Abstract: A method for transfer of heat between a first and a second fluid, wherein the first and the second fluid circulate respectively on both sides of a thermally conductive wall of a monobloc assembly formed in a single piece. The monobloc assembly, which is arranged in the interior of a device, includes: a first, three-dimensional, cellular, thermally conductive structure through which the first fluid can pass; at least the thermally conductive wall; and a second, three-dimensional, cellular, thermally conductive structure through which the second fluid can pass. The first and second three-dimensional, cellular structures are situated on both sides of and integral with the wall such that heat transfer is carried out from the first to the second fluid through the wall, and both first and second fluids are under liquid phases and under gaseous phases, with the liquid phases circulating in a direction opposite that of the gaseous phases.