5-Axial fan

INTERNAL CHARACTERISTIC OF AN AXIAL FAN

1


1 GOALS This paper presents the experimental procedure used to establish the internal characteristic for an

axial fan, for a constant rotating speed. The process is assumed as isothermal. In addition:

- using of the orifice meters for measuring of the mass flow rate,

- measuring of the total pressure using a Pitt tube. 2 THEORETICAL APPROCH The fans are pneumatic generators that are working with gases. In this way, they transform

mechanical energy supplied by an electrical motor in pneumatic energy. Between inlet and outlet,

there will be an increase of pressure. A particular characterisc of the axial fans is that they are

used for significant volumetric flow rates of gases at small pressure. Internal characteristic of a fan represent the dependency between total pressure totp of the fan and the mass flow rate mQ (or volumetric flow rate) of this, )Q(fp mtot = (or )Q(fptot = ). It characterizes the working behaviour of the fan. The mass flow rate (or volumetric flow rate) is defined as flux of velocity through the inlet section

iS (or outlet section oS ) per unit time.

i ,oSno ,im dS )Q(

= (1)

where: n is velocity of fluid through a control section, that is normal to the flow direction. Total pressure totp of the fan is the pressure change of the gas through fan (the difference of average total pressure between inlet and outlet):

idinstodinstitototottot )pp()pp()p()p(p ++== (2) where: i ,ost )p( static pressure into inlet section and outlet section;

i ,odin )p( average dynamic pressure in the same sections. Seldom, in practical applications, total pressure can be determined with equation:

i

2

sto

2

sttot )2p()

2p(p ++= (3)

where: i ,o)( mean velocities into inlet and exit sections;


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Taking into consideration that useful power is defined as real power transferred to the gas, we can

consider that, energetically totp represent the hydraulic power per flow rate unit. totu pQP = (4)

3 DESCRIPTION OF USED FAN The following components are composing the fan (see figure 1):

Case a cylindrical tube 1 with two flanges at extremities and equipped in interior with a

fixed stator blades 2, placed in downstream from rotor blades, for a minimum

turbulence of the air into exit section;

Stator blade: is composed from four stationary blades 3;

Rotor blade: is composed from a hub and an assembly of four aerodynamically shaped blades 4;

Electrical motor 5: is fixed on a support 6, in interior of the case.

Inlet nozzle 7 and outlet nozzle 8: they are linked to case through the flanges, and have conical shape.

Fig. 1 - Axial fan, 3D view 4 THE LABORATORY PRINCIPLE

The purpose of this practical work is to establish the dependency between total pressure totp of the air delivered under pressure by the axial fan 9 (see figure 2) through the duct 10 and the mass


3

Fig.

3. -

Exp

erim

enta

l set

-up


4

flow rate mQ . All these are performed for some air stationary flowing cases (flow rates), which are

established with the aid of the valve 14 (a device used to control the flow rate). Later on, for any

cases partly there will be calculated the magnitude of the useful power for fan. In order to determine the mass flow rate, there will be used an orifice meter with a known

calibration curve (for any other details see the paper CALIBRATION OF THE ORIFICE METERS). Total pressure is measured with the aid a Pitt tube.

5 COMPUTATIONAL PROCEDURE In order to determine the mass flow rate of the air through the exhaust duct, we can find the

magnitude of mQ [kg/s] from calibration curve of the orifice meter (see figure 4). This is the

dependency between the vertical deflection of the piezometric liquid, dh [mm], which is indicated by the manometer connected at the orifice meter, and the mass flow rate.

ddd klh = [m] (5) where: dl [m]: the length of the piezometric liquid;

dk [ - ]: a constant which is function of angle of manometer leg and the nature of

piezometric liquid. The next equation gives us the magnitude of the volumetric flow rate:

aer

mQQ = [m3/s] (6)

The following equation can be used to determine the density of the air aer :

aer

aer0

aer0

aeraer0aer T

Tpp = [kg/m3] (7)

The magnitude of the total pressure can by computed using the equation (8):

tlptot h gp = [N/m2] (8) where: lp [kg/m3] density of the piezometric liquid; th [m] the vertical deflection of the piezometric liquid, tl [mm], which is

indicated by the manometer connected at the Pitt tube which is placed

in the center of the exhaust duct. Same as in the equation (5):

ttt klh = [m] (9) where: tl [m] the length of the piezometric liquid, on leg of the manometer;


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tk [ - ] a constant which is function of the angle of manometers leg and the

nature of piezometric liquid. Equation (4) give us the magnitude of useful power of the fan

6 EXPERIMENTAL PROCEDURE Step 1. Check the horizontal planes of the manometers.

Step 2. Because the device used for measuring of total pressure is a Pitt-Prandtl tube, it is

necessary to remove the static pressure connection (if necessary);

Step 3. With magnitudes of ,paer aert compute aer for the moment of practical work, Eq. (7). Step 4. With the aid of valve, establish a flow case. Start the fan.

Step 5. Read the values of the lengths of piezometric liquid, dl and tl (on the leg of

corresponding manometer).

Step 6. Compute dh , Eq. (5) and th , Eq. (9). Step 7. Determine the mass flow rate from calibration curve of the orifice meter, as function of

dh . Step 8. Compute the total pressure totp , Eq. (8) and uP , Eq. (4). Step 9. Repeat the anterior operations for another minimum seven flow cases;

Step 10. Establish the dependence )Q(fp mtot = ; figure 3 show us a general representation of it.

Fig. 3 - General representation of internal characteristic of an axial fan

7 PHISICAL FACTORS AND CONSTANTES USED

air0 density of the air, for standard conditions of temperature and pressure: air0p , air0T . air0t = 0 C ( =air0T 273,15 K); air0p = 760 mmHg; air0 = 1,293 kg/m3;

=lp 1000 kg/m3 for water; =lp 800 kg/m3 for alcohol.


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TABLES FOR RESULTS

airp airT air [mmHg] [K] [kg/m3]

dk dl dh mQ Q tk tl th tp uP Reglaj clapet [ - ] [mm] [m] [kg/s] [m3/s] [ - ] [mm] [m] [N/m2] [W]

1 2 3 4 5 6 7 8 9

10 11 12

Fig. 4 - Calibration curve of the orifice meter

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5-Axial fan