The combined jet effect, which consists of a impinging jet and cross flow, has an effect that increases the cooling performance of electronic elements. In this study, heat transfer and performance analysis from cube and trapezoidal surfaces by using water and 2% volumetric concentration Diamond-Water nanofluid in channels without fin and 30o fin angled, N=2D fin distance and H=3D and 4D channel heights with combined jet flow were numerically researched. Numerical analysis was carried out steady and in three dimensions with the k-ε turbulence model Ansys-Fluent program. Considering the studies in the literature, three models were placed in the channels in accordance with the channel dimensions. The Re number range of the fluids in the channel is 5000-9000. The results were matched with the Nu results of the experimental study in the literature and they were determined to be compatible. The results of the study were examined comparatively for water and nanofluid as mean Nu number and surface temperature changes for each model in channels at different channel heights in finless and finned cases. In addition, velocity and temperature contour distributions of the combined jet nanofluid flow were presented. However, performance evaluation numbers (PEC) at different Reynolds numbers and average Nu number (Num) and surface temperature values (Tm) were evaluated for Re=7000 for all three model surfaces in the channels. The use of Diamond-Water nanofluid in the H=3D and finned channel resulted in 24.14% and 18.91% higher Num values on cube and trapezoidal surfaces compared to the finless and water-fluid channel, respectively.