Flow visualization in a laboratory vaned diffuser

An experimental model of a vaned diffuser with rectangular flow cross-sections was constructed of clear plastic for flow visualization studies. A swirl generator was used to induce fluid rotation without subjecting the diffuser to any unsteady and irregular impeller flow phenomena. The blades were of a thin circular arc shape. The clear plastic construction allowed large-scale flow visualization with tufts attached to the diffuser wall and dye injected into the separation regions. Four conditions were tested: a vaneless, a four-vaned, a six-vaned, and eight-vaned diffuser. Each test was conducted at an average Reynolds number of 20 000, based on passage thickness. In the absence of diffuser blades the flow angle was not radially constant, as a result of the viscous effects, varying as much as 11° from the ideal 16°. With four blades installed, separation began at 23% of the blade length from the leading tip. At the peak development of the separation regions 34% of the flow area was blocked. Separation began at 27% from the leading edge when six blades were used. Finally, with eight blades in place, separation began at 50% of the blade length from the leading tip; at the peak development of the separation regions 64% of the flow area was blocked.