In this study, we investigated the atomization characteristics of rapid internal mixing injector (RIM injector) developed in our laboratory. RIM injector successfully emulsifies base fuel without any surfactant just before fuel injection. The diameter of droplet discharged from RIM injector was evaluated based on processing of shadowgraph images. It was found that Sauter mean diameter (SMD) of droplet is determined by the gas to liquid ratio (GLR) and viscosity of emulsified fuel. The increasing GLR decreases SMD value. As water content ratio is increased, the inner structure of droplet changes to W/O type emulsion. The emulsification increases its viscosity, which deteriorates the atomization characteristics. We proposed an empirical formula as functions of GLR and Reynolds number reproducing the deterioration resulting from increasing viscosi ty. The formula successfully predicts the SMD variation with respect to GLR and water content ratio. Finally, we examined th e ef fect of atomization air ratio on NOx and PM em issions. The quantity of atomization air significantly influences the PM emission because the increasing air improves the mixing of fuel vapor with combustion air. 1 I n t r o d u c t i o n In recent decades, combustion technology based on water emulsification has been studied [1]. Micro-explosion is characteristic phenomena that flash evaporation of water particles dispersed in fuel droplet generates fine fuel droplet s, which promotes the evaporation of fuel as a result of the incre ase in total surface area of droplets [2]. In addition, the micro-explosion also promotes the mixing of fuel vapor and combustion air, resulting in the reduction of soot emission [3] [4]. On the other hand, water contained in emulsified fuel decreases flame temperature of emulsified fuel combustion. This is due to latent heat, and dilution of combustible mixture with water vap or. The lower flame temperature reduces NOx emission [5]. In contrast, it was reported in the previous studies that emuls ified fuel has some drawbacks. First, the emulsification needs expensive surfactant to mix oil with water. Second, although th e surfactant is doped to emulsified fuel, the quality of the emulsified fuel temporally deteriorates [6]. Third, excessive water in emulsified fuel often induces unstable combustion due to heat sink resulting from latent heat [7]. To overcome the drawbacks, we have developed a fuel injector (Rapid Internal mixing injector, RIM injector) incorporating a small mixing chamber [8]. Water, fuel and atomizing air are separately introduced into the RIM injector, and then the three fluid are mixed in the mixing chamber, resulting in emulsification of fue l with water. The emulsified fuel made is discharged though holes drilled on the mixing chamber into a furnace. By using our injector, we can make emulsified fuel without any surfactant ju st before combustion in a furnace, and adjust its water content in response to thermal condition in furnaces. In our previous study [8], we have investigated the droplets discharged from RIM injector, and demonstrated the successful e m u l s i f i c a t i o n i n t h e w i d e r a n g e o f w a t e r c o n t e n t . E m i s s i o n s from the laboratory-scale furnace with our RIM injector has bee n also examined, which proved the simultaneous reduction of NOx and soot emission as when using conventional emulsified fuels. The aim of this study is to investigate the detail of atomizati on characteristics of RIM injector when high viscosity fuel such a s vegetable oil is used. We measured Sauter mean diameter (SMD) of droplets discharged from RIM injector using the processing of spray images captured by high-speed shadowgraph method, and investigated the effects of gas to liquid ratio (GLR) and viscosity on SMD. From these results, we proposed an empirical formula predicting SMD based on GLR and Reynolds number reproducing effect of viscosity. Finally, the effects of atomiz ing air ratio on NOx and PM emissions were examined by combustion test using a laboratory-scale furnace. 