AIAA 2009-3964
27th AIAA Applied Aerodynamics Conference
22 - 25 June 2009, San Antonio, Texas
Aerodynamic Design of a Part-Span Tandem Bladed Rotor
for Low Speed Axial Compressor / Fan
Bhaskar Roy*,
Srivatsava V Puranam#
Aditya S. Mulmule^
Aerospace Engineering Department, Indian Institute of Technology, Bombay, Mumbai 400076, India
Tandem blades provide a very effective option to overcome the blade loading limitations.
The flow from the pressure surface is used to energize the flow on the rear blade suction
surface to avert impending flow separation. Based upon earlier experimental and CFD
studies, as well as on results from other studies, a design philosophy was formulated to
attempt the design of the hybrid tandem blades. This paper reports the design of a novel
concept of hybrid rotor where the tandem configurations have been applied to only over the
part of the span of the rotor blades. An aerodynamic design and experimental study of such
a rotor was undertaken. The study indicated a remarkable performance improvement in the
regions where tandem modifications were applied, which also promoted overall rotor
performance enhancement. The hybrid tandem configuration provides a way to enhance the
blade loading capacity and further optimize the aircraft engine compressor.
Keywords: axial compressor, tandem blade, part-span, hybrid tandem
Nomenclature
AR = Aspect ratio;
c= Chord length;
Ca = Absolute axial velocity, m/s;
CDA = Controlled diffusion airfoil;
Cp=Coefficient of pressure;
Cw = Absolute swirl velocity, m/s;
DF = Diffusion Factor = 1-
cosβ1 ∆Cw cos β1
;
+
cosβ 2 2 × Ca × σ
i = minimum loss incidence angle = β1-κ1;
LE = leading edge;
P = Pressure, Pa;
PS = Pressure side;
SS = Suction terrors;
TE = trailing edge;
β =relative air angle, degrees;
δ =angle of deviation = β2-κ2, degrees;
κ = blade setting angle, degrees;
θ = camber angle = β1-β2-i+δ, degrees;
ξ =stagger angle =β1 − i − (θ / 2 ) , degrees;
Subscripts / superscripts:
1, 2 = entry, exit to the blade;
FB, RB = Front, Rear blade;
* Professor and Senior Member AIAA ;
# B.Tech IIT, Bombay, presently Gradate Research Assistant,@ UC., Irvine, USA, and Student-member AIAA
^ Gradate Student, non-member-AIAA
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Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
1. Introduction
The pursuit for improvement in the power output and efficiency of the aircraft engine has led to the design
of high performance and compact aircraft engine components. In the modern aircraft engines the design of axial
compressor plays a very vital role. The required size of the compressor has been reduced along with the aircraft
engine size, which has put the onus of generating required pressure rise within minimum number of stages on the
designer. Reduction in the compressor size requires one to increase its suction capacity, aerodynamic loading
capacity, decrease the aerodynamic losses and ultimately reduce the number of stages and number of blades in every
blade row. Such design optimizations have often led to the development of innovative blade shapes e,g, computer
generated airfoils like CDA, swept blades and leaned blades, which produce efficient and stable work output.
Although these advances have improved the compressor performance and efficiency a great deal (today, it is of the
order of 90%) further improvement has become necessary due to the unending demand of higher blade loading
without increasing overall size of the engine components [1, 2].
Tandem blades (fig.1a) provide a very effective option to overcome the present stage loading (high turning)
limitations, as has been reported by some existing literature [1-3]. A tandem blade has airfoils in tandem across its
chord-wise cross-section. The airfoil at the front forms the front blade and the airfoil at the rear constitutes the aft
blade. In case of airfoils in tandem, the boundary layer on the suction surface, which is prone to separation, is
energized by the airflow coming from the front blade and hence the separation is averted [2].
Fig. 1(a) Fundamentals of tandem blades
1(b) Single & Tandem CDA blades studied at IITB
2. Earlier Studies with Tandem Blades
Bammert and Beelte [3] developed a 5-stage tandem bladed axial compressor where the middle three stages
had tandem bladed rotors. The characteristic compressor performance map, pressure and velocity measurements
downstream of the stages of this compressor indicate the improved stage loading capacity due to tandem bladed
rotors as compared to the single bladed one (a blade having only one airfoil rather than having two in tandem). Their
study also revealed a limited operating range for tandem blades as a result of the close proximity of surge line to the
design point which might have resulted from absence of optimum blade geometries of rotor as well as stator. Their
study does not give enough qualitative basis for performance comparison of different tandem blade shape. Such a
comparison was reported by Saha and Roy [1] (Fig.1b). They studied the coefficient of pressure, stage diffusion
factor, mean loss coefficient and wake profiles for a single blade cascade (CDA 43) as well as two different tandem
blade cascades (CDA 21-21 and CDA 32-21). The passage formed between the gaps due to the near-overlap of the
front blade over aft blade was changed in case of CDA 32-21 by providing cusped shaped trailing edge. More flow
turning without separation was extracted out of CDA 32-21 cascade by providing extra 50 of camber.
