1435 - uotechnology.edu.iqseptember 14 2014 lab. of fluid machineries electromechanical eng. dept ä...

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Republic of Iraq

Ministry of Higher Education

and Scientific Research

University of Technology-Electromechanical

Department

1435 2014

September 14 2014 Lab. of Fluid Machineries Electromechanical Eng. Dept

Title Page No.

Experiment No. (1): Characteristics of Pelton turbine

Experiment No. (2): Turbine characteristic at constant head

Experiment No. (3): Turbine characteristic at constant speed

Experiment No. (4): Turbine characteristic at constant efficiency

Experiment No. (5): Single Pump Operation

Experiment No. (6): Series Pump Operation

Experiment No. (7): Parallel Pump Operation

Experiment No. (8): Fan Constant Speed Characteristics

Experiment No. (9): Dimensional Analysis

Experiment No. (10): Fan System Characteristics


[1]

Objective:

To determine the operating characteristics of a Pelton Turbine.

Method Using a brake dynamometer in order:

1) To vary the speed of the turbine rotor from maximum speed (zero,torque) to minimum speed (rotor stalled / maximum torque) in stages

2) To measure the torque produced by the turbine rotor at each stage Using a spear valve to change the volume flow rate through the turbine, allowing the above tests to be repeated at different flow rates.

From the readings obtained to plot graphs of Torque, Brake power and Overall Efficiency against rotor speed to show the operating characteristics of the Pelton Turbine.

Equipment In order to complete the demonstration we need a number of pieces of equipment.

• The Fl-10 Hydraulics Bench which allows us to measure flow by timed volume collection.

• The FI-25 Pelton Turbine Apparatus. • A stopwatch to allow us to determine the flow rate of water. • A non-contacting type tachometer to measure the rotor speed.


[2]

Theory The Pelton turbine (Fig 1 ) is the most visually example of an impulse

machine. A spear valve directs a jet of water at a series of buckets which are mounted on the periphery of a rotor. As the water exiting the spear valve is at atmospheric pressure, the force exerted on the rotor is entirely due to changes in the direction of the flow of water. The Pelton turbine is therefore associated with considerable changes of kinetic energy. The spear valve allows the jet diameter to be varied which allows the flow rate to be varied with a constant velocity of jet.

Note that large turbines may include more than one spear valve around the periphery of the rotor to improve the power available from the turbine.

Fig. 2 Example characteristics of a turbine at different flow rates

Fig. 1 Rotor and Spear Valve arrangement of the Pelton turbine.

Rotational Speed (rpm)


[3]

The operating characteristics of a turbine are normally shown by plotting the Torque T, the Brake Power Pb, and the Overall Turbine Efficiency o against the turbine rotational speed N for a series of volume flow rates Qv, as shown in Fig 2. It is important to note that the Brake Power and the Overall Efficiency of the turbine rise to a maximum and then fall back to zero, whilst the Torque changes constantly and linearly with speed.

Because turbines are normally used at fixed speed, e.g. when generating electricity, a turbine must be carefully designed to ensure that the maximum efficiency coincides with the normal operating speed. As the load on the turbine changes, the flow of water is regulated via the spear valve to maintain the turbine at the required operating speed. Note that the peak efficiency changes slightly with load / flow rate so the turbine can only be optimized at one condition. If a turbine is optimized for operation at full load then the efficiency will fall slightly as the load reduces (while maintaining a constant speed).

The basic parameters that define the turbine performance are :-

1) Volume flow rate (Q) 2) Inlet Head (Hi) 3) Hydraulic Power (Ph) 4) Torque (T) 5) Brake Power (Output Power) (Pb)

6) Overall Turbine Efficiency ( o )

Each of these is considered in term:-

The flow rate of fluid through the turbine is the volume passing through the system per unit time.

tVQ [m3/s] (1)

The term 'Head' refers to the elevation of a free surface of water above or below a reference datum. In the case of a Pelton Turbine we are interested in the head of the water entering the spear valve, which of course has a direct effect on the characteristics of the unit. In this apparatus the head of water is generated by the


[4]

pump on the hydraulics bench rather than an elevated reservoir.

The Bourdon pressure gauge measures the inlet pressure, pi, in relation to atmospheric pressure. As the runner and the outlet of the turbine are at atmospheric pressure, it can be assumed that the reading given by the gauge is the pressure difference across the turbine. For the purpose of calculating the performance of the turbine the measured

pressure is converted to an equivalent head of water, Hi, as follows:

gpH i

i m H2o where pi, is measured in N/m2 (2)

The hydraulic power supplied by the water, Ph, can be calculated as

Ph = gHiQ [ Watt ] (3)

The mechanical power, Pb, produced by the turbine in creating a torque T on the brake at rotor speed N is given by

602 TNPb [ Watt ] (4)

The torque itself is given by the equation:

T = Fb * r [N.m] (5)

where r is the radius of the brake pulley and Fb is the Brake force

Where Fb=(W2-W1) and W2 and W1 are the readings on the two spring balances.

