Matrix for a heat exchanger, the matrix comprising: an envelope with an inner surface spatially delimiting a path for the flow for a first fluid, and an outer surface; the envelope being generally arcuate in shape, defining an axial direction which is the main direction of the flow of the first fluid, and a radial direction and a circumferential direction; the outer surface defining an outer outline of the matrix, seen in a plane perpendicular to the main direction; and a network extending within the path and within which the second fluid flows; characterized in that in the axial direction, the outer outline varies radially and circumferentially, these variations being reflected in the fact that any point on the external surface which axially travels on the external surface while retaining its circumferential position has a radial position which varies; and any point on the external surface which axially travels on the external surface while retaining its radial position has a circumferential position which varies; and more specifically, the circumferential variation is manifested by the fact that the outer outline has a circumferential width, the circumferential width varying between a minimum circumferential width (L(B')) and a maximum circumferential width (L), the maximum circumferential width (L) being at least 10% greater than the minimum circumferential width (L(B')), and the radial variation is manifested by the fact that the outer outline has a radially outer midpoint (A, A'), the radial coordinate of this midpoint (R, R(A')) varying between a minimum coordinate (R(A')) at an extreme upstream position of the matrix and a maximum coordinate (R), the maximum radial coordinate (R) being at least 10% greater than the minimum radial coordinate (R(A')). an envelope with an inner surface spatially delimiting a path for the flow for a first fluid, and an outer surface; the envelope being generally arcuate in shape, defining an axial direction which is the main direction of the flow of the first fluid, and a radial direction and a circumferential direction; the outer surface defining an outer outline of the matrix, seen in a plane perpendicular to the main direction; and a network extending within the path and within which the second fluid flows; characterized in that in the axial direction, the outer outline varies radially and circumferentially, these variations being reflected in the fact that any point on the external surface which axially travels on the external surface while retaining its circumferential position has a radial position which varies; and any point on the external surface which axially travels on the external surface while retaining its radial position has a circumferential position which varies; and more specifically, the circumferential variation is manifested by the fact that the outer outline has a circumferential width, the circumferential width varying between a minimum circumferential width (L(B')) and a maximum circumferential width (L), the maximum circumferential width (L) being at least 10% greater than the minimum circumferential width (L(B')), and the radial variation is manifested by the fact that the outer outline has a radially outer midpoint (A, A'), the radial coordinate of this midpoint (R, R(A')) varying between a minimum coordinate (R(A')) at an extreme upstream position of the matrix and a maximum coordinate (R), the maximum radial coordinate (R) being at least 10% greater than the minimum radial coordinate (R(A')).