NACA

RESEARCH MEMORANDUM

PERFORMANCE OF MIXED-FLOW IMPELLER, MODEL MFI-IB, WITH

DIFFUSER VANES AT EQUIVALENT IMPELLER SPEEDS

FROM 1100 TO 1700 FEET PER SECOND

By Walter M. Osborn

Lewis Flight Propulsion Laboratory Cleveland, Ohio

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

WASHINGTON

June 16, 1954 Declassified April 15, 1958

https://ntrs.nasa.gov/search.jsp?R=19930088262 2020-07-23T03:01:52+00:00Z

NACA RM E54D23

NATIONAL ADVISORY COMMI1TEE FOR AERONAUTICS

RESEARCH MEMORANDUM

PERFORMANCE OF MIXED-FLOW IMPELLER, MODEL MFI-1B, WITH DIFFUSER VANES AT

EQ.UIVALENT IMPELLER SPEEDS FROM 1100 TO 1700 FEET PER SECOND

By Walter M. Osborn

SUMMARY

An investigation was made of the performance characteristics of a mixed-flow impeller, model MFI-1B, in combination with a vaned diffuser. The experimental results indicated that the peak pressure ratio and maxi­mum efficiency were not being obtained at overdesign speeds because of premature surge. An increase in the inlet angle at the leading edge of the vanes from 610 (as designed) to 660 improved the pressure ratio and efficiency at overdesign speeds with little or no effect at underdesign speeds. The peak pressure ratio and maximum efficiency at an equivalent speed of 1700 feet per second with a vane inlet angle of 660 were 5.39 and 0.733, respectively. An analysis of the data indicates that a pressure ratio of 6.0 and an efficiency of 0.72 can be obtained at an equivalent impeller speed of 1800 feet per second with a vane inlet angle of 690 •

INTRODUCTION

Two mixed-flow impellers, models MFI-l and MFI-2, have been designed and tested at the NACA Lewis laboratory in order to develop a reliable aerodynamic design procedure for centrifugal compressors (re1s. 1 to 4). The over-all performance characteristics of the two impellers, when tested wi th a vaneless diffuser} indicated good range, high efficiency, and high pressure ratlo per stage for this type compressor over a range of equi va­lent impeller speed from 700 to 1600 feet per second. However, it was recognized that the over-all performance characteristics of an impeller in combination with a vaned diffuser would be of more practical value to the designer. Therefore, the more efficient of the two impellers, the MFI-l (configuration B, ref. 4), was selected for testing ~th a vaned diffuser. A vaned diffuser, incorporating 24 vanes, was designed by the method of reference 5 and is shown in figures 1 and 2. The vanes were designed for an equivalent impeller speed of 1400 feet per second (design speed for the impeller) with a resulting inlet angle (fig. 1) at the leading edge of the vanes of 610.

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2 NACA RM E54D23

Three overdesign speeds, 1500, 1600, and 1700 feet per second, were

investigated with the vaned diffuser to enable the designer interested

in high pressure ratio per stage to extrapolate the performance curves to

a pressure ratio of 6.0 with a higher degree of accuracy. At the over­

design speeds, it was indicated that the maximum efficiency and peak

pressure ratio were not being obtained because of premature surge. To

alleviate this condition, the inlet angle at the leading edge of the vanes

was increased from 610 to 660 .

This report presents the performance characteristics of the MFI-lB

impeller .nth a vaned diffuser consisting of 24 vanes having an inlet

angle of 660 • In addition, portions of the performance curves for the

impeller with the vaned diffuser as designed (inlet angle of 610 ) are

presented to indicate the change in performance obtained by increasing

the inlet angle of the vanes and to enable the designer to extrapolate

the results to obtain the performance at additional settings of the inlet

angle.

u

w

SYMBOLS

The following symbols are used in this report:

actual impeller speed based on 7.00-in. radiUS, ft/sec

actual air weight flow, lb/sec

angle between camber line of vane and a conic element through

leading edge of vane (fig. 1)

ratio of inlet total pressure to NACA standard sea-level pressure,

29.92 in. Hg abs

~ad adiabatic temperature-rise efficiency

e ratio of inlet total temperature to NACA standard sea-level tempera-

ture, 518.40 R

APPARATUS, INSTRUMENTATION, AND PROCEDURE

Apparatus

The impeller used in this investigation was of the mixed-flow type

with 21 complete blades and 21 splitter blades and is identified as con­

figuration B of model MFI-l (ref. 4). In order to smooth out several

damaged blades at the inlet, the leading edges of this impeller were

swept backward from the hub to a point on the shroud 0.25 inch in the

axial direction from the original leading edge as shown in figure 1.

