A 3D conjugate heat transfer model has been developed for the simulation of the heat transfer problem in rectangular cooling channels characterized by high roughness surfaces. The main objective of the model is a complete and detailed description of coupled solid/coolant behaviour characterizing the liquid rocket engine cooling systems. Different objectives have been considered for the model development: to describe the behaviour of coolant within the channel (three-dimensional effects and

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... ble supercritical behaviour of the coolant) and to describe the solid thermal behaviour. The development of this model combines the industrial necessity to obtain proper results in reasonable times with a detailed description of the involved phenomena and specific characteristics of the configuration. One of the most important aspects involved in this process is the introduction of a proper high roughness model able to describe the effects of high roughness wall on the internal flow and on the cooling channel performance in terms of thermal behaviour. The defined model also represents a proper tool to obtain a good description of possible propellant stratification close to the hot wall surface. Indeed, only the coupled description of fluid stratification and roughness effects allow the correct evaluation of thermal behaviour of the cooling systems because they directly affect the wall temperature, the corresponding heat flux coefficient, and the fluid bulk temperature. The analyses have been performed by Ansys Fluent 2021 R2. The model is a 3D conjugate heat transfer model coupling a RANS Spalart-Allmaras turbulence model for the coolant flow with Fourier equations for the thermal conduction in the solid volume. The model is completed by a high roughness wall modelling that has been properly set to be fully representative for the case of interest. The model validation has been realized by means of two different test cases requiring the modelling of two different cooling fluid, water and methane, characterized by different [...]