Topology optimization is a computational approach for designing systems by optimizing material distribution within a given design domain. This study focuses on the application of multi-physics and multi-fluid topology optimization in the design of conjugate heat transfer systems, particularly heat exchangers for hydrogen. The fluid flow is modelled using the incompressible Navier Stokes equations, including the addition of a Darcy term that allows the incorporation of the effects of the presence of material in the design domain, as in the work of Othmer[1]. The thermal problem solves the conjugate heat transfer, that considers the convective heat transfer in the fluid, while turning into Fourier’s law in the solid. The thermophysical properties of the solid and the fluid are interpolated depending on the pressure, temperature and the domain type (i.e. solid or fluid). The multi-physics, multi-fluid topology optimization is implemented in OpenFOAM using the discrete adjoint formulation, as presented in the work of Dilgen et al[2] and the wok of Høghøj et al[3], considering the minimization of the power loss between entry and exit and the maximization of the heat transfer between fluids, as in the work of Subramaniam et al[4]. After verification with benchmark problems from the literature, our contribution was applied to an industrial application for designing heat exchangers for hydrogen applications in the context of a research project named HYPSTER (HYdrogen Propulsion System : Thermique Et Régulation). REFERENCES [1] C. Othmer. A continuous adjoint formulation for the computation of topological and surface sensitivities of ducted flows, Int. J. Numer. Methods Fluids, Vol. 58, pp. 861-877, 2008. [2] S.B.Dilgen, C.B.Dilgen, D.R.Fuhrman, O.Sigmun, B.S.Lazarov, Density based topology optimization of turbulent flow heat transfer systems, Struct. Multidiscip. Optim., Vol. 57(5), pp. 1905–1918, 2018. [3] L.C.Høghøj, D.Ruberg Nørhave, J.Alexandersen, O.Sigmund, C. Schousboe Andreasen, Topology optimization of two fluid heat exchangers, Int. J. Heat Mass Transf., Vol. 163, 2020. [4] V. Subramaniam, T. Dbouk, J.-L. Harion, Topology optimization of conjugate heat transfer systems: A competition between heat transfer enhancement and pressure drop reduction, Int. J. Heat Fluid Flow, Vol. 75, pp. 165-184, 2019.