2 Experimental setup Figures 1(a) and 1(b) shows cross-sectional and exploded-view diagrams, respectively, of RIM injector used in the experiments . The injector consists of a main body, a swirler, a mixing chamb er, and a cover clamping the mixing chamber onto the main body, all of which are made of brass. The length of the mixing chambe r is 33 mm, and the inner diameter is 12.6 mm. Figure 1: (a) Cross-sectional and (b) exploded-view schematics of proposed fuel–water RIM injector Corresponding author. Yuzuru Nada E-mail address: ynada@tokushima-u.ac.jp The base fuel, water, and atomizing air are respectively introduced into the mixing chamber through a coaxial, a central channel, and a four-slit swirler, generating rotational flow wi thin the chamber. The introduced fuel and water are mixed by the swirling air, resulting in the emulsification of the fuel withi n the chamber. The emulsified fuel is subsequently discharged through eight holes of 1.0 mm in diameter arranged in a circular patter n in the upper section of the chamber. Each injection hole is inclined by 45° relative to the vertical direction. In this study, we investigated Sauter mean diameters (SMDs) and inner structure of droplets under cold conditions. In addit ion, we measured NOx and soot emission during hot combustion tests. The experimental apparatus used in this study are same a s those in our previous study [8]. Here, we will briefly introduc e the apparatus used. The SMDs of droplets in sprays discharged from the injector were evaluated using shadowgraph images. Figure 2 shows a schematic of the shadowgraph system, which consists of a spray chamber and an injector as well as a nano- spark photography unit [9] and a digital camera (Nikon D7000). The chamber has two optical windows composed of silicate glass on opposing lateral sides to allow parallel light emitted by a light source to pass through and illuminate the spray in the chamber for short durations of 30 ns. The spatial resolution of shadowgraph images is 4.1 µm/pixel. The diameters of droplets were evaluated through the image processing based on the luminosity gradient around the droplets. Figure 2: Experimental apparatus for droplet size measurements using shadowgraph method Figure 3: NO x and soot emission measurements To examine the fuel emulsification resulting from the mixing in the chamber, the internal structure of the droplets discharged from the injector was investigated using an immersion method. During the droplet sampling trials, blue coloring agents were added to the water in the injector to allow for the easy identification of water and oil droplets in the microscopic ima ges. The combustion test was carried out on the laboratory-scale furnace. Figure 3 shows a schematic of the combustion test apparatus, which consists of a combustion furnace and an injector as well as a NO x and so o t m ea su rem en t sy stem . R I M injector was mounted on the center of a flame holder. Secondary air flows into the holder through holes on the bottom. A probe was installed from the exit of the combustion furnace to sample flue gases. NOx concentration was measured by a gas analyzer (Horiba, PG-250). PM in the flue gas was collected on a piece o f filter paper, and the weight of the PM on the paper was determined using an electronic scale. During the assessment of the spray and emulsification characteristics, the mass flow rate of base fuel M f w a s v a r i e d from 0.48 to 1.61 kg/h while atomizing gas flow rates Ma were set to 3.94 kg/h and 5.40 kg/h. The accuracies of the rates wer e respectively ± 0.02 kg/h in fuel mass flow, and ± 0.07 kg/h in atomizing gas. Soybean oil was used as fuel, and nitrogen was the atomizing gas rather than air for safety reasons. In the pr esent study, the extent of water introduction is represented by the w ater content ratio Y w = Mw/(Mf + Mw), where Mw is mass flow rate of water. During the experimental trials, this ratio was varied fr om 0 to 0.7 with a maximum error of ±2.0%. Gas to liquid ratio (GLR), which is used as a measure of atomization, is defined as GLR = M a/(Mf + Mw). During the measurements of NO x and soot emissions, the total air flow rate Mt (sum of atomizing air and secondary air) was maintained at 15.42 kg/h. The atomizing gas flow rate Ma were set to 3.94 kg/h ( Ma / Mt = 0.26), and 5.40 kg/h ( Ma / Mt = 0.35). The fuel mass flow rates were 1.13 kg/h and 1.31 kg/h, which correspond to an equivalence ratio  of 0.9, and 1.05, respectively. The rich condition simulates the combustion conditions in a conventional furnace operating under high-load conditions. 