The Cp distribution across the tandem and single blades in cascade indicate (fig.2) the increase in the
suction capacity along with the absence of flow separation in case of tandem blades as opposed to single bladed
cascade, where flat Cp distribution on the rear half of the suction surface already shows signs of impending
separation and stall. The continued diffusion indicates a greater loading capacity for the tandem bladed cascades and
the loss coefficient as well as wake thickness profiles (Fig.3) indicate controlled growth of boundary layer where
flow separation was observed for single blade cascade on its suction surface. The initial cascade CFD study was
done at the camber angle, and boundary conditions of CDA 43 profile [5]. Mach contour of solid CDA 43 and
tandem blade on Fig.3 shows thinner wake and separation delayed for the tandem blade as compared to the solid
blade for same camber angle of the blades. The flow in the gap shows acceleration, which increases the kinetic
energy of the flow on the suction surface of the rear tandem blade
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Separation bubble
Fig.2. C Distributions: Single (43
p
0
0
Camber) & Tandem (48 Camber) [both at 220 stagger) [1]
Fig 3 Mach contour of solid and tandem blade blades at M1=0.0576, camber angle of 480 [5]
Separation bubble
o
o
Fig.4. C Distributions: with and without tip clearance : Single (43 ) & Tandem (48 ) [6]
p
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The analysis of cascade parameters with (WTC) and without (ZTC) tip clearance, measured at mid-span
region, was initiated for the design of axial compressor rotor with part-span tandem blades. The tandem blades were
compared with the earlier solid CDA blades and showed better control of the Cp distribution. (Fig.4]. The flow
through the gap thus helps to suppress both separation bubble on the early part and the separation tendency on the
later part of the front blade, and then it helps delay flow separation on the rear blade. With 2% tip clearance (WTC)
the diffusion on the rear blade is less strong than without (zero) tip clearance, though the separation tendency is still
suppressed
Apart from these advantages, due to a thicker and more turbulent boundary layer the tandem blades may
suffer large aerodynamic losses [1]. It indicates that the design of the compressor with tandem blades is probably a
trade off between the higher pressure rise per stage and higher aerodynamic losses. This particular angle has not
been explored for compressors with tandem blades. The objective of the current work is to explore this trade off in
details and figure out a possible way to minimize tandem cascade losses. There aren’t sufficient studies which would
establish guidelines for designing axial compressor with tandem blades. The aim of the current work is to establish
further these guidelines through low speed testing which could be used to design tandem blades compressors to
begin with for handling equipments, ventilation fans, etc and later on may qualify for use in aircraft engines.
Owing to these highly encouraging results a recent study was conducted with design of compressor rotor
with part-span tandem blades [2], and tests were conducted at the low speed axial flow fan facility at IIT Bombay.
At present, efforts have been made at this axial flow fan research facility to develop a rotor with hybrid
blades (fig 5). This concept, which can be used to improve the blade loading and rotor performance, has not been
reported before. The performance of axial flow fan with an earlier solid blade rotor (old rotor) is then compared with
the new part-span tandem bladed axial fan.
3. Part-Span Tandem Bladed Axial Fan
LE
LE
PS
NBS
PS
TBS
Fig.5. Part-span tandem rotor blade
(LE–Leading edge, PS–Pressure surface, NBS–Normal blade shape, TBS–Tandem blade shape) [2]
The design of blades for the single bladed rotor was accomplished by the blade element design procedure
using free vortex. With a view to use tandem blades high blade loading was designed into the lower half of the rotor
blades. First airfoils at different radial blade height are designed, and then blade cross-sections in-between them are
smoothly extrapolated from these airfoils. The tandem airfoils were designed according to following guidelines:
a) The single airfoil was converted into a set of tandem airfoils as shown in figure 2. The front blade was
created along the first half of the original camber line while rear blade was created along the second half ;
b). The chord lengths of both the blades (cFB and cRB) were chosen such that the axial gap between the
airfoils was kept at 2 per cent of the total chord and the tangential gap was 1 percent of the total chord ;
c) Front blade consisted of leading edge to 50 % c and rear blade consists of 50 %c to the trailing edge and
the total blade loading was distributed amongst the two blades;
d) From camber and known CDA (modified C4) blade profiles, camber line slopes at leading edge, midchord and trailing edge were calculated;
e) From these slopes cambers of front blade and aft blade were calculated. It was assumed that the air outlet
angle from the front blade is same as the air inlet angle to the rear blade. The total camber of the airfoil was
modified in such a way that the total flow deflection attempted by the tandem blades would increase;
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Tandem blades are not likely to be advantageous below camber of 40 degrees. Also, tandem (or split)
blades near the rotor tip is not good idea for the complex tip flow situations. Thus the blade upper half was designed
to retain single airfoil shape (solid airfoil).. The blade sections, where design blade camber is greater than 40 degrees
were replaced by the tandem blade arrangement. Baseline design details are shown in the following Table.1.
