In the present work, a numerical investigation on the Chilldown process under constant heat flux has been made. An attempt is made to study the variation of different hydrodynamic properties like, gauge pressure and multiphase flow pattern as well as temperature at the different cross section with time. The present work focuses exclusively on the transient dynamics of the chilldown process where the transfer line is nearly at the saturation temperature whereas the previous work were predominantly focused on the drop in temperature of the transfer line from ambient to cryogenic temperature. Initially, the transfer line is predominantly filled with liquid cryogen. The working fluid was chosen as nitrogen, and it was supplied as a saturated liquid into the transfer line. The variation of different parameters were noted at plotted against time and the drawn conclusion was that with time the vapour fraction of Nitrogen vapour increases and it accumulates at the top of the transfer line due to buoyancy effect