Praktikumsanleitung Engl

7/28/2019 Praktikumsanleitung Engl

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Otto-von-Guericke-University of Magdeburg

Institute of Fluid Dynamics and Thermodynamics

Professorship Thermodynamics and Combustion

Professorship Technical Thermodynamics

LABORATY WORK

Measurement of Heat Transfer Coefficients

by infrared thermography

Contents

1. THE TOPIC / PROBLEM..................................................................................................................................2

2. THE MEASURING PRINCIPLE AND TEST FACILITY............................................................................2

3. THE WAY OF ANALYSIS................................................................................................................................5

4. THE EXPERIMENTS........................................................................................................................................8

4.1 CONNECTIONAND ADJUSTINGTHE INFRARED CAMERA.....................................................................................8

4.2 ADJUSTMENT THE AIRVOLUME FLOW RATE....................................................................................................8

4.3 MORE ADJUSTINGAND MEASURING DATAS.......................................................................................................9

4.3.1 Electrical Power Supply.........................................................................................................................9

4.3.2 More Measuring Datas..........................................................................................................................9

5. EVALUATION OF THE TESTS....................................................................................................................10

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1. The Topic / Problem

The aim of the laboraty work is to use the infrared-technique to determine the heat transfer

coefficients. In case of an air flowed pipe, the local heat transfer coefficients should be

determined after a sudden change of diameter by measuring the outlet surface temperaturewith the help of a infrared-camera.

The essential advantage of the infrared thermography is, its possible to measure the

temperature without a contact and with a high resolution till 0.1 K. Thus quasi on every axial

point of the flowed pipe a temperature could be measure.

The local heat transfer coefficient of flowing air in the pipe is always relating to the

temperature difference between inner pipewall and airflow. In the following comments could

be shown, that the infrared thermography is a very accurate determination for this difference.

Compared with other methods the measuring error reduced.

For the local heat transfer of a turbulent flow in a pipe the VDI-Wrmeatlas [1] give the

following determining equation:

+

+

= 32

z

D

3

11

132

Pr8

7,121

PrRe8

zNu

(1)

with

( ) 25,1Re10log8,1= (2)

This equation shows the influence of the pipe length z and of the diameter D on the local heat

transfer. This classic correlation strictly applies for the sudden contraction in a pipe base. But

its not to use by increasing the diameter. The measurements arranged while the laboraty work

will show that the measured heat transfer coefficient and the Nusselt number respectively is a

multiple higher than the calculated values, which are determined by the classic equation

(equation 1).

2. The Measuring Principle and Test Facility

The investigations should be done for a sudden pipe extension. Figure 1 shows a principle

plan of such a pipe extension. At first the air flows in a pipe with the diameter d. Than a

sudden pipe extension happens till the diameter D. As a result inside the pipe turbulence and

backflow are generated, which influenced the boundary layer near the wall. Now the local

heat transfer through the pipe coordinate z in the range z = 0 z = 27 D have to be measured.

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The plan of the test facility is shown byfigure 2. The test facility is feeded by

compressed air. With a modulating valve

connected with pressure on a pressure

reducer its possible to set up the volume

flow rate. On this way volume flow rates

in a range 0.2 300 m/h are adjustable.

For measuring the volume flow rates are

used rotameter. The rather test section is

vertical arranged and consists of an acryl

feeder pipe with a precipitation stainless

steel pipe. On the transition range(between acryl and stainless steel pipe) is

the sudden extension of the cross section.

The stainless steel pipe isolated with a 30

mm thick mineral wool and the air flows

downstairs from the top.