3 Results and discussion 3.1 Spray characteristics of RIM injector Figure 4 shows probability profiles of droplet diameter evaluat ed by the shadowgraph method. During the measurements, GLR was set to a constant of 2.47. Figure 4: Probability profiles of droplet diameter at condition s with various water content ratios When pure fuel was only applied ( Y w = 0), the probability shows the peak value at a diameter of approximately 25 m. However, there are some huge droplets with a diameter beyond 100 m even though its probability is much lower. In our previous stud y [8], to investigate the mechanism of the huge droplets formatio n, we observed flow pattern appearing in the mixing chamber using a high-speed camera. The flow pattern in the mixing chamber is classified as annular flows that the liquid film flows on the s ide 16 14 12 10 8 6 4 2 0Probability (%) 140 120 100 80 60 40 20 0 Droplet diameter ( m) Yw = 0.0 Yw = 0.3 Yw = 0.5 Yw = 0.7GLR = 2.47 wall of the chamber. When thick film reaches the hole of the chamber, the huge droplets is discharged because the quantity o f the liquid increases. As the water content ratio increases, probabilities of droplets with diameters over 40 m increase. Figure 5 shows the variation of SMD of droplet with water content ratio. At Yw = 0, SMD value is approximately 80 m, which is greater than the mode value shown in Fig. 4. This is d ue to the appearance of the huge droplets. With increasing water content ratio, SMD value increases, and subsequently reaches 130m at Y w = 0.5. However, a further increase in water content ratio decreases SMD value at Yw = 0.7. Figure 5: V ariation of SMD with water content ratio The trend of the SMD variation shown in Fig. 4 and Fig. 5 has been already reported in our previous study [8], in which it wa s found that the increase in SMD with water content ratio in the range of 0 < Y w < 0.5 results from changing inner structure of droplets. Figure 6 shows the droplet sampled by immersion method. At Yw = 0, we can observe transparent droplets in this image, which corresponds to pure fuel droplets. On the other hand, emulsified fuel containing small water particles appears at Y w = 0.5, which demonstrates that the structure of droplets discharged is classified as W/O type emulsified fuel. Figure 6: Microscopic images of emulsified fuel droplets sampled by immersion method at (a) Yw = 0, (b) Yw = 0.5 Yaron and Gal-Or [10] have proposed complicated expression representing the variation of viscosity with water content rati o. Assuming that continuous phase is soybean oil, we can estimate the viscosity of emulsified fuel using their expression. Figure 7 shows the variation of the viscosity with respect to water cont ent r a t i o . T h e v i s c o s i t y o f e m u l s i f i e d f u e l i n c r e a s e s a s t h e w a t e r content ratio is increased. Li et al. [11] discussed the effect of viscosity on the primary atomization, and concluded that high viscosity result in the production of much larger droplets. Fro m these results demonstrated in the previous studies, the increas e i n S M D v a l u e s h o w n i n F i g . 5 i s d u e t o t h e i n c r e a s e i n t h e viscosity resulting from the changing structure from pure fuel to emulsified fuel. In the range of Y w > 0.5, SMD value decreases with the increase in water content ratio. Under this condition, our injector incompletely emulsifies base fuel because of large amount of water. Therefore, pure water with lower viscosity remains in th e mixing chamber, and it is subsequently discharged as smaller droplets. The existence of small droplets decreases SMD value shown in Fig.5. Figure 7: V ariation of viscosity with water content ratio T o inv est ig ate ef f ec ts o f G L R o n S MD v alu e, w e v ar ie d G L R while keeping water content ratio constant. Figure 8 shows the probability profile of droplet diameter at Y w = 0 . 