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A design recommendation for truly reversible axial flowfan design

Bahuz Can Osso

To cite this version:Bahuz Can Osso. A design recommendation for truly reversible axial flow fan design. 16th In-ternational Symposium on Transport Phenomena and Dynamics of Rotating Machinery, Apr 2016,Honolulu, United States. �hal-01894402�

Article Title — 1

A DESIGN RECOMMENDATION FOR TRULY REVERSIBLE AXIAL FLOW FAN DESIGN Bahuz Can Osso

ISROMAC 2016

International Symposium on

Transport Phenomena and

Dynamics of Rotating Machinery

Hawaii, Honolulu

April 10-15, 2016

Abstract Truly reversible axial flow fans have a wide range of usage area in industry. These type of fans are aerodynamically same for both flow directions. They are especially used for emergency ventilation such as underground car park and tunnel ventilation systems which require the same aerodynamic performance in both directions. Different blade variations can be obtained according to motor and hub diameters, motor shaft and rotor hub connections. These type of blade geometries can be designed as “S-Shape Blade Profiles” which are aligned symmetrically or dis-symmetrically around a radial axis. In this study, optimum values for a dimensionless number called “Pressure Correction Number” is recommended according to different initial guessed pressures, rotational speeds and blade quantities so that we can effectively shorten the design and optimization process of S-Shaped Blades. In the first step of rotary machine designs, more specially fan designs, one needs several initial values such as volumetric flow rate, flow diameter and total pressure. If one does not know the total pressure, we have to assume an initial total pressure and we will obtain different design points. At that point, a Pressure Correction Number is used to assume a total pressure value. At the beginning of the study, velocity triangles are obtained according to “free vortex” assumption which recommends quasi-equal pressure and work distributions. After this step an initial CAD geometry will be generated and this geometry will be used as a computational domain for further ANSYS-CFX (RANS) runs. At the end of the first solution step, we will be searching for the consistency of the assumed Pressure Correction Number and analysis results. Several CFD analysis will be run until obtaining an acceptable convergency. Keywords Axial Turbomachines — Truly Reversible Axial Fans — Computational Fluid Dynamics — ANSYS-CFX 1 Department of Heat & Fluid, Istanbul Technical University, Istanbul, Turkey and Bahcivan Engineering Ltd., Istanbul, Turkey

*Corresponding author: osso@itu.edu.tr and c.osso@armovent.com

INTRODUCTION “S-Shaped Blades” are used with several NACA profiles which are generated by means of constant twist angles in industry. Especially NACA – 6512 series are one of the frequently used type [1]. However “S-Shaped Blades” created according to half of the chord length of chosen NACA series, more generally NACA series are created according to the fluid which flows from leading edge to trailing edge. Thus we have a blade seems like having two leading edges.

The author thought the flow rate of tunnel jet fans, which products are frequently used in industry, as boundary condition. In this study, the axial velocity of the fluid assumed constant as an initial guess because of the assumption of the constant cross-sectional flow areas for all prototypes. This assumption is valid for same rotational speed fans. The author referenced some initial total pressures for the designs, after then by means of “free vortex” assumption, different velocity triangles created both hub and tip profiles and so that the blade profile, which has formed according to constant twist angles, generated in commercial CFD programme calls as ANSYS CFX (RANS) until obtaining an acceptable convergency [6]. The results will show us we will obtain different “pressure

correction numbers” for different type of blades, blade quantities, rotational speeds, thrusts and thrust efficiencies.

SYMBOLS D : Fan diameter (m) d : Hub Diameter (m) Aflow : Flow area (m2) cax : Axial velocity component of the fluid (m/s) Q : Volumetric flow rate (m3/s) U : Blade velocity (m/s) n : Rotational speed (rpm) w1 : Relative inlet velocity of the fluid (m/s) Δptot : Total pressure (Pa) ρ : Density of the fluid (kg/m3) U2 : Tangential outlet velocity component of the fluid (m/s) cu2 : Tangential absolute outlet velocity component of the

fluid (m/s) U1 : Tangential inlet velocity component of the fluid (m/s) cu1 : Tangential absolute inlet velocity component of the fluid

(m/s) wu2 : Tangential relative outlet velocity component of the

fluid (m/s) w2 : Relative outlet velocity of the fluid (m/s) wm : Relative mean velocity of the fluid (m/s) β1 : Inlet angle of the fluid (o) β2 : Outlet angle of the fluid (o) βm : Mean angle of the fluid (o) : Stagger Angle (o)

