US2873142A

US2873142A - Diffuser for sub-critical flow

Info

Publication number: US2873142A
Application number: US534859A
Authority: US
Inventors: Zetterstrom Karl-Axel
Original assignee: Svenska Flygmotor AB
Current assignee: GKN Aerospace Sweden AB
Priority date: 1955-09-16
Filing date: 1955-09-16
Publication date: 1959-02-10
Anticipated expiration: 1976-02-10

Description

Feb. 10, 1959 1 KARL-AXEL ZETTERSTRM DIFFUSER FOR SUB-CRITICAL FLOW Filed Sept. 16. 1955 Z}? mentor Ker/"A xe/ Z c wars tram of. the efficiency of the diffuser.

United States Patent ""ce 2,873,142 DIFFUSER FOR SUB-CRITICAL FLOW Application September '16, 1955, Serial No. 534,859 2 to]. 299-141 The present invention relates :to diffusers for subcritical flow and has for its object toprovide a diffuser the length of the diffuser channel of which is considerably reduced at the cost of but a comparatively slight decrease It is well known that in transforming the kinetic energy of a gas current into pressure energy'by effecting a reduction of the speed of flow of the gas current and a corresponding increase of the pressure thereof, the channels required for carrying this transformation into effect must have a very great length owing to the centrifugal forces appearing inside the gas mass. This, in its turn, often causes technical drawbacks, and the long channels may also give rise to large losses due to friction against the walls of the channels, especially at high rates of flow.

The reason for said comparatively long length of a standard type of diffuser is the action of centrifugal force and the resulting pressure variation appearing inside the gas current owing to the mass inertia of the gas particles, as the gas current is caused to change its direction of flow so as to follow the wall surface of the diffuser. An additional reason is the turbulence appearing in the layer of the gas immediately adjacent the diffuser wall.

If the diffuser pipe is too short with resulting too small radius of curvature of the surface of separation, a vortex will appear adjacent said surface. As a result, the paths of the particles of the regularly flowing gas will deviate from the rigid wall surface so as to form a surface of discontinuity within the gas mass proper, thereby nulifying the diffusing action of the pipe.

If the flow along the limit surface of the rigid wall were regular and lamineferous altogether, considerably shorter radii of curvature could be used, resulting in a much shorter length of the diffuser pipe than is really possible owing to the turbulence appearing adjacent the rigid wall surface.

The turbulence appears at every point of the surface of separation of the rigid wall, and upon the flow of the easily moving medium along said surface, a layer of turbulent medium the thickness of which increases in the direction of flow, will thus continuously exist along said surface. The average rate of flow of such a turbulent layer is comparatively low with the result that, within such a layer only a very slight mass inertia mayappear along the direction of flow, as long as said layer remains comparatively thin. According as the layer grows thicker it will be influenced by pressure variations in a gradually increasing degree, thereby accelerating the particles of said layer in the direction of pressure fall. Consequently, the turbulent medium will be correspondingly displaced in said direction.

If the frictional forces appearing in the gas adjacent the wall surface of the diffuser are no longer sufficient to remove the turbulent medium, the layer thereof will rapidly grow thicker, thereby reducing the radius of curvature of the regularly flowing medium so that said medium 2,873,142 Patented Feb. 10, 1 95 9 loses its contact with the diffuser wall and the diffuser action gradually ceases.

In order to prevent to the largest extent possible the formation of cavities along the wall surface of the diffuser it is, therefore, of importance to effect what may be termed a drainage of turbulent medium from the wall surface. To this end the action of the frictional forces on that portion of the wall surface where the :turbulent layer is thickest should be increased in the highest degree possible and, in addition, rthe difference in pressure appearing in the turbulent medium along the direction of flow within the same portion of the wall surface should be reduced as far as possible. In respect to verylow rates of flow a comparatively shortdiffuser may be used without the formation of cavities :at the wall surfaceof the diffuser pipe. If .in such a short diffuser the rate of .flow beincreased, cavities will be formed at the wall surface of the diffuser pipe. Where the formation of .cavities starts, depends on, thecurvature of the wall surface along the length of the diffuser.