The Overall Efficiency of the turbine is determined from several separate efficiencies as follows:

Fluid friction 'losses' in the turbine itself, require a hydraulic efficiency h that is defined as:


[5]

pliedsuppowerFluid

rotorthebyabsorpedPowerh *100% (6)

Mechanical losses in the bearings etc. require a mechanical efficiency m that is defined as

rotorthebyabsorpedPowerrotorthebypliedsupPower

m *100% (7)

The Armfield FI-25 Demonstration Pelton Turbine does not include direct measurement of mechanical power output Pm, but instead measures brake force that is applied to the rotor via the band brake. A further efficiency is therefore required, expressing the friction losses in the brake assembly b that is defined as:

rotorthebypliedsupPowerbrakethebyabsorpedPower

b *100% (8)

The Overall Efficiency of the Pelton Turbine is the product of these individual efficiencies Et = Eh Em Eb therefore:

bmho .. (9)

pliedsuppowerFluid

brakethebyabsorpedPowero *100% (10)

QHgTN

o2 *100% (11)

Table of Measured and Calculated variables :-


[6]

Application of Theory Comment on the shape of the graphs obtained.

At what speed is the maximum Torque obtained?

At what speed is the maximum power output obtained from the turbine?

Is the maximum efficiency at the same speed?

What happens to the power output and the maximum efficiency when the flow is reduced?

Suggest optimum conditions for operation of a Pelton Turbine.

Measured variables Calculated variables

Hi

m

Time (t)

S

N

rpm

W1

N

W2

N

Q

m3/s

Fb

N

T

N.m

Pb

W h o


[7]

Nomenclature

Column Heading

Units Nom. Type Description

Radius of Brake Drum

m r Constant Radius of drum on which brake band operates (r = 0.030 m)

Tachometer Reading

RPM Measured Rotor speed measured in RPM. Convert to Hertz for calculations (divide reading by 60)

Rotor Speed Hz N Calculated Rotational speed converted to Hz Spring Balance 1 N W1 Measured Force reading from spring balance 1 Spring Balance 2 N W2 Measured Force reading from spring balance 2 Brake Force N Fb Calculated Difference between readings on two

spring balances i.e. Fb = (W2-W1) Volume Collected

m3 V Measured Volume of water collected in a known time period (t). Note: Convert to cubic metres for calculations (divide litres by 1000)

Time to Collect s t Measured Time taken to collect a known volume of water (V)

Volume Flow Rate

m3/s Qv Calculated Qv =V/t Volume Collected/ Time to Collect

Inlet Pressure N/m2 Pi Measured Pressure at inlet measured by Bourdon gauge

Inlet Head Mh2O Hi Calculated HI=PI/Pg

Hydraulic Power Watts Ph Calculated Available power from the fluid (kinetic + potential energy) Ph=PgHiQv

Torque Nm T Calculated T = Fbr

Brake Power Watts Pb Calculated Power absorbed by the brake Pb = 2mT

Overall Turbine Efficiency

% E, Calculated Et =Pb /Ph* 100% = 2 /Ph PgH,Qv*100%


[8]

Experiment No. 2

Turbine characteristic at constant head

Objective To obtain the characteristics curves for a turbine operating at a range of fluid flow rates in constant head. In order to obtain the characteristics curves (main characteristic curves ) the tests are performed on the turbine by maintaining a constant head and a constant gate opening and the speed is varied by changing the load on the turbine .

Theory The basic terms used to define, and therefore measure ,turbine performance in relation to rotational speed include :

Volume flow rate The volume flow rate of fluid through the turbine ,Q is the volume passing through the system per unit time . This is expressed in liter per minute (lit/min) but converted to cubic meters per second (m3/sec) for further calculations.

Head H The term head refers to the elevation of a free surface of water above or below a reference datum. In the case of turbine we are interested in the head of water entering the rotor, which of course has a direct effect on the characteristics of the unit . The input head to the turbine (H)is the head used by the turbine in performing work. The inlet pressure sensor in the FM60 measures a gauge pressure. As the out let of the turbine is at atmospheric pressure, it can be assumed that the reading given by (Pis) is the pressure difference across the turbine. Therefore ,the inlet head ,H, is given by :

H=P/ g (1)


[9]

Power outlet and efficiencies: The brake drum on the FM60 is free to rotate but restrained by a torque arm which is connected to a load cell. The force measure by the load cell is converted to a torque:

T=F * r (2)

Where r is the length of the torque arm (0.045m).

The brake power ( Pb ) produce by the turbine in creating a torque T on the brake at a rotor speed N is given by :

602 TNPb (3)

The hydraulic power of the fluid is define by :

Ph= g Q H (4)

Therefore an overall efficiency can be defined as :

%*PP

pliedsuppowerfluidUsefulbrakebyabsorbedPower

hb

h 100 (5)

Results In order to obtain the characteristics curves (main characteristic curves ) for a turbine at constant head draw Qu on (y-axis) v/s Nu on (x-axis) , Pu on (y-axis) v/s Nu on (x-axis)and o on (y-axis) v/s Nu on (x-axis)

Table of measured and calculated variables


Gat

e op

enin

g

P

N/m2

Q

m3/s

N

rpm

T

N.m

H

m

Qu Pu o

1


[10]


At which reaction or impulse turbine the flow rate is constant for each value of turbine speed.