Article Title — 2

ϕ : Angle of attack (o) : Glide Angle (o)

s : Blade space (m) cL : Lift coefficient () cD : Drag coefficient () L/D : Lift drag coefficient () : Thrust efficiency ()

dt : Tip diameter (m) F : Thrust (N) P : Shaft power (W) 1. METHODS

The fans designed according to the ideal gas conditions which are in atmospheric pressure and 20 oC constant temperature. Tunnel fans –especially tunnel jet fans- works in the principle so called “low pressure and high flow rates” therefore the author limited the hub-tip ratio in the value of 0,5 as Bleir specified [4]. According to this information the corresponded hub diameter to 1,4 m fan diameter specified as 0,5 m as applied in industry. The tip clearance is % 1 for all models. Other specifications for designed fans are as follows;

Table 1. Basic Specifications of Designing Fans Design

No Flow Rate

(m3/s)

Initial Total Pressure

(Pa)

Rotational Speed (rpm)

Blade Quantity

() 1 40 1500 1485 9 2 40 500 1485 9 3 40 250 1485 9 4 40 1000 1485 9 5 40 1500 1485 10 6 40 500 1485 10 7 40 250 1485 10 8 40 1000 1485 10 9 20 375 375 9

The flow area of the fan and axial velocity of the fluid calculated as follows;

Aflow= π (D2 - d2) (1) 4 cax= Q (2) Aflow

We will specify NACA 6512 profiles with different

stagger angles for hub and tip profiles in next steps. For axial flow fans absolute inlet velocity to cascade is equal to axial velocity and also blade velocity is tangential component of relative inlet velocity to cascade.

U= π n D (3) 60 w1= √(U2 + cax2) (4) The total pressue rise is as follows;

Δptot= ρ U2 cu2 - ρ U1 cu1 (5)

Tangential components of inlet and outlet flows to cascade are equal to each other for axial flow fans (U2 = U1 = U) so that we can find the tangential component of the outlet absolute velocity as follows;

cu2= Δptot (6) ρ U2

The values for tangential component of the outlet

relative velocity and outlet relative velocity can be determined as follows;

wu2= U - cu2 (7) w2= √(wu2 + cax2) (8) The mean relative velocity; wm= (w1 + w2) (9) 2

Figure 1. NACA 6512 Chart

The inlet and outlet angles of the fluid can be

determined as follows; β1= arccos(U / w1) (10) β2= arccos(wU2 / w2) (11) The mean angle of the fluid;

Article Title — 3

βm= arctan((tan(β1) + tan(β2))/2) (12)

We will determine an attack angle from NACA Report

460 [1]. The author chose attack angle as the value of 0o because lift-drag coefficient has a maximum value for this range. After this step we will determine stagger angle as follows. This angle is relative to flow axis and it will equally raise from hub to tip because twist angle determined constant before.

= 90o – (βm + ϕ) (13)

We will apply following equation to define “glide angle”

by means of lift and drag coefficients which has determined before according to NACA 6512. This the angle which is between lift and drag forces [3].

= arctan(cD/cL) (14) The chord length can be determined as follows for all

sections of blade [3]; c= 2 Δptot s cos( ) cax (15) cL ρ wm2 U sin(βm + ) Daneshkhah and Sheard has described an efficiency

coefficient calls as “thrust efficiency” to have an optimum design for tunnel jet fans [5].

= 0,515 F1,5 (16)

dt P where the shaft power determined from torque and

rotational speed of the fan. Torque and thrust values will obtain by means of numerical analysis. Now we can calculate these values for the prototypes as follows;

Table 2. Calculated Specifications of Designing Fans

Design No

1 2 3 4 5 6 7 8 9

Hub wm 39,8 45 46,9 41,7 39,8 45 46,9 41,7 19,9

Tip wm 107 111 112 109,2

107 111 112 109,2 53,7

Hub βm 68,8 42,3 39,6 51 68,7 42,3 39,6 51 68,8

Tip βm 16,1 15,6 15,4 16 16,1 15,6 15,4 16 16,1

Hub θ 21,3 47,8 50,4 39 21,3 47,8 50,4 39 21,3

Tip θ 73,8 74,4 74,6 74 73,8 74,4 74,6 74 73,8

cL 1,02

L/D 21,5

ζ 2,66

Hub c 218 76 36,75

155 196 68,4 33 139,5 218

Tip c 87,8 28,22

13,97

57,5 79 25,4 12,6 51,72 87,8

According to these information CAD geometries can

create as following sample pictures;

