This study presents a numerical investigation on the effect of the U-shaped flow routing plate to laminar mixed convection heat transfer from protruded heat sources at the side walls of a horizontal channel. The air was used as a cooling fluid and protruded heat sources were equipped as 4 x 8 rows into the rectangular channel with insulated walls. Numerical investigations are carried out for the plate width/channel width ratio (L-P/W) of 3/20, 1/10, 1/20 and plate angles (alpha) of 0 degrees, 30 degrees, 60 degrees at different Reynolds (Re), modified Grashof (Gr*) and Richardson (Ri) numbers. In the study, periodic plate placement was analyzed numerically and the effects of a plate placement array on heat transfer enhancement were investigated. The highest heat transfer enhancement (180%) was observed for the values of alpha = 30 degrees L-P/W = 3/20 at all the Re, Gr* and Ri number values. The predominance of natural convection increases the use of the flow routing plate effectiveness but causes a decrease in the heat transfer enhancement after a certain number of Ri. Therefore, the increase in natural convection needs to be controlled. Theoretical fan power (N-fan) requirements due to pressure loss are also investigated. Depending on the parameters used in the numerical study, as a result of the plate usage, the pressure losses increased according to the case without a plate. It is observed that the most important factor affecting the pressure loss was the Re number and this case indicated that the plate placement was more appropriate for low values of Re. The findings obtained during the numerical studies were presented in detail as graphics of the row averaged Nusselt number (Nu(row ave.)) and the heater temperatures, surface stream lines, and temperature contours.