Anderson, Robert J

Ritter, William K

English

NASA Technical Reports Server

" 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
TECHNICAL NOTE 
No, 1313 
AN INVESTIGATION OF THE EFFECT OF BLADE CURVATURE 
ON CENTRIFUGAL-IMPELLER PERFORMANCE 
By Robert J. Anderson, William K. Ritter 
and Dean M. Dildine 
Flight Propulsion Research Laboratory 
Cleveland, Ohio 
Washington 
May 1947 
GOVT. DOC. 
JU~ 12 IJ 7 
BUSINE~S, srrFNCE 
& I ECH~\ L0( / ...JE.I-"T. 
https://ntrs.nasa.gov/search.jsp?R=19930082027 2020-06-17T21:11:30+00:00Z
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.. 
NATIONAL ADVISORY COMMITTEE FOR AERONAu~ICS 
TECHNICAL NOTE NO . 1313 
AN INVESTIGATION OF THE EFFECT OF ELADE CURVA1'URE 
ON CENTRIFUGAL-IMPELLER PERFORMANCE 
By Robert J . Anderson, Wiiliam K. Ritter 
and Dean M. Dildine 
Three centrifugal impellers, the same except for angular blade 
curvature, were investigated to determine tbe effect of the ilistribu -
tion of blade load ing on impeller performanc e. The blade curvatures 
are geometrically definable shapes and the differences in blade 
loading were compared. on tr.e basis of par ticle travel with constant 
axial velocity alor~ the blade surface. Impeller A has a parabolic 
blade curvature and a constfulw blade loading. Impeller E ha.s an 
elliptical blade curvature with a loading that decreases 'fith 
increasing impeller depth and that has a higher initial value than 
that of impeller A. Impeller C has a circular blade curvature with 
the highest initial blade loading, which also decreased with 
increasing impeller depth. The blade curvatures of impellers A 
and E extend the full depth of the impellers ; the blade curvature 
of impeller C extends 0,60 of the impeller depth. The blades of 
impeller C have a 7.50 forward inclination at the impeller discharge . 
The t~ee impellers were investigated with a vaneless diffuser i n 
a variable-component compressor test rig with the same conditions 
and instrum.entation. 
Impeller E had the hiGhest peak adiabatic efficiency at all 
e~uivalent tip speeds except 1400 and 1600 feet per second. 
L,rpeller C had the lowest peak adic.ba.tic efficiency of all three 
impellers at equivalent tip speeds above 1000 feet per second but 
the largest slip factor of all three impellers at all equivalent 
tip speeds . The variatlon of pressure ratio at peale adiabatic 
efficiency vrith equivalent tip speed was more nearly the same for 
the three impellers than the variation of slip factor and peak 
adio.batic efficiency. J:m.pell -::r C had the largest maximum specific 
capacity at all s;:leedsj impeller E had nearly the same maximum 
specific capacity as impeller C; and impeller A had from 6 to 11 per -
cent lovrer maximum specific capacity than impeller C. Impeller B 
had the largest operatinG range above an adiabatic ei'ficiency of 
0 . 70 at a pressure rati o of LBO, and impeller A had the largest 
NACA TN No . 1313 
This report presents a comparison of the 'gerformance of the 
three impellers over a range of tip si,eeas and volume flows. The 
rer:tormaDce characteristics compared include the efficiency, the 
slip factor) the maximum I'lov capacity) the range of flow, and the 
pressure ratio . 
IMPELLER DESIGN 
Each of the three impellers used in this investigation has 
3 
18 blades, the 8ame blade -entrance angles, and the same entrance 
and discharge diameters. The blade~entrance ansle is defined as 
the complement of the acute angle between blades at the entrance 
edge and a plane perpendicular to the axis of' rotation. The blade-
entrance angle for each of these imllellers is 60° at the blade tip 
and the tane:;ent of the angle varies linearly wlth impeller radius. 
An ax"La-:'.-plane section for the three impellers (A) B, and C) J 
'{i th the pertinent dimensions given, is shown in figure 1. Impel -
lers A and B have a radial entrance edge and impeller C has an 
entrance edge that deviates from the others as shown by the dotted 
line in figure 1. 'rhe indicated difference in the entrance edge 
of impeller C from that of impellers A and B is the result of the 
necessity for keeping the entrance angles of all three impellers 
the same at all radii . The entrance edges of all three impellers 
were sharpened to a maximum radius of 0.024 inch. Front views of 
:impellers A, B, and C a1'0 shown in figure 2. 
