Diffusers - are important components of many machines. Considerable effort has been exerted in investigating their properties and they are also subject of extensive available literature.

Their task is to convert kinetic energy to pressure energy. Basically, they are ducts with cross sectional area increasing in flow direction.

Fig.D-34 Schematic representation of a planar
rectangular diffuser (shown without cover plate). The in-
crease in cross-sectional area in the flow direction
- from inlet X to outlet Y - is due to the diverging
mutual position of two opposing walls.
Although exisiting in various more complex variants, most important are their two simplest forms - the rectangular and the axisymmetric one.
The rectangular diffusers are mostly planar, nominally two-dimensional ones - as shown in Fig.D-34. There are two plane walls placed opposite each other so that the space between them has a shape of a wedge. The geometry is basically determined by the divergence angle of the two walls. The other geometric factor of importance is the relative length of the diffuser / b. Behaviour is also dependent upon aerodynamic blockage by boundary layer at the inlet - and weakly influenced also by other entrance flow
Fig.D-35 Axisymmetric diffusers: Simple
straight-walled conical diffusers are usually the best
choice; experience with "trumpet" and "bell" shapes
has beendisappointing.
parameters like Reynolds number, Mach number, inlet velocity profile shape, and inlet turbulence level. Since the main feature of diffusers is the increase in cross-sectional area, from inlet cross section to the outlet cross section, the basic geometric factor characterising all diffusers is the area ratio . If the divergence angle is gradually increased, flow passes through a range of regimes shown in Fig.D-36. At small divergence angles and in short diffusers, the flow is firmly attached to both walls. Once the limit curve a - a is crossed, reverse flow may be detected somewhere on the diffuser walls. This separation (stall) is usually not permanent and varies in time - this is why it is called transitory stall. A concept of appreciable stall was introduced to describe conditions in which reverse flow (fixed or transitory) exists over a majority of the length on at least one side of the diffuser. Diffusers usually achieve highest pressure increase when operating near the appreciable stall curve. Line b - b is defined as the transition to a regime in which flow remains fully attached to only one of the two divergent walls (and separated from the opposite
Fig.D-36 Flow regimes in planar diffusers. The optimum
performance is usually achieved near to the appreciable stall
line b - b.
one). When crossing the line b - b, diffuser performance decreases substantially, flow is erratic and gross fluctuations of the entire flow pattern are observed. Line c - c denotes the transition to the jet flow regime, separated from both walls. The transition to regime of jet flow is hysteretic - it does not follow the same path when the direction of changes is reversed. Line d - d represents the locus of states in which transition from jet flow regime to the one-sided attachment takes place as the divergence angle is decreased. An overlap region, the hysteresis zone, exists between the lines c - c and d - d.
It is an important fact that there is not a single parameter characterising the performance - we may strive at achieving maximum pressure coefficient , but it may be even more useful to achieve maximum value of the transformation efficiency


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This is page Nr. D12 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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