Liquid flows incorporating small-size bubbles play a vital role in many industrial applications. In this work, an experimental investigation is conducted on bubble formation during gas injection from a microtube into the channel of a downward liquid cross flow. The tip of the air injector has been located at the wall wall orifice and also at several locations from the wall to channel centerline nozzle injection. The size, shape, and velocity of the bubbles along with liquid velocity field are measured using a shadow-particle image velocimetry/particle tracking velocimetry system. The process of bubble formation for the wall orifice and the nozzle injection configurations is physically explained. The effect of variation in water and air flow rates on the observed phenomena is also investigated by considering water average velocities of 0.46, 0.65, and 0.83 m/s and also air average velocities of 1.32, 1.97, 2.63, and 3.29 m/s. It was observed that shifting the air injector tip toward the center of the channel resulted in the coalescence of some of the preliminary bubbles and the formation of larger bubbles termed secondary and multiple bubbles. Increase in air flow rate and reduction in water flow rate also intensify the rate of bubble coalescence. A correlation-based model is also suggested to overcome the shortcoming of the available models in the literature which are developed to only estimate the size of the preliminary bubbles. The model predicts the percent of the preliminary, secondary, and multiple bubbles along with the average size of secondary and multiple bubbles as a function of nozzle position within a cross flow. 德國LaVision PIV/PLIF粒子成像測速場儀 陰影法激光粒徑分析儀