US 3289921 A
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Description (OCR text may contain errors)
Dec. 6, 1966 SHAO 1.. $00 3,
VANELESS DIFFUSER Filed Oct. 8, 1965 5 Sheets-Sheet 1 INVENTOR.
SHAO L. 500
ATTORNEYS Dec. 6, 1966 SHAO L. $00 3,289,921
VANELESS DIFFUSER Filed Oct. 8, 1965 5 Sheets$heet 5 2 o G I 5 -0.08 0.06 o.o4 .o.oz 0 0-0! INVENTOR 5 HA 0 L- 5 00 United States Patent 3,289,921 VANELESS DIFFUSER Shao L. $00, Urbana, 111., assignor to Caterpillar Tractor (10., Peoria, Ill., a corporation of California Filed Oct. 8, 1965, Ser. No. 494,225 3 Claims. (Ql. 230127) This application is a continuation-in-part of my copending application for Vaneless Diffuser, Serial No. 328,03 8, filed December 4, 1963, now abandoned.
The present invention relates to the diffuser section of a turbomachine, such as a compressor or turbine and particularly to an improved contour of diffuser walls for increasing the etficiency of a compressor or the like.
In the past, vaneless diifusers have been made with parallel wall-s, diverging walls and constant flow area configurations, without regard to specific control of boundary layer separation and attendant wall friction losses.
It is the object of the present invention to provide a vaneless diffuser specifically contoured to obtain maximum efliciency with a minimum of loss due to friction and heat.
The invention is disclosed herein in combination with a radial flow compressor but is equally useful with axial and mixed flow compressors or turbines for fluid pumpmg.
The manner in which the invention is carried into practice will best be understood from the following specification wherein reference is made to the accompanying drawings.
In the drawings:
FIG. 1 is a central sectional view through a typical radial compressor and diffuser therefor showing a diffuser embodying the present invention;
FIG. 2 is a fragmentary view in section showing a modified diffuser profile;
FIGS. 3 to 6, inclusive, are schematic views illustrating typical flow patterns of fluid through a confined space such as a diffuser; and
FIG. 7 is an enlarged view in cross section of the diffuser profile of FIG. 1 showing flow patterns at different positions therein.
FIG. 8 is a graph of boundary [layer parameter G as a function of boundary layer parameter I,.
The compressor shown in FIG. 1 includes a housing 10, a shaft 12 driven by means not shown to rotate an impeller 14 thereon, the impeller having blades 15. Fluid to be compressed enters the housing axially of the shaft from the left as shown in FIG. 1 and is compressed by the impeller and directed radially outwardly at its periphery through an annular vaneless diffuser consisting of a pair of axially spaced annular walls shown at 16 in FIG. 1 into a collector, 17. In FIG. 1 both walls of the diffuser are contoured according to the teaching of the present invention. A modification is shown in FIG. 2 where a similar diffuser is shown as having one flat wall and the other wall contoured to obtain a similar result.
The present invention is predicated upon producing a diffuser profile by applying known formulae to produce a condition of optimum flow throughout the greater part of its length. The efliciency of fluid flow through a confined space is determined largely by boundary layer conditions for the action of the fluid adjacent the walls of the space. Some boundary layer conditions are illustrated in FIGS. 3m 6, inclusive, which show velocity profiles of a fluid flowing within passage walls. As shown in FIGS. 3, 4 and 5, the flow at the main or central portion of the fluid stream is relatively uniform, while the boundary layer or flow adjacent the passage walls varies considerably under different conditions of velocity, pressure, temperature, density and other factors.
Under certain conditions, there exists an unseparated boundary layer having a velocity gradient adjacent to the walls of less than as illustrated in FIG. 4. This produces high friction and conversion of kinetic energy to heat. FIG. 5 represents a condition producing a separated boundary layer where there is a greater than 90 gradient and a reverse curve in the velocity profile. This reverse curve is shown by the enlargement in FIG. 6 illustrating the reversal of flow in the velocity profile and representing an even greater loss in elliciency and flow range than the condition illustrated in FIG. 4.
The optimum conditions are represented in FIG. 3 which illustrates imminent boundary layer separation with a gradient adjacent the wall of approximately 90.
The present invention resides in the provision of a diffuser which produces the efiicient velocity profile of the type illustrated in FIG. 3 throughout the greater part of its length. This is accomplished through a convergingdiverging-converging diffuser wall contour as represented by the areas 18, 19 and 20 of FIG. 7.