Figure 2. Model-1 CAD Geometry – 9 Blade

Figure 3. Model-5 CAD Geometry – 10 Blade

Thanks to the interface type of rotational periodicity for

each slices of the blades the author performed one slice of the blade for the numerical analysis. Thus our mesh quality, consistency of analysis and so that the convergence will be better. The mesh characteristics as follows. The reason for different mesh quantities is the possibility of different volumes for each computational domains, mesh qualities and to obtain acceptable convergence for each models.

Table 3. Mesh Characteristics

Design No 1&9 2 3 4 5 6 7 8

# Mesh Elements

113. 671

208. 776

37. 488

87. 680

112. 182

399. 372

59. 200

160. 136

The mesh topology method was chosen as H/J/C/L-

Grid for the computational domains and also added O-type grid. The sample for the mesh as follows;

Article Title — 4

Figure 4. Model-7 Computational Domain

Air ideal gas used in analysis as fluid. The rotational

speed for the fan models from 1 to 8 is 1485 rpm and 742,5 rpm for Model 9. The author does not require to solve energy equation because the problem is incompressible flow problem so that the flow is isothermal for all models. The total pressure is 0 Pa relative to ambient for inlet boundary of the models and also turbulence intensity assumed as % 1 for this boundary. The mass flow rates for the models from 1 to 4, from 5 to 8 and 9 are respectively 5,351; 4,816 and 2,6755 kg/s which are predefined in calculations before. Turbulence intensity for outlet boundary is % 5. The reason for difference of turbulence intensities between inlet and outlet boundaries can be explained as following: “Turbulent kinetic energy of the fan inlet is lower than outlet’s.” “Shear Stress Transport” is used as turbulence model for all models. The convergence target is 10-5. The samples of convergence curves are as follows.

Figure 5. Model-3 Convergence Curve– Momentum

Figure 6. Model-3 Convergence Curve– Turbulence

The resulting pressure value, thrust, thrust efficiency

and pressure correction number for all models are shown as the following section.

2. RESULTS AND DISCUSSION

Table 4. Numerical Analysis Results

Design No

Initial Total

Pressure [Pa]

Resulting Total

Pressure [Pa]

Thrust [N]

Thrust Efficiency

[]

Pressure Correction

Number []

1 1500 305,4 1729,7 0,58229 4,911591356

2 500 29,773 861,52 2,61 16,79373929

3 250 24,673 736,16 5,5572 10,13253354

4 1000 55,559 1237,8 0,93147 17,99888407

5 1500 340,63 1673,4 0,55254 4,403605085

6 500 26,299 844,69 2,1167 19,01212974

7 250 23,419 746,26 4,9049 10,67509287

8 1000 55,305 1227,8 0,94287 18,08154778

9 375 75,031 435,83 0,2929 4,997934187

In an attempt to better understand the variation of the

resulting values with initial total pressure which has assumed as initial guess before, the charts can be obtain as follows;

Article Title — 5

Figure 7. Initial Total Pressure – Resulting Total Pressure Curve

Comments for Figure-7:

1. There is no significant difference for resulting pressure values which are obtained from the chosen initial total pressure values from 250 to 1000 Pa but after the value of 1000 Pa sudden rise obtained for the resulting pressures. It can be recommended that the designers should be sensitive to obtain their target pressure values.

2. It can be understood smoothly that the resulting pressure differs according to 9 blade and 10 blade designs have a tolerance as the maximum value of ± 12%.

3. If we observe the difference between Model 1 and 9, we can obtain a comment as following: “If the rotational speed decreases its half value, the resulting pressure decrease to its ¼ value.” This shows us that the fan laws also can be used for “S-Shaped Axial Flow Fans”.

Figure 8. Initial Total Pressure – Thrust Curve

Figure 9. Resulting Total Pressure – Thrust Curve

Figure 10. Initial Total Pressure – Thrust Efficiency Curve

Comments for Figure-8 to 10:

1. For both chosen two blade quantities, the relationship between thrust and initial total pressure is directly proportional and have a curve nearly linear.

