The fuel-water internally rapid mixing type of injector has been developed to reduce NOx and soot emissions from combustion furnaces operating under high-load conditions.The injector allows spray injection of water emulsified fuel originating from base fuel and water without any surfactants.The aim of present study is to elucidate the mechanism of emulsification occurring in the injector and the atomization characteristics of the injector.We measured the sizes of fuel droplets discharged from the injector by means of a high-speed shadowgraph method combined with image processing.Soybean oil was used as the base fuel.The flow patterns of the fuel and water in a transparent mixing chamber of the injector were also visualized.In addition,we investigated the inner structure of the large droplets sampled by an immersion droplet sampling method.The base fuel,water and air are separately introduced into the injector.In the mixing chamber of the injector,fuel and water are blown by swirling air,and then impinge on the inner wall of the chamber.The base fuel is emulsified through the mixing of fuel with water resulting from the impingement.The emulsified fuel moves to injection holes along the inner wall,and is finally discharged through the injection holes with atomizing air.The probability profiles of droplet size exhibit that the existence probabilities of coarse droplets with diameters greater than approximately 35 μm are increased when the fuel is emulsified.Although the emulsification deteriorates the atomization capability of the injector,the secondary atomization including the micro-explosion occurring in combustion furnaces would form fine droplets,and thus reducing the soot emission from the furnaces.The microscope observations revealed that the emulsified fuel filling in a large droplet sampled corresponds to W/O type.