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22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Gas-liquid two-phase chemical reaction model of reactive plasma inside a bubble for water treatment T. Hayashi1, S. Uehara2, H. Takana2 and H. Nishiyama2 1 Graduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan 2 Institute of Fluid Science, Tohoku University, Sendai, Miyagi, Japan Abstract: Numerical simulation of chemical reactions inside a bubble is conducted. The model is developed based on the experiment. The numerical model is a zero-dimensional model in order to conduct simulations of a repetitive discharge for relatively long duration. OH produced in a gas phase is transported into liquid phase by a diffusion trough the bubble interface. The concentrations of chemical species in gas- liquid phases are obtained. Keywords: chemical reaction model, water purification, discharge inside a bubble 1. Introduction Water pollution is urgent issue to be solved recently. Ozone has been widely utilized as for water purification systems since ozone has high oxidation potential and long lifetime. However, the oxidation potential of ozone is not strong enough to decompose persistent organic substances such as dioxin and acetic acid completely. Recent years, it has been paid attention to advanced oxidation processes (AOPs) using non-thermal plasma [1-2]. Hydroxyl (OH) and oxygen (O) radicals generated by non-thermal plasma have stronger oxidation potentials than that of ozone. Due to the short lifetime of these radicals, initiating plasma in water or near water surface is effective method to decompose the pollutant by radicals. One of the effective methods is discharge inside bubbles injected in water [3]. The advantage of this method is that it requires less energy to initiate the discharge than the direct discharge in water since the discharges can occur easily in a gas phase. Another positive aspect is that the method does not release by-products to the environment. Decomposition of acetic acid by OH radical generated by nano-second pulse discharge inside bubbles is reported in the previous experiments [4]. For further improvements of the decomposition performance, it is essential to clarify the radical generation process by the discharge inside bubbles and obtain the physicochemical factors which increase the OH radical generation in water. In this paper, a gas-liquid two-phase chemical reaction model of numerical simulation for discharged bubble was originally developed to provide the fundamental data for the improvement of the water treatment process. This model includes the chemical reactions in gas and liquid phases and the diffusion through the bubble interface. The numerical simulations were conducted under various conditions based on the experiments [4]. P-I-3-27 2. Gas-liquid two-phase numerical model of chemical reaction inside a bubble The schematic image of developed zero-dimensional two phase (gas and liquid) model of radical generation is shown in Fig. 1. Fig. 1. Schematic of the chemical reaction model in a Ar gas bubble. In the model, a domain where a streamer progressing along a bubble interface is assumed to be a domain as gas phase [5]. The reaction of O 2 is assumed to be able to neglect since the operating gas is Argon gas. After second pulse is applied, oxygen is generated by the chemical reaction which originate from H 2 O and OH. However, in order to operate the calculation longer to clarify the physicochemical characteristics of OH when the pulse is applied repetitively, chemical reaction of O 2 is not considered. Chemical model is developed based on the one proposed by Matsui et al. [6]. Chemical species are diffused into liquid phase through the interface of bubble based on the Henry’s law with Henry’s constant H i for each species. The governing equation is as follows: 1 dC gas ,i dt dC liq ,i dt = −k mtWL C gas ,i + k mtWL = k mt C gas ,i − k mt 1 C liq ,i + G gas ,i − L gas ,i H i RTg 1 C liq ,i + Gliq ,i − Lliq ,i H i RTg (1) (2) where, C i , and R, T g denote a concentration of species i, gas constant and a temperature, respectively. k mt is the mass transport coefficient which is determined by the bubble form. G and L denote the term which related to a generation and a loss of species, respectively. The temperature (295 K) and pressure (1 atm) is constant both in liquid and gas phases. W L , the ratio of liquid volume to the bubble volume is taking into account for the effect of the volume difference. Applied electric potential is obtained by the numerical iterative calculation using the value from the previous experiment [4]. Firstly, the simulations for a single nano-pulse discharge were conducted. The influences of vapor concentrations and applied voltages on the time evolution and the diffusion fluxes of chemical species were evaluated. Secondly, the simulations for repetitive nanopulse discharges were conducted. OH radical in liquid phase which transported from a gas phase by a diffusion trough the bubble interface was investigated. 3. Results and discussion 3.1. Single discharge inside a bubble Fig. 2 shows the production rates of OH in a gas phase. OH is generated rapidly by H 2 O dissociation through the electron impact during the discharge in a gas phase. The OH concentration decays rapidly with the generation of H 2 O 2 and HO 2 . Fig. 3. Time evolution of OH, H 2 O 2 , HO 2 and O 2 in liquid phase when applying single pulse. 3.2. Repetitive discharges inside a bubble Fig. 4 shows the time evolution of OH, H 2 O 2 , HO 2 and O 2 in gas phase at V = 5 kV, f = 2000 Hz until 18 μs when the bubble is dispatched [4]. The electron increases from 10-9 mol/l to 10-5 mol/l and changes periodically under discharge. OH, H 2 O 2 , HO 2 change periodically with production and decay corresponding to the electron behaviour. However, O 2 increases gradually due to the neglect of chemical reaction with electron. O2 H2O2 OH HO2 electron Fig. 4. Time evolution of electron OH, H 2 O 2 , HO 2 and O 2 in gas phase when applying repetitive pulse. Fig. 5 shows the time evolution of same chemical species in liquid phase. The concentration of OH in liquid phase decreases 10-4 down compared with that in gas phase. All chemical species increases gradually with time. Fig. 2. Time evolution of production rates of OH in a gas phase when applying single pulse. Fig. 3 shows the concentrations of chemical species in liquid phase. OH increases gradually with respect to time by the diffusion from gas phase trough the bubble interface. However, the final concentration of OH in liquid phase is 10-4 times lower than the one produced in a gas phase during the discharge. O2 H2O2 OH HO2 Fig. 5. Time evolution of OH, H 2 O 2 , HO 2 and O 2 in liquid phase when applying repetitive pulse. 2 P-I-3-27 4. Conclusions Numerical simulation of chemical reaction inside a bubble in water is conducted with using the value obtained in experiment. 5. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 26249015. 6. References [1] T. Shibata, et al. J. Phys. D: Appl. Phys., 47, 105203 (2014) [2] T. Shibata, et al. Plasma Chem. Plasma Process., 34, 1331-1343 (2014) [3] H. Nishiyama, et al. J. Fluid Sci. Technol., 8, 65-74 (2013) [4] H. Nishiyama, et al. Plasma Chem. Plasma Process., online (2015) [5] W. Tian, et al. J. Phys. D: Appl. Phys., 47, 055202 (2014) [6] Y. Matsui, et al. Plasma Sources Sci. Technol., 20, 034015 (201) P-I-3-27 3