfaces, generating secondary products. The chemical reaction equations and reaction rate (k) described below are applied to the CFD simulation. The reaction rate is the value at the room temperature (25 [degrees]C (77 [degrees]F)). The chemical reaction rate of OH radical with VOCs ([k.sub.OH*VOCs]) is the sum of each reaction rate between OH and over two hundred different kinds of gaseous organics indoors (Kowal et al. 2017 and Atkinson & Arey. 2003).

HONO + hv [right arrow] OH + NO [k.sub.HONO] = 1.26-[10.sup.-4] [s.sup.-1] (1)

OH + VOCs [right arrow] Products [K.sub.OH*VOCS] = 6.8-[10.sup.-13] [cm.sup.3][molecule.sup.-1][s.sup.-1] (2)

After calculating spatial distributions of species, the generation mechanism of OH radical in the solar radiation zone is observed. The average and breathing zone concentrations of HONO, OH, NO, VOCs, and products are measured to investigate the effects of chemical reactions on VOCs concentration reduction.

Standard k-[epsilon] model is utilized to simulate turbulence generated by supply air. The model is one of the common turbulence models, which uses kinetic energy [kappa] and turbulent dissipation [epsilon]. These two variables determine the energy and the scale of the turbulence (Launder et al,. 1974). The CFD model uses the following mass transport equation for species considering chemical reactions:

[[partial derivative]/[partial derivative]t]([rho][C.sub.i]) + [[partial derivative]/[partial derivative][x.sub.j]]([rho][u.sub.j][C.sub.i]) = [[partial derivative]/[partial derivative][x.sub.j]([rho][D.sub.i])[[partial derivative]/[C.sub.i]/[partial derivative][x.sub.j]]+[R.sub.i]+[S.sup.i] (3)

Where [??] is air density, [u.sub.j] is air velocity, [C.sub.i] is the mass fraction of chemical species, [R.sub.i] is the reaction source term, [D.sub.i] is the molecular diffusion coefficient, and [S.sub.i] is source term. Convection, diffusion, chemical reaction, and source generation influence in the concentration of species. In the species transport equation, chemical reaction source term is modeled as source term shown below:

[R.sub.i] = [M.sub.w,i][[N.sub.r].summation over (r=1)][[??].sub.i,r] (4)

Where [N.sub.r] is the number of chemical species in reaction r, [M.sub.w,i] is the molecular weight of ith species and [R.sub.i,r] is the molar rate of creation and destruction of ith species in reaction r. The net source of chemical reactions is calculated as the sum of species which is generated and reduced by different chemical reactions at each cell in CFD model (Uherek E. 2004).

The photochemical reaction is sensitive to ventilation and lighting conditions. Therefore, the different CFD simulations are conducted under different environmental conditions. The parameters are solar radiation incidence angle, air diffuser position, air exchange rate, and chemical reaction rate between OH radicals and VOCs. As changing the above parameters, spatial distributions and average concentrations of different species are analyzed. When air conditioning system is off, air exchange rates vary from 0.5h-1 to 1h-1 depending on temperature difference between indoor and outdoor (Wallace et al. 2002). At the condition of operating air conditioning, the air exchange rates increase about 4 to 5 h-1 in residential buildings. Thus, air exchange rates changes from 0.5[h.sup.-1] to 5[h.sup.-1]. Two representative indoor ventilation conditions (displacement and mixing ventilation) is applied. To investigate the effects of the amount of the solar radiation indoors on the formation of OH radicals, olar incidence angles varies from 0 to 60 degree. When the solar incidence angle is 0 degree, the solar lay is incident in the whole room which represent the glass room or skylight room. The chemical reaction rates between OH radical and VOCs change 1,000 [ppb.sup.-1][h.sup.-1] to 10,000 [ppb.sup.-1][h.sup.-1].

RESULTS

Spatial distribution of OH radicals, HONO, Products

Figure 2 indicates that spatial distributions and concentrations of species. The solar radiation zone is the under dashed line. Emitted HONO from the sphere is transported by airflow indoors. When it moves into the solar radiation zone, HONO molecules starts to be decomposed by solar radiation into OH radicals and NO. Figure 2c shows that the NO is generated from the photolysis of HONO in the solar radiation zone and diffused to the ambient air. As the results, the concentration of NO is the highest in the solar radiation zone. Figure 2b indicates concentrations of OH radicals. The same yields of OH radicals are produced with the NO by photolysis of HONO. However, OH radicals disappear by reacting with VOCs right after the generation in the solar radiation zone. The average concentration of OH radical is 0.004 ppt in solar radiation zone and the value of non-solar radiation zone is almost zero indicating blue as shown in Figure 2b before it diffuses to the ambient zone. Figure 2e shows that the concentration of products is the peak in the solar radiation zone and it is transported to the ambient zone.

