Tunnel kilns are used in the burning of Sanitaryware. The width can be up to 6 meters and the height up to 3 meters, they sometimes reach a length of 180 meters. The middle zone, where fuel is injected, is called burning zone. The combustion gas flows in a countercurrent direction to heat the products in the preheating zone. Due to complex structure of the Sanitaryware, the gas flows along the gap between products and tunnel kiln wall and not between the Sanitaryware itself. Therefore, it is of great importance for thorough mixing in the cross section area of the kiln. In order to improve the mixing and the heat transfer in the cross section of the kiln, air is injected along the preheating zone from the side of the kiln. The aim of the present work is to establish a computational fluid dynamics (CFD) simulation for the Sanitaryware kiln to study the gas circulation in the cross section. 3D simulations were carried out for the preheating zone and the flow field which is distinctly visualized by images of velocity vectors. The results show the effect of nozzle axial velocity and its arrangement. Moreover, the cross sectional velocity is much higher than the axial velocity. As a consequence, the radial velocity produced by the burner and the nozzles is necessary to increase the heat transfer. The results also demonstrate that the average velocity inside the studied domain is low, around 1 m/s. Using the velocity the convective heat transfer was estimated. A mathematical model has been developed to show how the increasing of heat transfer can decrease the energy consumption.