CFD ANALYSIS OF CHEMICALLY REACTIVE POLLUTANTS IN 2D TEST ROOM

Recently, theoretical analyses have begun to evaluate some free radicals and other products that are generated by chemical reactions. Especially, the products of ozone / terpene reactions cause greater airway irritation in mice than would be predicted based on the known response of mice to ozone or terpenes. This paper presents an analysis of chemically reactive pollutants in indoor air. We have used CFD technique to simulate bimolecular reactions occurring indoors. In this research, the Second Order Rate Constant model which is a simple and fundamental model reproducing bimolecular Reaction is adopted. In order to demonstrate the performance of the Rate Constant models, CFD simulations are carried out for a 2D test room. Three sets of chemical compounds, ozone / terpene, are used in this study. The quantity of production substance Cprod through the chemical reaction is proportional to the concentration of source substances (Ozone and terpene). INDEX TERMS CFD, VOCs, Diffusion, Deposition, Chemical Reaction INTRODUCTION The concentration distribution of the chemical pollutants (e.g. VOC) in a room was analyzed and reported by modeling the diffusion within the building material and within the room air; this included the sorption on the building material surface [Murakami et al., 2001]. The results of a numerical prediction of the concentration distribution and time history of the emission rates were reported [Murakami et al., 2000]. These results were sufficiently consistent with the results from experiments in a chamber. However, in actual measurements of VOC concentrations in a room, the measured concentration and the values estimated from the emission flux from each material in the room and the ventilation rate are not necessarily consistent with each other, and the existence of a phenomenon known as "lost TVOC" has been confirmed [Wolkoff, 1995]. One of the causes of this is believed to be the generation and decomposition of VOC due to chemical reactions in the air. In particular, it has been confirmed that ozone [O3] in the room air actively generates various free radicals by reacting with the organic and inorganic compounds existing in the air. The free radicals and other products of reactions are often more irritating than their precursors. Regarding this background, simplified equations for the chemical reactions were modeled in this research, with the aim of establishing a method of estimating the concentration in the room that incorporates these phenomena. * Contact author email: ito@arch.t-kougei.ac. jp MODELING THE EQUATIONS FOR THE CHEMICAL REACTION Various chemical reactions are assumed to take place in room air. In particular, various free radicals exist as intermediate products from these chemical reactions; however, these free radicals are in unstable states, posing great difficulties in both qualitative and quantitative estimation. In addition, it imposes a considerable additional burden on the computational process to accurately compute the behavior of these free radicals. Therefore, the purpose of the modeling was to estimate the total concentrations of the final products. In this research, the Second Order Rate Constant model which is a simple model reproducing bimolecular Reaction is adopted [Charles et al., 2000]. Bi-molecular Reaction and Reaction Modeling using the Rate Constant A bi-molecular reaction is a chemical reaction involving multiple substances, and in order to numerically analyze this, it is necessary to solve multiple scalar equations corresponding to the number of reaction substances and products. The generation of unstable substances in a room during a bi-molecular reaction with highly reactive ozone and terpenes has been drawing attention. The simplest chemical reaction modeling uses the rate constant [Nazaroff et al., 1986]. Assuming the concentration of the reaction substance "A" and "B" at a point in space to be C1 (i,j,k) [ppb] and C2 (i,j,k) [ppb] respectively, the transportation of the reaction substances are expressed by equation (1)and (2), and the bi-molecular reaction by equation (3). A j j j j S x C D x x C U t C + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ 1 1 1 (1) B j j j j S x C D x x C U t C + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ 2 2 2 (2) 2 1 C C k S S b B A ⋅ ⋅ − = = (3) Here, S is the source term; kb is the second order rate constant [1/ppb h]. From equation (3), changes over time in the concentration of substance "A" and "B" due to the bi-molecular reaction are computed. In addition, assuming the concentration of the hypothetical products (all products) by reaction to be Cprod [ppb], the amount of change over time is expressed by equation (4) and (5). prod j prod j j prod j prod S x C D x x C U t C + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ (4) 2 1 C C k C k S b prod prod prod ⋅ ⋅ = ⋅ = (5) Here, kprod is the first order rate constant [1/h] of the hypothetical reaction product Cprod. EXISTING RESEARCH ON RATE CONSTANTS A lot of second order rate constants have been measured in research by Atkinson et al. [Atkinson et al., 1990]. Data on the second order rate constants for ozone at 296K (23°C) and terpenes have been collected for reactions that are believed to exist in the room air. Some of the results of measurements made by Atkinson et al., are shown in Table 1. In this research, the data shown in Table 1 were used to estimate the chemical reactions and concentration distribution in the room air. WALL SURFACE DEPOSITION FLUX MODELING OF OZONE This research accounted for ozone as one of the reaction substances in the room. In accounting for the phenomena of the transportation of ozone in a room, it is necessary to consider not only its transportation with ventilation, and chemical reaction, but also the phenomena of sorption by the solid walls. We proposed simple adsorption/desorption models for a sorptive material surface using the adsorption isotherms described in the previous report [Murakami et al., 2001]. In this report, a model of deposition using the deposition velocity vd [m/s] was adopted, taking into account the deposition of ozone on the solid wall surfaces [Seinfeld, 1985]. The deposition flux against a solid surface using the deposition velocity vd is expressed by the following equation: ( ) 0 − = Cs v Js d (6) Here, Js is the deposition flux. Cs is a reference concentration. The deposition velocity vd correspond to the convective mass transfer coefficient there the surface concentration assumed to be zero. According to Nazaroff et al., ozone deposition rates in general building materials are estimated to be from 2.5×10 to 7.5×10 [m/s] [Nazaroff et al., 1986]. OUTLINE OF THE NUMERICAL ANALYSIS Flow fields and diffusion fields were analyzed, targeting the room models used in the precise room model experiments [Ito et al., 2000]. An outline of the space to be analyzed is shown in Figure 1. When the air supply slot width is the representative length (L0=20mm), the space is a 2-dimensional room of 75 L0 (x)×50L0 (z) (=1500mm×1000mm). Flow fields were analyzed using the low Reynolds type k-e model [Murakami et al., 1996]. Numerical conditions are shown in Table 2. The bi-molecular reactions in the room were analyzed, assuming that stable and equal amounts of contaminant 1 (concentration C1), here assumed as ozone, and contaminant 2 (concentration C2) were generated from source 1 and source 2. Also, both the left and right walls were assumed to be contaminant 1, ozone, and deposition surfaces. The deposition velocity vd [m/s] was taken as the value for general building materials, which is 4.0×10 [m/s]. The room temperature was assumed to be a constant 25°C. Analysis cases are shown in Table 1. The amount of chemical pollutants generated from the contamination sources and the amount of supply opening velocity were assumed to be representative scales, and the results of the analyses were all Table 1 Cases Analyzed and Rate constant Table 2 Numerical Conditions C1 C2 kb [[1/ppb・h] vd [m/s] Turbulence model Low Re type k-emodel (MKC model) Case0 d-Limonene Mesh 68(x) × 64(z) Case1 d-Limonene 0.0184 Scheme Convection Term : Up wind (first order) Case2 α-terpinene 0.756 Case3 α-pinene 0.00756 Inflow condition Uin = 3.0m/s, kin =3/2・(Uin×0.015) 2 , ein =Cμkin/lin, Cμ=0.09, lin =L0/7, Case4 d-Limonene 0.0184 Outflow condition Uout , kout, eout = free slip Case5 α-terpinene 0.756 Re number U0L0/ν = 4200 Case6 Ozone α-pinene 0.00756 4.0×10 Wall treatment no slip 200 200 100 900mm inlet velocity Uin=3.0m/s adsorbent wall vd vd adsorbent wall