The fuel-water internally rapid mixing type of injec tor has been developed to reduce NOx and soot emis- sions from combustion furnaces operating under high -load conditions. The injector allows spray injectio n of water emulsified fuel originating from base fuel an d water without any surfactants. The aim of present study is to elucidate the mechanism of emulsification occurring in the injector and the atomization characteristic s of the injector. We measured the sizes of fuel droplets di scharged from the injector by means of a high-speed shadow- graph method combined with image processing. Soybea n oil was used as the base fuel. The flow patterns o f the fuel and water in a transparent mixing chamber of t he injector were also visualized. In addition, we i nvestigated the inner structure of the large droplets sampled b y an immersion droplet sampling method. The base fu el, water and air are separately introduced into the injector . In the mixing chamber of the injector, fuel and w ater are blown by swirling air, and then impinge on the inne r wall of the chamber. The base fuel is emulsified t hrough the mixing of fuel with water resulting from the im pingement. The emulsified fuel moves to injection ho les along the inner wall, and is finally discharged thr ough the injection holes with atomizing air. The pro bability profiles of droplet size exhibit that the existence probabilities of coarse droplets with diameters gr eater than ap- proximately 35 µm are increased when the fuel is emulsified. Althou gh the emulsification deteriorates the atomi- zation capability of the injector, the secondary at omization including the micro-explosion occurring i n combus- tion furnaces would form fine droplets, and thus re ducing the soot emission from the furnaces. The micr oscope observations revealed that the emulsified fuel fill ing in a large droplet sampled corresponds to W/O t ype. Keywords: Fuel-water internally rapid mixing inject or, Emulsification, Atomization, Combustion, Soot em ission Introduction Over the last several decades, combustion researche rs have made effort to reduce NOx and soot emission s from combustion furnaces. The combustion of emulsif ied fuel consisting of base fuel and water doped wi th surfactants, is a combustion technology able to sim ultaneously reduce the emissions [1]. When water em ulsified fuel is injected into hot gases, the flash evaporat ion of the water dispersed in the fuel droplets cau ses the secondary atomization, namely micro-explosion [2-4] and puffing [4, 5], generating a large number of f ine droplets [6], and increasing the combustion efficie ncy under conditions of optimum water contents [7, 8]. The soot emission from the emulsion combustion is mainl y reduced by the enhanced mixing of fuel vapor with combustion air resulting from the flash evaporation [7, 9, 10]. Furthermore, OH radical originating fr om water dispersed in emulsified fuel oxidizes both soot and its precursors, leading to the soot reduction [10, 11]. The introduction of water into the flame via emulsified fuel injection also reduces the NOx emission [12], which is due to the pure dilution effect [13] and the chemic al dilution effect associated with H 2O [14]. Our research group has developed a fuel-water inter nally rapid mixing injector which has some advantag es regarding fuel emulsifications [15]. Base fuel, wat er, and atomizing air are separately introduced int o the injector, and then the base fuel is emulsified with the water in the mixing chamber of the injector. Because the injector allows the emulsification without surfactants, the other emulsification equipment as well as surfactan t is not required. Kajitani and Sawa [16] reported the aging deterioration of emulsified fuel, that is, increas e in the Sauter mean diameter of water droplets dispersed in base f uel due to its coalescence. The deterioration incre ases the viscosity of the emulsified fuel, which probably de grades the NOx and soot emissions. In contrast, the base fuel introduced into our injector is emulsified just bef ore the injection into furnaces, and hence it is po ssible to supply flesh emulsified fuels into furnaces via our inject or. In addition, the water content can be changed i n response to combustion situation during furnace operation. Spad accini and Pelmas [8] reported that the combustion efficiency has a peak value at an optimum water con tent of emulsified fuel, and the use of emulsified fuel with excessive water content destabilizes the flame. The adjustment of water content by our injector could c ontribute to stable operations of combustion furnaces. In our previous study [15], it was demonstrated tha t the