Influence scope of local loss for pipe flow in plane sudden expansions

Abstract

A pipe with sudden expansions are used in a wide variety of engineering applications such as hydraulic, civil, and nuclear industries. Flow through a pipe with sudden expansion about its head loss becomes local head loss as the boundary changes. The dynamic water pressure for the rapidly varied flow corresponding to the local head loss has remained inconclusive and inaccurate. It is partly due to the complexity of the vortex involved and partly because of limitations of the traditional experiments. Therefore, in this study, the methods based on theoretical analysis and numerical simulation were employed for the prediction of the influence scope of a plane pipe with sudden expansions. A parametric study is performed for the expansion ratios are 1.5, 2, 3, 4, 5 and in a Reynolds number range of 10 - 100000. On the basis of numerical results, an existing correlation of the influence scope is also extended to take into account the effect of the scale effect additionally. The simulation results show that the influence scope of local head loss is affected greatly by expansion ratio and Reynolds number of the plane pipe, and expansion ratios occurring from the sudden expansions are less sensitive to the Reynolds number. The obtained research results have shown that for the cases when Reynolds number is relatively small (Re < 2000), the recirculation degree (L/D, where L is the length of recirculation zone, D is the diameter of the latter pipe) increases significantly with the increase of the Reynolds number; and for the cases when the Reynolds number is larger (Re > 10000), the recirculation degree changes slowly with the Reynolds number. In general, the length of the influence scope is logarithmically related to the Reynolds number.