Figure 1: Principle profile amplification

For determination the local heat transfer coefficient between the stainless steel pipe and the

flowing air in it (figure 3) an electric direct current was made to pass through the tube wall

(till 6 V and 400 A). So a constant heat flux to 20 kW/m is generated. The air flows from top

to bottom and is warmed up thereby. The surface temperature of the pipe under the insulation

has to be measured. When the steady state is reached, an axial segment of mineral wool is

removed for a short period of time during which the outer wall temperatures are recorded. The

infrared camera can take up pictures at intervals of 0.15 seconds. In this way a measurement

period of 5 seconds is sufficient. With this developed measuring method its possible to

contain the external heat losses at air flowed pipes on a passable dimension and also to inhibit

the influence of free convection and radiation, which cant calibrate because of the changing

axial temperature profile. Simultaneous its easy to control the emission ratio of the coated

pipe surface by comparison measurement with thermocouples after a few test series. The

maximum pipe wall temperature is regulated by the direct current, there 200 C dont have to

be overrun. The temperature difference between pipe wall and air lies in a range 60 K 140

K.

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D

d

Z


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Figure 2: Plan of the test plant

Figure 3: Detail measuring route

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3. The Way of Analysis

At first a defined air volume rate has to be adjusted on the plant. For this air volume rate the

heat transfer has to be investigate. After adjusting steady state the surface temperature of the

stainless tube is to measure with the infrared camera.In order to calculate the local heat transfer coefficients, the tube length is divided into finite

sections of length z beginning from the location of the cross-sectional change. The deter-

mination of the local heat transfer coefficient is then based on the energy balance for each

discrete section.

Figure 4: Dividing a measuring pipe into section length

For a discrete section j the convective heat flux jQ is equal to the input electric power Pel j

minus the heat flux of losses to the ambient QL,j and minus the heat flux by axial conduction

QC,j.

jCjLjelj QQPQ ,,, = . (3)

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The local heat transfer coefficient*

j is defined by the equation:

[ ]j,Kaljjj zDQ = , (4)

using the bulk temperature bulk,j of the fluid. The bulk temperature is determined from the

enegy balance of a discrete section.

The internal tube wall temperature j is assumed to be equal to the measured external tube

wall temperature since the differences are only in the range of 0.05 to 0.5 K under the given

experimental conditions with air flow.

With equation. (3) and (4) the j

is calculated by:

[ ]j,kaljj,Lj,Vj,el

jzD

QQP

=

. (5)

The electric power j,elP is given by: zIP j,el

2j,el =

, (6)

where I is the amperage, *el,j is the specific electric resistance in /m of the stainless steel

for jth section j and z is the length of the discrete section. The specific electric resistance for

the stainless steel used was determined experimentally as a function of the temperature. It was

found the correlation *el,j =f().

The convection and radiation on the external surface of tube insulation lead to lower heat

losses. By the chosen insulation thickness (30 mm) the heat loss decreases till 80 % in

comparison to a not insulated tube. The generated heat loss is under steady state conditions a

function of the external tube surface temperature j and the ambient air temperature a.

[ ]ajjjL kzsDQ += )2(, (7)

The over all heat transfer coefficient kj includes conduction through the insulation and free

convection and radiation on its outer surface. This coefficient was determined experimentally

in calibration test as a function ofj and z.

in which the stainless steel filled with insulation material and so the inner convection could

be eliminated. For a Reynolds number of 10 000 the heat loss j,VQ is nearly 20 % of the

electric power and decreases under 5 % for Re = 100 000.

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The axial heat conduction jCQ , depends on the axial temperature profile of the stainless

steel tube. This influence is very low in comparison with the electric power and can neglected.

In the case of contraction the Reynolds number is defined by diameter D, in the case of

extension it could be defined with the diameter d. The viscosity of the flowing medium has to

be insert on the area of cross-section changing (z = 0). Therefore the Reynolds number is alsorelated top the cross-section changing.

0z,Fluid

dd

dwRe

=

= ,

0z,Fluid

DD

DwRe

=

= . (8)

In both cases (extension and contraction) the Nusselt number

j

Nu is defined with the

diameter D of the stainless steel tube:

z,Fluid

j

j

DNu

=

, (9)

because the heat transfer in this tube is observed. The heat conductivity of the flowing

medium, which is determined by the temperature on z, must be inserting by z.

