Boiling crisis under transient heat input is a phenomenon of major interest for the safety of research nuclear reactors. Recently, a model for highly subcooled flow was developed by Nop et al. (2021), which assumes the existence of a homogeneous mantle exchanging with the heating wall and the bulk flow until its saturation. We present an extension of this approach, taking into account the increase of the bulk temperature during the transient. We demonstrate that, if the assumption of an isothermal bulk is valid under the experimental conditions of Nop et al. (2021), it is no longer the case for configurations with high aspect ratios (L/Dh), and thus for the application of the model to reactor geometries.