The blade curvature of inpeller A is illustrated in figure 3. 
Figure 3(a) shows a cylindrj.cal section that has a diameter equal 
to the outside diameter of tlJEJ impeller and a length equal to the 
axial length of the impoller . The blade surface (shaded) consists 
of radial elements and the intersection of the cylinder with the 
blade surface defines the blade curvature . The portion of the 
blade surface used by the impeller is ShOiVl1 in figure 3(b). The 
bounding axial··plane curvatures 'vere chosen to prevent excessive 
adverse pressure gradients at equivalent tip speeds below 
1200 feet per second. 
The passage area, taken perpendicular to a mean -flOiv-path 
line formed by rotating each radial blade element into an axial 
cross-sectional plane of the impeller , was cons~ant. 
The blade curvature of impeller A (fig, 2(a) ) corresponds to 
th~t of a parabola on the developed surface of the cylinder. This 
.. 
J 
NACA TN No. 1313 5 
schematic view of this apparatus is shown in figure 4. The collector 
casing is enclosed in an insulating box, the walls of which are 
composed of one-quarter inch of hard asbestos, 1 inch of insulating 
board, and one-half inch of plywood. The impellers were drive~ 
with a 1500-horsepower induction motor through a step-up gear. The 
collector casing was mounted separately from the gear box to reduce 
heat transfer between it and the gear drive. The arrangement of the 
eqUipment is shown in figure 5. 
The impellers 'vere tested i.n conJunction with a 24-inch-diameter 
vaneless diffuser of constant area, Every component part except the 
impeller was the same for all tests. 
Instrumentation 
The equipment was instrumented with thermocouples and pressure 
taps and tubes located in conformance vith reference 3. This 
instrumentation provides an entrance measuring station 2 pipe 
diameters upstream of the impeller, and tHO discharge measuring 
stations 12 pipe diameters downstream of the collector casing. In 
addition to the standard instrumentation, pressure and temperature 
measurements were taken at the diffuser discharge. Three shielded 
total-pressure rakes composed of five tubes each ~ere equally 
spaced around the diffuser circumference for the pressure measure-
ments. These rakes w·ere insensitive to angle of yaw up to ±44°. 
~fO calibrated thermocouples were placed midway between the front 
and rear liiffuser surfaces 1800 apart for the temperature measure-
ment. These thermocouples were a high-recovery type, insensitive 
to angle of yaw up to ±20o and similar to the Pratt & Whitney probe 
described in reference 4. The to~al pressures and temperatures 
measured at the diffuser discharge Were used fOT t~le comparative 
performance of the impellers in this investigatj.on .. 
Each impeller is charged with the pressure loss that occurs in 
the diffuser but not wlth the loss in dyna'D.ic pressure that occurs 
in the large collector chamber. The temperature measurement at the 
dlffuser discharge vras used because it has been found that this 
procedure allows the performance to be correlated at all tnlet-air 
temperatures, whereas the performance based upon discharge-pipe 
temperatures does not correlate at all inlet-air temperatures. At 
ambient inlet-air temperatures, the efficiencies based on the 
diffuser-discharge temperatures ,vere as much as 0.04 lower than 
those based on temperatures measured in the discharge pipe. 
NACA TN No . 1313 7 
RESULTS AND DISCUSSION 
Impeller Efficiency 
The variation of peak adiabatic efficiency with equivalent tip 
speed for the three impellers is compared in figure 6. At an equiv-
alent tip speed of 900 feet per second, impeller A had a peak adia-
batic efficiency of 0.78, and the peak adiabatic efficiency decreased 
to 0,77 at an equivalent tip speed of 1100 feet per second, to 0 . 69 
at 1400 f'eet per second and to 0.56 at 1600 feet per second . The 
rate of change in peak adiabatic efficiency was greatest between 
equivalent tip speeds of 1400 and 1500 feet per second for impeller A. 
