MEAN FLOW CHARACTERISTICS OF 16 CONICAL AND SQUARE DIFFUSERS

Abstract

An accurate knowledge of the flow phenomena and the energy losses In diffusers to important in several +cnginoering problems. An oxper&mental study of these asp- octs in conical and square diffusers having a constant area ratio equal to 4 and an overall angle of divergence 160 is presented In this thesis. For both types.,, three boundary geometries were tested eorrespor(Jing to tho strewn line of a potential flow„ Gibson's profile and a plane profile. The Reynolds number was varied from 323000 to 738000 and Its effect on the performance of diffusers is studied. Turbulent boundary layer is ensured in all cases by placing a proper stimulator at the entrance of the diffuser. Growth of the turbulent boundary layer along one side of the diffuser is traced by measuring the mean velocity pro files along it Air was used as the fluid. The diffuser section discharges into the square duct. Tirre-average values of total and static heads had been measured at the inlet and exit sections and also in the boundary layer along the centre line of one side of the diffuser. the case of square diffusers, meanur ments were also taken at the corner at a distance of 1.5 cm, from the side wall at the entrance and the exit section. Dimensionless plots of the wall pressure for the square and circular d ffusers at different Reynolds numbers are given. The energy eorr- action factors were obtained by graphical integration and the results are tebulatcd. The coefficient of energy loss as a function of Reynolds number for aU the dtffu- sers is pre%ented on a log-log graph# The results indicate that the values of this coefficient .en.t are relatively high for the diffusers with straight geometries The results also indicate that the effect of Reynolds number on the coefficient of energy loss is small# The plots of wall pressure d stri.bution within the diffuser indicate that the pressure rises gradually except just at the entrance section. Weightless fibers were attached in the diffuser to visualise the flow pattern near the wall. The motion of threads indicated flow asymmetry across the section. They indicated alternating flow at the top near the exit end though at the bottom they were eiIcned only in the fortsord direction. This alternating tendency of the separated flow is found to reduce with the increase in Reynolds number,