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In numerical research about two-phase flow in tube bundles, mainly two trends are distinguishable. On the one hand, some studies start from the assumption that only relatively small bubbles occur in a continuous liquid flow, for which typically Eulerian-Lagrangian modelling is appropriate. This approach is not suitable when modelling flow-induced vibrations in tube bundles, as the most severe vibration conditions are achieved in intermittent, churn or slug flow. On the other hand, some studies focus on accurate numerical modelling of the bubble shape. In that case, it is customary to start from a bubble which is already present inside the domain. However, in order to simulate a sufficiently long flow time which is of interest in the current research, the bubbles should enter the domain through an inlet boundary. In this paper, a new inlet model is proposed which defines a transient inlet boundary condition to be applied in a subsequent Eulerian simulation, more specifically using the Volume-Of-Fluid method. In the first part of this paper, the inlet model is described. The model guarantees the introduction of a userspecified amount of gas during a set time-interval at locations in space and time that are chosen randomly. Subsequently, the model is tested on a 3x5 tube bundle subjected to an axially flowing air/water mixture. The computational time required to complete the inlet model is reported for different values of the most important model parameters. Finally, the computational effort of the new inlet model is compared to that of a simulation with a precursor-domain where bubbles are created by break-up of air jets imposed at the inlet.