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NAeA RM E54D23 3

For the purposes of this investigation, a new diffuser was fabricated (fig. 1) incorporating a 2.40-inch vaneless section followed by a 3.50-inch vaned section composed of 24 vanes which were designed by the method presented in reference 5 for a diverging annulus. The divergence is con­tinued for an additional 0.5 inch, at which point the passage becomes a constant-area duct for the remaining 6.0 inches. A schematic diagram and photograph of the impeller and vaned diffuser are shown in figures 1 and 2, respectively. The vanes were cast from a 0.90-0.10 tin-zinc alloy having a melting point of 3~00 F and were attached to the lower diffuser Wall. A brass strip embedded in the casting for a distance of 2.0 inches from the leading edge (fig . 2) enabled the inlet section of the vanes to be bent a small amount in order to make changes in the inlet angle !3 .

The remainder of the experimental setup i s the same as that des­cribed in reference 2.

I nstrumentation

The instrumentation is the same as that in reference 2 with the exception that unshielded-type total-pressure probes were used for the outlet instrumentation. The outlet measuring station is located at a 13.0-inch radius (0.882 in. upstream from the end of the passage). The outlet instrumentation consisted of eight static-pressure taps, 12 total­pressure rakes, four thermocouple rakes, and eight thermocouple probes distributed throughout the 24 passages in such a manner as to give a minimum. blockage effect. The outlet instrumentation gave the equivalent of four passages instrumented as shown in figure 1.

Procedure

This investigation was carried out at a constant inlet air pressure of 14 inches of mercury absolute. Ambient air at 750 F was used at an equivalent impeller speed of 1100 feet per second. For speeds from 1300 to 1700 feet per second, an inlet air temperature of -600 F was used be­cause of the low melting point of the tin- zinc alloy used in the con­struction of the vanes and because of impeller stress considerations at speeds above 1400 feet per second .

The test and computation procedures are the same as those used in reference 2.

EXPERTh1ENTAL RESULTS

The over-all performance characteristics for the MFI-1B impeller wi th a vaned diffuser with the vane inlet angle !3 set at 660 are

---- - - - - - - - --- - - - -

4 NACA RM E54D23

presented in figure 3 for a range of speed from 1100 to 1700 feet per

second. The over-all pressure ratio is plotted against weight flow for

the range of speed, with contours of constant efficiency superimposed in

figure 3(a). The adiabatic temperature-rise efficiency and Mach number

at the outlet measuring station are plotted against weight flow for the

range of speed in figure 3(b). The peak pressure ratio and maximum

adiabatic efficiency at the design equivalent speed (1400 ft/sec) were

3 .71 and 0.771, respectively. At an overdesign speed of 1700 feet per

second, the peak pressure ratio was 5.39 and the maximum efficiency was

0.733. The average Mach number at the outlet measuring station for the

maximum efficiency points for the range of speed was between 0.27 and

0.30 as shown in figure 3(b).

In figures 4 and 5, the performance of the impeller with vaned

diffuser is compared at two settings of ~, 610 (as designed) and 660 .

The peak total -pressure ratiO, maximmn adiabatic efficiency, and maximum

and minimum weight flow are plotted against equivalent speed in figures

4 (a), (b), and (c), respectively. It is indicated in figure 4(a) that

the increase i n ~ to 660 caused no appreciable effect upon the pressure

ratio at speeds of 1300, 1400, and 1500 feet per second. However, at

speeds of 1600 and 1700 feet per second, there are substantial gains in

pressure ratio with a 7-percent increase at 1700 feet per second.

Figure 4 (b) indicates a similar trend in the maximum efficiency with an

inc.cease of approximately 3 points in efficiency at 1700 feet per second.

The weight-flow range shown in figure 4 (c) indicates a gain in range at

all speeds except 1300 feet per second as ~ is increased from 610 to

660 • Figure 5 shows a comparison of the over-all performance charac­

teristics at the two settings of ~ for the overdesign speeds and

summarizes the curves presented in figures 4(a) and (c).

