Fuel oil industrial burners are mainly formed by the atomizing nozzle and the air diffuser. The diffuser is responsible to condition the air flow so that it satisfies the requirements for combustion. A widely used technique to achieve this overall objective in an efficient way is to induce a swirling motion to the primary air to create a recirculation zone. The objective of this work is to modify the original design of the diffuser currently in use in a Cuban power plant to improve its performance. Experiments have been conducted in a wind tunnel with a 1/3 scaled down diffuser model, seeding the air with tracer particles and illuminating the flow with a laser sheet. The existing configuration has been compared to a modified one that adds an outer finned swirler ring which generates rotation in the combustion air in the same direction of that imposed by the diffuser swirler plate. First, visualization images have been acquired to have an initial estimate of the flow structure. In a second step, the three components of the average velocity have been measured with stereo particle image velocimetry (SPIV) in a complete diametric plane. To have a better resolution, the plane has been divided in forty 48 mm x 33 mm zones. The analysis of the results indicates that the presence of the outer swirler ring enhances the rotation of the air flow, shortening the recirculation zone and moving it closer to the exit nozzle. (C) 2013 Elsevier Ltd. All rights reserved.