Abstract The increased performance of close series stacked tube axial fans due to the introduction of a diffuser element between the primary and secondary fan is investigated in this paper. Experimental results showed that there was a 15% increase on average in the performance of two stacked Delta AFB0412VHN 40 mm fans across their operating range when a diffuser element was introduced between the two fans. It was also observed that the performance approximately increased by an additional 4% when the space between the diffuser and primary fan was increased from 0.3 D h to 0.5 D h. Numerical simulation of the flow between the primary and secondary fans was used to probe the cause of the increased performance. The area-averaged secondary kinetic energy (SKE) was used in the presented study as an indicator of the swirl decay. The numerical results showed that the introduction of a diffuser element reduced the SKE/SKE inlet ratio dramatically. It was also found that the SKE/SKE inlet ratio was further decreased when the distance between the diffuser and primary fan was increased to 0.5 D h, which is consistent with the experimental observation.