3 . A s G L R increases from 2.47 to 3.35, the probability of smaller droplet s increases. Figure 9 shows the relation between GLR and SMD value under conditions with different water content ratio. The result shown in this figure includes those shown in Fig. 5. At each water content ratio, SMD value decreases with GLR. Figure 8: Probability profiles of droplet diameter at condition s with different GLR Figure 9: V ariation of SMD with GLR In the previous study [12], the effects of atomizing air veloci ty, surface tension, and viscosity are expressed using empirical formula. In this study, we focus on viscosity (see Fig. 5) and GLR (Fig. 9) to express the SMD value of droplets discharged from our injector. The effect of viscosity is represented using Reynolds number based on the estimated viscosity (see Fig. 7) and velocity of atomizing air. The empirical formula proposed i n this study is shown as follow; 𝑆𝑀𝐷 ൌ 2421൫𝐺𝐿𝑅 ሺ𝑅𝑒 ଶሻ൯ି଴.ଶଵ ( 1 ) In above equation, unit of SMD is m. Figure 10 shows the 160 140 120 100 80 60 40 20 0SMD (m) 1.0 0.8 0.6 0.4 0.2 0.0 Water content ratio 600 500 400 300 200 100 0Kinematic viscosity (mm2/s) 1.0 0.8 0.6 0.4 0.2 0.0 Water content ratio 16 14 12 10 8 6 4 2 0Probability (%) 140 120 100 80 60 40 20 0 Droplet diameter ( m) GLR = 2.47 GLR = 3.35Yw = 0.3 160 140 120 100 80 60 40 20 0SMD (m) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 GLR Yw = 0.0 Yw = 0.3 Yw = 0.5 Yw = 0.7 prediction of SMD value using Eq.1, in which plots are experimental results shown in Fig. 5 and Fig. 9, and solid line is prediction obtained from Eq.1. The SMD predicted shows a good agreement with experimental results, which demonstrates tha t v isc o sity and G L R a re do m ina nt p ar am et er s f o r t he S MD value of droplets discharged from our injector. Figure 10: Prediction of SMD value using Eq. 1 3.2 Emissions Figure 11 shows the NOx emission index (EINOx) with respect to the water content ratio under conditions with equivalence ra tio  = 0 . 9 . T h e E I N O x values were estimated according to a definition proposed by Turns and Myhr [13]. As shown in Fig. 11, NOx emission is reduced by water addition. This is likely d ue to lower flame temperatures resulting from the latent heat of t he added water [5] and the pure dilution effect from the increase in the concentration of water vapor [14] and the increase in OH radical [15]. Although M a/Mt is increased, which corresponds to conditions shown in Fig. 8, NOx emission is kept a constant. Figure 11: NO x emission as a function of the water content ratio Figure 12: Mass concentrations of PM as functions of the water content ratio Figure 12 shows the PM emission with respect to water content ratio. PM concentration significantly decreases with increasing water content ratio. This is attributed to the improved mixing of fuel vapor and combustion air. Nazha and Crookes [3] reported that the high-equivalence-ratio region that is present in emulsified fuel spray disappears as a result of the improved mixing of fuel vapor with combustion air, which suppresses soot formation. Tsue et al. [4] also reported that the enhanced mixi ng caused by the puffing behavior decreases the volume of the sooting region. When M a/Mt is increased, PM emission is significantly reduced. This is because the high velocity of atomizing air improves the mixing of fuel vapor with combustion air. 4 Conclusions In this study, we investigated the atomization characteristics of our RIM injector enabling the emulsification of a base fuel without any surfactant. The mean diameter of droplets discharged from RIM injector is determined by the GLR and Reynolds number. The Reynolds number represents the variation of viscosity resulting from the change of inner structure of droplet from pure fuel to W/O type emulsified fuel. A simple expression based on GLR and Reynolds number is proposed, and successfully predicts SMD value of droplets. Finally, we e x a m i n e d t h e e f f e c t o f a t o m i z i n g a i r r a t i o o n N O x a n d P M emissions. Under the conditions during experimental trials, the increasing atomizing air significantly reduces the PM emissions because of the improved mixing of fuel vapor with combustion air.