When, as is usually the case in respect to diffusion methods, it is desired to obtain a flow including a rectilinear parallel movement with equalized speed of particles at the diffuser outlet, where the highest pressure prevails, all paths of current will have an inflection point where the curvature of the paths of current changes its direction. In such a point the path of current and and the line of current coincide, the locus of the inflection points of all paths of current will thus be a level surface, that is to say, the same pressure exists at all points of this surface.

When it is desired with the aid of a partial diffusion at the wall surface of the main diffuser to protect this surface against the formation of cavity, the wall surface at the smallest sectional area of the partial diffuser must change its direction of curvature, since otherwise cavity would appear in the partial diffuser. A partial diffusion at the wall surface of the main diffuser may be obtained by placing an annular shield inside the main diffuser which in conjunction with the wall surface of the main diffuser forms a partial diffuser.

By the provision according to this invention of a perforated shield at the locus of the inflection points of the paths of current, where also the wall surface of the diffuser along the direction of flow changes its radius of curvature, and by so shaping this shield at its periphery as to cause it to form a differential diffuser in conjunction with the wall of the main diffuser, a pressure increasing impulse action in a direction towards the wall surface of the main diffuser as well as a partial diffusion adjacent said wall surface may be obtained, with a resulting destroying of any cavity in the neighbourhood of the smallest cross section area of the partial diffuser.

In order to secure a reliable pressure and speed equalization at the outlet end of the main diffuser, the perforated shield should be bodily situated at or in the neighbourhood of the locus ofthe inflection points of the paths of flow of the particles.

The shortened diffuser according to the invention is considered to satisfy in the best way the conditions above discussed. An embodiment of such a diffuser is shown in the drawing in a diagrammatic longitudinal section.

The diffuser illustrated in the drawing comprises a tube shaped member the wall 3 of which is widened at its outlet end as shown. Inserted in. the widened portion of the wall 3 is a perforated shield 1 in the shape of a spherical cup having its convex surface facing the outlet of the diffuser. The peripheral, circular, usually imperforated portion 2 of said perforated cup is curved towards said outlet in such a way as to form between itself and the surrounding widened end portion amen-1a of the wall 3 an annular passage 4 the cross section area of which increases in the direction of flow of the medium passing through the diffuser. Said direction is indicated by arrows in the drawing The shield '1- as a whole is positioned in the neighbourhood of the level surface indicated'at 5 in the diffuser 3 which represents the locus of-the inflection points of the paths of flow of the particles of the medium. I claim:

' 1. A diffuser for sub-critical flow comprising a tubelike wall forming a main ditfuser' channel closed all around its circumference, said wall having an end'portion which is widened in cup-like form, a perforated dished shield disposed in said widened portion, said perforated dished shield comprising a substantially spherically curved perforated central portion and a nonfperforated annular circumferential portion curved substantially in conformity with the curvature of the sur' rounding portion of the ditfuser wall though of a small radiusof curvature as compared with that of said end portion or" said wall; said shield being so positioned with relation to the surrounding portion of the main wall as to 4 divide the diffuser channel as a whole into a central passage of a cross sectional area gradually increasing in the direction of flow and an annular passage of a cross sectional area also gradually increasing in the direction of flow so as to form a partial diffuser concentrically surrounding said inner passage.

2. A diffuser as claimed in claim 1, having the further feature that the substantially spherically curved, perforated central portion of the shield is so positioned along the longitudinal direction of the difiuser wall as to generally coincide with the level surface inside the main diffuser, channel at which the radii of curvature of the paths of flow of the particles of the medium passing through the diffuser "change their direction or become infinitely long.

References Cited in the file of this patent UNITED STATES PATENTS n ant