The 1.74 ppb HONO out of 9.7 ppb is decomposed into OH radicals and NO. The same amount of secondary products are generated from the chemical reaction between VOCs and OH radicals. Because the chemical reaction coefficient between OH radical and VOCs ([k.sub.OH-VOCs]) is much larger than the value of photolysis of HONO ([k.sub.HONO-hv]), all produced OH radicals are reacted with VOCs. Although the concentration of generated secondary products is 1.74 ppb which is less than the concentration VOCs (100 ppb), the chemical species can affect human health adversely (Finlayson-Pitts and Pitts 1997).

Effects of solar incidence angles

The photochemical reaction is sensitive to the amount of light. Figure 3 indicates three CFD simulation models which change the solar incidence angles (0, 30, and 60 degrees). As the solar altitude angle decreases, the more amount of solar radiation penetrates into the room. Thus, in the 0-degree case, about 5 times more OH radicals are produced compared to the 60-degree case. Also, the average concentration of products in the 0-degree case is 2.96 ppb, which is about 5 times the value of the 60-degree case.

In the 0-degree case, because photolysis of HONO occurs in the whole room, right after generation of HONO, OH radical is produced from the source and diffused to the ambient air. Unlike the 0-degree case, the photochemical reaction occurs in the solar radiation zone as shown in Figure 3b and 3c. As emitted HONO is transported and decomposed in the solar radiation zone, and the zone becomes the origin of OH radicals.

Effects of air exchange rate

Regardless of air exchange rates, the concentration of HONO and VOCs are 10ppb and 100ppb, respectively. It means that the 0.5 and 5 times mass fluxes of HONO and VOCs are emitted on the surfaces in 0.5 [h.sup.-1] and 5 [h.sup.-1] cases compared to the 1 [h.sup.-1] case. Figure 4 shows concentration variations of OH radical, HONO, and products under different air exchange rate. Concentrations of products inversely proportion to air exchange rates. When it comes to the chemical reaction between VOCs and OH radical, because the concentrations of VOCs are 30 to 70 times higher than the concentration of OH radicals, the generated products are influenced not by the concentration of VOCs but by the concentration of OH radical. Although the volume fraction of produced OH radicals at the 1[h.sup.-1] case is the same as the value of 5 [h.sup.-1] case, OH radicals in the 5 [h.sup.-1] case are diluted by 5 times more outdoor air. Therefore, in the 5 [h.sup.-1] case, the concentration of OH radical before reacting with VOCs is 5 times lower compared to the 1 [h.sup.-1] case, and the concentration products show also 5 times lower values.

Effects of chemical reaction rate between OH radical and VOCs

In the baseline case, the chemical reaction rate between OH radicals and VOCs is set as 10,000 [ppb.sup.-1][h.sup.-1]. According to ambient air temperature and the composition of VOCs, the chemical reaction rate can be changed. As the reaction rate ([K.sub.OH,VOC]) changes from 10,000 [ppb.sup.-1][h.sup.-1] to 1,000 [ppb.sup.-1][h.sup.-1], the concentration OH radical which is the reactant in the equation (2) is ten times less than the baseline case. However, the average room concentrations and spatial distributions of other species are the same as the baseline case. The order of chemical reaction rate ([K.sub.OH,VOC]) is higher than the HONO decomposition rate([K.sub.HONO]). Thus, all OH radicals generated by photolysis of HONO are consumed by reacting with VOCs in the lower chemical reaction rate case, 1000 [ppb.sup.-1][h.sup.-1]. It is evident that the average concentrations and spatial distribution of species are influenced not by the VOC-OH chemical reaction rate ([K.sub.OH,VOC]) but by HONO decomposition rate([K.sub.HONO]).

CONCLUSION

This study investigates photochemical reaction and spatial distributions and concentrations of reaction products under varied lighting and ventilation conditions by using CFD simulations. HONO emitted from indoor combustion source is decomposed into OH radical and NO in the solar radiation zone of the room. The produced OH radical quickly reacts with VOCs and generate reaction products in the room. The products concentrations are relatively high in the solar radiation zone than the bulk air of the room.

The amount of OH radical influences mainly the concentration of reaction products. The concentration of reaction products shows higher values with a larger solar radiation at a lower air exchange rate due to greater OH radical formations. However, chemical reaction rate between OH radicals and VOCs marginally influence the spatial distribution and concentration of species.