At what speed is the maximum power output obtained from the turbine?

Is the maximum efficiency at the same speed?

What happens to the power output and the maximum efficiency when the flow is reduced?

Suggest optimum conditions for operation of a Pelton Turbine.

2


[11]

Experiment No. 3

Turbine characteristic at constant speed

Objectives To investigate the performance of the turbine at constant speed.

Principle The turbines are generally designed to work at particular values of H, Q, P, N and

o which are known as the designed conditions. But often the turbines are required to work at conditions differs from those for which they have been designed. Therefore, it is essential to determine the exact behavior of the turbines under the varying conditions by carrying out tests either on the actual turbines or on their small scale models. The results of these tests are usually graphically represented and the resulting curves are known as characteristic curves. One of these curves is the constant speed curves.


[12]

Procedure In order to draw these curves following procedure is adopted:

1. The constant speed is attained by regulating the gate opening thereby varying the discharge flowing through the turbine as the load varies . The head may or may not remain constant

2. The power developed corresponding to each setting of the gate opening is measured and the corresponding values of o are computed.

3. The total load capacity of the turbine, the percentage of full load may be computed from the measured power.

Observation 1. For each percentage of load read the following parameters: Q, p, N, T and

developed power P. 2. From the above reading parameters, the following parameters can be

calculated: unit flow rate Qu, unit speed Nu, unit power Pu and overall efficiency.

Results 1. In order to obtain the characteristics curves at constant speed plot o ( on

y-axis ) v/s percentage of full load ( on x-axis ). 2. Plot an other graphs , output power ( on y-axis ) v/s discharge ( on x-axis

) and o ( on y-axis ) v/s discharge ( on x-axis ). Both these graphs are plotted from a certain minimum discharge Qo which is required to initiate the motion of the turbine runner from its state of rest.



Perc

enta

ge o

f ful

l lo

ad

P

N/m2

Q

m3/s

N

rpm

T

N.m

H

m

Qu Pu o

1


[13]

Application of Theory 1. Discuss the plotted graphs. 2. At which percent of full load the efficiency is a maximum?

Experiment No. 4

Turbine characteristic at constant efficiency

Objectives To investigate the performance of the turbine at constant efficiency.

Principle The turbines are generally designed to work at particular values of H, Q, P, N and

o which are known as the designed conditions. But often the turbines are required to work at conditions differs from those for which they have been designed. Therefore, it is essential to determine the exact behavior of the turbines under the varying conditions by carrying out tests either on the actual turbines or on their small scale models. The results of these tests are usually graphically represented

2


[14]

and the resulting curves are known as characteristic curves. One of these curves is the constant efficiency curves.

Procedure In order to draw these curves following procedure is adopted:

4. Operating the turbine at about 8 to 10 gate openings. 5. The corresponding number of o v/s Nu and Nu v/s Qu ( or Pu) curves are

plotted as described in the constant head test. 6. On the o v/s Nu curves set of horizontal lines ( each line representing the

same efficiency ) are drawn which will cut the curves corresponding to each gate opening at different points.

7. These points are projected on the corresponding Nu v/s Qu ( or Pu) curves for each gate opening and the points of the same efficiency are joined by smooth curves which are the iso-efficiency curves.

Observation 3. For each gate opening read the following parameters: Q, H, N and developed

power P. 4. From the above reading parameters, the following parameters can be

calculated: unit flow rate Qu, unit speed Nu, unit power Pu and overall efficiency.

Full G.O 0.75 G.O 0.5 G.O 0.25 G.O

H=constant

Iso-efficiency


[15]




Gat

e op

enin

g

P

N/m2

Q

m3/s

N

rpm

T

N.m

H

m

Qu Pu o

1

2


[16]

At which inner or outer iso-efficency curves the efficiency is maximum.

At given unit flow rate , explain the way to find maximum efficiency.

What is the meaning of pest performance curve and how can be obtained?


[17]

Experiments No. 5,6 and 7

Characteristics of centrifugal pump Objective To determine the head/flow rate characteristics of a centrifugal pump for a number of different configurations

Method By measurement of pressure at pump inlet and outlet and discharge flow rate.

Equipment In order to complete the demonstration we need a number of pieces of equipment.

• The Hydraulics Bench, which provides- one of the two pumps used during this experiment, and allows the volume flow rate to be measured by timed volume collection • The Fl-26 Test Accessory. • A stopwatch to allow us to determine the flow rate of water .

Technical Data The following dimensions from the equipment are used in the appropriate calculations. If required these values may be checked as part of the experimental procedure and replaced with your own measurements.

