Heat exchangers operating under severely humid conditions can be subject to excessive frost accumulation on the finned surfaces. At these conditions the leading edges of the fins are increased in thickness due to frost buildup. The increased leading-edge thickness can alter the heat transfer and frosting behaviour of the heat exchangers in the ongoing operation. In this study the frosting behaviour on protruded flat surface is numerically studied. Protruded flat surface is selected for its geometrical similarity with the fins with frost covered leading edges. Frosting is simulated using a transient, two-dimensional Eulerian-Eulerian multiphase model in ANSYS Fluent. Frost accumulation is modelled through mass source terms implemented with user-defined functions. Forcing between the humid air and frost phases was calculated by a drag-correlation approach where frost phase is modelled as spherical particles. Predictions of the frosting model were compared against experimental frost thickness and heat flux data. Two sets of simulations were conducted. The simulation with the constant frost particle diameter overpredicted heat flux decay on the surface compared to the measurements. However, the simulation with position-dependent particle size distribution predicted both the frost thickness and heat flux level much more closely to the experimental measurements than the constant diameter case does. The position-dependent particle size distribution is considered to account for the possibly different frost morphologies at the leading edge and at the top surface of the protruded surface. The improved predictive capability of the model with the position-dependent particle diameter method signifies the importance of accounting for the differences in frost morphologies.