Axial fan design

YEDITEPE UNIVERSITY – SPRING 2010

1

AXIAL FAN DESIGN

COURSE: APPLIED FLUID MECHANICS

INSTRUCTOR: DR. ESRA SORGÜVEN

MEMBERS

Ercan ÖZGÜN

Mert GÖNÜL


2

ABSTRACT

In this experiment many kinds of axial fans which are commomnly used in industry

are investigated over journals and related references; and then designed by our decisions,

also the proper calculations are done due to the given information range. Using the mass

balance, Euler’s turbomachinery equations and axial fan velocity equilibriums; a mean line

analysis had been done. Also a 3-D model is created to visualize the design geometry. In

addition to this, MATLAB codes are developed to overcome the necessary iterative

calculations. Considering air flow, the detailed information and calculations are going to be

shown in the following pages.


3

THEORY

A fan is a powered device used to create flow within a gas, usually air that consists of

a rotating arrangement of vanes or blades which act on the air. Fans produce air flows with

high volume and low pressure, as opposed to compressors which produce high pressures at

a comparatively low volume.

There are three main types of fans used for moving air, axial, centrifugal (radial) and

crossflow (tangential). The one that is used in this design is the axial fan.

The axial-flow fans have blades that force air to move parallel to the shaft about

which the blades rotate. Axial fans blow air along the axis of the fan, linearly, hence their

name. This type of fan is used in a wide variety of applications, ranging from small cooling

fans for electronics to the giant fans used in wind tunnels.

http://en.wikipedia.org/wiki/Machine

http://en.wikipedia.org/wiki/Gas

http://en.wikipedia.org/wiki/Gas_compressor

http://en.wikipedia.org/wiki/Parallel_(geometry)

http://en.wikipedia.org/wiki/Wind_tunnel


4

1. INTRODUCTION

1.1 Problem Statement

We are assigned to design a fan, having a mass flow rate of 1 kg/s, a pressure change

of 3000Pa and a tip diameter range of 10-16 cm.

1.2 Assumptions: We decided to have more flow area at the outlet by choosing a small

thickness value and the maximum tip diameter at the range.

No prewhirl.

Incompressible flow.

N is chosen 5000 rpm to have a high speed fan.

Thickness is chosen 2 mm by investigating industrial data.

The number of blades is chosen 9 by investigating industrial data.

Medium: air, ρ=1.2 kg/m3

Rtip=8 cm

Rhub=3 cm

u1=u2 ( because the inlet and outlet diameters of the axial fan are same )

ca1=ca2 ( because of mass balance )

The design is seperated into 3 steps:

STEP 1: Meanline analysis

STEP 2: Calculation and analysis of losses

STEP 3: CAD design of the project


5

2. STEP 1

The formulas that are used for mean line analysis:

Mass equation: mdot=caAflowρ

Aflow=πrtip2-πrhub

2-z (rtip-rhub)

Euler’s turbomachinery equation: W= =gH=u2cw2

(Assuming no prewhirl)

Power formulae: P= gH= Δp

Specific speed: NS=

Mean radius: rmean=

Rotor speed: umean= 2πrmeanN/60

Absolute velocity: ca= (without occlusion)

Area without occlusion: A0=


6

3. CALCULATIONS

Firstly the power is found to have a chance to compare it with the industrial designs.

The volumetric flow rate: = = 0.83 m3/s

The head is calculated from which is 255 m.

From the power equation P= 2490 watt (assuming no losses).

The specific speed is then found to be NS= 71.4 from the specific speed equation.

The mean radius is calculated to be; rmean= 0.065 m, by that the mean rotor speed can be

found: um= 28.8 m/s .

Applying Euler’s turbomachinery equation; cw2= 86.8 m/s .

The outlet axial absolute speed is found to be ca=48 m/s , considerin no occlusion.