At an equivalent tip speed of 900 feet ?er second, impeller B 
had a peak adiabatic eff ici ency of 0.82 and the peak adiabatic effi-
ciency decreased to 0.78 at an equivalent tip speed of 1100 feet per 
second, to 0.67 at 1400 feet per second, and to 0 . 53 at 1600 feet 
per second. The rate of' change in peak adiabatic efficiency for 
impeller B was greatest between equi va l ent tip speeds from 1300 to 
1400 and from 1500 to 1600 feet per second. 
At an equivalent tip speed of 800 feet per second, impeller C 
had a peak adiabatic efficiency of 0 .80 and the peak adiabatic effi-
ciency decreased to 0 . 75 at an equivalent tip speed of 1100 feet per 
second, to 0.59 at 1400 feet per second, and to 0 .54 at 1500 feet 
per second. The peak adiabatic efficiency of impeller C decreased 
abruptly between equivalent tip speeds of 1200 and 1300 feet per 
second. 
Impeller B, with the elliptical blade curvature, had the highest 
peak adiabatic efficiency at all equivalent tip speeds except 1400 and 
1600 feet per second . Impeller A, with the parabolic blade curvature, 
decreased in peak adiabatic efficiency at approximately the same rate 
as impeller B, and the greatest diffe rence between the peak adiabatic 
eff iciency of the two impellers was 0. 04 at an equivalent tip speed 
of 900 feet per second. At equivalent tip speeds of 1400 and 
1600 f eet per second, impel ler A had the highest peak adiabatic effi-
ciency of the three impellers. Impeller C, with the circular blade 
curvature, had the lowest peak adiabatic efficiency at equivalent 
tip speeds higher than 1000 feet per se cond. Up to an equivalent 
tip speed of 1200 fee t per second, the peak adiabatic ef ficiency of 
impeller C was no more than 0.03 lower than that of imJ?eller B, but 
at equivalent tip speeds of 1300 f eet per second and hiEher) the peak 
adiabatic efficiency of impeller C was 0.08 to 0.10 belcw that of 
impeller B. 
NACA TN No . 1313 9 
The rate of increase in pressure ratio with oqu:i-valent t ip 
speed dropped to zero for impeller A be G'YJeen 1400 and 1500 feet per 
s, cond; it decr.ased in valne for impe l hr C at 1200 feet per 
s'-'cono;arh). it d!'o~)l)Eld off jIlcre~_J~':l':-lj at cqui7alent tip speeds 
alove 1300 feet per second for impeller ]3. Tho vari a,tion of pres -
sure rati) at pt3a;~ ad1-abatic e::'ficiency with equivalent tip speed 
was more nearly the same for the three impellers than was the varia-
tion of sUp fact')r or peak adia,bati c e~ fi c iency 1-rith equivalent 
tip speed. 
Flow Chal.'uctol'istics 
'rhe maximum specific cs,paci t;r increased with increasing equi v-
alent tip speed at neal'ly the sc.me rate ror all three impellers 
(fig. 9) . Impeller C had the highest maximum specific capacity, 
1;hich varied from GGSO to 7925 cu~ic foet per minute per square 
foot at eq,ui\ralent tip spseds from 600 to 1500 feet per secor.d) 
reflpectively . Impel:,r 13, which had nearly the same maximum 
specific capacity as impeller C J varied fl'om 6650 to 7904 cubic 
fE."3t per minute per sq,uare foot at equivalent tip speeds from 
80l) to 1500 f8et p 'lr f~ecNld) respectivoly, and to 7984 cnbic feet 
per minute ".:1er s-1uare feot at 1600 feet :ger se~ond, Impeller A 
!12,d, the Slll.D 1 h'st maximum specific capacity J which varied from 
6100 to 7500 cnbic feet per minute per sq,uare foot at eq,uivalent 
tip 2pee.is from 800 to 1600 feet per second, respectively . 
Impeller- C 'Y7i th the highest initial blade loading had the highest 
:li1.ximl1ill specific capacity of the three impellers and impener A 
,\-7ith the lowest initial blade 108.d ing had the lowest ;vith a maximum 
s~ecific capacity 6 to 11 pel'cent smaller than that of impeller C. 