ACKNOWLEDGMENTS

This project is funded by Alfred P. Sloan Fuoundation as MOdeling Consortium for Chemistry of Indoor Environments (MOCIEE).
NOMENCLATURE

[??]        =   density
[u.sub.j]   =   air velocity
[C.sub.i]   =   mass fraction of chemical species
[R.sub.i]   =   reaction source term
[D.sub.i]   =   molecular diffusion coefficient
[N.sub.r]   =   number of chemical species in reaction r
M           =   molecular weight


REFERENCES

Alvarez, E. G., Amedro, D., Afif, C., Gligorovski, S., Schoemaecker, C., Fittschen and Wortham, H. 2013. Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid. Proceedings of the National Academy of Sciences, 110(33), 13294-13299.

Atkinson, R., & Arey, J. 2003. Atmospheric degradation of volatile organic compounds. Chemical reviews, 103(12), 4605-4638.

Colome, S., McCunney, R. J., and Samet, J. M. 1994. Indoor air pollution: an introduction for health professionals. In Indoor air pollution: an introduction for health professionals. EPA.

Finlayson-Pitts, B. J., and Pitts, J. N. 1997. Tropospheric air pollution: ozone, airborne toxics, polycyclic aromatic hydrocarbons, and particles. Science, 276(5315), 1045-1051.

Kowal, S. F., Allen, S. R., & Kahan, T. F. 2017. Wavelength-Resolved Photon Fluxes of Indoor Light Sources: Implications for HO x Production. Environmental science & technology, 51(18), 10423-10430.

Klepeis, N. E., Nelson, W. C., Ott, W. R., Robinson, J. P., Tsang, A. M., Switzer, P., ... & Engelmann, W. H. 2001. The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants. Journal of Exposure Science and Environmental Epidemiology, 11(3), 231.

Launder, B. E., & Sharma, B. I. 1974. Application of the energy-dissipation model of turbulence to the calculation of flow near a spinning disc. Letters in heat and mass transfer, 1(2), 131-137.

Wallace, L. A., Emmerich, S. J., and Howard-Reed, C. 2002. Continuous measurements of air change rates in an occupied house for 1 year: the effect of temperature, wind, fans, and windows. Journal of Exposure Science and Environmental Epidemiology, 12(4), 296.

Weschler, C. J. 2016. Roles of the human occupant in indoor chemistry. Indoor Air, 26(1), 6-24.

Zhou, S., Young, C. J., VandenBoer, T. C., Kowal, S. F., and Kahan, T. F. 2018. Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home. Environmental science & technology.

Youngbo Won

Student Member ASHRAE

Donghyun Rim, PhD

Associate Member ASHRAE
Table 1. Parametric analysis for photolysis chemical reactions by solar
radiation

                          Solar        Chemical      ACH     Diffuser
                          Incidence    Reaction      (h-1)   Position
                          Angle        Rate
                                       (ppb-1h-1)

Case1   Baseline model    30[dagger]   10,000        1       upper
Case2   Lower diffuser    30[dagger]   10,000        1       lower
Case3   ACH: 0.5          30[dagger]   10,000        0.5     upper
Case4   ACH: 5            30[dagger]   10,000        5       upper
Case5   Solar angle 0o     0[dagger]   10,000        1       upper
Case6   Solar angle 30o   60[dagger]   10,000        1       upper
Case7   KOH,VOC           30[dagger]   1,000         1       upper

Table 2. Summary of simulation results

Case  Inlet     ACH          solar        Concentration (ppb)

      Position  ([h.sup-1])  Angle        HONO   NO     OH
1     upper     1            30[degrees]  7.98   1.67   1.61x[10.sup.-6]
2     lower     1            30[degrees]  7.55   1.35   1.68x[10.sup.-6]
3     upper     0.5          30[degrees]  6.76   2.61   1.13x[10.sup.-6]
4     upper     5            30[degrees]  9.     0.43   1.32x[10.sup.-6]
5     upper     1            0[degrees]   6.69   2.96   3.14x[10.sup.-6]
6     upper     1            60[degrees]  9.03   0.62   0.58x[10.sup.-6]
7     upper     1            30[degrees]  7.98   1.67   1.61x[10.sup.-7]

Case    Concentration (ppb)

        VOCs      Products
1        98.25    1.67
2        89.87    1.35
3        95.40    2.61
4       105.7     0.43
5        95.8     2.96
6        99.29    0.62
7        98.25    1.67
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Author:Won, Youngbo; Rim, Donghyun
Publication:ASHRAE Transactions
Date:Jan 1, 2019
Words:2937
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