For the last years, several studies have been conducted by our research group in order to determine an optimal configuration of a twin-fluid nozzle with an internal mixing chamber, especially suitable to atomize highly viscous fuel oils in large-capacity, steam boilers. It has been demonstrated that this type of nozzle is advantageous compared to the traditional "Y" type twin fluid approach because it can produce smaller droplets with a lower gas mass flow rate. It seems that the most important point is to assure a thorough mixing inside the nozzle to achieve a homogeneous bubbly flow. To obtain it, the roles of the mixing chamber design and size, and the diameter of the gas inlet have to be clarified. Flow patterns in the internal chamber have been visualized in a scaled-up device specifically designed for this set of experiments using both laser-induced fluorescence and a high-speed camera. Results show that different flow patterns, for similar air-to-liquid mass flow ratios injected, can be generated when the internal mixing chamber volume is increased. The characteristics of the internal flow have been correlated with the spray droplet Sauter mean diameter, determined with a laser diffractometer.