injector allows simultaneous reductions of NO x and soot emissions from furnaces fueled with gas oil an d soybean oil. However, the characteristics of fuel spray discharged from the injector, including the droplet diameter and the inner structure of droplets, have not been 13th ICLASS 2015 Spray Characteristics of Fuel-Water In ternally Rapid Mixing Injector for Burner Combustio n 2 elucidated. The aim of the present study is to clar ify the spray characteristics of emulsified fuel di scharged from the injector, as well as the effects of the water c ontent of emulsified fuel on NOx and soot emissions under high- load conditions. Experimental Setup Figure 1 shows a schematic cross-sectional diagram of the fuel-water internally rapid mixing injector. The injector is composed of a body having some flow cha nnels and a co-axial slit, swirler, and mixing cham ber with injection holes. The length of the injector is 107 mm. The water is introduced into the mixing chamber through the central channel while the base fuel is through the co-axial slit. The atomizing air flows into the chamber through some channels and the swirler providing a r otational flow in the chamber. The fuel and water i ntroduced are blown by the swirling air, and then the base fu el is emulsified by the mixing of the fuel with the water. The emulsified fuel is discharged through eight holes w ith a diameter of 1.0 mm, circularly arranged in th e upper section of the chamber. The direction of each injec tion hole inclines by 45 degree to the vertical dir ection. We evaluated Sauter mean diameter (SMD) of droplets in sprays discharged from the injector by using shadowgraph images. Figure 2 shows schematics of ex perimental apparatus consisting of a spray chamber, injec- tor, nano spark photography unit [17], and digital camera. The spray chamber has a cuboid shape with a height of 190 mm, and a square cross section of 180 mm × 1 80 mm. The injector is installed at the top surface of the chamber obliquely downward by 45 degree, such that a single spray is discharged into the chamber verti cally downward while the other sprays are into a separate d room. The atomizing gas and emulsified fuel in th e cham- ber are withdrawn by a vacuum pump. Commercially ob tainable soybean oil was used as the base fuel, whi ch was supplied by a pump at designated mass flow rate s controlled by needle valves and a mass flow meter (Keyence, FD-SS02A). Nitrogen was used as the atomi zing gas alternative to air for safety reasons, the flow rate of which was also controlled by a mass flow meter ( SMC PFA 721) and a valve. The volume flow rate of w ater introduced was measured by a calibrated rotameter. The chamber has two optical windows made of silicate glass on opposed lateral sides, through which the parallel light emitted by the light source is sent to spray in the chamber at a short duration of 30 n s. Shadow- graph images of sprays were captured by a digital c amera (Nikon D7000) with a ISO speed of 100 and a s patial resolution of 4.1 µm/pixel. By using this system, static images of dro plets moving with a high velocity of the order of 100 m/s were obtained. The luminous intensi ties of the images are represented by 16-bit data. The detail of image processing evaluating SMD values will be e xplained in the following section. In order to investigate the mechanism of fuel emuls ification occurring in the mixing chamber of the in jector, we observed fuel and water streams in a transparent mixing chamber by using a high-speed camera (NAC, MEMRECAM GX-1) with a frame rate of 5000 fps. In addi tion, we investigated the inner structure of emulsi - fied fuel droplets discharged from the injector. A small shallow dish with a depth of 4.6 mm was inser ted into a spray stream at a downstream location from the inje ction hole by 143 mm. Some large droplets in the sp ray im- merged into the pool of silicon oil in the dish. Wi thin one minute after the end of the sampling, we b egan to ob- serve the droplets in the pool with a digital micro scope (Yashika, Mk-UMS1). During the sampling of dr oplets, Figure 1 Schematics of a fuel-water internally rapid mixing injector. Figure 2 Experimental apparatus for droplet size measurements based on a shadowgraph method. ICLASS 2015, 13th Triennial International Conference on Liquid Atomi zation and Spray Systems, Tainan, Taiwan, August 23 -27, 2015 3 the water introduced into the injector was colored by red for easy identification of water in microsco pe images. In our previous study [15], the effects of water in troduction on NOx and soot emissions have been inve sti- gated. In this study, we newly measured the NOx and soot emissions from a