## 1 Introduction  
Metal cutting fluids are classified into three main categories: straight oil (insoluble), water-based, and synthetic cutting fluids (grinding fluids). Among the various cutting oils, water-based cutting fluids, which are mainly composed of water, have always played an important role in metal processing and are widely used. They overcome the risks associated with straight-cutting oils and provide excellent cooling effects [1]. However, water-based cutting fluids may decay due to the breeding of numerous bacteria or fungi that may decompose the additives in the cutting fluids, causing the cutting fluid to fail and even endangering the health of workers [2]. They absorb nutrients such as hydrocarbons, SO\({}_{4}\) ions, and sulfur compounds from soluble cutting oil, forming compounds such as hydrogen sulfide, thus generating odors [3]. This consequently results in the discoloration and complex odor of water-based cutting fluids because of the growth of microorganisms and putrefying bacteria, which significantly reduce cutting performance, prevent rust, and deteriorate the environment for workers [4]. Therefore, water-based cutting fluids are replaced frequently [5,6]. Exposure to water-based cutting fluids has been reported to cause health problems among workers; the methodologies used to assess the health risks from its exposure were discussed in the 2002 National Safety Council report. According to the previous investigations, it has been reported that exposed workers had elevated respiratory and digestive issues, along with cough and phlegm. Several case studies have indicated that occupational asthma can be caused by odor exposure to the fluids [7]. Ozone is a strong oxidizing agent that efficiently sterilizes microorganisms [8]. Recently, ozone treatment technologies have been developed to purify polluted water [9; 10]. Compared with chlorine, ozone is a very effective chemical species that readily oxidizes organic matter, pesticides, diverse microbes, and chemical residuals at low concentrations and contact times. Moreover, it rapidly disintegrates into oxygen molecules in water without creating secondary contaminants [5]. Therefore, ozone treatment is an appropriate technique for sterilizing water-based cutting fluids [5]. However, the ozone treatment methods used in industry suffer from technical and economic limitations, such as high installation costs, high energy dissipation, inconsistent treatment efficiency, and the need for a large space. To overcome these disadvantages, ozone generators have been studied in various industrial fields [11]. Recently, corona discharge and UV radiation have been used in many studies to generate ozone [12]. Corona discharge has the disadvantages of high energy consumption and a bulky power supply. UV irradiation is inefficient in generating ozone in terms of energy consumption. To overcome these drawbacks, a compact air-cooled coaxial dielectric barrier discharge (DBD) was developed. An alternating current (AC) high-voltage power supply to the electrodes in the DBD system provides a strong electric field between the electrodes, and atmospheric air is broken down to fire the discharge. The proposed system, with its compact structure, features a low energy density and low manufacturing cost, making it easy to generate high concentrations of ozone and install anywhere else. In addition, this system uses the air generated in the blower as the discharge and cooling gases rather than compressed air or oxygen gas, which decreases the running cost. In this system, the most significant reaction for ozone production is as follows [13; 14; 15; 16]:  
\[\text{O}+\text{O}_{2}+\text{M}\rightarrow\text{O}_{3}+\text{M} \tag{1}\]  
where M represents the nitrogen and oxygen molecules in air as third collision partners. Ozone is produced in a compact air-cooled coaxial DBD via a three-body reaction, as shown in Equation (1). A general disadvantage of DBDs is that the temperature of the high-voltage electrode is increased by Joule heating, and the insulator is easily damaged.