A heat exchanger comprising: a first fluid inlet; a first fluid outlet fluidly coupled to the first fluid inlet via a core section; a second fluid inlet; a second fluid outlet fluidly coupled to the second fluid inlet via the core section; the core section including: a plurality of first fluid passages configured to convey the first fluid flow from the first fluid inlet toward the first fluid outlet; and a plurality of second fluid passages configured to convey the second fluid flow from the second fluid inlet toward the second fluid outlet such that the first fluid flow exchanges thermal energy with the second fluid flow at the core section; and characterised by: one or more first inlet vanes disposed between the first fluid inlet and the plurality of first fluid passages to direct the first fluid flow from the first fluid inlet to the plurality of first fluid passages, a gap present between each first inlet vane and the core section; and one or more second inlet vanes disposed between the second fluid inlet and the plurality of second fluid passages to direct the second fluid flow from the second fluid inlet to the plurality of second fluid passages, a gap present between each second inlet vane and the core section; one or more bypass passages extending from the first fluid inlet toward the second fluid inlet, bypassing the core section, the one or more bypass passages configured to direct a bypass portion of the first fluid flow from the first fluid inlet past the second fluid inlet, the one or more bypass passages extending across the second fluid inlet, the second fluid inlet and the one or more bypass passages spaced apart from the core section by a second fluid inlet manifold fluidly connecting the second fluid inlet to the core section; wherein each first fluid passage of the plurality of first fluid passages has a circular cross-section. a first fluid inlet; a first fluid outlet fluidly coupled to the first fluid inlet via a core section; a second fluid inlet; a second fluid outlet fluidly coupled to the second fluid inlet via the core section; the core section including: a plurality of first fluid passages configured to convey the first fluid flow from the first fluid inlet toward the first fluid outlet; and a plurality of second fluid passages configured to convey the second fluid flow from the second fluid inlet toward the second fluid outlet such that the first fluid flow exchanges thermal energy with the second fluid flow at the core section; and characterised by: a plurality of first fluid passages configured to convey the first fluid flow from the first fluid inlet toward the first fluid outlet; and a plurality of second fluid passages configured to convey the second fluid flow from the second fluid inlet toward the second fluid outlet such that the first fluid flow exchanges thermal energy with the second fluid flow at the core section; and characterised by: one or more first inlet vanes disposed between the first fluid inlet and the plurality of first fluid passages to direct the first fluid flow from the first fluid inlet to the plurality of first fluid passages, a gap present between each first inlet vane and the core section; and one or more second inlet vanes disposed between the second fluid inlet and the plurality of second fluid passages to direct the second fluid flow from the second fluid inlet to the plurality of second fluid passages, a gap present between each second inlet vane and the core section; one or more bypass passages extending from the first fluid inlet toward the second fluid inlet, bypassing the core section, the one or more bypass passages configured to direct a bypass portion of the first fluid flow from the first fluid inlet past the second fluid inlet, the one or more bypass passages extending across the second fluid inlet, the second fluid inlet and the one or more bypass passages spaced apart from the core section by a second fluid inlet manifold fluidly connecting the second fluid inlet to the core section; wherein each first fluid passage of the plurality of first fluid passages has a circular cross-section.