laboratory-scale furnace to clarify the effects of water content on the emissions. Figu re 3 shows schematics of experimental apparatus con sisting of a combustion furnace, burner, and NOx and soot meas urement system. Whereas a simple cylindrical tube w ith a length of 700 mm and an inner diameter of 160 mm wa s used as a furnace in the previous study [15], the furnace used in this study is extended to 1400 mm length, w hich is more than twice luminous flame length appea ring in the furnace. As shown in Fig. 3, the furnace exit i s contracted so as to ensure a uniform distribution of NOx con- centration [18]. In the lower section of the cylindrical furnace a b urner, which is composed of the fuel-water internal ly rapid mixing injector, a flame holder, and a pilot nozzle , is mounted. The detail of the burner has been alr eady de- scribed in our previous paper [15]. We used a propa ne pilot flame at an ignition timing of the soybean oil flame. Secondary air is supplied from a blower into the fu rnace. The flue gases were sampled by a sampling pro be in- serted into the furnace through the exit. The inner diameter of the probe is 10 mm, and the flue gases were suc- tioned at a volume flow rate of 0.4 L/min by a vacuu m pump. Since the lengths of flames were less than 700 mm, the temperatures of flue gases released from the ex it were measured to be lower than 1000 K. Hence, NO x pro- duction through the Zel’dovich mechanism probably ce ased until the flue gas reached to the exit. Theref ore, the sampled gases in the probe were not cooled. The NOx concentration was measured by a gas analyzer (HORI BA, PG-240). Particulate matter (PM) existing in the fl ue gas was sampled by a filter inserted into the tu be connect- ing the probe to the pump. The weight of sampled PM on the filter was measured by electronic scale (MET - TLER, AE240). After the soluble organic fraction (SOF) in the PM was eliminated by Soxhlet extractor, we measured the weight of the residue corresponding to dry soot. The weight of SOF component is defined a s the difference between weights of PM and soot. Image Processing for Evaluation of Droplet Diameter We measured diameters of droplets in sprays by mean s of processing of images captured by shadowgraph method. Figure 4 (a) shows a shadowgraph image with a size of 20 mm × 13 mm, and the top of the image re- gion is shifted by 10.5 mm downstream from the inje ction hole. Many droplets with various diameters ca n be found in this image, and some droplets seem to be i n the breakup process. We evaluated a diameter of e ach drop- let in the following manner [17, 19, 20]. 1. Candidates for droplets are extracted from binar ized images, and the position of the luminosity cen troid of each candidate is determined. 2. The characteristic length of each droplet is eva luated on the distribution of luminous intensity gr adient around the centroid. 3. Actual diameter of each droplet is obtained with a correlation between actual diameter and characte ristic length. Figure 4(b) shows a close-up image extracted from t he image shown in Fig.4 (a). The image was binarize d with a threshold value IT defined as following equation. Figure 3 Experimental apparatus for NOx and soot measurements. Figure 4 shadowgraph images. (a) shadowgraph im- age. (b) close-up image exhibiting the region indic ated by red square in Fig 4 (a). (c) binarized image bas ed on Fig. 4(b) in which red cross symbols denote the pos i- tions of luminosity centroid. 13th ICLASS 2015 Spray Characteristics of Fuel-Water In ternally Rapid Mixing Injector for Burner Combustio n 4 ( )min min IIIImean T +− =α (1) where Imean and Imin are mean and minimum luminosities throughout the c lose-up region, respectively. Parameter α is set to 0.8. Figure 4 (c) shows the binarized im age of the close-up region, in which black regions correspond to candidates for droplets, and the positions marke d by cross symbol coincides with those of luminosit y cen- troids of the candidates. Figure 5 shows radial distributions of luminosity a nd its gradient azimuthally averaged around the cen troid of a droplet candidate. Some obscure candidates cor responding to droplets existing off the focal surfa ce were excluded on the basis of relation between minimum a nd maximum values of luminosity gradient. The chara cter- istic length of the approved droplet r0 is evaluated on following equation. ( ) ( )dr rG dr rGr rr r∫∫=0 12 1 21 (2) In Equation (2), r denotes the radial distance from the centroid, and G(r) represents the radial profile of lumi- nosity gradient. r1 and r2 denote the radial distances of the positions where the value of luminosity gradient matches to the averaged