NACA-TN-4080

Abstract

Tests on two aspects of the behavior of wide-angle, plane-walled, two-dimensional diffusers with essentially incompressible flow have been conducted in the Mechanical Engineering Laboratory at Stanford University. First, a thorough study of the flow mechanism has been made using dye injection in a water table. The four regimes of flow found are delineated on graphs in terms of the three important parameters. Test data from a large water-table unit and a small water-table unit are given. Second, data are presented which demonstrate means for producing efficient diffusers for total included angles up to at least 45⁰ by use of simple, short, flat vanes. <br>In the absence of vanes, or other means of boundary-layer control, all of the following parameters are important in determining the behavior of the flow: (a) Divergence angle, (b) ratio of throat width to wall length, and (c) free-stream turbulence; divergence angle alone is definitely insufficient. Variations in Reynolds number and aspect ratio seem to have little effect on the flow regime for the range of aspect ratios normally encountered and for all Reynolds numbers in excess of a few thousand. Inlet-boundary-layer shape and thickness probably also have an effect on performance but have not been investigated in the present tests. <br>Starting from very low divergence angles and maintaining other conditions constant, the following four entirely different regimes of flow are found as the divergence angle is increased from zero: (a) Unstalled flow, (b) transient, three-dimensional stalls, (c) steady, two-dimensional stalls, and (d) jet flow separated from both walls. With the turbulence level held constant, increasing the ratio of wall length to throat width from 4 to 20 decreases the angles at which both three-dimensional transient and two-dimensional steady stall occur by a factor of the order of 2 or 3 to 1. Increasing turbulence level, with the ratio of wall length to throat width held constant, increases the angle at which transition occurs from three-dimensional transient separation to two-dimensional