Baoqing Deng

A unified conjugate mass transfer model of VOC emission

This paper develops a unified conjugate mass transfer model for VOC (Volatile Organic Compound) emission, which implies conjugate boundary condition for mass transfer at the material-air interface. Thus, no special treatment is needed at the material-air interface and numerical methods for conjugate heat transfer problem can be applied directly. The material-air partition coefficient has been taken into account and its effect is the same as specific heat in the energy equation. The equivalent diffusion coefficient in the material KmaDm instead of Dm characterizes the rate of mass transfer. The ratioKmaDm/Da indicates whether VOC emission is controlled by the internal diffusion or not. The equivalent air-phase initial concentration C0/Kma determines the order of maximum concentration in the air. VOC emission contains two stages: the initial stage and the pseudo-steady stage when the emission rate nearly equals mass rate through the outlet of the air. Diffusion coefficient of VOC in the material has a significant effect on VOC emission in the two stages. The effect of partition coefficient on VOC emission depends on the value of KmaDm/Da.

Transient Characteristics of VOCs Removal by an Air Cleaner in Association with a Humidifier Combined with Different Ventilation Strategies in an Office

This paper investigated the effects of an air cleaner and ventilation strategies on the removal of VOCs in an office in association with the influence of moisture on the air cleaner performance. Here, a transient three dimensional air flow field and concentration distribution of VOCs emitted from a desk were considered. Commercial software FLUENT 6 was utilised to solve the continuity and momentum equations together with the low Reynolds number k-ε turbulence model. Numerical results showed that both the moisture and ventilation strategies have notable impacts on the VOCs concentration distribution. Cases under stronger ventilation take less time for the VOCs concentration to reach the steady state. But, in the cases with the function of the air cleaner, VOCs could be removed more considerably. The combination of the use of the air cleaner and enhanced ventilation strategies could yield more desirable indoor air quality within a relatively short time. The results showed that moisture can have a slight effect on the performance of the air cleaner, but lower moisture would be helpful to the effectiveness of the air cleaner. Thus, based on this work, suitable concentrations of indoor moisture, together with appropriate ventilation strategies have been recommended for control of indoor air quality.

Analytical Solution for Spatially Dependent Solute Transport in Streams with Storage Zone

The transient storage model with space-variable coefficients is solved by using the generalized integral transform technique (GITT) coupled with Laplace transform method. The spatially dependent flow velocity and solute dispersion are considered in the governing equations of the transient storage model. The correctness and accuracy of the derived analytical solution is verified by comparison with the analytical solution for constant flow velocity and the dispersion coefficient. Some illustrative examples are presented to demonstrate the application of the derived analytical solution. The increasing flow velocity and solute dispersion cause the faster transport of solute. Moreover, the faster the flow velocity and solute dispersion increase, the faster the solute transports.

Simulations to Determine the Disinfection Efficiency of Supplementary UV Light Devices in a Ventilated Hospital Isolation Room

A three-dimensional simulation on biological pollutant concentrations in a hospital isolation room with ultraviolet (UV) light devices has been carried out using computational fluid dynamics software, Fluent 6.3. The effects of the positions of the UV lamps, light intensity, and ventilation rates on the microorganism removal efficiency were examined by this study. The UV light intensity in the air flow domain was obtained with the modified P-1 model. Numerical results showed that the airflow could be very complicated in the room. Two different arrangements of the UV lamps were considered and the microorganism removal efficiency with the UV devices set on the wall was shown to be higher than that set in the corner of the room. The microorganism removal efficiency was shown to increase with an increase in the light intensity. Ventilation could enhance the microorganism removal efficiency, but would reduce the disinfection efficiency.A three-dimensional simulation on biological pollutant concentrations in a hospital isolation room with ultraviolet (UV) light devices has been carried out using computational fluid dynamics software, Fluent 6.3. The effects of the positions of the UV lamps, light intensity, and ventilation rates on the microorganism removal efficiency were examined by this study. The UV light intensity in the air flow domain was obtained with the modified P-1 model. Numerical results showed that the airflow could be very complicated in the room. Two different arrangements of the UV lamps were considered and the microorganism removal efficiency with the UV devices set on the wall was shown to be higher than that set in the corner of the room. The microorganism removal efficiency was shown to increase with an increase in the light intensity. Ventilation could enhance the microorganism removal efficiency, but would reduce the disinfection efficiency.