The Nusselt number Nuj* calculated from the experimental results are valid for the case of

heating-up the gas (j>bulk) through the tube wall, since the temperature of the tube wall is 60to 140 K higher than the air temperature. A transfer to the case of isothermal wall (TbulkTj) ismade with the Gnielinski's approximation [7]:

45,0

)//( jbulk TTNuNu= . (10)

The Nusselt numbers, which determined on this way, are the basis for comparison with classic

solutions like in the VDI-Wrmeatlas.

The error of the calculated heat transfer coefficients is a function of many measuring values

and could be determined with law of propagation. By experiments with air the error in the

turbulent area is under 5 %.

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4. The Experiments

4.1 Connection and Adjusting the Infrared Camera

The camera is to assembly on a stand and the object lense has to show to the tube . Thedistance between camera and tube is to set by hand. A temperature is assigned every pixel

point of the infrared image with the coordinate (x, y). By marking points on the flowed tube

the vertical pixel points y could be assigned a axial tube length z.

Marked point 1: Marking by wire

Pixel position on the display: y1

Distance from wire to cross-section changing: z1

Marked point 2: Marking by wire (under point 1)

Pixel position on the display: y2

Distance from wire to cross-section changing: z2

Scale factor (gradient): m = (z2-z1)/(y2-y1) in [cm/pixel point]

Absolute member a from linear equation: a= z2- m y2=z1-m y1 in [cm]

Equation for the calculation the tube length z with pixel points y as input data:

z = m y +a (11)

The height of the camera is to adjust that ah useful. Therefore the pixel

number y, where the camera has to read out the temperature, could be determined.

h = m y + a

y>(h-a) / m.

After steady state the segment insulation is to removed and the surface temperature of the pipe

is to measure by the infrared camera.

4.2 Adjustment the Air Volume Flow Rate

In the case of cross-section extension the tests are to do. The characteristic diameters are:

d= 8 mm

D=29 mm

The thickness of wall is 0,5 mm.

The local heat transfer coefficients have to be investigated for 3 different air volume flows.

Though this 3 different volume flows Vindication must adjusted and the fitted air pressures pV in

front of the rotameter must be written.

Vindication volume flow on rotameter [m3/h]pV - pressure in front of the rotameter [bar] (overpressure against atmospheric pressure)

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4.3 More Adjusting and Measuring Datas

4.3.1 Electrical Power Supply

The tube should be flowed by a direct current. On the electrical power supply the amperage I

must be adjusted so that a tube wall temperature of 200C on the end of pipe not overrun.

Measuring datas: voltage Uamperage I

4.3.2 More Measuring Datas

Temperature of room airroom air

Temperature of ambient (all solid walls) a

Temperature of pressure airpressure air

Temperature of the collar on inlet collar

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5. Evaluation of the Tests

Assignment of tasks:

Calculate the Reynolds number for the 3 adjusted volume flows related to the diameter d

and diameter D. Therefore the viscosity of air on the place of cross-section extension(z=0) must insert.

Give the transformation equation (11) based on the adjusting of infrared camera. So you

have to convert the vertical pixel point y into axial length z with unit cm.

After readout surface temperature area, for the pixel points with same y a mean arithmetic

temperature has to generate. These temperatures have to be illustrated in a diagram

depended on axis length z for 3 volume flows and discussed accordingly.

For all infrared measurements the camera has to adjusted on a emission ratio of =0.92. In

calibration measurements a dependence of emission ratio on surface temperature could be

investigated:

=0,9809 -9,28*10-4 +1,93*10-62 + 2,18*10-93

Which of the readout (by infrared camera) temperature conforms with the really

temperature of pipe surface? Discuss in which direction the other readout temperatures

have to be corrected!

Approximate the measured surface temperatures =f(z) in excel by a polynom departure

(=A0 + A1*z + A2*z2 + A3*z3 + .......). The degree of polynom is to choose for a good

approximation. If there is no good approximation with one polynom, so the measured tube

section is to be selected in 2 or 3 parts. For this parts separated polynoms have to be

determined. Determine polynoms for all 3 volume flows and illustrate this polynoms

together with measuring values in a diagram!

