At present, to reduce the energy consumption of buildings and thereby decrease carbon emissions, the research on the internal flow mechanism of local components represented by elbows has been paid more and more attention in heating, cooling and water supply systems. In this paper, the study delves into the mechanisms behind fluid flow pattern changes in elbows, examining factors such as the causes of fluid flow pattern changes, the dynamic mechanisms of uneven pressure distribution, and mechanical properties. Comparative analyses were conducted on the changes in local resistance coefficient, center section velocity, and wall shear stress of elbows under various conditions. The results revealed that, the local resistance coefficients of bending pipes with different pipe diameters do not completely change according to the power function law at the same inlet flow velocity. Further analysis of the mechanical properties and dynamic mechanisms in elbows with these pipe diameters indicates that the observed results are attributed to various factors, including fluid velocity and wall shear stress. It is found that the result is caused by many factors such as fluid velocity and wall shear stress. With the increase of pipe diameter, the influence of the turning section on the fluid decreases, and the curvature effect of the pipe also weakens. This study offers theoretical insights for drag reduction optimization of local components in heating, cooling, and water supply pipelines.