Figure 6 gives an indication of the losses that might be expected

from dumping the air at the end of the diffuser vanes into a collector

by comparing the pressure ratio and efficiency based on measurements

taken at the outlet measuring station with the pressure ratio and ef­

ficiency based on static-pressure measurements taken in the collector.

Figure 6(a) indicates a 5- to 6-percent decrease in pressure ratio over

the speed range, while figure 6(b) indicates l osses of 3 to 4.5 points

in efficiency with the greater losses occurring at the lower speeds.

DISCUSSION OF RESULTS

The experimental results obtained with the diffuser vanes as de­

signed indicated that the peak total-pressure ratio and the maximum

adiabatic efficiency at speeds above design speed (1400 ft/sec) were

not being obtained because of premature surge, as shown by the surge

line for ~ = 610 in figure 5 . Therefore, the leading edges of the

vanes were bent slightly to increase ~ to 660• This change in ~

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NAeA RM E54D23 5

caused little or no loss in maximum flow but increased the range by shift­ing the surge line, as shown in figure 5 for ~ ~ 660 , thus producing a peak pressure ratio of 5.39 and a maximum efficiency of 0.733 at a speed of 1700 feet per second.

The performance at 1700 fee t per second gave indications that a pressure ratic of 6.0 wi t h an efficiency of 0.70 or more could be obtained by increasing the equivalent impeller speed. With the assumption that the pressure ratio will increase as the square of the speed, a speed of 1800 feet per second is necessary to obtain a pressure ratio of 6.0. However, figure 4(a) shows that a pressure ratio of 6.0 could not be obtained with vane inlet angles of either 6lo or 660 for a speed of l800 feet per second. In figure 4(c), zero range of weight flow is indicated for ~ = 610 and a very small range for ~ = 660 at 1800 feet per second. In figure 4(b), efficiencies below 0.70 are indicat ed for both angle settings.

A comparison of the over-all performance characteristics at 1600 feet per second (fig . 3(a)) with those of the impeller with a vaneless diffuser (ref. 4) indicated that, with the vaned diffuser, surge waS caused by the vanes. Thus, it may be assumed that the vanes could be bent an additional 50 from ~ = 660 to ~ = 710 with an increase in performance resulting at overdesign speeds. Hypothetical performance curves, obtained by extrapolating the performance curves of figures 4 and 5 by direct proportion, were determined for the minimum value of ~

required to obtain a pres sure ratio of 6.0. Only the minimum required angle is considered here in order to minimize any detrimental effects upon the compressor performance which may result at lower speeds. The inlet vane angle required was 690 , and the hypothetical curves of figures 4 and 5 for this angle indicate that a pressure ratio of 6.0 and an efficiency of 0.72 can be obtained at an equivalent impeller speed of 1800 feet per second.

SUMMARY OF RESULTS

An investigation was made of the performance characteristics of a mixed-flow impeller, model MFI-IB, in combination with a vaned diffuser. In order to improve the performance characteristics at speeds above design speed, the inlet angle of the vanes was increased from 610 (as designed) to 660 . The experimental data were analyzed to see if a pressure ratio of 6.0 could be obtained at a higher speed. The following results were obtained:

1. At a vane inlet angle of 660 , the peak pressure ratio and maximum adiabatic efficiency at a design equivalent speed of 1400 feet per second were 3.71 and 0.771, respectively. At an overdesign speed of 1700 feet per second, the peak pressure ratio was 5.39 and the maximum efficiency was 0.733.

6 NACA RM E54D23

2. Increasing the vane inlet angle from 610 to 660 had no appreciable effect upon the pressure ratio and efficiency at a design speed of 1400 fee t per second and at lower speeds . However, there were gains of 7 per­cent in pressure ratio and 3 points in efficiency at 1700 feet per second. There were gains i n wei ght -flow range at nearly all speeds.

3. Losses of 5 to 6 percent in pressure ratio and 3 to 4 .5 points in efficiency may be expected by dumping the air into a collector at the end of the diffuser vanes.

4. An analysis of the experimental data for vane inlet angles of 610

and 660 indicated that a pressure ratio of 6 .0 and an efficiency of 0.72 can be obtained at a speed of 1800 feet per second with a vane inlet angle of 690

.

