Conceptual design of a bayonet tube steam generator with heat transfer enhancement using a helical coiled downcomer

Bayonet tube heat exchangers are a proposed solution for advanced nuclear reactor steam generators and heat removal systems, in particular in liquid metal cooled reactors. However, the performance of this heat exchangers is not very high due to the limited heat transfer surface; thus, many long bayonet heat exchangers are needed to remove significant power values. In the present paper, the use of a helical coiled downcomer inside the bayonet is proposed, instead of the traditional straight one, as a passive heat transfer enhancement method. The helical coiled downcomer acts on the annulus as a swirl flow device; for the evaluation of heat transfer and pressure drop it can be considered as a helical coiled wire or a helical coiled tape. The global thermal resistance between the heat source (external flow of primary fluid) and the heat sink (secondary fluid) is reduced, while the pressure drop is not excessively increased. The component behavior has been simulated using the thermal-hydraulic code RELAP5-3D, with lead and water respectively as primary and secondary coolants. A one-dimensional model has been adopted, modifying the heat transfer coefficient and the frictional pressure losses to properly simulate the helical coil effect with two different methods: the use of a heat transfer coefficient multiplication factor and the adoption of the swirl tube heat transfer mode. The component has been operated as a steam generator considering the parameters and the boundary conditions of ALFRED reactor proposed design. Two different downcomer tube dimensions have been considered (6–8mm and 8–10mm ID-OD) with five helical pitches (5, 7.5, 10, 15 and 20 cm), which have been selected taking into account the manufacturing constraints. The results show a significant improvement of the bayonet heat exchanger performance. The removed power increases more than 8% and the steam superheating increases around 23 K for the best configuration.