Numerical Simulation And Pressure Loss Analysis Of Sudden Enlargement In Circular Pipes At Low Reynolds Number

The calculation of the head loss is very important in the pipe systems. The head loss can be divided into two types: the frictional loss and the local loss. In general, the local loss coefficient is found to be just the function of enlargement ratio at sudden enlargement in circular pipes in turbulent flow. And the value of the coefficient is determined by a series of experiments. However, in other instances, where high-viscosity fluid or tiny geometries are involved, the Reynolds number is low and the flow feature is laminar. Unlike at high Reynolds numbers, the local loss coefficient varies significantly not only with enlargement ratio but also with Reynolds number. Therefore, the existing results of local loss coefficients lack of accuracy in the laminar regime.In literatures, some information is presented about local loss coefficients of laminar flow through sudden enlargement, but they are limited to the only case of a uniform inlet velocity profile. In this thesis, numerical simulation of the laminar flow has been carried out at enlargement ratios E=1.5, 2, 3, 4 and 5 for Reynolds numbers below 200. And local loss coefficients have been analyzed.Then the laminar flow of a Newtonian fluid in axisymmetric sudden enlargement is studied by solving the Navier-Stokes equation using the finite volume method. The SIMPLER algorithm has been implemented with Fortran 90. The results of the numerical analysis have been quantified in terms of the reattachment length and compared with available experimental data in literatures. Good agreement is found. So the comparisons deem sufficiently validate all aspects of the calculation procedure and convince us its capacity to analyze the local loss coefficient with these simulation results.It can be concluded from the result that at low Reynolds numbers the local loss coefficient is inversely proportional to the Reynolds number and inconspicuously with the enlargement ratio since the flow are dominated by the viscous forces. As the Reynolds number increases, local loss coefficient varies conspicuously with the enlargement ratio. The local loss coefficient tends to a constant value when Reynolds numbers is high. Simulation results from CFD software FLUENT show that within the certain range of Reynolds number namely Re<200, there exist only one recirculation domain which is axisymmetric and in which there is only one eddy after the enlargement section till the flow achieves steady state. As the Reynolds number increases, more than one eddy exists in the gradually increased expanded recirculation domain and the flow is unstable and not axisymmetric any longer.