Flow conditioner device (40), for use in a heat exchanger system (10). The flow conditioner device includes a honeycomb structure (42) and a mesh (44). The honeycomb structure is configured for rectifying an incoming gas flow (26), and is formed by walls that border channels extending in a flow direction (X) from inlet apertures at a leading surface, to respective outlet apertures at a trailing surface of the honeycomb structure. The mesh is formed by a plurality of wires that extend along further directions (Y, Z) transverse to the flow direction, and which are mutually spaced to define openings. The mesh is attached directly to the honeycomb structure and abuts the second surface, and cross-sectional areas of the openings defined along the further directions vary as a function of position along at least one of the further directions.