2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 133 -
TABLE
I
E
XPERIMENTAL CONDITIONS
Working fluid
Test section
Reynolds number
water temperature(ºC)
water
Multiport-minichannel
d
h
= 1.2 mm ,
n
= 24
450 to 1800
20
,25, and 30
TABLE
II
U
NCERTAINTY
Temperature
Pressure transducer
Mass flow rate of refrigerant
±0.1 ºC
±0.1 kPa/m
±0.1 % of full scale
IV. D
ATA REDUCTION
The data reduction of the measured results is
summarized in the following procedures. The heat
transfer rate from the DC heater to the water in the test
section:
VI Q
(1)
The water heat transfer rate can be calculated as
follows:
,
,
(
)
p w out
w in
Q mC T T
(2)
where
m
is the mass flow rate of water,
p
C
is the
specific heat of water, and
in
T
and
out
T
are inlet and outlet
water temperatures, respectively.
The heat flux of the test section can be received from
s
Q q
A
cc
(3)
where
s
A
is the outer surface area of the test section
From the definition of the local heat transfer
coefficient, it is defined as the ratio of the local heat flux
and the local temperature differences of wall surface and
working fluid, therefore the local heat transfer coefficient
can be calculated from as follows:
,
,
"
(
)
local
wall i
w i
q
h
T T
(4)
where
l
A
is the local surface area of multiport tube,
,
w l
T
is the local water temperature,
,
s l
T
is the local surface
wall temperature of multiport tube, and
l
q
cc
is the local
heat flux on multiport tube.
The average heat transfer coefficient (h
avg
) can be
calculated as follows:
,
,
"
(
)
avg
wall avg w avg
q
h
T
T
(5)
where
h
avg
is the heat transfer coefficient averaged on
the entire 95 mm long tube. A total of ten thermocouples
are soldered on the outer wall of the tube at the middle,
left, and right sides (as presented in Fig. 2 ) to measure
the wall temperatures (
T
wall
). The average wall surface
temperatures (
T
wall
,
avg
) of the test section are taken as the
arithmetic mean of the 10 measurement positions as
follows;
10
,
,
1
i
wall avg
wall i
i
T
T
¦
(6)
V. R
ESULTS AND DISCUSSION
A. Cooling capacity
The cooling capacity of water flowing through the
multiport minichannel is plotted in terms of the variation
of Reynolds number with the heat fluxes difference as
presented in Fig. 3. The results are obtained at five
different of heat flux in the ranges of 6.3 -35.1 kW/m
2
,
and water temperatures 20 °C (Fig. 3a) and 25°C (Fig.
3b). As indicated in Fig. 3, the cooling capacity is
strongly affected with heat flux, it increase with
increasing heat flux. Figure 3 also clearly shows that
mass flux has a significant effect on the cooling capacity.
When the results at different water temperature are
compared, it is found that the cooling capacity decreases
with increasing the water temperature.
Reynolds number
400
600
800 1000 1200 1400 1600 1800
Cooling capacity (W)
20
40
60
80
100
120
140
160
180
200
6.3-16.8
13.0-21.3
17.5-23.5
21.8-28.5
24.7-35.1
Heat Flux : kW/m 2 D h = 1.2 mm , n = 24 channel
T w = 20 o C
(a)
Reynolds number
400 600 800 1000 1200 1400 1600 1800 2000
Cooling capacity (W)
20
40
60
80
100
120
140
160
180
200
7.5-10.5
10.9-16.1
17.7-21.4
19.0-23.5
24.4-30.4
Heat Flux : kW/m 2 D h = 1.2 mm , n = 24 channel
T w = 25 o C
(b)
Fig. 3. The cooling capacity for heat flux for water temperature for (A)
20°C and (B) 25
°
C.
B. Local heat transfer coefficient
As shown in Fig. 4, the test section is soldered with
three multiport mininichannel: left, middle, and right
