US 3861826 A
A cascade diffuser providing high pressure recovery in radial flow turbomachinery has first and second vaned diffuser stages arranged between an impeller and collector. The first and second stages are formed by thin, straight vanes which contribute substantially to the pressure recovery rate. The first stage vanes are preferably spaced apart from the impeller to provide a vaneless region for initially conditioning fluid flow from the impeller. The leading edges of the first stage vanes are preferably tapered to provide a semi-vaneless transition region between the vaneless region and first diffuser stage. The diffuser walls form a flow path between the impeller and collector between which the first and second stage vanes are disposed.
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Description (OCR text may contain errors)
United States Patent 9] Dean, Jr.
[ Jan. 21, 1975 CASCADE DIFFUSER HAVING THIN,
STRAIGHT VANES  US. Cl. 415/211, 415/207  Int. Cl. F04d 17/08, F04d 29/44  Field of Search ..415/211,181, 207,163
 References Cited UNITED STATES PATENTS 1,187,428 6/1916 Homershan 415/163 2,111,136. 3/1938 Bauer ...415/211 3,069,070 12/1962 Macaluso et al. 415/211 3,184,152 5/1965 Bourguard 415/211 3,424,372 1/1969 Blattner et al. 415/211 3,442,441 5/1969 Dettmering 415/181 FOREIGN PATENTS OR APPLICATIONS 317,623 1/1957 Switzerland 415/211 971,224 7/1950. France 415/211 Primary Examiner-Henry F. Raduazo Attorney, Agent, or Firm-Phillips, Moore, Weissenberger, Lempio & Strabala  ABSTRACT A cascade diffuser providing high pressure recovery in radial flow turbomachinery has first and second vaned diffuser stages arranged between an impeller and collector. The first and second stages are formed by thin, straight vanes which contribute substantially to the pressure recovery rate. The first stage vanes are preferably spaced apart from the impeller to provide a vaneless region for initially conditioning fluid flow from the impeller. The leading edges of the first stage vanes are preferably tapered to provide a semivaneless transition region between the vaneless region and first diffuser stage. The diffuser walls form a flow path between the impeller and collector between which the first and second stage vanes are disposed.
1 Claim, 11 Drawing Figures sum NF 4 6 PATENIEB JANZI I975 CASCADE DIFFUSER HAVING THIN, STRAIGHT VANES BACKGROUND OF THE INVENTION The present invention relates to radial flow diffusers for arrangement between an impeller and a collector of radial flow turbomachinery. Internal duct losses, which contribute to inefficiency, are proportional to the square of the velocity of the fluid employed in the operation of the turbomachine. It is the purpose of a diffuser to receive the relatively high velocity fluid from the impeller and convert its velocity pressure head to static pressure head. The static pressure recovery coefficient Cp is commonly used to measure effectiveness of this conversion process. The parameter Cp is defined as the ratio of static pressure rise to inlet velocity head.
The simplest way to convert velocity pressure head to static pressure in a radial flow turbomachine is by a vaneless diffuser. A diffuser of this type is merely an open annular area radially displaced about the outer periphery of the compressor impeller. This arrangement permits the fluid to flow in a natural spiral path from the impeller blade tip to the outer circumference of the vaneless diffuser. However, the circuitous flow path characteristics of the vaneless diffuser arrangement is relatively long, and therefore subject to generally higher friction losses which will reduce the static pressure recovery, Cp.
To overcome this disadvantage inherent in vaneless diffusers, various attempts have been made to construct flowdirecting passages in the diffuser annulus. It is the purpose of such passages to guide fluid from the impeller tip to the outer circumference of the diffuser by a shorter path than permitted by the vaneless arrangement and thereby reduce the aforementioned friction losses associated with the longer flow path. These passages have heretofore been formed by a row of vanes, either curved or straight, or by vane islands. An example of a diffuser employing curved vanes may be seen in US. Pat. No. 2,819,837 to W. A. Loeb.
' However, each of these arrangements has disadvantages which limit realization of the greater static pressure recovery permitted by the present invention.
Another parameter of significant importance in such vaned diffusers is the Area Ratio for a given passage which is defined as the ratio of the area (height times width) at the passage exit to a similar area at the passage inlet. For a normal divergent passage the Area Ratio will be greater than unity. Also, it is generally accepted that for a given passage length to inlet width ratio, there is a finite Area Ratio which will yield an optimum Cp. Unfortunately, for single vane rows the Area Ratio required to accomplish the desired reduction in fluid velocity is greater than the Area Ratio which would yield the optimum Cp. Furthermore, vane islands which attempt to establish an Area Ratio which will yield a favorable Cp reduce the amount of velocity pressure head conversion possible for a diffuser of a given radius due to blockage by the annular volume displaced by the islands.
Therefore, a need still exists for a diffuser that will permit optimum static pressure recovery Cp while allowing substantial design freedom as to both reduction of the fluid velocity and overall size of the diffuser en velope.
SUMMARY OF THE INVENTION The present invention has been found to provide such an advance over the prior diffuser art by employing first and second vaned stages in a diffuser, each of the stages comprising a plurality of thin, straight vanes. Various embodiments of such a diffuser are described below and are illustrated in the accompanying drawings.
