A radial diffuser for a heat exchanger system, the radial diffuser comprising: a diffuser inlet section (110, 208, 308, 402, 502) which is upstream of the heat exchanger system and oriented along a central flow axis of the radial diffuser, the diffuser inlet section (110, 208, 308, 402, 502) having a cylindrical shape which extends along the central flow axis and a circular cross section along an inlet section plane perpendicular to the central flow axis; a diffuser outlet section (112, 210, 310, 404, 504) which is downstream of the diffuser inlet section (110, 208, 308, 402, 502) with respect to the central flow axis; a diffuser wall comprising a diffuser wall inner surface defining a fluid flow path which is non-linear, the diffuser wall inner surface comprising: a convex curved section adjacent to the diffuser inlet section (110, 208, 308, 402, 502) and having a convex curvature with respect to the central flow axis; and a concave curved section adjacent to the diffuser outlet section (112, 210, 310, 404, 504) and having a concave curvature with respect to the central flow axis, wherein the concave curved section is adjacent to and downstream of the convex curved section with respect to the central flow axis; and a movable diffuser hub which is configured to move with respect to the central flow axis relative to the diffuser wall, thereby altering a fluid flow through the radial diffuser. a diffuser inlet section (110, 208, 308, 402, 502) which is upstream of the heat exchanger system and oriented along a central flow axis of the radial diffuser, the diffuser inlet section (110, 208, 308, 402, 502) having a cylindrical shape which extends along the central flow axis and a circular cross section along an inlet section plane perpendicular to the central flow axis; a diffuser outlet section (112, 210, 310, 404, 504) which is downstream of the diffuser inlet section (110, 208, 308, 402, 502) with respect to the central flow axis; a diffuser wall comprising a diffuser wall inner surface defining a fluid flow path which is non-linear, the diffuser wall inner surface comprising: a convex curved section adjacent to the diffuser inlet section (110, 208, 308, 402, 502) and having a convex curvature with respect to the central flow axis; and a concave curved section adjacent to the diffuser outlet section (112, 210, 310, 404, 504) and having a concave curvature with respect to the central flow axis, wherein the concave curved section is adjacent to and downstream of the convex curved section with respect to the central flow axis; and a convex curved section adjacent to the diffuser inlet section (110, 208, 308, 402, 502) and having a convex curvature with respect to the central flow axis; and a concave curved section adjacent to the diffuser outlet section (112, 210, 310, 404, 504) and having a concave curvature with respect to the central flow axis, wherein the concave curved section is adjacent to and downstream of the convex curved section with respect to the central flow axis; and a movable diffuser hub which is configured to move with respect to the central flow axis relative to the diffuser wall, thereby altering a fluid flow through the radial diffuser.