2. For both chosen two blade quantities, the relationship between thrust and resulting pressure is proportional and have a parabolic curve.

3. For both chosen two blade quantities, the relationship between thrust efficiency and resulting pressure is inversely proportional and have a parabolic curve.

4. More generally tunnel jet fans are used for obtaining thrust as explained before. If we have a comment for thrust efficiency as

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000

Res

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Tota

l P

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[P

a]

Initial Total Pressure [Pa]

9 Ka atlı Tasarı lar Ka atlı Tasarı lar

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800

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]

Initial Total Pressure [Pa]

9 Ka atlı Tasarı lar Ka atlı Tasarı lar

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]

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Article Title — 6

having the aim of achieving optimum thrust in response to optimum energy consumption, the designer should more sensitive for low pressure fans and tunnel jet fans across these resulting pressure ranges.

5. If we observe the difference between Model 1 and 9, we can obtain a comment as following: “If the rotational speed decreases its half value, the resulting pressure decrease to its ¼ value as similar as pressure”

Figure 11. Initial Total Pressure – Pressure

Correction Number Curve

The Comments for Figure-11:

1. For both chosen two blade quantities, the relationship between pressure correction number and reference pressure is bell-shaped curve.

2. It can be understood smoothly that the pressure correction number differs according to 9 blade and 10 blade designs have a tolerance as the maximum value of ± 13%.

3. If we observe the difference between Model 1 and 9, we can obtain a comment as following: “If the rotational speed decreases its half value, pressure correction number still equal to each other.”. It means that whichever initial total pressure is chosen by designers, they nearly ensure the same pressure correction number for different rotational speed ranges. The error margin is 1,75% and it can be acceptable.

Figure 12. Model-3 Vectors from Meridional View – Colorizing according to Total Pressure

Figure 13. Model-3 Vectors from Frontal View – Colorizing according to Total Pressure

Figure 14. Model-3 Vectors from Meridional View –

Colorizing according to Mach Number

0

5

10

15

20

25

0 500 1000 1500 2000

Pre

ssu

re C

orr

ecti

on

N

um

ber

Initial Total Pressure [Pa]

9 Ka atlı Tasarı lar Ka atlı Tasarı lar

Article Title — 7

Figure 15. Model-3 Vectors from Frontal View – Colorizing according to Mach Number

The Comments for Figure-12 to 15: 1. The point where the pressure gradient begins

to fall below zero (dp/dxi ≤ 0), that is to say the point where the pressure gradient begins, becomes very close distances. That can be explained following reasons;

a) Over limits of attack of angle for this type ofblade and resulting lift drag coefficient.

b) The wake region where begins from half chordlength of blade because of S-Shape.

2. For these reasons, the author have three morerecommendations as below;

a) The blade design which has an attack anglebelow 0o

b) Investigating alternative blade form and NACAprofiles.

c) Dis-symmetrical S-Shaped blade profiles bymeans of designing Inlet Guide Vanes and/orStators.

ACKNOWLEDGMENTS The author wish to express his gratitude to Bahcivan

Engineering Ltd., Istanbul, Turkey. He are also indebted to Mr. Erkan Ayder who is a Professor of Mechanical Engineering at Istanbul Technical University and Mr. Ayhan Nazmi İlikan who is a Assistant Professor of Mechanical Engineering at Isik University because he learned and will learn a lot of things about Turbomachinery.

REFERENCES

1. Eastman N. JACOBS, Kenneth E. WARD, Robert M.PINKERTON, “NACA Report 460 – The Characteristics of 78 Related Airfoil Sections from Tests in The Variable – Density Wind Tunnel”.

2. S. L. Dixon and C. A. Hall, 2010, “Fluid-Mechanics andThermodynamics of Turbomachinery Sixth Edition – Chapter-3 Two Dimensional Cascades”.

3. S-C Lin, M-L Tsai, 2011, “An integrated study of thedesign method for small axial-flow fans, based on the airfoil theory”, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 2011 225: 885

4. F. P. Bleir, 1997, “Fan Hand Book Selection, Applicationand Design” by MC GRAW HILL

5. K. Daneshkhah, A. G. Sheard, 2013, “A ParametricStudy of Reversible Jet-Fan Blades Aerodynamic Performance” Proceedings of Journal of Engineering for Gas Turbines and Power by ASME

6. ANSYS CFX, Commercial Release, Version 12.1, AnsysInc., Canonsburg, PA