CHAPTER the slope of the boiling curve. In this

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 5

RESULTS AND DISCUSSION

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5.1  PERFORMANCE OF PLAIN COPPER SURFACE WITH DI
WATER

 

Figure 5.1: Pool boiling curve for
plain copper surface

Figure
5.1 explains the pool boiling characteristics of plain copper surface with DI
water. Boiling incipience occurs at about 102.675°C. The curve shows that at
lower wall superheat, the heat transfer from the test surface was very less.
The heat flux varied from 1.5 W/cm2 to 3.9 W/cm2 as the
wall superheat varied from 2.7°C to 5.1°C. After the temperature reaches above
5.1°C of wall superheat, the rate of bubble formation increases. This increases
the heat transfer from the surface and also the slope of the boiling curve. In
this region of the curve, for a small change in wall superheat a large change
in heat flux takes place. The critical heat flux is obtained at a wall
superheat of 27.9°C and its value was recorded to be 111.64 W/cm2.
The boiling HTC was found out to be 3.99 W/cm2 °C.

 

 

Figure 5.2: Boiling HTC versus Heat
flux curve

5.2 
PERFORMANCE OF SANDPAPER ROUGHENED COPPER SURFACE

Figure 5.3: Boiling curve for
sandpaper roughened surface

Figure
5.2 explains the pool boiling characteristics of surface roughened with
sandpaper. In this boiling incipience occurs at 102.1°C. Compared with plain
copper surface, the curve clearly shows an enhancement in heat flux. The reason
for enhancement of heat flux is because of the increase in surface roughness,
which increases the number of nucleation site and also the rate of bubble
formation. The CHF in this case was obtained at a much lower wall superheat of
26.05°C and value was obtained as 129.675 W/cm2. The maximum value
of boiling HTC was obtained as 4.98 W/cm2°C. The experimental
observations showed that although there is enhancement in CHF but it is
obtained at a lower wall superheat. Thus, it can be suggested that the slope of
the boiling curve has increased and the lower wall superheat indicates that the
whole boiling curve has shifted to the left.

Figure 5.4: Boiling HTC versus Heat
flux for sandpaper roughened surface

5.3  PERFORMANCE OF ACID ETCHED COPPER SURFACE

Figure 5.5: Boiling HTC versus Heat
flux for acid etched surface

Figure 5.3: Boiling curve for acid
etched surface

The
above boiling curve was observed for acid etched copper surface. In this case,
the boiling incipience occurred at 102.6°C. Again, the observations showed that
there is enhancement in heat flux compared to plain as well as sandpaper
roughened copper surface. It is a well known fact that the heat transfer
decreases at higher values of wall superheat due to the formation of a stable
film of vapour over the heating surface. In acid etching, the surface is made
uneven so that there is less chance of formation of stable film of vapour. As
the surface becomes uneven, the surface area also increases and thereby heat
transfer enhances. The CHF was obtained at a wall superheat of 25.425°C, which
again is lower than that obtained for plain and sandpaper roughened copper
surface and the CHF was obtained as 135.04 W/cm2. The highest value
of CHF was obtained with acid etching and same was the case with boiling HTC.
The value of boiling HTC obtained with acid etched surface was 5.31 W/cm2°C.
The comparison between the CHF obtained for each test surface is done in the
next section.

 

 

 

 

5.4  COMPARISON
OF CHF FOR DIFFERENT SURFACES

Figure 5.4: Comparison of CHF for
various surfaces

The
result in the above figure illustrates the enhancement of CHF with surface
modifications. It can be seen that the values of CHF for the plain, sandpaper
roughened and acid etched surface are 111.64, 129.675 and 135.04 W/cm2
respectively. Thus, the tailoring of surface is a good way to enhance the heat
transfer rate in boiling and also the CHF.