Nuclear Science and Engineering / Volume 156 / Number 1 / May 2007 / Pages 24-39
The behavior of an evaporating thin liquid film on a nonuniformly heated cylindrical rod with both parallel and cross vapor flow has been numerically investigated. The aim is to develop a mechanistic model for local dryout in boiling water reactors (BWRs). The liquid film on a full-length BWR fuel rod may experience significant axial and azimuthal heat flux gradients and cross flow due to variations in the thermal-hydraulic conditions in surrounding subchannels caused by proximity to an inserted control blade tip and/or the top of part-length fuel rods. Such heat flux gradients coupled with localized cross flow may cause the liquid film on the fuel rod surface to rupture by hydrodynamic instability, thereby forming a dry hot spot. These localized dryout phenomena cannot be accurately predicted by traditional subchannel analysis methods in conjunction with empirical dryout correlations. To this end, a numerical model based on the level contour reconstruction method has been developed. The model includes a ghost-cell extrapolation technique to handle the complex interface geometry. Additionally, a sharp interface temperature technique has been implemented. Application of the model to BWR fuel rods shows that localized cross flow coupled with heat flux gradients can lead to liquid film rupture and dry spot formation.