National Advisory Committee for Aeronaut ics Lewis Flight PropulSion Laboratory

Cleveland, OhiO, April 22 , 1954

REFERENCES

1. Osborn, Walter M. , and Hamrick, Joseph T.: Design and Test of Mixed­Flow Impellers . I - Aerodynamic DeSign Procedure . MCA RM E52E05 , 1952 .

2. Withee , Joseph R . , Jr . , and Beede, William L. : DeSign and Test of Mixed-Flow Impellers . II - Experimental Results, Impell er Model MFI - lA . NACA RM E52E22, 1952 .

3 . Hamrick, Joseph T. , Osborn, Walter M' J and Beede, William L.: DeSi gn and Test of Mixed-Flow Impellers. III - DeSign and Experimental Results for Impeller Model MFI-2A and Comparison with I mpeller Model MFI- lA . NACA RM E52L22a, 1953 .

4 . Hamrick, Joseph T. , Beede, William L., and Withee, Joseph R.) Jr. : Design and Test of Mixed-Flow Impellers. IV - Experimental Results f or Impeller Models MFI- l and MFI- 2 with Changes in Blade Height . NACA RM E53L02, 1954 .

5 . Stanitz, John D.: Approximate Design Method for High- Solidity Blade Elements in Compressors and Turbines. NAeA TN 2408, 1951 .

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Cross-sectional view .

Figure 1 . - Schematic diagram of MFI- lB impeller with vaned diffuser showing location of outlet instrumentation .

Plane of outlet instrumentation

area duct

Pl ane of out l et instrumentationj7

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8 W 12 M 16 Equivalent weight flow, w~/D, Ib/sec

(a) Performance characteristics.

Figure 3 . - Over-all performance characteristics of MFI-LB impeller with vaned diffuser. Angle at inlet to vanes, 66° ; inlet-air pressure, 14 inches of mercury absolute.

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Equivalent weight flow, W.[ej'6, lb/sec

(b) Over-all efficiency and Mach number .

Figure 3 . - Concluded. Over-all performance characteristics of MFI-lB impeller with vaned diffuser . Angle at inlet to vanes, 66°; inlet­air pressure, 14 inches of mercury absolute.

NACA RM E54D23 11

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(a) Over-all pressure r atio.

Figure 4 . - Comparison of per for mance of MFI-lB impell er wi th vaned diffuser with t wo settings of i nlet angl e at leading edges of vanes. (Curve s are extended for impeller speeds greater than 1 700 ft/se c. )

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Impeller equi valent speed, U/~ f t/sec

(b ) Maximum ef f iciency.

NACA RM E54D23

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Figure 4 . - Continued . Comparison of per f ormance of MFI- 1B

impeller with vaned diffuser with t wo settings of inlet

angle at l eading edge s of vanes . (Curves are extended for

impel ler speeds greater than 1700 ft/sec.)

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Impeller eqUivalent speed, U/..re, ft/sec

(c) Weight-flow range.

Figure 4. - Concluded. Comparison of performance of MFI-lB impeller with vaned diffuser with two settings of inlet angle at leading edges of vanes . (Curves are extended for impeller speeds greater than 1700 ft/sec.)

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NACA RM E54D23

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1' , deg impeller -

61 ( experimental) speed, U/~,

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~ ~ 1 1500

14 15 16 17 18 Equi valent weight - flow rate, W~O , l b/ sec

Fi gure 5 . - Comparison of over-all performance chara cteri st ic s of MFI- lB i mpeller wi th vaned diffuser at two set t i ngs of vane i nlet angle. (Resul t s are extr apol ated to obtain performance curve at 1800 ft/se c wi th vane i nlet angle of 69° . )

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Figure 6. - Comparison of performance of MFI-lB impeller with vaned diffuser (vane inlet angle , 66 0 ) based on measurements t aken at the outlet measuring station and on static- pressure measurements taken in the col lector .

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. 66 1300 1400 1500 1600 1700 1800

Impe l ler equi valent speed J U/~ ft/sec

(b) Maximum ad iabat ic efficiency .

Figure 6 . - Concl uded. Comparison of performance of MFI- 1B impeller with vaned di ffuser (vane i nlet angl e , 66 0 ) based on measurement s taken at the outl et measuring stati on and on static-pre s sure measurements taken in the collector .

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