Bulk AlN crystal growth by Physical Vapor Transport (PVT) is studied both experimentally and numerically. This paper presents the analysis of heat and mass transport mechanisms in closed and partially open crucible geometries. The heat transfer in the growth system used at North Carolina State University (NCSU) is simulated. The computed temperature profiles are used in the analysis of mass transport in the growth cell to gain understanding of the effect of species exchange between the crucible and environment on the AlN growth rate. The model predictions are in reasonable agreement with observations.