Use of thin, straight vanes in multiple rows to form a cascade has been determined as a particularly important feature contributing to a high rate of pressure recovery in a diffuser constructed according to the present invention.
Additional features of the preferred embodiments described below further contribute to the pressure recovery rate and design versatility of the present invention. The trailing edges of the first stage vanes are preferably tapered to reduce mixing losses for fluid flow between the vaned stages of the diffuser. A vaneless region between the impeller and first vaned stage has been found effective for initially conditioning fluid flow from the impeller prior to its entering the first vaned stage. Further, the leading edges of the first stage vanes are preferably tapered to coincide with the flow direction providing a transition zone for fluid entering the diffuser passages formed between the first stage vanes.
Because of the radially offset arrangement of the vanes, each adjacent pair of vanes provides for a single downstream divergence in the passage formed therebetween. The diffuser walls forming a flow path between the impeller and collector provide support for the vanes. The walls are generally parallel but may also diverge slightly with relation to each other in a downstream direction providing double divergence in the diffuser passages which may further contribute to the pressure recovery rate.
The present invention has also been found to contribute to increased design freedom by making possible a diffuser with vanes arranged in tandem rows, wherein each row is constructed with an Area Ratio that will yield the optimum Cp. The successive blade rows, or cascades, serve to reduce the fluid velocity to a desirable rate.
Additional features of the present invention contributing to unique and advantageous characteristics of a cascade diffuser having thin, straight vanes are set forth in the following description of various embodiments having reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings FIG. 1 is a sectioned view of a turbomachine including an impeller, a diffuser constructed according to the present invention and collector means;
FIG. 2 is a fragmentary section view similar to FIG. 1 and more clearly illustrating the diffuser;
FIG. 3 is a fragmentary view in section taken along one side of the diffuser flow path as seen for example in FIGS. 1 and 2;
FIG. 4 is a view taken along section lines IV -IV of FIG. 3;
FIGS. 5, 6 and 7 illustrate pressure velocity profiles at indicated locations in the turbomachine of FIG. 3;
FIGS. 8, 9 and 10 are fragmentary sectioned views similar to FIG. 3 illustrating respectively three additional embodiments of a diffuser constructed according to the present invention; and
FIG. 11 is a fragmentary view similar to FIG. 2 and illustrating a diffuser flow path formed by diverging walls.
DETAILED DESCRIPTION OF THE INVENTION The turbomachine illustrated in FIGS. 1 and 2 includes a housing 12, a shaft 13 driven by means not shown to rotate an impeller 14 having blades 15. Fluid to be compressed enters the housing axially of the shaft from the left as shown in FIG. 1. The fluid is compressed by the impeller and directed radially outwardly at its periphery through an annular diffuser l6 including a pair of axially spaced annular walls 17 and 18 forming a flow path 19 through the diffuser. Fluid exits the diffuser into a collector 21.
The present invention is directed toward construction of the diffuser within the above combination and more particularly to a cascade vaned diffuser adapted for radial flow. All of the figures illustrate a diffuser having two vaned stages and other features as an optimum arrangement for selected impeller exit conditions. However, a diffuser constructed according to the present invention may have a varying number of such stages depending, for example, on engine cycle requirements establishing the compressor conditions.
The diffuser 16, illustrated in greater detail in FIGS. 3 and 4, includes a first vaned stage 22 formed by vanes 23 and a second vaned stage 24 formed by vanes 26. The vanes 23 and 26 extend between the walls 17 and 18. The first stage vanes 23 are arranged in radially offset relation at a selected angle of incidence with relation to the impeller to form diffuser passages 27 between the adjacent pairs of vanes. It may be noted that the centerlines of the vanes 23 are disposed along or close to a vector line representing the direction in which fluid leaves the impeller and impinges the first stage vanes. This arrangement prevents or minimizes undesirable shock waves which might otherwise develop from interaction of the high velocity gas flow with the vanes. It is further noted that each adjacent pair of first stage vanes diverges uniformly in a downstream direction at an angle determined by the number of vanes in the first stage. In the illustrated diffuser, the first stage includes 32 vanes 23.
The second stage vanes 26 are arranged between the first stage vanes 23 to have substantially the same angle of divergence therebetween and to similarly form second stage diffuser passages 28.
As noted above, a particularly important feature of the present diffuser contributing to high pressure recovery is the thin, straight configuration of each of the vanes 23 and 26. Referring particularly to FIG. 3, each of the vanes has a substantially constant cross-section, at least in width, except for the leading and trailing edges 31, 32 of the first stage vanes 23 and the leading edges 33 of the second stage vanes 26 which are tapered for reasons discussed below. It may be noted that, in the embodiment of FIG. 11, the height of the vanes varies somewhat because of the slight divergence in the diffuser walls.