Head Correction Values:

Datum to manifold gauge: hd = 0.960m

Datum to FI-26 outlet gauge: ha = 0.170m

Datum to FI-26 inlet gauge: ha = 0.020m

Datum to Bench pump inlet: hd = 0.240m

Theory

In this type of pump (Fig. 1), the fluid is drawn into the centre of a rotating impeller and is thrown outwards by centrifugal action. As a result of the high speed of rotation, the liquid acquires a high kinetic energy. The pressure difference between


[18]

the suction and delivery sides arises from the conversion of this kinetic energy into pressure energy.

The centrifugal pump is a radial flow rotodynamic machine, wherein fluid enters the rotor or impeller at one radius and leaves at a larger radius. In so doing, changes in kinetic, potential and pressure energy occur, and any understanding of pump behavior and performance assessment requires measurement or calculation of these quantities.

The general relationship between the various forms of energy, based on the 1st Law of Thermodynamics applied to a unit mass of fluid flowing through a 'control volume' (such as the pump itself) is expressed as:

FdPdz.gvdWs 2

2 (1)

where -Ws is the mechanical shaft work performed on the fluid, d(v2/2) is the change in kinetic energy of the fluid, g.dz is the change in potential energy of the fluid, F is the frictional energy loss as heat to the surroundings or in heating the fluid itself as it travels from inlet to outlet and (dP/ ) is the change in pressure energy.


[19]

The first three terms of the right hand side represent the useful work, W0, i.e.

1212

122

2PPzzg

vvWo (2)

Where subscript 2 refers to the pump outlet and subscript 1 to the inlet.

The term W0 represents the actual work performed in changing the energy stages of a unit mass of the fluid. This may alternatively be expressed as the total dynamic head, H of the pump, by converting the units from work per unit mass to head expressed as a length, which involves dividing by the acceleration per unit mass, g

gPP

zzgvv

H 1212

21

22

2 (3)

On this apparatus the pipe diameters are similar, and so we can assume that the (v22

- v12)/2g term is insignificant, hence:

gPPzzH 12

120 (4)

The gauges measure the inlet and outlet pressures in terms of a head, h, where h = p/pg, giving:

12120 hhzzH (5)

The relative vertical positions of the inlet and the outlet are represented by the (z2 – z1) term. Each head measurement is at a different relative vertical position. The positions are therefore taken relative to a datum position, the horizontal plane running through the centre of the FI-26 pump impeller. Each position is given a datum head correction factor, hd, as the examples shown on the diagram below:


[20]

The relative vertical positions of a pump inlet and outlet will therefore be:

)inlet(d)outlet(d hh (6)

The relative vertical distance between the inlet and outlet may then be expressed as a head difference, Hd

)inlet(d)outlet(dd hhzzH 12 (7) Substituting this into equation (5) finally gives the head generated across the pump

12 hhHH d (8)

The datum head correction factor for each measurement position can be found in the Technical Data section of this experiment. If the pump support is not positioned on the same base level as the hydraulics bench, these figures will need adjusting accordingly.

The basic terms used to define, and therefore measure, pump performance include:

i. discharge ii. head

i. Discharge ( Qt )

The discharge (or flow rate, or capacity) of a pump is the volume of fluid pumped per unit time.

ii. Head ( H )

The term 'head' refers to the elevation of a free surface of water above or below a reference datum. The useful work generated by the pump may be given in terms of a head, as shown earlier in this section.

Single Pump Operation

The best way to describe the characteristics of a Centrifugal Pump is through the use of a head / flow characteristic curve (Fig.2)


[21]

Series Pump Operation Should the head of a single pump not be sufficient for an application, pumps can be combined in series to obtain an increase in head at the same flowrate as the single pump.

As shown in figure 3, when two pumps having similar head-flowrate characteristics are operated in series the combined pump head-flowrate curve is obtained by adding the heads of the single pump curves at the same flowrate. In practice the theoretical combined head is not quite achieved because of additional losses in the fittings between the two pumps.

Parallel Pump Operation Should the flowrate of a single pump not be sufficient for an application, pumps can be combined in parallel to obtain an increase in flowrate at the same head as

Figure 3: Series pumps operation

Qt


[22]

the single pump.

As shown in figure 4, when two pumps having similar head-flow rate characteristics are operated in parallel the combined pump head-flow rate curve is obtained by adding the flow rates of the single pump curves at the same head. In practice the theoretical combined flow rate is not quite achieved because of additional losses in the fittings between the two pumps.

Procedure - Equipment Set Up

Three test configurations are available; single pump operation (fixed speed), two pumps in series (fixed speed) and two pumps in parallel (fixed speed).

To set up these demonstrations we need to modify the configurations of the flexible tubing as shown below.

Figure 4: Parallel pumps operation

Qt


[23]

Experiment No. 5 • Single Pump Operation

For single pump operation the inlet of the FI-26 (side connection on pump) should be connected to the sump drain valve on the FI-10 hydraulics bench, which must be fully opened while performing the experiment. The outlet on top of the pump should be connected to the discharge manifold.