A first area represented by the distance 18 in FIG. 7 has an entry portion of converging cross section which is provided to produce acceleration up to the point of a throat, indicated at 21, which acceleration reduces turbulence created by the vanes 15 of the compressor. A second area 19 has a diverging cross section and causes diffusion up to the point 22 where imminent boundary layer separation is reached. In communication with and extending downstream from this point and to the point of discharge as represented by a third area 20, the walls of the diffuser are contoured to maintain imminent boundary layer separation as represented by the three typical velocity profiles 2'5, 26 and 27 illustrated in this area.
The following presents one application of known formulae to illustrate the detailed computation procedure followed in producing the diffuser profile for optimum flow. Other procedures could be followed to produce similar profiles.
A. Nomenclature Pressure, 17
Dynamic viscosity, u
Kinematic viscosity, 1/
Boundary layer thickness, 5 Turbulent radial wall shear stress, 'r Radial Mach number, M,
B. Equations of fluid mechanics Beginning with the Navier-Stokes equations and the equation of conservation of mass in cylindrical coordi nates, the assumptions of steady, compressible, axismymetric, boundary layer flow results in the following momentum integral equation:
dimensional incompressible flow, the following is derived from Equation 1:
a e] p u d1" 1 u Upon inserting the definition of I in Equation 2 a linearequation of first order in Y results:
1 du 2... r; u dr and K is a constant of integration.
C. D 'fiuser profile The solution of Equation 4 may be obtained by specifying u, v, and G as functions of r and performing the required integrations. Constant K may be obtained by specifying a certain value of Y at r=r As an example, let u be given by:
where L is the diffuser wall spacing. Let v be given by:
(conservation of mass) T (free-vortex) G may be taken from FIG. 8. At a specified r, trial and error yields 1 for specified L. Using Buris data, I, is restricted to the range:
(stagnation) (separation) 4 The difi'user profile illustrated in FIG. 7 may be obtained by specifying the wall spacing L in region 18 in accordance with the following equations:
where, from NACA Technical Note 1426,
r =radius to throat 21 L =wall spacing at throat 21 The dimensions r and L are given by the required design conditions of the diffuser, such as design mass flow and pressure ratio.
The difiuser profile in region 19 may also be proportioned in accordance with NACA Technical Note1426, as follows:
i tan \V 1'1- v2 Sir/2A1 where #1 may vary between 4 and 10 degrees.
Calculation of L in region 19 is terminated when I Equation 4, reaches the imminent separation value of approximately .055. The Wall spacing in region 20 is computed by keeping l =.055=constan-t so as to maintain imminent boundary layer separation to the diffuser discharge.
The above example is presented as an illustration only. Other procedures could be used to obtain the non-separating vaneless diffuser shape shown generally by FIG. 7. The diffuser flow could be unsteady or non-symmetric. The boundary layers could be laminar as well as turbulent. The fiow could be axial, as the discharge from an axial turbine, or at any intermediate angle with respect to the shaft 12.
(1) H. Schlichting, Boundary Layer Theory, Mc- Graw-Hill, 1955.
(2) S. P-ai, Viscous Flow Theory, Van Nostrand, 1957.
(3) NACA Technical Note 1426, 1947.
1. A vaneless diffuser for a radial fiow turbomachine, said ditfuser comprising a pair of axially spaced annular walls about the periphery of an impeller and defining a vaneless diffuser fioW passage thereabout, the radially inner portion of the axially spaced walls defining an entry portion for pumped fluid to said diff-user passage, the radially outer portion defining a discharge point wherein said pumped fluid enters a surrounding collector in an unrestricted manner, said Walls of said entry portion defining a first converging area to a throat for accelerating fluid and reducing turbulence in the fluid flow, said axially spaced walls defining a diverging second portion of said flow passage to define an area of increasing cross-section to a point in the downstream flow where boundary layer separation is imminent from said axially spaced Walls in said second portion of said fiow passage, said axially spaced walls defining a third portion of said flow passage in communication With said second portion and extending downstream of said second port-ion of said flow passage to the point of discharge therefrom, said axially spaced walls of said third portion defining a means for maintaining the condition of im- 5 6 minent boundary layer separation from said walls through- References Cited by the Examiner out the third portion of said flow passage. FOREIGN PATENTS 2. A vaneless diffuser for a radial flow turbomachine according to claim 1 wherein said axially spaced annular walls are arranged symmetrically about a plane midway 5 between said Walls.
3. A vaneless diffuser for a radial flow turbornachine according to claim 1 wherein one of said axially spaced annular walls is substantially flat. HENRY F. RADUAZO, Examiner.
855,124 2/ 1940 France.
26,368 1907 Great Britain. 336,840 10/ 1930 Great Britain.
DONLEY J. STOCKING, Primary Examiner.