O~Jera t j nc; Range 
The performance clla~acteri sti cs of impelle!'s A, 13 J and C are 
pr esented in figure 10 , where the pressure ratios at each eq,uiva-
lent tip speed arc plotted against specific capacity . Contour 
lines of cOl1sta:J.t adiabatic efL.ciency are also shown on these 
figures. For a cOLlpm'ison of the performance of these impellers , 
the useful operating range is defined as the range of specifi c capac-
i ty in which the adiabatic efficiel1cies dre greater than 0 . 70. These 
operating ranses o.2.'e determined from figure 10 . The useful operating 
range at any constant pressure ratio will lie within the 0.70 adiabatic-
efficiency lines or be limited on the minimlnil end by the occurr ence 
of surge . The operating ranges at adiabatic efficiencies above 
.i 
NACA TN No. 1313 11 
an unstable operating region that was similar in lOOny r espects to 
the unstable operating regions encouni:,ered with impellers B anll C. 
Probably stable operating ranges at pressure ratios hignor than 
those limited by surge also exist for the impellers 01 this investi-
gation . Both the oDerational instability and the lack of an 
appreciable operating range occurred only at high equ i valent till 
speeds; the velocity gradients along the mean flm<1 path of the 
impeller at these speeds were greater than the maximUm. value assumed 
in the design of the passage area of these impe11e::ts . 
SUMMARY OF RESULTS 
m1 investigation of the effect of angular curvature, or rate 
of adding angular vebci ty, of impeller b~_ades has shown for the 
three blaJe f orms tested: 
1. Impeller B, with the elliptical bla&e curvature, had the 
highest peak adiabatic ef1icionc at all of the equivalent tip 
speeds except 1400 and J.600 feet per s~cond. Impeller A, with the 
')arabolic blade curvature, had the highest peak adiabatic efficiency 
at equivalent tip s,eeds of 1400 and IGOO feet per second. 
Impeller C, with the circular blade curvature, had the lowest peak 
adiabatic efficiency at equivalent tip speeds higher than 1000 feet 
per second. For the range of tip speeds investigated) all tlITee 
impellers decreas 'd 1.n adiabatic effici.ency l'li th increasing tip 
speed. 
2. Impeller C had the highest slip factor of a]l three impellers. 
3. The variation of pressure ratio at peak adiabatic efficiency 
with equivalent tip speed was more nearly the same for the three 
impellers than the variation of slip factor and peak adiabatic 
efficiency. JJllpeller C had the hiBhest pressure ratios at equiva -
lent tip speeds as high as 1200 feet per secon1, impeller B had the 
highest at 1300 and 1500 feet per second,and impeller A had the 
highest at 1400 feet per s econd. 
4. Im~ello~ C had the highest maxllllUID specific capacity at all 
equivalent tip speeds up to 1500 faet per second, I:rn:psEer B had 
nearly the same naximum specific capacity as impeller C and 
impeller A hael a maximum 3pecific capacity 6 to U percent Sr..laller 
than that of irupe:_ler C. 
NACA TN No. 1313 
NATIONAL ADVISORY 
COMMITTEE FOR AERONAUTICS 
7.468 
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Figure I. - Axial- plane section of impellers A, 8, and C. A ll dimensions in 
inches. 
.' 
NACA TN No. 131 3 Fig. 2 
I a) Impell er A, parabol ic Ib} Impeller S, elliptical 
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NACA TN No. 1313 
NATIONAL ADVISORY 
COMMITT£E FOR AERONAUTICS 
Inlet duct 
Collector chamber 
r Diffu~~r-djsch~rge + measuring statIon 
Dead-ai r space 
I mpe I Ie r 
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Figure 6. - Comparison of peak adiabatic efficiencies , 
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Figure 7. - Comparison of sl ip factors at peak adiabatic efficiency. 
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NACA TN No. 1313 
NATIONAL ADVISORY 
C~MITTEE FOR AERONAUTICS 
Equivalent 
tip speed 
Uln 
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temperature 
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stagnation 
pressure 
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2 Specific capacity, Qt,1/IllrD2 , cu ftllmln"sq ftl 
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Figure 10. - Concluded . Perfo rmance characteristics of I mpellers . 
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An Investigation of the Effect of Blade Curvature on Centrifugal-impeller Performance

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