The leading edges of the first stage vanes are tapered, preferably by beveling, to form pressure surfaces 34, each terminating generally opposite the leading edge 31 of an adjacent first stage vane 23 to form a throat indicated at B in FIG. 3. The throat B, together with the tapered or beveled pressure surface 34, forms a semivaneless region or zone of rapid adjustment for conditioning fluid flow prior to its entry into the first stage diffuser passages 27.
To minimize a wake effect tending to be caused by the first stage vanes in fluid flow from the first stage vanes to the second stage,'the trailing edges 32 of the first stage vanes are tapered or beveled as may be best seen in FIG. 3. The leading edges of the second stage vanes are also tapered to reduce inlet flow blockage.
To further enhance pressure recovery in the diffuser, the leading edges 31 of the first stage vanes are radially spaced apart from the periphery of the impeller to form a vaneless region annularly arranged around the impeller and indicated at 36. The vaneless region 36 and the semi-vaneless region formed by the pressure surfaces 34 have two primary objectives of mixing fluid flow from the impeller to obtain a more uniform velocity profile and reducing the Mach number of the fluid as much as possible prior to its entering the passages of the first vane stage. The two vane stages then provide for additional conversion of the fluid velocity pressure head to a static pressure head.
The conversion effect of the diffuser is illustrated by the velocity profiles illustrated in FIGS. 5-7 representative of different locations in the turbomachine. FIG. 5 illustrates the uneven velocity profile at the periphery of the impeller, taken along line A-A of FIG. 3, and indicates the need for uniformity or mixing in the fluid. FIG. 6 illustrates the velocity profile at the throat B and shows the effect of the vaneless and semi-vaneless regions in accomplishing their objectives stated above. FIG. 7 illustrates the velocity profile in one of the second stage diffuser passages downstream of the first stage vanes, at line C-C.
In the turbomachine of FIG. 3, the impeller has a design speed for example 51,900 rpm, developing a fluid velocity at its periphery of almost sonic speed, for example Mach 0.973. The fluid velocity is reduced through the various portions of the diffuser in the man- I ner discussed herein to obtain optimum conversion of Variations possible in the diffuser vaned stages are illustrated in FIGS. 8-10. Features of the diffuser variations in these figures are indicated by primed numerals corresponding to those employed in FIGS. 1-4.
In FIGS. 8 and 9, fluid exits straight off the second stage diffuser passages rather than through a turn as in FIGS. l-4 and 10.
The second stage vanes 26' in FIG. 8 are similarly arranged with relation to the first stage vanes 23' and each other as in FIG. 3.
However, in FIGS. 9 and 10, an equal number of vanes are employed in both the first and second stages. The trailing edges of the first stage vanes 23' and the leading edges of the second stage vanes 26' are similarly beveled as in FIGS. 3 and 8. Unlike the other embodiments, the leading edges of the second stage vanes 26' are disposed substantially upstream from the trailing edges of the first stage vanes 23. The overlapping relation of the first and second stage vanes tends to further minimize wake effects at the trailing edges of the first stage vanes. In addition, the overlapping vane arrangement illustrates a capability of effectively increasing the length to width ratio for the diffuser passages without increasing the overall diameter of the diffuser.
An additional variation of the present invention is illustrated in FIG. 11. The vane arrangement of the two diffuser stages 22 and 24' may be similar to any of the preceding figures. As noted above, the diverging arrangement of the thin, straight vanes provides single divergence in each of the diffuser passages. However, in FIG. 11, the diffuser walls 17' and 18' diverge slightly in a downstream direction to provide double divergence in the passages of either or both diffuser stages. It has been determined that, under various diffuser operating conditions, the double divergent configuration of the passages contributes to increased pressure recovery in the diffuser.
1. A cascade diffuser for providing high pressure recovery in radial flow turbomachinery having an impeller and a collector, the diffuser including generally parallel spacedapart walls normal to the axis of said impeller providing a flow path between the impeller and collector, the diffuser comprising first and second stage regions, the first and second regions each being formed by a plurality of thin vanes of constant cross-section along to length extending between the walls and being radially offset and diverging slightly with respect to each other in a downstream direction through at least a portion of the flow path with relation to the impeller to form diffuser passages between adjacent ones of the vanes which diverge relative to each other in a downstream direction toward the collector, each of the vanes being straight and having a leading edge and trailing edge, the leading edges of the first stage region vanes being spaced apart in radial relation from the impeller to provide an initial vaneless stage formed by the diffuser walls for conditioning fluid flow from the impeller prior to its entry into the first diffuser stage and intersecting fluid flow from the impeller with the leading edge of each first stage vane being tapered to provide a beveled pressure surface extending in a downstream direction and terminating generally'opposite the leading edge of an adjacent first stage vane to provide a throat for the respective diffuser path through which fluid from the impeller passes to the respective diffuser paths and the trailing edges of the first stage vanes being tapered to reduce mixing losses for fluid flow between the first and second stages and the leading edges of the second stage vanes being tapered and located generally adjacent the trailing edges of the first stage vanes to reduce inlet flow blockage and being arranged in pairs between adjacent ones of the first stage vanes an axial opening in one of said parallel walls between each pair of said second region vanes at the radial outer most portion thereof providing communication between said diffuser and said collector.