Numerical Investigation of the Campbell Diffuser Concept

Campbell diffusers have a unique feature in that they lack axial symmetry in the end walls. It has been proposed that the asymmetry of the end wall in a vaned diffuser can be tailored to improve its performance. This concept is applied to the redesign of two vaned diffuser configurations: (1) diffuser with straight leading edge; (2) diffuser with parabolic leading edge. In the redesign, the original axisymmetric end wall configuration is modified so that the resulting end walls on the pressure side converge from the leading edge to the throat and then diverge back to the original passage depth. Thus to maintain the same flow area distribution as the original vaned diffuser, the end walls on the suction side would have to be tailored so that they diverge from the beginning of the channel region to the throat before converging back to the original passage depth at the exit. A three dimensional inviscid Euler code is used to evaluate the performance of these diffusers. The performance of Campbell diffusers and two dimensional diffusers have been compared at an optimized operating point. The operating point has been chosen such that the Mach number at the throat is as high as possible (with a limit of one). Important parameters that govern the extent of the flow response to end walls changes are studied. These are (i) the pitch-chord ratio; and (ii) the ratio of passage depth to chord. When these two ratios are small (of the order of 0.1), which is typically found in practical vaned diffusers, the flow is essentially one dimensional so that its response to the end wall contouring is not as dramatic. However, when either of the two ratios is large enough, the effect of end wall contouring can be more significant. In the case where the passage depth to chord ratio is sufficiently large, Campbell diffusers can potentially outperform two-dimensional diffusers in term of pressure recovery based on the present computational results. Thesis Supervisor: Dr. Choon S. Tan Title: Principal Research Engineer