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Verification of Additional Losses in Complex Flow Fields

Author(s): F. Valentin

Linked Author(s): Franz Valentin


Abstract: Additional loss coefficients are commonly used to simplify the calculation of losses to an appropriate level. The most important disadvantage of this method is the lack of tranparency with regard to the quality of this kind of parameters. Based on a series of experiments the flow in combining manifolds is considered to estimate the total losses for this type of flow. The experimental set-up was changed to different configurations of the pipe itself like the number and the spacing of openings. The evaluation of the numerous data is based on a simulation model with describes in detail the lateral inflow to the pipe and the axial flow in the pipe itself. The model used for this investigation separates the total losses into the friction losses for a fully developed turbulent flow and the above mentioned additional losses which are created by the disturbance of the velocity profile by the lateral inflow. From the experiments only an information on the total losses was available. One of the main problems during the evaluation was to question the physically correct interpretation of the results of various assumptions which describe the pressure distribution along the pipe and, therefore, the inflow to the pipe at different ports. The variation of the additional loss coefficient for the discontinuous flow with lateral inflow depends on a number of parameters. There is a main dependence on the ratio of the inflow velocity to the mean pipe velocity. This leads to a first assumption with respact to the variation of the local loss coefficients ζ i along the pipe which is characterised by a nonlinear decrease with increasing number of ports with a maximum value ζ 1 z for the first port. To show the influence of the port spacing best fit simulations of the experimental data were done by varying the discharge coefficientum for the inflow and the additional loss coefficient ζ 1 z. These tests resulted in increasing values ofum with an increasing number of ports which is in contradiction to reality. Smaller values of the discharge coefficients can be obtained by increasing differences in pressure head at the ports. Therefore, the additional losses have to be increased at the beginning of the pipe and decreased towards the end of the pipe. Simulations with this typ of variation showed only small variations of the inflow distribution which allowed the assumption of constant losses due to pipe friction under these conditions. A constant value of the total sum of additional losses enables the reduction the variation ofζz in dependence of the spacing ratio to mean values ofζ Combining manifolds find an increasing application in discharging purified water from final settling tanks in waste water treatment plants[1]. The design of such an outflow system requires detailed knowledge of the resistance behavior to ensure an equally distributed outflow for instance in circular tanks. One of the crucial points in the calculation of discontinuous flow in pipes is the choice of correct values for the additional losses created by the lateral inflow. Over the years a series of experiments have been carried out in in this context at the Laboratory of the Institute of Hydraulics and Hydrology, Technische Universitat Munchen.


Year: 1999

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