Three dimensional Eulerian-Eulerian simulation on hydrodynamics in dissolved air flotation tank with different turbulence models

The Eulerian-Eulerian approach is used to simulate the two phase flow of air and water in a dissolved air flotation (DAF) tank. Unsteady simulation is adopted to obtain the quasi-steady result. The two dimensional model cannot capture the three dimensional feature of flow pattern in the tank. The three dimensional model reproduces the stratification structure observed in the experiment and obtains the satisfactory volume fraction of air bubbles in the separation zone. To account for the turbulence effect, four two-equation turbulence models are evaluated. Although all turbulence models capture the stratification structure in the separation zone, the re-normalization group theory (RNG) k-ɛ model obtains the best agreement with the experimental data. The effect of bubble diameter on the performance of DAF tank is simulated. Small air bubbles can form a big range of high volume fraction of air bubbles, which helps to remove the suspended matters.

CFD simulation of an unbaffled stirred tank reactor driven by a magnetic rod: assessment of turbulence models.

A simulation of an unbaffled stirred tank reactor driven by a magnetic stirring rod was carried out in a moving reference frame. The free surface of unbaffled stirred tank was captured by Euler-Euler model coupled with the volume of fluid (VOF) method. The re-normalization group (RNG) k-ɛ model, large eddy simulation (LES) model and detached eddy simulation (DES) model were evaluated for simulating the flow field in the stirred tank. All turbulence models can reproduce the tangential velocity in an unbaffled stirred tank with a rotational speed of 150 rpm, 250 rpm and 400 rpm, respectively. Radial velocity is underpredicted by the three models. LES model and RNG k-ɛ model predict the better tangential velocity and axial velocity, respectively. RNG k-ɛ model is recommended for the simulation of the flow in an unbaffled stirred tank with magnetic rod due to its computational effort.

Use of P-1 model with the additional source term for numerical simulation of ultraviolet radiation in a photoreactor

Radiation distribution in a photoreactor with multiple mediums is solved by the P-1 model. The UV lamp is included in the computational domain. An additional source term method is presented to describe the generation of UV light, which can avoid the use of analytical lamp emission model at the lamp surface. The boundary condition for incident radiation at the semi-transparent wall between adjacent mediums has been derived to link the P-1 equations in adjacent mediums, which enables the present model to adapt to the photoreactor with complicated structure. The predicted incident radiation agrees well with the experimental data in literature. The effects of absorption coefficient, scattering coefficient and phase function of lamp plasma on the radiation distribution are discussed in detail.

Modeling volatile organic compounds sorption on dry building materials using double-exponential model

A double-exponential surface sink model for VOCs sorption on building materials is presented. Here, the diffusion of VOCs in the material is neglected and the material is viewed as a surface sink. The VOCs concentration in the air adjacent to the material surface is introduced and assumed to always maintain equilibrium with the materialphase concentration. It is assumed that the sorption can be described by mass transfer between the room air and the air adjacent to the material surface. The mass transfer coefficient is evaluated from the empirical correlation, and the equilibrium constant can be obtained by linear fitting to the experimental data. The present model is validated through experiments in small and large test chambers. The predicted results accord well with the experimental data in both the adsorption stage and desorption stage. The model avoids the ambiguity of model constants found in other surface sink models and is easy to scale up.

Analytical Solution for Solute Transport in Semi-Infinite Heterogeneous Porous Media

Solute transport in porous media concerns advection, dispersion, sorption, and reaction. Since porous media is commonly heterogeneous, the properties of porous media are spatially and temporally variable. In this paper, one dimensional unsteady solute transport in semi-infinite heterogeneous porous media is investigated. Both linear and nonlinear decay is considered. Analytical solution is obtained for linear decay with spatially and temporally diffusion coefficient and velocity by using generalized integral transform technique. The inverse integral transforms are developed for the problems in semi-infinite space based on some weighted functions. Some examples are given to show the application of the method and analytical solutions.