Table 1: Blade Element Design at hub, mean, tip & other intermediate blade sections
Location Hub
Mean
Tip
0.22
0.24
0.26
0.28
0.32
0.34
0.36
0.38
Station 0.2
0.3
0.4
32.67
35.19
40.21
42.73
45.24
47.75
U 25.13 27.65 30.16
37.7
50.27
42.06
44.82
49.12
50.89
52.49
53.96
β1
28.93 34.31 38.64
47.13
55.32
0.22
8
21.6
27.19
32.05
36.27
β2 ‐20.67 ‐14.93 ‐7.67
15.19
39.94
41.84
36.82
27.52
23.69
20.44
17.69
∆β
49.6 49.24 46.31
31.93
15.38
0
0
0
0
0
0
0
0
α1 0
0
0
41.94
40.47
37.21
35.61
34.08
32.64
α2 42.92 43.5 43.05
38.85
31.3
36.21
35.41
34.81
34.74
34.73
34.75
Ca 45.48 40.51 37.73
35
34.78
1.24
1.18
1.12
1.06
0.94
0.88
0.82
0.76
index 1.3
1
0.7
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0.23
0.23
m 0.23
0.76
0.83
0.9
0.97
1.11
1.17
1.24
1.31
sqrt(c/s) 0.69
1.04
1.38
10.22
8.66
6.05
5.05
4.24
3.57
δ
13.81 13.07 11.77
7.26
3.02
52.05
45.48
36.57
32.75
29.67
25.26
θ
63.41 62.31 58.08
39.19
22.4
0
0
0.01
0
‐3
‐4.01
‐4.99
‐4
i
0
0
‐4
9.6
16.025
22.08
45.33
stager ‐2.775 3.155
27.535 33.835 38.525 42.645
48.12
The gap geometry was decided on the basis of the earlier experimental and CFD cascade studies [1,5,6].
The single blade sections (mid span to tip) for these blades were kept unchanged, whereas the tandem blades were
introduced in the lower half. The single blade sections used known CDA blade profiles [1] and the tandem blade
airfoils were redesigned according to the above guidelines and the cascades studied earlier. Smooth 3-D blade
shapes were obtained through digital modelling and blade fabrication process. Designing blade in this way, it was
expected that around 25 % more total pressure would be generated in the tandem bladed portion as compared to
single blade portion of the same blade. The fan rotor was designed for operation at 2400 rpm at 30o C and 1 bar.
4. Rig Test Results
Fig. 6 Experimental test rig for low speed axial flow compressor research
The existing single-stage, low-speed axial flow fan test facility at IIT Bombay is shown in Fig 6. The setup consists
of inlet bell mouth, constant area duct, fan stage, diffuser, constant area duct and throttle cone. Maximum mass flow
rate is about 4.0 kg/s at 2400 rpm. The fan has a diameter of 0.400 m. Rotor is driven by the 15 KW AC motor with
a variable frequency drive controller. A 1:2.6 ratio pulley belt drive is provided for running the axial fan at higher
rpm of the order of 4500. At the exit of the test tunnel duct a conical throttle controls the airflow rate, which is
actuated by a screw mechanism. The rpm and throttle cone can be independently varied to achieve required mass
flow rates or rotational speeds. The rotor comprises of eleven blades and the stator consists of thirteen blades. Tip
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clearance is maintained close to 0.5 percent of the rotor diameter. The pressure readings are obtained using a
o
shielded kiel probe ( ±55 directional insensitivity) behind the rotor and a standard pitot-static probe upstream of
the rotor, on a digital micro manometer ( ±1% error), giving time averaged readings. The speed of the rotor was
measured in RPM using an infra-red non-contact digital sensor ( ±1% error) fitted on the rotor shaft.