The gauges used for measurement of inlet and outlet heads for this experiment are the FI-26 inlet gauge, hi, and the FI-26 outlet gauge. Ho.

Experiment No. 6

• Series Pump Operation

For series pump operation the inlet of the FI-26 (side connection on pump) should be connected to the water outlet on the hydraulics bench, using the screw on


[24]

adapter supplied. The outlet on top of the pump should be connected to the discharge manifold.

The gauge used for measurement of outlet head is the FI-26 outlet gauge, h0. The inlet to the hydraulics bench pump is assumed to be at atmospheric pressure, modified by the datum head correction factor given in the Technical Data section.

Experiment No. 7 Parallel Pump Operation

For parallel pump operation the inlet of the FI-26 (side connection on pump) should be connected to the sump drain valve on the hydraulics bench, which must be fully opened while performing the experiment. The outlet on top of the pump should be connected to the supplied Tee connector. The outlet from the hydraulics bench pump should also be connected to the Tee connector using the screw on adapter supplied. Finally the remaining outlet on the Tee connector should be connected to the discharge manifold.

The gauges used for measurement of inlet and outlet heads for this experiment are the FI-26 inlet gauge, hi (both pumps are assumed to be at similar inlet heads), and the discharge manifold gauge, hm.

Volume of water V(m3)

Time to collect t (s)

Inlet head 0 or hj (m water)

Inlet head correction

hm (m water)

Outlet head correction

Datum head correction

Pump power input

Q Total head H (m)

Qt


[25]

Application of Theory Comment on the graphs obtained. Do we get double the head for two pumps

in series? Do we get double the flow rate for two pumps in parallel? Give reasons for any differences observed between the theoretical head and the head obtained.

What is the effect of inlet (suction) head on the performance of the pump?

What is the effect of change in kinetic energy of the fluid if the inlet and outlet pipe diameters change from 25mm to 32mm?

Experiment No. 8

Fan Constant Speed Characteristics

Objective To obtain the fan characteristic curves for an axial fan operating at constant speed.

Method


[26]

By taking sensor readings over a range of flow rates at constant fan speed setting, for a series of constant fan speeds.

Equipment Required

Armfield FM41 Axial Fan Demonstration Unit

Armfield IFD7 Interface Device

Compatible PC running Armfield FM41-304 software

Theory

Pump manufacturers and fan system designers require a method for indicating fan performance, to allow the correct fan to be selected for any given system and performance requirement. Designers will wish to select a fan that will be able to produce the required flow rate and pressure differential under typical system conditions, and also to select a fan that will operate efficiently under normal use. A common method of presenting the sort of information required is to plot the fan

power, efficiency, and total pressure produced against the flow rate across the full

range of the fan for a constant fan speed:


[27]

When lines of constant efficiency are superimposed on such a graph for a range of fan rotational speeds, a comprehensive illustration of pump performance is obtained:

Equipment Setup Check that the outlet aperture is fully open.

Check that the sensor and power leads from the FM41 are connected to the sockets on the front of the IFD7.

Q

Typical axial fan characteristics at constant speed


[28]

Check that the IFD7 is connected to a suitable mains supply, and that the USB socket on the front is connected using the lead provided to a suitable PC. Check that trie red and green indicator lights on the front of the IFD7 are illuminated.

Run the Armfield FM41-304 software on the PC, and check that the software ndicates IFD: OK’ in the bottom right-hand corner of the window.

Switch on the mains supply to the IFD7, and switch on the IFD7 using the power switches on the front. Check that the power switch on the IFD7 is illuminated.

On the software mimic diagram screen, select the ‘Fan On’ button to switch on the FM41. Check that the green watchdog indicator on the mimic screen is illuminated.

Check that the sensor readings on the software screen give sensible values. The air velocity and the pressure readings should be zero when the fan is not moving. Zero the pressure readings if required using the ‘Zero’ buttons. The temperature should be sensible given the ambient conditions in the room (typically between 15 and 30 °C).

Procedure Read through the experiment before starting, to familiarise yourself with the

procedure.


[29]

Set the fan speed to maximum (100%) and note the rotational speed of the fan.

Take readings for air temperature, orifice differential pressure, fan differential pressure, and motor power by selecting the icon on the software toolbar.

Using the maximum air velocity as a guide, select incremental values for air velocity that will give 10-15 individual steps between minimum and maximum velocity.

Close the aperture to increase the air velocity by approximately one step. Select the icon. Repeat for the next flow velocity increment, adjusting the aperture and fan

setting to give the required values and selecting the icon to record the data once the settings are correct.

Continue in steps until the aperture is fully closed, recording the data each time.

NOTE: There will always be a small indicated discharge even when the aperture is fully closed, as a result of the fan blades acting on the air within the inlet and outlet ducts. Always monitor the aperture visually at low discharge rates, and do not twist the aperture beyond the point at which it is fully closed. Over-twisting the aperture device will damage the aperture.

Create a new results table using the icon on the software toolbar.

Select a new fan rotational speed, for example half the value for the first set of data. Adjust the fan speed to give this new rotational speed, and note the maximum flow velocity. Select the icon.