Calculate with excel in sections the heat transfer coefficient * and the Nusselt number

Nu*, which result from the measuring values, and illustrate this in a diagram as a function

of dimensionless axis length z/D. The calculation should be simplified under following

estimates:

Neglect the axis heat conduction (QL=0).

In the area of the upper collar h=1.6 cm is no heat transfer.

For the calculation the section length z=0.5 cm is to choose.

The section1 should start by z > 2.0 cm.

For all sections z is to calculate with cP=1,007 kJ/kg K.

For the calculation the following approach is to use:

Specific electrical resistance el* [Ohm/m]:

el* = 0,013531 + 17,339*10-6 -6,59*10-92

Over all heat transfer coefficient k [W/mK] of calibration measurements

k=C0 + C1*

if z


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D2=0,479 E2=-0,00184

D3=-0,0493 E3=0,000215

D4=0,00198 E4=-0,00000932

if z=>7,85 than: C0=1,8232

C1=0,0024

Heat conductivity airLuft [W/mK]

air = 24,343*10-3 +71,323*10-6L -19,237*10

-9L2 +4,536*10-12L

3

The results of calculation have to be discussed!

Annex: Paper of measuring data

Measuring values emission rate

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Measuring Data - Practical Pipe Inlet flow

(Paper 1)

I. Adjusted values on infrared camera

Adjusted values of camerah=1.6 cm (visual covered height in inlet section)

marked point on pipe Pixel point in camera Distance of marked point to

cross-section changing

Marked point 1 y1= 37 z1= 1.6 + 1 = 2.6

Marked point 2 y2= 145 z2= 1.6 + 24.5 = 26.1

Calibration of basic units on camera under:

Image>Settings>Units

Abstand (Distance): m

Temperatur (Temperature): C

Calibration of objects parameter on camera under:

Image>Settings>Objekt Parameter

Parameter Dimension Value

Emissionsgrad

(Emissivity)

- 0.9

Abstand Kamera - Objekt

(Distance)

M 1.8

Temp. d. Umgebung (Wand)

(Ambient Temperature)

C 20

Luft Temperatur(Atmospheric Temperature)

C 20

Relative Raumfeuchte

(Relative humidity)

% 50

Name of saved picture data:

II. Measuring volume flow

Measuring apparat: Krohne Typ FA20/air; 4 - 40Nm3/h

Physical data noted on flowmeter

Paramter Dimension Value

Temperature C 20

Pressure bar 1.013

Density kg/m3 (bei =C) 1.293

Viscosity mPa 0.0181

Compr. - Factor - 1

Min. volume flow Q-Min m3/h 4

Max. volume flow Q-Max m3/h 40

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Measuring Data - Practical Pipe Inlet flow

(Paper 2)

Tyopical data from apparat for FA20; 4-40 Nm3

/hType N

Konus dimension 40

Konus N41.13

Suspension body shape AIII 41

Suspension body material Steatit

Display type mm+Pl-Skal

Measuring values

Parameter Dimension Value

Really air temperature before going

into flowmeter

C 24.5

Pressure in front of the flowmeter pV

(as overpressure)

bar 0.16

Notified volume flow VAnzeige on

flowmeter

Nm3/h 7

With the program "KroValCal 3.3.0" calculated volume flow under normal conditions (STP);

(0C, 101325 Pa):

V0 air= 7.501 Nm3

/h

Mair= V0 air x 1,29 kg/m3= 9.698 kg air/ h

Same procedures for each case:

Case (Nm3/h) 7 12 18

Mair (kg/m3) 9.698 18.994 35.885

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Measuring values emission rate of kiln lacquer coated measuring pipe (Di=29 mm)

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0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 50 100 150 200

Oberflchentemperatur [C]

Em

issionsgrad

[-]

Messwerte

Polynomisch

(Messwerte)

= 0,9809 - 9,28*10-4

+ 1,93*10-6

2

+ 2,18*10

-9

3

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Praktikumsanleitung Engl