Convective heat transfer in non circular channels are of interest in many industrial applications. In the present work, fluid flow in the space between a square channel and a circular tube with different positions of the holder rigid plate is investigated numerically. The use of holder plates is applicable in industrial applications. Holder plates allow different flows with different thermal and hydrodynamic behaviors in a channel at constant Reynolds numbers. Six geometries selected to explore the effect of the position of holder plate on heat transfer rate. Results demonstrated that the plate position has significant effects on fluid flow behavior. It is found that hydrodynamic and thermal behavior affected by the plate position for different Reynolds numbers. For example, for the case that the circular tube is positioned in the center of the square channel with two inclined plates, average convective heat transfer outside and between the two plates is 344 [wm2k] and 465 [wm2k], respectively.