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CHAPTER 4

EXPERIMENTAL PROCEDURE

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The
experiment consisted of a series of predetermined steps to obtain the results.
The entire procedure is outlined as shown below:

4.1    LEAKAGE TEST

The
boiling chamber is initially filled with DI water at 1 atm pressure to carry out
a leakage test. After it is ensured that the setup is leak free, further
experimental work is  carried out.

4.2    DEGASSING

500
ml of DI water is filled in the boiling chamber. It is then boiled with the
help of auxiliary heater at moderate heat flux for about 20 minutes. This is
done to remove any non condensable gases, which went out of the boiling chamber
through the opening present in the reflux condenser.

Figure 4.1: Degassing

 

4.3    SURFACE PREPARATION

The
test surfaces were prepared according to the experimental requirements as
follows:

4.3.1   Sandpaper roughened surface

The
surface is roughened with sandpaper P60. The test specimen is placed on a hard
surface and sandpaper is moved on the top surface of the test specimen in both
vertical and horizontal direction, 25 times in each direction. The test surface
is then cleaned with a soft cloth to remove any fine particles that may be
present.

Figure 4.2: Sandpaper roughened surface

4.3.2    Acid etched surface

The
test specimen is prepared using copper. The top surface of the test specimen is
polished using very fine grade emery paper. It is then cleaned with soft cloth.
The etchant is then prepared and the top surface of the test specimen is dipped
in the etchant for sufficient time so that proper etching takes place.

Material used

Copper

Etchant

50% Nitric acid by volume

Distilled water

50% by volume

Etching time

5 minutes

Table 4.1: Specifications of acid
etching

Figure 4.3: Acid etched surface

4.4    TEST SURFACE INSTALLATION

The
test surface is installed into the central opening of the boiling chamber
bottom plate. Any gap between the test specimen and teflon insulation and
between the test specimen and heater block is properly sealed using gasket maker
red sealant. The sealant is left to dry in atmospheric air for three hours.

4.5    DATA ACQUISITION

The
power supply from the auto transformer is varied in steps of 5 V and the
readings of the data logger for different thermocouples were recorded after
every 10 minutes. When the surface temperature reached close to 90°C, the power
supply is varied in steps of 3 V to prevent the burnout of cartridge heaters.
The system is allowed to reach the state of critical heat flux which is
characterized by an abrupt rise in surface temperature and also a visible layer
of vapour covering the boiling surface.

4.6    CALCULATIONS

The
figure below shows the position of the thermocouples in the test specimen.

Figure 4.2: Position of
thermocouples

The
temperature gradient, dT/dx is calculated by using three point backward Taylor
series approximation.

where
T1= Top thermocouple reading in °C

          T2= Middle thermocouple
reading in °C

          T3= Bottom thermocouple
reading in °C

          ?x= distance between each of the
thermocouple in mm.

The
heat flux is calculated using Fourier 1-D heat conduction equation which is
given by:

Heat
flux,

 

where

= temperature gradient in °C/m

           KCu = Thermal
conductivity of copper in W/mK

The
surface temperature is obtained using the equation given below:

                        Surface temperature,

where x1=
distance between the boiling surface and top thermocouple in mm.

The heat transfer
coefficient (HTC) is calculated using Newton’s law of cooling as shown below:

                                                
HTC,

where
TW = Bulk temperature of DI water in °C.

 

 

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