Three-dimensional CFD model for the coking of supercritical n-decane in circular and elliptical tubes.

• Normal motion with hybrid mesh construction is proposed for three-dimensional coking simulation. • Competing effect between flow speed and temperature governs the pyrolysis of fuel. • Higher thermal risk and pressure drop are the penalties of coking. • Buoyancy effect is reflected and analyzed by Gr-based criterion. • The method is applicable in other deposition-involved fields. Coking is an inevitable phenomenon in regenerative cooling system of hypersonic vehicles using endothermic hydrocarbon fuels (EHF) as coolant, which affects cooling performance and even clogs cooling channels. This work proposes an upgraded dynamic mesh method, normal motion with O-type-based hybrid mesh construction, which successfully breaks 2D dimension barrier of normal elementary numerical method and realizes 3D simulation. The framework is tested in two circular tubes with different diameters (Tube A and Tube B) and one elliptical tube (Tube C) under supercritical pressure of 4 MPa. Under heavy deposition (the cross-sectional area reduces to 70 % of its initial), the circumferential and axial distribution of the deposit are simulated and its effect on flow, heat transfer, pyrolysis, and cooling capacity is discussed. Results indicate that the deposition elevates the maximum temperature and pressure drop, leading increased thermal risk and flow resistance. The acceleration effect from reduced cross-sectional area outweighs the thermal effect from deteriorated heat transfer, manifested as increased outlet temperature but decreased n -decane conversion and cooling capacity. The buoyancy effect is significant in Tube B, resulting in non-uniform circumferential distribution of coke deposit. It is proved that the dimensionless Gr q / Gr th with criterion of 1.0 can well explain the buoyancy effect quantitatively. The upgraded dynamic mesh method well achieves the coking effect evaluation in real 3D regenerative cooling, which also shows great potential to be applied to other field involving deposition phenomenon such as petroleum industry. [ABSTRACT FROM AUTHOR]