Model Investigation of Subcooled Flow Boiling in Narrow Rectangular Channel Using CFD Method

The plate-type fuel assembly with the advantages of high specific power of active zone and compact structure has been widely adopted in many research reactors and material irradiation test reactors. The narrow rectangular channel in platetype fuel assembly has the geometric characteristics of large aspect ratio, resulting in large gradient and steep distribution of velocity in the height direction. When the subcooled boiling occurs, the bubble behavior near heated wall will be affected, and the bubble sliding phenomenon has a great influence on the boiling heat transfer. In this paper, a wall heat flux distribution model considering the sliding heat flux was built and quenching heat flux was redefined as the sensible heat released by the solid wall under the dry spot. The bubble stress analysis model and sliding model based on theoretical and semi-empirical expressions were established to calculate essential auxiliary parameters including bubble departure diameter, lift-off diameter and sliding distance. A wall boiling model suitable for upward flow boiling in the narrow rectangular channel was developed. After that, the numerical simulations under different operating conditions were performed against the Nuthel flow boiling experiments in narrow rectangular channels for model verification. The results show that the model developed in this paper can better predict the wall superheat degree of upward flow boiling in the narrow rectangular channel under medium and low pressure conditions in the range of 1-4 MPa. Compared with the RPI model, the wall temperature calculated by the developed model in this paper is much higher, and the maximum error of wall superheat in the boiling region decreases from 80% to 17% in the simulated conditions, indicating that the wall boiling model considering the sliding heat flux can describe the flow and heat transfer characteristics of upward flow boiling in the narrow rectangular channel more accurately. As for the heat flux distribution, the sliding heat flux and single-phase convective heat flux calculated by developed model take up significant fraction, and evaporation heat flux is less significant. The quenching heat flux, defined by the reconstructed expression, is almost negligible, which demonstrates that the sensible heat released by the solid wall under the dry spot has little influence on the overall heat transfer process. In RPI model, however, evaporation heat flux accounts for the majority of the total heat flux in the boiling region. Lower fraction of heat flux allocated to evaporation heat flux leads to less evaporation rate and predicted void fraction.