Open the aperture in steps as before, recording each set of data with the icon.

Fully open the outlet aperture after recording the last set of data.

If time permits, additional rotational speed settings may be investigated to give a full series of performance data. Remember to create a new results sheet for each set of data. Alternatively, each student or set of students may take data for different rotational speeds, and the results can then be combined.


[30]

Results The software logs the following variables:

Inlet Temperature T °c Orifice Pressure Pi Pa Fan Diff. Pressure P2 Pa Fan Setting - %

Fan Speed n Hz Mechanical Power (input)

Pm W

From these the software calculates the following values:

Air Density kg/m: Inlet Velocity Vi m/s Outlet Velocity V2 m/s Discharge Qv m3/s

Experiment No. 9 Dimensional Analysis

Introduction to Scaling

Objective To predict the performance of a fan at a given speed from data obtained at a different speed.

Method By using equations obtained from dimensional analysis to calculate the performance characteristics for the fan at the required speed, using data obtained at


[31]

a different speed.

Equipment Required Armfield FM41 Axial Fan Demonstration Unit Armfield IFD7 Interface Device Compatible PC running Armfield FM41-304 software

Theory It is not practicable to test the performance of every size of fan in a manufacturer’s range at all speeds at which it may be designed to run. Hence a mathematical solution is required whereby assumptions can be made as to the operating characteristics of a fan running at one speed, impeller size, etc from experimental results taken at another.

The use of dimensional analysis reduces the large number of variables involved in describing the performance characteristics of rotodynamic machines to a number of manageable dimensionless groups. The methods used for forming these dimensionless groups will not be entered into here, but the groups themselves are known by the following names:

PDN

P53 The power coefficient

3DNQ The flow coefficient

22DNHg The head coefficient

Use of these affinity laws allows performance of geometrically similar fans of different sizes or speeds to be predicted accurately enough for practical purposes. Exact accuracy would require that effects of surface roughness of the fan, the viscosity of the gas, etc. to be taken into account. Any two fans may be considered geometrically similar when:- • They have the same number of blades or vanes • Their angular dimensions are the same • Their linear dimensions are proportional For this exercise the same fan will be used throughout, which ensures geometric similarity.


[32]

The affinity laws are most often used to calculate changes in flow rate, pressure and power of a fan when the size, rotational speed or gas density is changed. Therefore, in the following affinity laws the suffix ‘1’ has been used for initial known values and the suffix ‘2’ for the changed values and the resulting calculated table:

where Qv is the volume flow rate,

PiF is the fan total pressure,

is the density,

n is the fan rotational speed,

D is the impeller diameter, and

Pu .is the fan power.

These laws can be simplified when variables remain unchanged, e.g. when only the

fan speed is varied:

D


[33]

More generally, the relationship between two geometrically similar machines with characteristic diameters D1 and D2 operating at rotational speeds N1and N2 is shown in the following diagram.

are termed corresponding points), it follows that:

PIF2=PIF1 )

and:


[34]

Qv2=Qv1 n2/n1(

It also follows that the power coefficient (P/ n3D5) and the efficiency gr must also have the same values at corresponding points.

Equipment Setup If results from Experiment No.8 are available then it is possible to use these for manual calculations in order to investigate scaling. If results are not available, or if computer analysis is required, set up the equipment as follows:

Check that the sensor and power leads from the FM41 are connected to the sockets on the front of the IFD7.

Check that the IFD7 is connected to a suitable mains supply, and that the USB socket on the front is connected using the lead provided to a suitable PC. Check that the red and green indicator lights on the front of the IFD7 are illuminated.

Run the Armfield FM41-304 software on the PC, and check that the software indicates; IFD: OK in the bottom right –hand corner of the window.

Switch on the mains supply to the IFD7, and switch on the IFD7 using the power switch on the front .check that the power switch on the IFD7 is illuminated.

On the software mimic diagram screen, select the 'Fan On’ button to switch on the FM41.Check that the green watchdog indicator on the mimic screen is illuminated.

Check that the sensor reading on the software screen give sensible values. The air velocity and the pressure readings should be zero when the fan is not moving. Use the zero buttons to zero the pressure sensor readings if required. The temperature should be sensible given the ambient conditions in the room (typically between 15and 30 C).

Procedure

If results from Experiment No.8 are available then it is possible to use these for manual calculations in order to investigate scaling. If data has been recorded and saved for The required fan rotational speeds (1050rpm and 2100rpm) it is also


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possible to load these saved results into the software without taking further measurements. If results are not available. or if a new set of results is preferred, then proceed as follows:

Read through the experiment before starting, to familiarise yourself with the procedure . To allow the software to perform the required calculations correctly, it is -:: ~3Ti to follow the procedure exactly and in the order given.

Two constant fan rotational speeds will be investigated, n = l000rpm and n = 2000rpm .In the first part of the exercise, data will be taken at n = l000rpm and the .are wi i use this data to predict the fan performance at n = 2000rpm. In the second part of the experiment, real data will be taken at 2000rpm so that this can then be compared to the predicted performance data.