1
1100
Conventional blade
Conventional blade
1050
Hybrid blade
Hybrid blade
0.95
1000
0.9
Efficiency
P re ssu re R ise , P a
950
900
850
0.85
0.8
800
0.75
750
0.7
700
3
3.5
4
4.5
5
3
5.5
3.5
4
4.5
5
5.5
mass flow rate, kg/s
mass flow rate, kg/s
Fig. 7: Pressure rise and efficiency characteristics of solid and part-span tandem bladed rotors [2]
Maximum increment in the total pressure rise obtained in the rig tests is about 15% in the tandem portion
(lower half) of the blade. A mass averaged overall 5% stage pressure rise and 4 % rise in stage efficiency is also
obtained for the entire rotor at the design point. Efficiency was obtained by comparing the measured shaft input
power and the computed aerodynamic fan output power. The addition of tandem blade improved the stall margin by
a large amount (Fig. 7). Also, the characteristic curves are flatter which indicates stable and safe off-design
performance capability [2]. However the curves have shifted a little to the left of an earlier solid bladed old
compressor characteristics, indicating that the tandem blades offer higher aerodynamic blockage to the flow through
them. Hence in future designs this would need to be compensated by proper aerodynamic tailoring of the blade
shape designs. Further optimization of the flows through 3-D CFD studies in the gap in the tandem configuration
may help improve the design further. Figure 8 shows the pressure rise attained by the tandem bladed rotor along the
blade height at the design point. The region where the tandem modifications were applied show 10 to 15 per cent
increase in the pressure rise.
120
Conventional blade
Hybrid blade
Tip
100
per cent span
80
60
40
20
Hub
0
750
800
850
900
950
1000
1050
1100
1150
Pressure rise, Pa
Fig 6: Pressure rise along the blade : single airfoil and the hybrid tandem rotors at design point
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5. Conclusions
1. The design of a hybrid blade with tandem configuration has been discussed. The tandem configuration design has
several controlling parameters such as axial and tangential gaps amongst the front and aft blades, the loading on the
front and aft blades. It also has some limiting parameters such as the total camber. Tandem configuration is not
beneficial below the camber of 40 degrees, compared to a well designed airfoil.
2. The design modifications based upon the above parameters gave birth to a novel concept of hybrid blades. The
tandem blade was applied to the blade elements where the camber was greater than 40 degrees. An experimental
study of such a rotor was undertaken at the axial flow fan facility at I.I.T., Bombay. The study indicated a
remarkable performance improvement in the regions where tandem modifications were applied and also overall
performance improvement is observed. The hybrid tandem configuration provides a way to enhance the blade
loading capacity and further optimize the aircraft engine compressor.
3. Recent studies indicated that for a tandem configuration optimization of axial and tangential gaps can provide
performance clearly superior to single airfoil blade. The change in the tangential gap was observed to influence the
performance of such a blade more as compared to the that of the axial gap. The optimum values of both these
configurations were within 2 to 5 per cent of the chord for various geometries reviewed..
4. Based on our experimental studies as well as on the results from various other studies, a design philosophy was
formulated to attempt the design of part-span tandem blades.
5. Further improvement in the tandem rotors is possible by active control of the blade angle of the rear blade. This
would open up the possibility of variable camber blades. However the flow through the gap under various operating
conditions would need to be analyzed in CFD in some detail before this is implemented.
8. Although a few studies have reported on flow over blades with airfoils in tandem, no reported research studies
have been devoted to the development of compressor rotor with part-span tandem blades.
References
[1] Saha, U.K., Roy, B., “Experimental investigations on tandem compressor cascade performance at low speeds”,
Experimental Thermal and Fluid Science (Elsevier), 1997, vol. (14), pp. 263-276.
[2] Srivatsava, P., “Design and Testing of a Tandem Bladed Rotor for Axial Compressors”, B. Tech project report,
2004, Aerospace Engineering Department, IIT, Bombay.
[3] Bammert, K., Beelte, H., “Investigation of an axial flow compressor with tandem cascade”, ASME Journal of
Engineering and Power, 1980, vol. (102), pp. 971-977.
[4] Wallis, R., “Axial flow fans and ducts” , New York : John Wiley, 1983.
[5] Melkia, Yilak; “CFD Analysis of tandem blades for axial flow compressor,” M.Tech Dissertation, 2003, IIT,
Bombay.
[6] Ujjwal K Saha ; Bhaskar Roy; “Effect of Tip Clearance on Blade Mid-span flow field in Variable Camber
XIV International Symposium on Air Breathing Engines, (16th ISABE),
Tandem Compressor Cascade”,
Cleveland, Ohio, USA, 2003
[7] Ed. by: Johnson I ; Bullock, R.O. “Aerodynamic Design of Axial Flow Compressor”, NASA SP 36, 1965.
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