Fig.D-37 Schematic representation of specific energy
changes in a diffuser and definition of coefficients used
as performance parameters. The expressions with area ratio
are to be used with caution - they are derived using
the assumption of one-dimensionality, which does not really hold
in diffusers - because of changes of velocity profile shape, Fig.D-35.
coefficient, as defined in Fig.D-37. In Fig.D-38, there are summarised data from experimental investigations of plane diffusers. It should be noted that the geometric conditions under which the maxima of the two coefficients are achieved do not coincide. Transformation efficiency is higher at smaller divergence angles, about 6 deg. This is a small angle and the corresponding diffusers tend to be inconvenmiently long. It should be mentioned that performance of diffusers decreases substantially below the data plotted in Fig.D-38 if the diffuser axis is not straight. Optimal conical diffusers (Fig.D-36) are generally shorter than their planar counterparts - both because of the lateral divergence in two transverse directions the area increase is achieved on shorter length and because of the optimum wall angle being somewhat larger: the highest effectiveness is obtained with 8 deg total divergence angle.
Notoriously unsuccessful are attempts at computational predictions of diffuser performance. In spite of the deceivingly simple geometry, there are problems with three-dimensional character of flow - not only cover
Fig.D-38 Map of the dependence between geometric parameters
and performance parameters for planar diffusers.
and bottom plate influence in rectangular variants, but three-dimensional character of stall, near to which the high-performance diffusers are operated. An even more serious problem is unsteadiness of the separation: in diffusers operated near the appreciable stall regime, the flow is actually unsteady, with intermittent backflow in the wall layer. This is very difficult to include in turbulence model and to compute.



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This is page Nr. D13 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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