gradient G0 in a region outside the droplet. The inner position (= r1) approximately co- incides with the centroid. For almost droplets, the characteristic length is close to a distance from the centroid to the position where the gradient profile exhibits a peak. By employing above definition of the length, we can avoid under-estimation of diameter frequently occur ring when the profile has several peaks. Finally, the actual diameter of droplet is evaluate d from the characteristic length. In order to obtai n a correla- tion between actual droplet diameter and characteri stic length, we prepared a glass plate on which app roximately 1000 ceramic balls with various diameters, simulati ng fuel droplets, were scattered. The characteristi c lengths of the ceramic balls were evaluated based on a shadowg raph image of the plate. On the other hand, the act ual diam- eter of every ball was manually measured with a sca le on enlarged images captured by a microscope. A l inear correlation between characteristic length D0 (= 2 r0) and actual diameter D was obtained as shown in Fig. 6. The slope of the correlation line is 1.12, indicating t hat the actual diameter is 1.12 times of the charac teristic length. The value of error introduced into the actual diamet er is within 12 %. Experimental Conditions During the investigation of spray characteristics i ncluding SMD of droplets, the mass flow rate of bas e fuel Mf was maintained at 19.2 g/min, corresponding to an equivalence ratio of 0.9 under our combustion condi tions. Nitrogen was selected as atomizing gas, the volume flow rate of which was also maintained at 56 NL/min . The mass flow rate of introduced water Mw was set to either zero or 10.5 g/min, such that th e mass water content, defined as Yw = Mf / ( Mf + Mw), was zero or 0.33. During the operation of the co mbustion furnace, the volume flow rates of atomizing air and secondary air were kept at 56 NL/min and 159 NL/min, respectively. The fl ow rate of atomizing air is equal to that of nitrogen used for the investigation of the spray characteris tics. When NOx and soot emissions from the combustion furnace were measured, the volume flow rate of base fuel was va ried from 19.2 g/min to 22.2 g/min, corresponding to the equivalence ratio ranging from 0.9 to 1.05. Figure 5 Radial distributions of luminosity and its gradient azimuthally averaged around the centroid. 30000 25000 20000 15000 10000 5000 0Luminosity gradient 30 20 10 0 Radial distance (pixcel) 50000 45000 40000 35000 30000 25000 20000 Luminosity Lunminosity gradient Lunminosity r0 r1 r2 G0 Figure 6 Correlation between characteristic length and actual diameter of ceramic ball. 250 200 150 100 50 0Actual droplet diameter D ( µm) 250 200 150 100 50 0 Characteristic length D0 (µm) D = D0 D = 1.12 D0 (fitting line) ICLASS 2015, 13th Triennial International Conference on Liquid Atomi zation and Spray Systems, Tainan, Taiwan, August 23 -27, 2015 5 Results and Discussion Spray Characteristics of Internally Rapid Mixing In jector Figure 7 shows existence probabilities of droplets with respect to diameter evaluated based on shadowg raph images. The number of sampled droplets extracted fro m the images is greater than 4700, and we confirmed that the probabilities shown in Fig. 7 have little corre lation with the number of sampled droplets. When wa ter is not introduced into the injector ( Yw = 0), the probability has a peak value at a diamet er of 20 µm. Although the prob- ability rapidly decreases with increasing diameter, several huge droplets with diameters greater than 100 µm can be found. As a result, the value of Sauter Mean Dia meter (SMD) is increased up to 77 µm. When the water is introduced ( Yw = 0.33), the peak probability is lower than that f or Yw = 0 even though the diameter exhibiting the peak is close to that for Yw = 0. Furthermore, the probabilities of droplets wi th diameters greater than 35 µm are higher than those for Yw = 0. The SMD value is therefore increased up to 18 7 µm, which is greater than twice that for Yw = 0. The increase in SMD value caused by the water introduction will be discussed later. The formation of the huge droplets can be explained in terms of fuel streams in the mixing chamber. Fig ure 8 shows a series of successive images of the transp arent mixing chamber when the base fuel is solely i ntroduced. The location of the image region is indicated by a b roken line in Fig. 8 (a). Red and blue arrows denot e the flow directions of fuel and of atomizing air, respective ly. We can observe the spiral stream of fuel with a shape of film attaching on the inner wall of the mixing cham ber. The fuel film is formed as follows; the