Urea SCR system, installed in diesel engine vehicles such as trucks and agricultural machinery, is widely used as an exhaust gas aftertreatment system that efficiently purifies NOx, an environmentally harmful substance. Furthermore, the Urea SCR systems may be installed in hydrogen/carbon-neutral fuel engines, and biofuel aircraft engines aiming to achieve carbon neutrality. However, an important problem is the degradation of NOx purification performance caused by urea crystallization due to an undesired reaction of urea water solution (UWS) and clogging of the exhaust pipe due to the formation of deposits caused by an unknown number of atomized UWS behaviors, mainly during idling and low-speed operation when the pipe temperature is relatively low. The problem is that the UWS behavior of the atomized UWS is not well understood. To solve these problems, it is necessary to clarify the complex two-phase flow phenomenon of gas and droplets in the exhaust pipe, which is still unknown. We believe that these approaches can be applied to CO2 absorption systems as well as to urea SCR systems [1]. Hence, a new CAE method is proposed in which visualization experiments simulating the inside of an exhaust pipe are conducted, gas and droplet distribution data are obtained by PIV (Particle Image Velocimetry) from the results, and these are applied to DDM (Discrete Droplet Model) as experimental coefficients. In this study, we attempted to clarify the behavior of UWS under a two-phase flow in more detail by visualization of injected behavior, PIV analysis, and distribution measurement using a high-speed camera with higher sensitivity than before to improve the accuracy of this CAE method.