Check that the outlet aperture is fully open, then set the fan to 100%. Check that the fan operates and that the sensor readings indicated on the mimic diagram change accordingly.

Reduce the fan setting until the rotational speed indicated on the mimic diagram is approximately l000rpm (The actual setting will probably be one or two rpm different; this is such a slight error that it should make no difference to the results. Allow a few moments for the fan speed to stabilise after making adjustments). Note the air velocity then fully close the outlet aperture.

Take readings for air temperature, orifice differential pressure, fan differential pressure and motor power by selecting the icon on the software toolbar.

The aperture will be opened to give increments in the air velocity until the aperture is fully open. Select a flow velocity increment that will give 10-15 separate readings. Open the outlet aperture to give a flow increase approximately equal to the increment

chosen, and select the icon again.

Continue to open the aperture to give step changes in the flow velocity, recording the sensor data at each increment.



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Increase the fan setting until the indicated rotational speed is approximately 2000rpm (allow a few moments for the fan speed to stabilize after making adjustments). Note the air velocity then fully close the outlet aperture.


The aperture will be opened to give increments in the air velocity until the aperture is fully open. Select a flow velocity increment that will give 10-15 separate readings. Open the outlet aperture to give a flow increase approximately equal to the increment chosen, and select the icon again.

Continue to open the aperture to give step changes in the flow velocity, recording the sensor data at each increment, with a final set of readings taken with the aperture fully open.

Set the fan back to 0% and switch it off using the ‘Fan On’ switch on the mimic diagram.

Save the results sheets using ‘Save As...’ from the ‘File’ menu, using a suitable filename for later retrieval such as the equipment code, exercise letter and date.

Results If using results from Experiment No.8 and results at 1000rpm and 2000rpm are available then the predicted discharge, predicted fan total head and predicted power will be useable. Follow the procedure as described below for ‘using a new set of data’.

If using results from Experiment No.8 that do not include both suggested fan speeds then the predicted results calculations given will not be useable. Instead select two sets of data for two different fan rotational speeds. From the first set of data, use the affinity laws to predict the values for fan total pressure and discharge at the second fan rotational speed. Plot these predicted values on the same graph as the measured values at the second fan speed.


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If using a new set of data then the software logs the following variables:


Air Density kg/m: Inlet Velocity Vi m/s Outlet Velocity V2 m/s Discharge Qv m3/s

Fan total pressure

Ptf Pa

Fan power output

Pu W

The software assumes the following constants:

Acceleration due to gravity g m/s2 From the first set of results, plot a graph of predicted fan pressure, and predicted

Power against predicted discharge.

From the second set of results, on a new graph plot the measured fan pressure, measured power and measured efficiency against measured discharge.

Conclusion Compare the predicted results with the measured results. How well do the two graph compare? Do the predicted values match across the full performance range

Inlet Temperature T °C Orifice Pressure P1 Pa Fan Diff. Pressure P2 Pa Fan Setting - % Fan Speed n Hz Mechanical Power (input)

Pm W


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of the fan? If not, then where in the range does the greatest deviation occur? How significant is any deviation compared to other sources of error, such as typical sensor accuracy? If time permits then it is possible to calculate the accuracy of the predicted results using standard statistical formulae.

Discuss the use of the affinity laws for predicting fan performance. What factors could potentially affect the accuracy of the results? Would these sources of error be present in every possible application? Suggest situations in which the use of the affinity laws would be particularly relevant. Give examples of applications in which the affinity laws should not be used, and give reasons for your choices.


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Experiment No. 10 Fan System Characteristics

Objective To obtain the fan characteristic curves for a centrifugal fan operating at constant speed.

Method By taking sensor readings over a range of flow rates at constant fan speed setting, for a series of constant fan speeds.

Equipment Required Armfield FM41 Axial Fan Demonstration Unit Armfield IFD7 Interface Device Compatible PC (not supplied) running Armfield FM41-304 software

Theory System analysis for a fan installation is conducted to select the most suitable fan units and to define their operating points. System analysis involves calculating pressure-capacity curves for the system (including all valves, pipes, fittings etc), and the use of these curves with those of available fans. These system curves are a graphic representation of all possible duty points: the fan total pressure is plotted against discharge from zero fan speed to the expected maximum speed.

Totai Static Pressure


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A typical set of results is shown below.

Volume Flow rata Qv

System capacity curves showing the effect of systems of increasing resistance

Fan characteristic curves illustrate the relationship between pressure, discharge, efficiency and power over a wide range of possible operating conditions, but they do not indicate at which point on the curves the fan will operate. The operating point (or duty point) is found by plotting the fan pressure-discharge curve with system pressure-discharge curve, as in the example below. The intersection of the two curves represents the pressure and discharge that the fan will produce if operated in the given piping system. It will be seen that the optimum operating condition is achieved if this operating point coincides with the maximum point in the efficiency- discharge curve of the fan.