fuel i ntroduced from the central channel is blown by the atomizing air with swirling motion, and then the blown fuel i mpinges on the inner wall of the chamber, forming the fuel film. As indicated by black arrows in Fig. 8 (b), t here appear some dark regions in the transparent chamber, where the thickness of film is larger than that in other regions. The fuel film spirally moves downward along the inne r wall. When the thick portion of the film reaches to the injection holes, a large amount of fuel is discharg ed from the holes with the atomizing air. Thus, the huge drop- lets responsible for the large SMD appear in the sp ray. The SMD value measured in the present study would be greater than those of droplets discharged from con - ventional injectors. The existence of large droplets destabilizes the flame and increases pollutant emi ssions. For 20 18 16 14 12 10 8 6 4 2 0Probability (%) 100 90 80 70 60 50 40 30 20 10 0 Droplet Diameter ( µm) Yw = 0 Yw = 0.33 Figure 7 Probabilities of droplets with respect to diameter . . Figure 8 Images of the transparent mixing chamber when the base fuel is solely introduced. Figure 8 (a) shows cross-sectional schematic diagram of the injector, in which the location of the image region is indica ted by a broken line. Figure 8(b) shows a series of successi ve images at time intervals of 18 ms. Atomizing air flow Fuel flow (a) (b) : Thick portion of fuel film Injection hole 13th ICLASS 2015 Spray Characteristics of Fuel-Water In ternally Rapid Mixing Injector for Burner Combustio n 6 the purpose of the reduction of droplet diameter, w e will try to uniform the film thickness by using a new type of swirler in our future works. Figure 9 shows the successive images of the transpa rent mixing chamber for Yw = 0.33. The mixing chamber seems to be filled with milky liquid since the film of emulsified fuel attaches on the inner wall. In the upper sec- tion of the mixing chamber, denoted by circle in th e image, we can find transparent particles attachin g on the inner wall, which are probably either pure fuel or water particles. It is evident from the images that the fuel mix- es with the water through the impingement on the in ner wall, resulting in the emulsification of the ba se fuel. The film of the emulsified fuel spirally moves to the i njection holes as in the case of Yw = 0. Since the emulsified fuel film is opaque, the spatial variation of film thick ness cannot be observed in the series of the images . However, we were able to find the periodic fluctuation of di scharge rate of emulsified fuel during experimental trials, which is exclusively due to the variation of film t hickness. Figure 10 shows the microscope images of droplets s ampled by the immersion droplet sampling method. The size of the image region is 0.8 mm × 1.05 mm. As shown in Fig. 10 (a), droplets seem to contain som e water particles colored by red, demonstrating that the em ulsified fuel discharged from the injector is class ified into the W/O type under our experimental conditions. In a pr evious study [21], the author reported that the vis cosity of W/O type emulsified fuel increases with increasing water content, suggesting that the deterioration of atomiza- tion characteristics as shown in Fig. 7 is caused b y the increase in the viscosity resulting from the emulsification. Since the dish used for sampling is filled with sil icon oil, the base fuel in the sampled droplets is solved in the oil. In a region enclosed by a broken circle sh own in Fig. 10 (b), there appear a lot of water bub bles originat- ing from the water particles dispersed in the emuls ified fuel droplet. The biggest bubble observed her e is formed through the coalescence among the particles in the sampling dish. Suzuki et al. [4] investigated the effect of the wa ter particle diameter on micro-explosion, and found that the increase in the water particle diameter relative to that of emulsified fuel droplet increases the occu rrence proba- bility of micro-explosion. Their results indicate th at the diameter of dispersed water particle is a ke y element determining the combustion characteristics includin g the NOx and soot emissions. Unfortunately, we wer e not Atomizing air flow Fuel flow (a) (b) Water flow Figure 9 Successive images of the transparent mixing chambe r at time intervals of 18 ms for Yw = 0.33. Figure 10 Microscope images of emulsified fuel droplet sampl ed by the immersion method. The water in- troduced into the injector is colored by red. (a) (b) Droplet A Droplet B Droplet C Droplet D ICLASS 2015, 13th Triennial International Conference on Liquid Atomi zation and Spray Systems, Tainan, Taiwan, August 23 -27, 2015 7 able to