The shell nozzle diameter plays a crucial role in determining heat transfer efficiency and pressure characteristics in shell and tube heat exchangers. In this study, we have conducted a numerical simulation to explore the effects of varying shell nozzle diameters on heat transfer performance, flow behavior, and pressure drop within a shell and tube heat exchanger. Ranging from 10 to 110 mm, we assessed how altering the shell nozzle diameter affected heat transfer, flow patterns, and pressure drop. Employing the k-ω shear stress transport turbulence model, we compared numerical Nusselt numbers to experimental data, observing a correlation. As the shell nozzle diameter decreased, inlet velocity surged, generating a stronger vortex. Shrinking from 110 to 10 mm, this reduction amplified vorticity by an average of 169%, notably intensifying heat exchange by 35%. Evaluating the overall heat transfer coefficient against pressure drop, we used the thermal enhancement factor, reaching a peak of 4.39 at a 70 mm shell nozzle diameter. This exploration lays groundwork for optimizing shell nozzle diameters in these heat exchangers.