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Equipment Setup Check that the sensor and power leads from the FM41 are connected to the

sockets or the front of the IFD7.

Check that the IFD7 is connected to a suitable mains supply, and that the USB socket on the front is connected using the lead provided to a suitable PC. Check that ne red and green indicator lights on the front of the IFD7 are illuminated.

Run the Armfield FM41-304 software on the PC, and check that the software indicates ‘IFD: OK’ in the bottom right-hand corner of the window.

Switch on the mains supply to the IFD7, and switch on the IFD7 using the power switch the front. Check that the power switch on the IFD7 is illuminated.

On the software mimic diagram screen, select the ‘Fan On’ button to switch

Fan Pressure Capacity Curve

Determining fan operational point (duty point)


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on the FM41.Check that the green watchdog indicator on the mimic screen is illuminated.

Check that the sensor readings on the software screen give sensible values. The air velocity and the pressure readings (which are relative to atmosphere) should be zero when the fan is not moving. Use the ‘Zero’ buttons to zero the pressure sensor reading if required. The temperature should be sensible given the ambient conditions in the room (typically between 15 and 30 °C).

Procedure Read through the experiment before starting, to familiarise yourself with the

procedure. Note that for this experiment the aperture will remain in a fixed position and the fan setting will be used to vary the air velocity. This differs from previous experiments.

Set the fan speed to maximum (100%).

Close the outlet aperture to give significant system resistance, for example % closed. Rename the current results sheet to match the aperture setting.


Using the discharge as a guide, select incremental values for discharge that will give 10-15 individual steps between minimum and maximum velocity.

In the software, reduce the fan setting gradually to reduce the discharge by approximately the increment chosen- the objective is to collect sufficient individual data points across the full range, not to obtain readings at specific values, so it does not matter if the exact discharge increment can be achieved. Allow the flow to stabilise then select the ® icon.

Repeat for the next flow velocity increment, adjusting the fan setting to give the required discharge and selecting the icon to record the data once the settings are correct.

Continue in steps until 0 m3/s discharge is reached. Note that this may not coincide with a fan setting of 0% due to the resistance within the system.



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Select a new aperture setting, for example 1/2 closed. Rename the results sheet to match the setting.

Repeat as before, starting with the fan set to 100% and reducing this in steps, and recording each set of data with the icon.

Additional aperture settings may be investigated if time permits. Sufficient time must be allowed to complete the last part of this procedure, however, and note that it is very difficult to obtain meaningful results with the aperture more than3/4 closed. Remember to create a new results sheet for each set of data, and to rename each sheet to match the setting used before taking any readings at that setting

Now a fan performance curve is required. Create a new results sheet and rename this ‘Fan Performance’.

Fully open the outlet aperture.

Set the fan to 100%. Select the icon. Note this maximum discharge and select a discharge increment that will give 10-15 separate readings between this maximum discharge and minimum (0 m3/s) discharge.

Close the aperture slightly to give approximately the first discharge increment. Allow the system to stabilise then select the icon.

Continue to close the aperture in steps to give a full set of data. Note that there will always be a small discharge reading when reducing the discharge using the aperture, and take care not to twist the aperture beyond the point at which it is fully closed.

After taking the last set of readings, fully open the outlet aperture and switch the fan to standby using the ‘Fan On’ switch on the software. Save the results by selecting

Save As...’ from the ‘File’ menu. Give the results a representative name, such as the equipment code, exercise letter and date.

Results The software logs the following variables:

Inlet Temperature T °C


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Orifice Pressure P1 Pa

Fan Diff. Pressure P2 Pa

Fan Setting - %

Fan Speed n Hz

Mechanical Power (input) pm W

Air Density pair kg/m3

inlet Velocity (V1) m/s

Outlet Velocity (V2) m/s

Discharge (Qv ) m3/s

Fan Total Pressure (ptf ) Pa

Fan Power, output (Pu) W

Fan Efficiency ( gr ) %


The software assumes the following constants:

Acc. due to gravity g m/s2

On the same set of axes, for each set of system characteristic results, plot the fan total pressure ptF against the discharge to produce a set of system curves. On the same axes, for the fan characteristic curve taken in the final part of the experiment, plot the p =. On the second y-axis plot the fan efficiency.


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Conclusion The system characteristic will vary depending on a range of factors such as cross- section. length, diameter and construction material of piping or ducting, bends and curves, valves and vents, and any additional obstructions such as rough joints, control vanes, grills or gratings, and debris. Each aperture setting investigated represents a different system resistance which could have resulted from these kinds of factors.

Examine and describe the graph obtained. What was the effect of increased system resistance (system resistance increases as the aperture is closed) on the shape of the curve obtained?

Note the points at which the system and fan characteristic curves cross, and determine the duty point for each aperture setting investigated. How does this duty point change with increasing system resistance? Using the efficiency curve, determine the aperture setting for which the duty point of the fan best suits the system characteristic.

1435 - uotechnology.edu.iqseptember 14 2014 lab. of fluid machineries electromechanical eng. dept ä...

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