The values ca, cw2, um are known; so the flow angle ( 2) at the outlet can be found from

traingular relations;

Cw2+um=v2*sinβ 2

V2*cos β2=ca2 => β2=50 degree

Same process is done to find 1;

V1*cos β1=ca

V1*sin β1=um => β1=30 degree


7

The iterations are done using MATLAB by finding new Aflow values; and putting them into the

old ones.

At last the absolute outlet axial velocity is found; which is the speed with occlusion:

ca= 70.58 m/s

4. STEP 2

4.1 Slip Losses:

The slip factor has to be calculated to find the absolute velocity with slip.

ς= <1

Stanitz equation is used to determine the slip factor.

ς= 1 -

Stanitz factor is found to be 0.78 which is <1 .

So = 67.7 m/s

4.2 Leakage:

Due to pressure difference between inlet and outlet of the impeller leakage occurs

through the gap. For the analysis, between the inlet and outlet bernoulli equation is applied,

assuming pressure distribution through the radius is constant.

Vleak= = 70.7 m/s

The gap area has to be calculated to determine the flow rate. (δ=5mm is predicted)

Agap=π*(rgap2-rtip

2)= 26mm2

Qleak=vleak*Agap= 1.83 liters/s

eff= = 99.82%


8

Drag and lift forces acting on the fluid are calculated as follows:

FD=ρ*ca* -ΔP*Aflow= 37.1 N

FL=ρ*cw2* = 36.6 N

The leakage percentage is negligable, so no analysis done to find the new ca.

The true power value that is supplied to the air:

PT=2490 W*0.78*0.9982=1940 W

5. Developed MATLAB codes for iterative solution:

Q=0.83;

ca=48 ; u=28.8 ; beta1=0.52 ; z=9; t=0.002; %blade thickness error=1 ; %error is set to 1 for loop initialization rtip=0.8 ; rhub=0.3; while (error > 0.0001) %relative error requirement Aflow=pi*(rtip^2-rhub^2)-z*t*(rtip-rhub)/sin(beta1); ca_new=Q/Aflow; beta1_new=atan(ca_new/u); error=abs((beta1-beta1_new)/beta1); %relative error beta1=beta1_new; end


9

6. Table of Parameters:

Given values Chosen parameters

Dimensionless numbers

Velocities Angles Loss parameters

Ρ=1.2 kg/m3 N=5000 rpm Ψ=8.26 Um=28.8 m/s β2=50 degree ς=0.78

Δp=3000 Pa t=2 mm

Φ=0.125 Cw2=86.8 m/s β1=30 degree =67.7m/s

mdot=1 kg/s z=9 Ns=71.4 Ca=48 m/s α2=61 degree Vleak=70.7 m/s

Dt=10-16 cm Rt=8 cm p=0.001 eff=99.82%

Rh=3 cm


10

7. THE FAN MODEL

7.1 Model View

7.2 Hub Dimensions


11

7.3 Tip Diameter

7.4 Blade Crossections


12

7.5 Blade Angles


13

8. Computational Flow Analysis


14

CONCLUSION

As a result of the above analysis, all the design parameters including flow angles,

power, velocities, dimensional and geometric parameters and also losses which effect the

efficieny of the fan are calculated. In addition to this a basic 3-D sketch and CFD analysis are

presented. In conclusion the blade angles are found so close to the suggested degree range.

The reason why cw2 is that much fast is the high power that needs to be achieved. The

efficiency of leakage has a negligible value, that means there is nearly no loss due to leakage.

However the slip factor is found to be 0.78 which significantly affects efficiency.

If the design done in this project is compared with currently used industrial axial fans,

one can visualize that it has been done suitabily due to the principles of applied fluid

dynamics and can be used in the areas where high mass rate flows is a necessary condition.


15

REFERENCES

ME 436-Applied fluid mechanics lecture notes-2010.

Turbomachinery, Design and Theory by Gorla and Khan.

The Elsevier journal database.

Wikipedia.

The science direct.

PROGRAMS

MATLAB software

Solidworks modelling software

Microsoft Word

http://me.yeditepe.edu.tr/courses/me436/Gorla_Khan.rar

Axial fan design

Design

Transcript of Axial fan design