evaluate the diameter of dispersed water pa rticle because the sampling method employed in the present study has a less accuracy for the evaluation. Almos t droplets sampled by the immersion method have a d iameter larger than SMD value, and hence, we have no images exhibiting the inner structure of fine droplets. F urther- more, the diameter of water particle increases with time through the coalescence among the particles b ecause the base fuel is emulsified without any surfactant. Ther efore, the diameters of water particles observed he re might be much larger than those when existing in a spray. Th e investigation of inner structure of droplets incl uding the size of dispersed water particles is also our futur e work. NOx and Soot Emissions from Combustion Furnace with the Internally Rapid Mixing Injector In this section, we will examine the emission chara cteristics of furnace combustion fueled with emulsi fied soybean oil discharged from our injector. Figure 11 shows emission index of NOx (EINOx) with respect t o the water content Yw measured during experimental trials at a global eq uivalence ratio of 0.9. EINOx is estimated according to a definition proposed by Turns and Myh r [22]. The value of EINOx decreases with increasing water content owing to the dilution effect [13] and the c hemical effect [14]. Figure 12 shows the mass concentrations of PM, SOLI D and SOF components measured at an equivalence ratio of 1.05. Since the measurements of the PM con centrations were made three times, this figure cont ains three plots at every experimental condition. Under the co ndition of an equivalence ratio less than unity, ou r measurement system was not able to collect measurab le amount of PM. The PM concentration significantly decreases as the water content increases because th e concentration of SOLID component corresponding to dry soot rapidly decreases. In contrast, the concentrat ion of SOF component slowly decreases, such that PM released from the furnace operating at a condition of Yw = 0.5 mostly consists of SOF component. Hayashi et al. [23] demonstrated that an increase i n SMD of droplets promotes soot formation in lamina r spray flames because of the enhanced diffusion flam e. As shown in Fig. 7, the emulsification by means of the injector increases SMD of droplets in the spray, wh ich would contribute to the soot formation. However , as shown in this figure, the PM emission is reduced ex clusively owing to the secondary atomization and OH radical attack. The simultaneous reductions of NOx and soot emissions indicate the excellent performance of ou r injector for the reduction of the emissions from fu rnaces operating at high load conditions. Summary and Conclusions In the present study, we investigated the character istics of spray discharged from fuel-water internal ly rapid mixing injector being developed in our research gro up. The diameter of droplet in the spray was measur ed by shadowgraph method combined with the image processi ng. The mixing process occurring in the mixing cham ber of the injector was investigated by using the high- speed camera and transparent mixing chamber. Furthe rmore, we observed the inner structure of emulsified fuel droplet sampled by immersion droplet sampling metho d, and finally examined the effects of the emulsification through the injector on the reductions of NOx and s oot emis- sions. The base fuel and water separately introduced into t he injector are mixed with one another in the mixin g chamber through the impingement of fuel and water o n the inner wall, resulting in the emulsification o f base fuel. The emulsified fuel discharged from the injector is W/O type such that the droplets in the discharged s pray con- tain some water particles. The Sauter mean diameter of the droplet when water is introduced into the in jector is much greater than that when base fuel is solely int roduced, because of the increase in the viscosity o f the emulsi- fied fuel. The NOx and soot emissions are simultane ously reduced by the emulsification through the inj ector Figure 12 The effects of water content on PM, SOLID, and SOF emissions. 200 150 100 50 0PM, SOLID, SOF (mg/m 3) 0.5 0.4 0.3 0.2 0.1 0.0 Water content Yw PM SOLID SOF Figure 11 The effect of water content on NOx emission. 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Emission Index of NOx (g / kg-fuel) 0.5 0.4 0.3 0.2 0.1 0.0 Water content Yw 13th ICLASS 2015 Spray Characteristics of Fuel-Water In ternally Rapid Mixing Injector for Burner Combustio n 8 even though the mean droplet diameter of emulsified fuel increases. The soot reduction is due to the s econdary atomization and OH radial attack to the soot.